Search

Surface topography is a tool to endow biomaterials with bioactive properties. However, the large number of possible designs makes it challenging to find the optimal surface structure to induce a specific cell response. The TopoChip platform is currently the largest collection of topographies with 2176 in silico designed microtopographies. Still, it is exploring only a small part of the design space due to design algorithm limitations and the surface engineering strategy. Inspired by the diversity of natural surfaces, it is assessed as to what extent the topographical design space and consequently the resulting cellular responses can be expanded using natural surfaces. To this end, 26 plant and insect surfaces are replicated in polystyrene and their surface properties are quantified using white light interferometry. Through machine-learning algorithms, it is demonstrated that natural surfaces extend the design space of the TopoChip, which coincides with distinct morphological and focal adhesion profiles in mesenchymal stem cells (MSCs) and Pseudomonas aeruginosa colonization. Furthermore, differentiation experiments reveal the strong potential of the holy lotus to improve osteogenesis in MSCs. In the future, the design algorithms will be trained with the results obtained by natural surface imprint experiments to explore the bioactive properties of novel surface topographies., (© 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Human mesenchymal stem cells (hMSCs) are widely represented in regenerative medicine clinical strategies due to their compatibility with autologous implantation. Effective bone regeneration involves crosstalk between macrophages and hMSCs, with macrophages playing a key role in the recruitment and differentiation of hMSCs. However, engineered biomaterials able to simultaneously direct hMSC fate and modulate macrophage phenotype have not yet been identified. A novel combinatorial chemistry-topography screening platform, the ChemoTopoChip, is used here to identify materials suitable for bone regeneration by screening 1008 combinations in each experiment for human immortalized mesenchymal stem cell (hiMSCs) and human macrophage response. The osteoinduction achieved in hiMSCs cultured on the "hit" materials in basal media is comparable to that seen when cells are cultured in osteogenic media, illustrating that these materials offer a materials-induced alternative to osteo-inductive supplements in bone-regeneration. Some of these same chemistry-microtopography combinations also exhibit immunomodulatory stimuli, polarizing macrophages towards a pro-healing phenotype. Maximum control of cell response is achieved when both chemistry and topography are recruited to instruct the required cell phenotype, combining synergistically. The large combinatorial library allows us for the first time to probe the relative cell-instructive roles of microtopography and material chemistry which we find to provide similar ranges of cell modulation for both cues. Machine learning is used to generate structure-activity relationships that identify key chemical and topographical features enhancing the response of both cell types, providing a basis for a better understanding of cell response to micro topographically patterned polymers., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Learning rules by which cell shape impacts cell function would enable control of cell physiology and fate in medical applications, particularly, on the interface of cells and material of the implants. We defined the phenotypic response of human bone marrow-derived mesenchymal stem cells (hMSCs) to 2176 randomly generated surface topographies by probing basic functions such as migration, proliferation, protein synthesis, apoptosis, and differentiation using quantitative image analysis. Clustering the surfaces into 28 archetypical cell shapes, we found a very strict correlation between cell shape and physiological response and selected seven cell shapes to describe the molecular mechanism leading to phenotypic diversity. Transcriptomics analysis revealed a tight link between cell shape, molecular signatures, and phenotype. For instance, proliferation is strongly reduced in cells with limited spreading, resulting in down-regulation of genes involved in the G2/M cycle and subsequent quiescence, whereas cells with large filopodia are related to activation of early response genes and inhibition of the osteogenic process. In this paper we were aiming to identify a universal set of genes that regulate the material-induced phenotypical response of human mesenchymal stem cells. This will allow designing implants that can actively regulate cellular, molecular signalling through cell shape. Here we are proposing an approach to tackle this question.

We previously found that surface topographies induce the expression of the Scxa gene, encoding Scleraxis in tenocytes. Because Scxa is a TGF-β responsive gene, we investigated the link between mechanotransduction and TGF-β signaling. We discovered that mesenchymal stem cells exposed to both micro-topographies and TGF-β2 display synergistic induction of SMAD phosphorylation and transcription of the TGF-β target genes SCX, a-SMA, and SOX9. Pharmacological perturbations revealed that Rho/ROCK/SRF signaling is required for this synergistic response. We further found an activation of the early response genes SRF and EGR1 during the early adaptation phase on micro-topographies, which coincided with higher expression of the TGF-β type-II receptor gene. Of interest, PKC activators Prostratin and Ingenol-3, known for inducing actin reorganization and activation of serum response elements, were able to mimic the topography-induced TGF-β response. These findings provide novel insights into the convergence of mechanobiology and TGF-β signaling, which can lead to improved culture protocols and therapeutic applications., (Copyright © 2020. Published by Elsevier Ltd.)

Tissue engineered constructs (TECs) based on spheroids of bone marrow mesenchymal stromal cells (BM-MSCs) combined with calcium phosphate microparticles and enveloped in a platelet-rich plasma hydrogel showed that aggregation of MSCs improves their ectopic bone formation potential. The stromal vascular fraction (SVF) and adipose-derived MSCs (ASCs) have been recognized as an interesting MSC source for bone tissue engineering, but their ectopic bone formation is limited. We investigated whether aggregation of ASCs could similarly improve ectopic bone formation by ASCs and SVF cells. The formation of aggregates with BM-MSCs, ASCs and SVF cells was carried out and gene expression was analysed for osteogenic, chondrogenic and vasculogenic genes in vitro. Ectopic bone formation was evaluated after implantation of TECs in immunodeficient mice with six conditions: TECs with ASCs, TECs with BM-MSC, TECs with SVF cells (with and without rhBMP2), no cells and no cells with rhBMP2. BM-MSCs showed consistent compact spheroid formation, ASCs to a lesser extent and SVF showed poor spheroid formation. Aggregation of ASCs induced a significant upregulation of the expression of osteogenic markers like alkaline phosphatase and collagen type I, as compared with un-aggregated ASCs. In vivo, ASC and SVF cells both generated ectopic bone in the absence of added morphogenetic proteins. The highest incidence of bone formation was seen with BM-MSCs (7/9) followed by SVF + rhBMP2 (4/9) and no cells + rhBMP2 (2/9). Aggregation can improve ectopic bone tissue formation by adipose-derived cells, but is less efficient than rhBMP2. A combination of both factors should now be tested to investigate an additive effect., (Copyright © 2017 John Wiley & Sons, Ltd.)

Chemical and mechanical cues are well-established influencers of in vitro chondrogenic differentiation of ATDC5 cells. Here, we investigate the role of topographical cues in this differentiation process, a study not been explored before. Previously, using a library of surface micro-topographies we found some distinct patterns that induced alkaline phosphatase (ALP) production in human mesenchymal stromal cells. ALP is also a marker for hypertrophy, the end stage of chondrogenic differentiation preceding bone formation. Thus, we hypothesized that these patterns could influence end-stage chondrogenic differentiation of ATDC5 cells. In this study, we randomly selected seven topographies among the ALP influencing hits. Cells grown on these surfaces displayed varying nuclear shape and actin filament structure. When stimulated with insulin-transferrin-selenium (ITS) medium, nodule formation occurred and in some cases showed alignment to the topographical patterns. Gene expression analysis of cells growing on topographical surfaces in the presence of ITS medium revealed a downregulation of early markers and upregulation of late markers of chondrogenic differentiation compared to cells grown on a flat surface. In conclusion, we demonstrated that surface topography in addition to other cues can promote hypertrophic differentiation suitable for bone tissue engineering.

Mesenchymal stromal cells (MSCs) are being experimentally tested in several biological systems and clinical settings with the aim of verifying possible therapeutic effects for a variety of indications. MSCs are also known to be heterogeneous populations, with phenotypic and functional features that depend heavily on the individual donor, the harvest site, and the culture conditions. In the context of this multidimensional complexity, a recurrent question is whether it is feasible to produce MSC batches as "standard" therapeutics, possibly within scalable manufacturing systems. Here, we provide a short overview of the literature on different culture methods for MSCs, including those employing innovative technologies, and of some typically assessed functional features (e.g., growth, senescence, genomic stability, clonogenicity, etc.). We then offer our perspective of a roadmap on how to identify and refine manufacturing systems for MSCs intended for specific clinical indications. We submit that the vision of producing MSCs according to a unique standard, although commercially attractive, cannot yet be scientifically substantiated. Instead, efforts should be concentrated on standardizing methods for characterization of MSCs generated by different groups, possibly covering a vast gamut of functionalities. Such assessments, combined with hypotheses on the therapeutic mode of action and associated clinical data, should ultimately allow definition of in-process controls and measurable release criteria for MSC manufacturing. These will have to be validated as predictive of potency in suitable pre-clinical models and of therapeutic efficacy in patients., (Copyright © 2016 International Society for Cellular Therapy. Published by Elsevier Inc. All rights reserved.)

Modular tissue engineering is a strategy to create scalable, self-assembling, three-dimensional (3D) tissue constructs. This strategy was used to deliver endothelial-like cells derived from bone marrow mesenchymal stromal cells (EL-MSCs) to locally induce vascularization. First, tissue engineered modules were formed, comprising EL-MSCs and collagen-based cylinders. Seven days of module culture in a microfluidic chamber under continuous flow resulted in the formation of interstices, formed by random packing of the modules, which served as channels and were lined by the EL-MSCs. We observed maintenance of the endothelial phenotype of the EL-MSCs, as demonstrated by CD31 staining, and the cells proliferated well. Next, collagen modules covered with EL-MSCs, with or without embedded MSCs, were implanted subcutaneously in immune-compromised SCID/Bg mice. After 7 days, CD31-positive vessels were observed in the samples. These data demonstrate the feasibility of EL-MSCs coated collagen module as a strategy to locally stimulate angiogenesis and vasculogenesis. Copyright © 2013 John Wiley & Sons, Ltd., (Copyright © 2013 John Wiley & Sons, Ltd.)

The combination of scaffolds and mesenchymal stromal cells (MSCs) is a promising approach in bone tissue engineering (BTE). Knowledge on the survival, outgrowth and bone-forming capacity of MSCs in vivo is limited. Bioluminescence imaging (BLI), histomorphometry and immunohistochemistry were combined to study the fate of gene-marked goat and human MSCs (gMSCs, hMSCs) on scaffolds with different osteoinductive properties. Luciferase-GFP-labelled MSCs were seeded on hydroxyapatite (HA) or β-tricalcium phosphate (TCP), cultured for 7 days in vitro in osteogenic medium, implanted subcutaneously in immunodeficient mice and monitored with BLI for 6 weeks. The constructs were retrieved and processed for histomorphometry and detection of luciferase-positive cells (LPCs). For gMSCs, BLI revealed doubling of signal after 1 week, declining to 60% of input after 3 weeks and remaining constant until week 6. hMSCs showed a constant decrease of BLI signal to 25% of input, indicating no further expansion. Bone formation of gMSCs was two-fold higher on TCP than HA. hMSCs and gMSCs control samples produced equal amounts of bone on TCP. Upon transduction, there was a four-fold reduction in bone formation compared with untransduced hMSCs, and no bone was formed on HA. LPCs were detected at day 14, but were much less frequent at day 42. Striking differences were observed in spatial distribution. MSCs in TCP were found to be aligned and interconnected on the surface but were scattered in an unstructured fashion in HA. In conclusion, the spatial distribution of MSCs on the scaffold is critical for cell-scaffold-based BTE., (Copyright © 2012 John Wiley & Sons, Ltd.)

While subcutaneous tissue has been proposed as a clinically relevant site for pancreatic islet transplantation, a major issue of concern remains, which is its poor vascular state. In an effort to overcome this limitation, we present an efficient and reproducible method to form human composite islets (CIs) with proangiogenic cell types in a controlled manner using nonadherent agarose microwell templates. In this study, we assessed the three-dimensional structure, function, and angiogenic potential of human CIs with human mesenchymal stromal cells (hMSCs), with or without human umbilical vein endothelial cells (HUVECs), and preconditioned hMSCs (PC-hMSCs) in EGM-2 under shear stress. Distinct cellular rearrangements could be observed in CIs, but islet functionality was maintained. In vitro angiogenesis assays found significantly enhanced sprout formation in case of CIs. In particular, the number of sprouts emanating from CIs with PC-hMSCs was significantly increased compared to other conditions. Subsequent in vivo assessment confirmed the proangiogenic potential of CIs. However, in contrast to our in vitro angiogenesis assays, CIs with hMSCs and HUVECs exhibited a higher in vivo angiogenic potential compared to control islets or islets combined with hMSCs or PC-hMSCs. These findings highlight the importance and necessity of verifying in vitro studies with in vivo models to reliably predict, in this case, revascularization outcomes. Regardless, we demonstrate here the therapeutic potential of CIs with proangiogenic support cells to enhance islet revascularization at a clinically relevant, although poorly vascularized, transplantation site.

The ability of human mesenchymal stromal/stem cells (hMSCs) to differentiate into various mesenchymal cell lineages makes them a promising cell source for the use in tissue repair strategies. Since the differentiation potential of hMSCs differs between donors, it is necessary to establish biomarkers for the identification of donors with high differentiation potential. In this study, we show that microRNA (miRNA) expression levels are effective for distinguishing donors with high differentiation potential from low differentiation potential. Twenty hMSC donors were initially tested for marker expression and differentiation potential. In particular, the chondrogenic differentiation potential was evaluated on the basis of histological matrix formation, mRNA expression levels of chondrogenic marker genes, and quantitative glycosaminoglycan deposition. Three donors out of twenty were identified as donors with high chondrogenic potential, whereas nine showed moderate and eight showed low chondrogenic potential. Expression profiles of miRNAs involved in chondrogenesis and cartilage homeostasis were used for the distinction between high-performance hMSCs and low-performance hMSCs. Global mRNA expression profiles of the donors before the onset of chondrogenic differentiation revealed minor differences in gene expression between low and high chondrogenic performers. However, analysis of miRNA expression during a 7-day differentiation period identified miR-210 and miR-630 as positive regulators of chondrogenesis. In contrast, miR-181 and miR-34a, which are negative regulators of chondrogenesis, were upregulated during differentiation in low-performing donors. In conclusion, profiling of hMSC donors for a specific panel of miRNAs may have a prognostic value for selecting donors with high differentiation potential to improve hMSC-based strategies for tissue regeneration.

Cells and tissues are intrinsically adapted to molecular gradients and use them to maintain or change their activity. The effect of such gradients is particularly important for cell populations that have an intrinsic capacity to differentiate into multiple cell lineages, such as bone marrow derived mesenchymal stromal cells (MSCs). Our results showed that nutrient gradients prompt the spatiotemporal organization of MSCs in 3D culture. Cells adapted to their 3D environment without significant cell death or cell differentiation. Kinetics data and whole-genome gene expression analysis suggest that a low proliferation activity phenotype predominates in stromal cells cultured in 3D, likely due to increasing nutrient limitation. These differences implied that despite similar surface areas available for cell attachment, higher cell concentrations in 3D reduced MSCs proliferation, while activating hypoxia related-pathways. To further understand the in vivo effects of both proliferation and cell concentrations, we increased cell concentrations in small (1.8 μl) implantable wells. We found that MSCs accumulation and conditioning by nutrient competition in small volumes leads to an ideal threshold of cell-concentration for the induction of blood vessel formation, possibly signaled by the hypoxia-related stanniocalcin-1 gene., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Stem cell fates on biomaterials are influenced by the complex confluence of microenvironmental cues emanating from soluble growth factors, cell-to-cell contacts, and biomaterial properties. Cell-microenvironment interactions influence the cell fate by initiating a series of outside-in signaling events that traverse from the focal adhesions to the nucleus via the cytoskeleton and modulate the sub-nuclear protein organization and gene expression. Here, we report a novel imaging-based framework that highlights the spatial organization of sub-nuclear proteins, specifically the splicing factor SC-35 in the nucleoplasm, as an integrative marker to distinguish between minute differences of stem cell lineage pathways in response to stimulatory soluble factors, surface topologies, and microscale topographies. This framework involves the high resolution image acquisition of SC-35 domains and imaging-based feature extraction to obtain quantitative nuclear metrics in tandem with machine learning approaches to generate a predictive cell state classification model. The acquired SC-35 metrics led to >90% correct classification of emergent human mesenchymal stem cell (hMSC) phenotypes in populations of hMSCs exposed for merely 3 days to basal, adipogenic, or osteogenic soluble cues, as well as varying levels of dexamethasone-induced alkaline phosphatase (ALP) expression. Early osteogenic cellular responses across a series of surface patterns, fibrous scaffolds, and micropillars were also detected and classified using this imaging-based methodology. Complex cell states resulting from inhibition of RhoGTPase, β-catenin, and FAK could be classified with >90% sensitivity on the basis of differences in the SC-35 organizational metrics. This indicates that SC-35 organization is sensitively impacted by adhesion-related signaling molecules that regulate osteogenic differentiation. Our results show that diverse microenvironment cues affect different attributes of the SC-35 organizational metrics and lead to distinct emergent organizational patterns. Taken together, these studies demonstrate that the early organization of SC-35 domains could serve as a "fingerprint" of the intracellular mechanotransductive signaling that governs growth factor- and topography-responsive stem cell states.

Surface topographies of materials considerably impact cellular behavior as they have been shown to affect cell growth, provide cell guidance, and even induce cell differentiation. Consequently, for successful application in tissue engineering, the contact interface of biomaterials needs to be optimized to induce the required cell behavior. However, a rational design of biomaterial surfaces is severely hampered because knowledge is lacking on the underlying biological mechanisms. Therefore, we previously developed a high-throughput screening device (TopoChip) that measures cell responses to large libraries of parameterized topographical material surfaces. Here, we introduce a computational analysis of high-throughput materiome data to capture the relationship between the surface topographies of materials and cellular morphology. We apply robust statistical techniques to find surface topographies that best promote a certain specified cellular response. By augmenting surface screening with data-driven modeling, we determine which properties of the surface topographies influence the morphological properties of the cells. With this information, we build models that predict the cellular response to surface topographies that have not yet been measured. We analyze cellular morphology on 2176 surfaces, and find that the surface topography significantly affects various cellular properties, including the roundness and size of the nucleus, as well as the perimeter and orientation of the cells. Our learned models capture and accurately predict these relationships and reveal a spectrum of topographies that induce various levels of cellular morphologies. Taken together, this novel approach of high-throughput screening of materials and subsequent analysis opens up possibilities for a rational design of biomaterial surfaces., (Copyright © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Angiogenesis and neovascularization are fundamental for the success of clinically relevant-sized tissue-engineered (TE) constructs. The next generation of TE constructs relies on providing instructive materials combined with the delivery of angiogenic growth factors and cells to avoid tissue ischemia. However, the majority of materials and cell types screened so far show limited clinical relevance, either due to insufficient number of cells or due to the use of animal-derived matrixes. Here, we investigated whether endothelial-like cells derived from mesenchymal stromal cells (EL-MSCs) can be used for vascular TE in combination with injectable dextran-hyaluronic acid (Dex-g-HA) hydrogels. These hydrogels can be easily modified, as demonstrated by the incorporation of vascular endothelial growth factor (VEGF). We examined in vitro the reciprocal influences between cells and matrix. Dex-g-HA enabled higher EL-MSC metabolic rates associated with optimal cell sprouting in vitro compared to human umbilical vein endothelial cells. In vivo evaluation demonstrated the absence of an acute inflammatory response, and EL-MSCs incorporated within Dex-g-HA formed a functional vascular network integrated with the host vascular system. This work demonstrates that Dex-g-HA is an efficient delivery method of VEGF to induce angiogenesis. Additionally, functional neovascularization can be achieved in vitro and in vivo by the combination of Dex-g-HA with EL-MSC.

The surface marker profile of mesenchymal stromal cells (MSCs) suggests that they can escape detection by the immune system of an allogeneic host. This could be an optimal strategy for bone regeneration applications, where off-the-shelf cells could be implanted to heal bone defects. However, it is unknown how pre-differentiation of MSCs to an osteogenic lineage, a means of improving bone formation, affects their immunogenicity. Using immunohistological techniques in a rat ectopic implantation model, we demonstrate that allogeneic osteoprogenitors mount a T cell- and B cell-mediated immune response resulting in an absence of in vivo bone formation. Suppression of the host immune response with daily administration of an immunosuppressant, FK506, is effective in preventing the immune attack on the allogeneic osteoprogenitors. In the immunosuppressed environment, the allogeneic osteoprogenitors are capable of generating bone in amounts similar to those of syngeneic cells. However, using osteoprogenitors from one of the allogeneic donors led to newly deposited bone that was attacked by the host immune system, despite the continued administration of the immunosuppressant. This suggests that, although using an immunosuppressant can potentially suppress the immune attack on the allogeneic cells, optimizing the dose of the immunosuppressant may be crucial to ensure bone formation within the allogeneic environment. Overall, allografts comprising osteoprogenitors derived from allogeneic MSCs have the potential to be used in bone regeneration applications., (© 2013 The Authors. Journal of Cellular and Molecular Medicine published by John Wiley & Sons Ltd and Foundation for Cellular and Molecular Medicine.)

Application of autologous cells is considered for a broad range of regenerative therapies because it is not surrounded by the immunological and ethical issues of allo- or xenogenic cells. However, isolation, expansion, and application of autologous cells do suffer from variability in therapeutic efficacy due to donor to donor differences and due to prolonged culture. One important source of autologous cells is mesenchymal stromal cells (MSCs), which can differentiate toward endothelial-like cells, thus making them an ideal candidate as cell source for tissue vascularization. Here we screened MSCs from 20 donors for their endothelial differentiation capacity and correlated it with the gene expression profile of the whole genome in the undifferentiated state. Cells of all donors were able to form tubes on Matrigel and induced the expression of endothelial genes, although with quantitative differences. In addition, we analyzed the effect of prolonged in vitro expansion on the multipotency of human MSCs and found that endothelial differentiation is only mildly sensitive to expansion-induced loss of differentiation as compared to osteogenic and adipogenic differentiation. Our results show the robustness of the endothelial differentiation protocol and the gene expression data give insight in the differences in endothelial differentiation between donors.

Human mesenchymal stromal cells (hMSCs) offer great potential for bone tissue engineering applications, but their in vivo performance remains limited. Preconditioning of these cells with small molecules to improve their differentiation before implantation, or incorporation of growth factors are possible solutions. Insulin-like growth factor-1 (IGF-1) is one of the most abundant growth factors in bone, involved in growth, development, and metabolism, but its effects on hMSCs are still subject of debate. Here we examined the effects of IGF-1 on proliferation and differentiation of hMSCs in vitro and we found that serum abolished the effects of IGF-1. Only in the absence of serum, IGF-1 increased proliferation, alkaline phosphatase expression, and osteogenic gene expression of hMSCs. Furthermore, we examined synergistic effects of bone morphogenetic protein-2 (BMP-2) and IGF-1 and, although IGF-1 enhanced BMP-2-induced mineralization, IGF-1 only slightly affected in vivo bone formation.

Calcium phosphate (CaP) based ceramics are used as bone graft substitutes in the treatment of bone defects. The physico-chemical properties of these materials determine their bioactivity, meaning that molecular and cellular responses in the body will be tuned accordingly. In a previous study, we compared two porous CaP ceramics, hydroxyapatite (HA) and β-tricalcium phosphate (TCP), which, among other properties, differ in their degradation behaviour in vitro and in vivo, and we demonstrated that the more degradable β-TCP induced more bone formation in a heterotopic model in sheep. This is correlated to in vitro data, where human bone marrow derived mesenchymal stromal cells (MSC) exhibited higher expression of osteogenic differentiation markers, such as osteopontin, osteocalcin and bone sialoprotein, when cultured in β-TCP than in HA. More recently, we also showed that this effect could be mimicked in vitro by exposure of MSC to high concentrations of calcium ions (Ca(2+)). To further correlate surface physico-chemical dynamics of HA and β-TCP ceramics with the molecular response of MSC, we followed Ca(2+) release and surface changes in time as well as cell attachment and osteogenic differentiation of MSC on these ceramics. Within 24 hours, we observed differences in cell morphology, with MSC cultured in β-TCP displaying more pronounced attachment and spreading than cells cultured in HA. In the same time frame, β-TCP induced expression of G-protein coupled receptor (GPCR) 5A and regulator of G-protein signaling 2, revealed by DNA microarray analysis. These genes, associated with the protein kinase A and GPCR signaling pathways, may herald the earliest response of MSC to bone-inducing ceramics.

Mesenchymal stromal cells (hMSCs) are advancing into the clinic but the therapeutic efficacy of hMSCs faces the problem of donor variability. In bone tissue engineering, no reliable markers have been identified which are able to predict the bone-forming capacity of hMSCs prior to implantation. To this end, we isolated hMSCs from 62 donors and characterized systematically their in vitro lineage differentiation capacity, gene expression signature and in vivo capacity for ectopic bone formation. Our data confirms the large variability of in vitro differentiation capacity which did not correlate with in vivo ectopic bone formation. Using DNA microarray analysis of early passage hMSCs we identified a diagnostic bone-forming classifier. In fact, a single gene, CADM1, strongly correlated with the bone-forming capacity of hMSCs and could be used as a reliable in vitro diagnostic marker. Furthermore, data mining of genes expressed correlating with in vivo bone formation represented involvement in neurogenic processes and Wnt signaling. We will apply our data set to predict therapeutic efficacy of hMSCs and to gain novel insight in the process of bone regeneration. Our bio-informatics driven approach may be used in other fields of cell therapy to establish diagnostic markers for clinical efficacy., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The repertoire of growth factors determines the biological engagement of human mesenchymal stromal cells (hMSCs) in processes such as immunomodulation and tissue repair. Hypoxia is a strong modulator of the secretome and well known stimuli to increase the secretion of pro-angiogenic molecules. In this manuscript, we employed a high throughput screening assay on an hMSCs cell line in order to identify small molecules that mimic hypoxia. Importantly, we show that the effect of these small molecules was cell type/species dependent, but we identified phenanthroline as a robust hit in several cell types. We show that phenanthroline induces high expression of hypoxia-target genes in hMSCs when compared with desferoxamine (DFO) (a known hypoxia mimic) and hypoxia incubator (2% O(2)). Interestingly, our microarray and proteomics analysis show that only phenanthroline induced high expression and secretion of another angiogenic cytokine, interleukin-8, suggesting that the mechanism of phenanthroline-induced hypoxia is distinct from DFO and hypoxia and involves the activation of other signaling pathways. We showed that phenanthroline alone was sufficient to induce blood vessel formation in a Matrigel plug assay in vivo paving the way to its application in ischeamic-related diseases., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Bone marrow-derived multipotent mesenchymal stromal cells (MSCs) are the most frequently investigated cell type for potential regenerative strategies because they are relatively easy to isolate and are able to differentiate into several mesenchymal lineages. Unfortunately, during ex vivo culture, MSCs present gradual loss of differentiation potential and reduced clinical efficacy. Reactive oxygen species (ROS) are associated with oxidative damage and accumulate during MSC expansion. Because ROS are believed to be involved in the loss of multipotency, we hypothesized that compounds with antioxidant activity have the capacity to scavenge ROS, prevent cellular damage, and rescue culture-induced loss of multipotency. In this manuscript, we show that antioxidant supplementation can partially rescue the loss of alkaline phosphatase expression induced by oxidizing agents and increases the yield of hMSCs, when supplemented to a fresh bone marrow aspirate. Concomitantly, oxidative DNA damage and ROS levels in hMSCs were reduced by antioxidants. We conclude that antioxidant supplementation during MSC expansion reduces the DNA damage load and increases the MSC yield.

There is a rise in the popularity of arthroscopic procedures in orthopedics. However, the majority of cell-based bone tissue-engineered constructs (TECs) rely on solid preformed scaffolding materials, which require large incisions and extensive dissections for placement at the defect site. Thus, they are not suitable for minimally invasive techniques. The aim of this study was to develop a clinically relevant, easily moldable, bone TEC, amenable to minimally invasive techniques, using human mesenchymal stromal cells (hMSCs) and calcium phosphate microparticles in combination with an in situ forming platelet-rich plasma gel obtained from human platelets. Most conventional TECs rely on seeding and culturing single-cell suspensions of hMSCs on scaffolds. However, for generating TECs amenable to the minimally invasive approach, it was essential to aggregate the hMSCs in vitro before seeding them on the scaffolds as unaggregated MSCs did not generate any bone. Twenty four hours of in vitro aggregation was determined to be optimal for maintaining cell viability in vitro and bone formation in vivo. Moreover, no statistically significant difference was observed in the amount of bone formed when the TECs were implanted via an open approach or a minimally invasive route. TECs generated using MSCs from three different human donors generated new bone through the minimally invasive route in a reproducible manner, suggesting that these TECs could be a viable alternative to preformed scaffolds employed through an open surgery for treating bone defects.

Osteogenic differentiation of human mesenchymal stromal cells (hMSCs) may potentially be used in cell-based bone tissue-engineering applications to enhance the bone-forming potential of these cells. Osteogenic differentiation and adipogenic differentiation are thought to be mutually exclusive, and although several signaling pathways and cues that induce osteogenic or adipogenic differentiation, respectively, have been identified, there is no general consensus on how to optimally differentiate hMSCs into the osteogenic lineage. Some pathways have also been reported to be involved in both adipogenic and osteogenic differentiation, as for example, the protein kinase A (PKA) pathway, and the aim of this study was to investigate the role of cAMP/PKA signaling in differentiation of hMSCs in more detail. We show that activation of this pathway with dibutyryl-cAMP results in enhanced alkaline phosphatase expression, whereas another cAMP analog induces adipogenesis in long-term mineralization cultures. Adipogenic differentiation, induced by 8-bromo-cAMP, was accompanied by stronger PKA activity and higher expression of cAMP-responsive genes, suggesting that stronger activation correlates with adipogenic differentiation. In addition, a whole-genome expression analysis showed an increase in expression of adipogenic genes in 8-br-cAMP-treated cells. Furthermore, by means of quantitative polymerase chain reaction, we show differences in peroxisome proliferator-activated receptor-γ activation, either alone or in combination with dexamethasone, thus demonstrating differential effects of the PKA pathway, most likely depending on its mode of activation.

The response of osteoprogenitors to calcium (Ca(2+)) is of primary interest for both normal bone homeostasis and the clinical field of bone regeneration. The latter makes use of calcium phosphate-based bone void fillers to heal bone defects, but it is currently not known how Ca(2+) released from these ceramic materials influences cells in situ. Here, we have created an in vitro environment with high extracellular Ca(2+) concentration and investigated the response of human bone marrow-derived mesenchymal stromal cells (hMSCs) to it. Ca(2+) enhanced proliferation and morphological changes in hMSCs. Moreover, the expression of osteogenic genes is highly increased. A 3-fold up-regulation of BMP-2 is observed after only 6h and pharmaceutical interference with a number of proteins involved in Ca(2+) sensing showed that not the calcium sensing receptor, but rather type L voltage-gated calcium channels are involved in mediating the signaling pathway between extracellular Ca(2+) and BMP-2 expression. MEK1/2 activity is essential for the effect of Ca(2+) and using microarray analysis, we have identified c-Fos as an early Ca(2+) response gene. We have demonstrated that hMSC osteogenesis can be induced via extracellular Ca(2+), a simple and economic way of priming hMSCs for bone tissue engineering applications., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Among the various types of cell-to-cell signaling, paracrine signaling comprises those signals that are transmitted over short distances between different cell types. In the human body, secreted growth factors and cytokines instruct, among others, proliferation, differentiation, and migration. In the hematopoietic stem cell (HSC) niche, stromal cells provide instructive cues to stem cells via paracrine signaling and one of these cell types, known to secrete a broad panel of growth factors and cytokines, is mesenchymal stromal cells (MSCs). The factors secreted by MSCs have trophic, immunomodulatory, antiapoptotic, and proangiogenic properties, and their paracrine profile varies according to their initial activation by various stimuli. MSCs are currently studied as treatment for inflammatory diseases such as graft-versus-host disease and Crohn's disease, but also as treatment for myocardial infarct and solid organ transplantation. In addition, MSCs are investigated for their use in tissue engineering applications, in which their differentiation plays an important role, but as we have recently demonstrated, their trophic factors may also be involved. Furthermore, a functional improvement of MSCs might be obtained after preconditioning or tailoring the cells themselves. Also, the way the cells are clinically administered may be specialized for specific therapeutic scenarios. In this review we will first discuss the HSC niche, in which MSCs were recently identified and are thought to play an instructive and supportive role. We will then evaluate therapeutic applications that currently try to utilize the trophic and/or immunomodulatory properties of MSCs, and we will also discuss new options to enhance their therapeutic effects.

Activation of the protein kinase A (PKA) pathway with dibutyryl cyclic adenosine monophosphate (db-cAMP) was recently shown to enhance osteogenic differentiation of human mesenchymal stromal cells (hMSCs) in vitro and bone formation in vivo. The major drawback of this compound is its inhibitory effect on proliferation of hMSCs. Therefore, we investigated whether fine-tuning of the dose and timing of PKA activation could enhance bone formation even further, with minimum effects on proliferation. To test this, we selected two different PKA activators (8-bromo-cAMP (8-br-cAMP) and forskolin) and compared their effects on proliferation and osteogenic differentiation with those of db-cAMP. We found that all three compounds induced alkaline phosphatase levels, bone-specific target genes, and secretion of insulin-like growth factor-1, although 8-br-cAMP induced adipogenic differentiation in long-term cultures and was thus considered unsuitable for further in vivo testing. All three compounds inhibited proliferation of hMSCs in a dose-dependent manner, with forskolin inhibiting proliferation most. The effect of forskolin on in vivo bone formation was tested by pretreating hMSCs before implantation, and we observed greater amounts of bone using forskolin than db-cAMP. Our data show forskolin to be a novel agent that can be used to increase bone formation and also suggests a role for PKA in the delicate balance between adipogenic and osteogenic differentiation.

Human mesenchymal stromal cells (hMSCs) are increasingly used in regenerative medicine for restoring worn-out or damaged tissue. Newly engineered tissues need to be properly vascularized and current candidates for in vitro tissue pre-vascularization are endothelial cells and endothelial progenitor cells. However, their use in therapy is hampered by their limited expansion capacity and lack of autologous sources. Our approach to engineering large grafts is to use hMSCs both as a source of cells for regeneration of targeted tissue and at the same time as the source of endothelial cells. Here we investigate how different stimuli influence endothelial differentiation of hMSCs. Although growth supplements together with shear force were not sufficient to differentiate hMSCs with respect to expression of endothelial markers such as CD31 and KDR, these conditions did prime the cells to differentiate into cells with an endothelial gene expression profile and morphology when seeded on Matrigel. In addition, we show that endothelial-like hMSCs are able to create a capillary network in 3D culture both in vitro and in vivo conditions. The expansion phase in the presence of growth supplements was crucial for the stability of the capillaries formed in vitro. To conclude, we established a robust protocol for endothelial differentiation of hMSCs, including an immortalized MSC line (iMSCs) which allows for reproducible in vitro analysis in further studies.

Human aging is associated with loss of function and regenerative capacity. Human bone marrow derived mesenchymal stromal cells (hMSCs) are involved in tissue regeneration, evidenced by their capacity to differentiate into several lineages and therefore are considered the golden standard for cell-based regeneration therapy. Tissue maintenance and regeneration is dependent on stem cells and declines with age and aging is thought to influence therapeutic efficacy, therefore, more insight in the process of aging of hMSCs is of high interest. We, therefore, hypothesized that hMSCs might reflect signs of aging. In order to find markers for donor age, early passage hMSCs were isolated from bone marrow of 61 donors, with ages varying from 17-84, and clinical parameters, in vitro characteristics and microarray analysis were assessed. Although clinical parameters and in vitro performance did not yield reliable markers for aging since large donor variations were present, genome-wide microarray analysis resulted in a considerable list of genes correlating with human age. By comparing the transcriptional profile of aging in human with the one from rat, we discovered follistatin as a common marker for aging in both species. The gene signature presented here could be a useful tool for drug testing to rejuvenate hMSCs or for the selection of more potent, hMSCs for cell-based therapy.

It is increasingly recognized that material surface topography is able to evoke specific cellular responses, endowing materials with instructive properties that were formerly reserved for growth factors. This opens the window to improve upon, in a cost-effective manner, biological performance of any surface used in the human body. Unfortunately, the interplay between surface topographies and cell behavior is complex and still incompletely understood. Rational approaches to search for bioactive surfaces will therefore omit previously unperceived interactions. Hence, in the present study, we use mathematical algorithms to design nonbiased, random surface features and produce chips of poly(lactic acid) with 2,176 different topographies. With human mesenchymal stromal cells (hMSCs) grown on the chips and using high-content imaging, we reveal unique, formerly unknown, surface topographies that are able to induce MSC proliferation or osteogenic differentiation. Moreover, we correlate parameters of the mathematical algorithms to cellular responses, which yield novel design criteria for these particular parameters. In conclusion, we demonstrate that randomized libraries of surface topographies can be broadly applied to unravel the interplay between cells and surface topography and to find improved material surfaces.

Human mesenchymal stromal cells (hMSCs) are able to differentiate into a wide variety of cell types, which makes them an interesting source for tissue engineering applications. On the other hand, these cells also secrete a broad panel of growth factors and cytokines that can exert trophic effects on surrounding tissues. In bone tissue engineering applications, the general assumption is that direct differentiation of hMSCs into osteoblasts accounts for newly observed bone formation in vivo. However, the secretion of bone-specific growth factors, but also pro-angiogenic factors, could also contribute to this process. We recently demonstrated that secretion of bone specific growth factors can be enhanced by treatment of hMSCs with the small molecule db-cAMP (cAMP) and here we investigate the biological activity of these secreted factors. We demonstrate that conditioned medium contains a variety of secreted growth factors, with differences between medium from basic-treated and cAMP-treated hMSCs. We show that conditioned medium from cAMP-treated hMSCs increases proliferation of various cell types and also induces osteogenic differentiation, whereas it has differential effects on migration. Microarray analysis on hMSCs exposed to conditioned medium confirmed upregulation of pathways involved in proliferation as well as osteogenic differentiation. Our data suggests that trophic factors secreted by hMSCs can be tuned for specific applications and that a good balance between differentiation on the one hand and secretion of bone trophic factors on the other, could potentially enhance bone formation for bone tissue engineering applications., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

For more than a decade, human mesenchymal stem cells (hMSCs) have been used in bone tissue-engineering research. More recently some of the focus in this field has shifted towards the use of embryonic stem cells. While it is well known that hMSCs are able to form bone when implanted subcutaneously in immune-deficient mice, the osteogenic potential of embryonic stem cells has been mainly assessed in vitro. Therefore, we performed a series of studies to compare the in vitro and in vivo osteogenic capacities of human and mouse embryonic stem cells to those of hMSCs. Embryonic and mesenchymal stem cells showed all characteristic signs of osteogenic differentiation in vitro when cultured in osteogenic medium, including the deposition of a mineralized matrix and expression of genes involved in osteogenic differentiation. As such, based on the in vitro results, osteogenic ES cells could not be discriminated from osteogenic hMSCs. Nevertheless, although osteogenic hMSCs formed bone upon implantation, osteogenic cells derived from both human and mouse embryonic stem cells did not form functional bone, indicated by absence of osteocytes, bone marrow and lamellar bone. Although embryonic stem cells show all signs of osteogenic differentiation in vitro, it appears that, in contrast to mesenchymal stem cells, they do not possess the ability to form bone in vivo when a similar culture method and osteogenic differentiation protocol was applied., (Copyright © 2010 John Wiley & Sons, Ltd.)

Human mesenchymal stromal cells are regarded as the golden standard for cell-based therapies. They present multilineage differentiation potential and trophic and immunosuppressive abilities, making them the best candidate for clinical applications. Several molecules have been described to increase bone formation and were mainly discovered by candidate approaches towards known signaling pathways controlling osteogenesis. However, their bone forming potential is still limited, making the search for novel molecules a necessity. High-throughput screening (HTS) not only allows the screening of a large number of diverse chemical compounds, but also allows the discovery of unexpected signaling pathways and molecular mechanisms for a certain application, even without the prior knowledge of the full molecular pathway. Typically HTS is performed in cell lines, however, in this manuscript we have performed a phenotypical screen on more clinically relevant human mesenchymal stromal cells, as a proof of principle that HTS can be performed in those cells and can be used to find small molecules that impact stem cell fate. From a library of pharmacologically active small molecules, we were able to identify novel compounds with increased osteogenic activity. These compounds allowed achieving levels of bone-specific alkaline phosphatase higher than any other combination previously known. By combining biochemical techniques, we were able to demonstrate that a medium to high-throughput phenotypic assay can be performed in academic research laboratories allowing the discovery of novel molecules able to enhance stem cell differentiation.

Human mesenchymal stem cells (hMSCs) are being considered for several areas of clinical therapy, due to their multipotent nature. For instance, osteogenic hMSCs are applied in bone tissue engineering, but current differentiation protocols need further optimization before they can be clinically applied. Protein kinase C (PKC) family members have been implicated in bone metabolism, which prompted us to use a pharmaceutical approach to manipulate PKC signalling in hMSCs. Inhibition of PKC resulted in a dose-dependent inhibition of dexamethasone-induced osteogenic differentiation. Surprisingly, PKC activation using phorbol 12-myristate 13-acetate (PMA) also resulted in inhibition of osteogenesis, although we observed that inhibition was more pronounced at low than at high concentrations of PMA. Furthermore, we observed that inhibition of PKCdelta blocked alkaline phosphatase (ALP, an early marker of osteogenic differentiation) expression, whereas inhibition of the conventional PKC subfamily and PKCmicro using Gö6976 resulted in an induction of ALP activity, collagen (I) expression and mineralization. In conclusion, inhibition of the conventional PKCs/PKCmicro and activation of PKCdelta could further benefit osteogenic differentiation of hMSCs in vitro and in vivo, which is currently under investigation., (Copyright (c) 2009 John Wiley & Sons, Ltd.)

Previously, we demonstrated that protein kinase A (PKA) activation using dibutyryl-cAMP in human mesenchymal stem cells (hMSCs) induces in vitro osteogenesis and bone formation in vivo. To further investigate the physiological role of PKA in hMSC osteogenesis, we tested a selection of G-protein-coupled receptor ligands which signal via intracellular cAMP production and PKA activation. Treatment of hMSCs with parathyroid hormone, parathyroid hormone-related peptide, melatonin, epinephrine, calcitonin or calcitonin gene-related peptide did not result in accumulation of cAMP or induction of alkaline phosphatase (ALP) expression. The only ligand that did induce cAMP, prostaglandin E2, even inhibited ALP expression and mineralization, suggesting that physiological levels of cAMP may inhibit osteogenesis. Furthermore, intermittent exposure of hMSCs to dibutyryl-cAMP inhibited ALP expression, whereas we did not observe an inhibitive effect at low dibutyryl-cAMP concentrations. Taken together, our results demonstrate that cAMP can either stimulate or inhibit osteogenesis in hMSCs, depending on the duration, rather than the strength, of the signal provided., (Copyright (c) 2009 John Wiley & Sons, Ltd.)

Fibrodysplasia ossificans progressiva (FOP) is a rare disabling disease characterized by heterotopic ossification for which there is currently no treatment available. FOP has been linked recently to a heterozygous R206H mutation in the bone morphogenetic protein (BMP) type I receptor activin receptor-like kinase 2 (ALK2). Expression of the mutant ALK2-R206H receptor (FOP-ALK2) results in increased phosphorylation of the downstream Smad1 effector proteins and elevated basal BMP-dependent transcriptional reporter activity, indicating that FOP-ALK2 is constitutively active. FOP-ALK2-induced transcriptional activity could be blocked by overexpressing either of the inhibitory Smads, Smad6 or -7, or by treatment with the pharmacological BMP type I receptor inhibitor dorsomorphin. However, in contrast to wild-type ALK2, FOP-ALK2 is not inhibited by the negative regulator FKBP12. Mesenchymal cells expressing the FOP-ALK2 receptor are more sensitive to undergoing BMP-induced osteoblast differentiation and mineralization. In vivo bone formation was assessed by loading human mesenchymal stem cells (hMSCs) expressing the ALK2-R206H receptor onto calcium phosphate scaffolds and implantation in nude mice. Compared with control cells FOP-ALK2-expressing cells induced increased bone formation. Taken together, the R206H mutation in ALK2 confers constitutive activity to the mutant receptor, sensitizes mesenchymal cells to BMP-induced osteoblast differentiation, and stimulates new bone formation. We have generated an animal model that can be used as a stepping stone for preclinical studies aimed at inhibiting the heterotopic ossification characteristic of FOP., ((c) 2010 American Society for Bone and Mineral Research.)

Human multipotent mesenchymal stromal cells (hMSCs) are multipotent cells that, in the presence of appropriate stimuli, can differentiate into different lineages such as the osteogenic, chondrogenic, and adipogenic lineages. In the presence of ascorbic acid, MSCs secrete an extracellular matrix mainly composed of collagen type I. Here we assessed the potential role of endogenous collagen synthesis in hMSC differentiation and stem cell maintenance. We observed a sharp reduction in proliferation rate of hMSCs in the absence of ascorbic acid, concomitant with a reduction in osteogenesis in vitro and bone formation in vivo. In line with a positive role for collagen type I in osteogenesis, gene expression profiling of hMSCs cultured in the absence of ascorbic acid demonstrated increased expression of genes involved in adipogenesis and chondrogenesis and a reduction in expression of osteogenic genes. We also observed that matrix remodeling and anti-osteoclastogenic signals were high in the presence of ascorbic acid. The presence of collagen type I during the expansion phase of hMSCs did not affect their osteogenic and adipogenic differentiation potential. In conclusion, the collagenous matrix supports both proliferation and differentiation of osteogenic hMSCs but, on the other hand, presents signals stimulating matrix remodeling and inhibiting osteoclastogenesis.

Since the discovery that bone morphogenetic proteins (BMPs) are able to induce ectopic bone formation, considerable effort has been devoted to apply it for bone regeneration. BMP activity needs to be temporally and spatially controlled and the organism has devised ways to achieve it. Here we show that the BMP inhibitor chordin-like 1 can interfere with BMP2 signalling thereby affecting the osteogenic differentiation of MC3T3-E1 cells. Besides its function as a BMP antagonist, chordin-like 1 enhanced the proliferation of human mesenchymal stem cells (hMSCs) in a BMP2-independent manner. When MC3T3-E1 cells were exposed to recombinant chordin-like 1 there was an inhibition of alkaline phosphatase (ALP) expression, whereas in the case of hMSCs no effect was observed. However, chordin-like 1 dose-dependently increased the proliferation of hMSCs. This effect is probably BMP2 independent because the chordin-like 1 concentration that stimulates proliferation does not interfere with BMP signalling monitored by a Smad-dependent reporter gene. Our data point towards a novel, BMP-independent role of chordin-like 1 in hMSC proliferation., (Copyright 2010 S. Karger AG, Basel.)

Collagen is the main protein component of the extracellular matrix of bone, and it has structural and instructive properties. Collagen undergoes many post-translational modifications, including extensive crosslinking. Although defective crosslinking has been implicated in human syndromes (e.g., osteogenesis imperfecta or Ehlers-Danlos syndrome), it is not clear to what extent crosslinking is necessary for collagen's instructive properties during bone formation. Here we report that inhibition of collagen crosslinking in the mouse pre-osteoblast cell line MC3T3-E1 impairs the osteogenic program. Genome-wide expression profiling of beta-aminopropionitrile-treated and control cells revealed that matrix deposition by MC3T3-E1 cells provides a feed back signal, driving cells through the differentiation process, that is strongly impaired when crosslinking is inhibited. Inhibition of crosslinking did not affect osteogenic differentiation of human mesenchymal stem cells (hMSCs), shown by the expression of alkaline phosphatase and genome-wide gene expression analysis, although it enhances matrix mineralization. In conclusion, collagen crosslinking harbors instructive properties in MC3T3-E1 differentiation but plays a more-passive role in differentiation of bone marrow-derived hMSCs.

Introduction: Large inter-donor differences exist in human mesenchymal stem cell (hMSC) yield and the response of these cells to osteogenic stimuli. The source of these differences may be clinical differences in stem cell characteristics between individuals or the aspiration procedure itself., Methods: From a total of 23 donors, we aimed to take 2 consecutive 10-mL aspirates from the same site in 17 donors and in 6 donors we aimed to take a 5-mL and a 20-mL aspirate. The aspiration was stopped either when the syringe was full or when no more bone marrow came through. Mononuclear cell yield (MNC), MSC yield, and differentiation capacity were analyzed for intra-donor and inter-donor variation. We analyzed the effect of the dilution with peripheral blood by drawing 20 mL at once., Results: There was a high correlation between the first and second aspiration volumes, and aspirates with a volume of less than 8 mL showed a large variation in cellular yield. The second 10-mL aspirate, and also 20-mL aspirates, contained a lower concentration of nucleated cells and yielded lower numbers of mesenchymal stem cells. No effect of the aspiration procedure on the biological characteristics of the mesenchymal stem cells was seen., Conclusion: We recommend collection volumes of bone marrow aspirates of at least 8 mL to reduce the risk of obtaining aspirates with low cell numbers. From the same site, a second aspiration or an aspirate of > 10 mL can be drawn without any loss of biological quality due to dilution with peripheral blood.

Tissue engineering of large bone defects is approached through implantation of autologous osteogenic cells, generally referred to as multipotent stromal cells or mesenchymal stem cells (MSCs). Animal-derived MSCs successfully bridge large bone defects, but models for ectopic bone formation as well as recent clinical trials demonstrate that bone formation by human MSCs (hMSCs) is inadequate. The expansion phase presents an attractive window to direct hMSCs by pharmacological manipulation, even though no profound effect on bone formation in vivo has been described so far using this approach. We report that activation of protein kinase A elicits an immediate response through induction of genes such as ID2 and FosB, followed by sustained secretion of bone-related cytokines such as BMP-2, IGF-1, and IL-11. As a consequence, PKA activation results in robust in vivo bone formation by hMSCs derived from orthopedic patients.

Bone tissue engineering using human mesenchymal stem cells (hMSCs) is a multidisciplinary field that aims to treat patients with trauma, spinal fusion and large bone defects. Cell-based bone tissue engineering encompasses the isolation of multipotent hMSCs from the bone marrow of the patient, in vitro expansion and seeding onto porous scaffold materials. In vitro pre-differentiation of hMSCs into the osteogenic lineage augments their in vivo bone forming capacity. Differentiation of hMSCs into bone forming osteoblasts is a multi-step process regulated by various molecular signaling pathways, which warrants a thorough understanding of these signaling cues for the efficient use of hMSCs in bone tissue engineering. Recently, there has been a surge of knowledge on the molecular cues regulating osteogenic differentiation but extrapolation to hMSC differentiation is not guaranteed, because of species- and cell-type specificity. In this review, we describe a number of key osteogenic signaling pathways, which directly or indirectly regulate osteogenic differentiation of hMSCs. We will discuss how and to what extent the process is different from that in other cell types with special emphasis on applications in bone tissue engineering.

The use of multipotent human mesenchymal stem cells (hMSCs) for tissue engineering has been a subject of extensive research. The donor variation in growth, differentiation and in vivo bone forming ability of hMSCs is a bottleneck for standardization of therapeutic protocols. In this study, we isolated and characterized hMSCs from 19 independent donors, aged between 27 and 85 years, and investigated the extent of heterogeneity of the cells and the extent to which hMSCs can be expanded without loosing multipotency. Dexamethasone-induced ALP expression varied between 1.2- and 3.7-fold, but no correlation was found with age, gender, or source of isolation. The cells from donors with a higher percentage of ALP-positive cells in control and dexamethasone-induced groups showed more calcium deposition than cells with lower percentage of ALP positive cells. Despite the variability in osteogenic gene expression among the donors tested, ALP, Collagen type 1, osteocalcin, and S100A4 showed similar trends during the course of osteogenic differentiation. In vitro expansion studies showed that hMSCs can be effectively expanded up to four passages (approximately 10-12 population doublings from a P0 culture) while retaining their multipotency. Our in vivo studies suggest a correlation between in vitro ALP expression and in vivo bone formation. In conclusion, irrespective of age, gender, and source of isolation, cells from all donors showed osteogenic potential. The variability in ALP expression appears to be a result of sampling method and cellular heterogeneity among the donor population.

During the past decade, there has been much interest in the use of human mesenchymal stem cells (hMSCs) in bone tissue engineering. HMSCs can be obtained relatively easily and expanded rapidly in culture, but for clinical purposes large numbers are often needed and the cost should be kept to a minimum. A rapid and efficient culturing protocol would therefore be beneficial. In this study, we examined the effect of different medium compositions on the expansion and osteogenic differentiation of bone marrow-derived hMSCs from 19 donors. We also investigated the effect of low seeding density and dexamethasone on both hMSCs expansion and their in vitro and in vivo osteogenic differentiation capacity. HMSCs seeded at a density of 100 cells/cm2 had a significantly higher growth rate than at 5000 cell/cm2, which was further improved by the addition of dexamethasone. Expanded hMSCs were characterized in vitro on the basis of positive staining for CD29, CD44, CD105, and CD166. The in vitro osteogenic potential of expanded hMSCs was assessed by flow cytometric staining for alkaline phosphatase. In vivo bone-forming potential of the hMSCs was assessed by seeding the cells in ceramic scaffolds, followed by subcutaneous implantation in nude mice and histopathologic assessment of de novo bone formation after 6-week implantation. Expanded hMSCs from all donors displayed similar osteogenic potential independent of the culture conditions. On the basis of these results we have developed an efficient method to culture hMSCs by seeding the cells at 100 cells/cm2 in an alpha-minimal essential medium-based medium containing dexamethasone.

Our approach to bone tissue engineering is the in vitro expansion and osteogenic differentiation of bone marrow-derived human mesenchymal stem cells (hMSCs) and their subsequent implantation on porous ceramic materials. Current osteogenic differentiation protocols use dexamethasone to initiate the osteogenic process, thus ignoring the multiple signaling pathways that control osteogenesis in vivo. Supporting osteogenesis at multiple stages might further enhance the bone-forming capacity of hMSCs. As reported previously, inhibition of so-called histone deacetylases (HDACs) stimulates osteoblast maturation, and in this report, we investigated whether trichostatin A (TSA), a widely used HDAC inhibitor, can be implemented in bone tissue engineering. We confirmed that TSA treatment of hMSCs results in increased expression of alkaline phosphatase (ALP) with concomitant increase in mineralization. Flow cytometry demonstrated that TSA increases the percentage of ALP-positive hMSCs as well as their average ALP expression level, but the robustness of the response differs between donors. Unfortunately, TSA has a profound negative effect on cell proliferation, so we investigated whether hMSCs respond to TSA after reaching confluence. Confluent hMSCs on tissue culture plastic displayed enhanced ALP expression. Therefore, we seeded TSA-treated hMSCs onto ceramic particles and analyzed ectopic bone formation upon implantation in immune-deficient mice. Unfortunately, TSA-treated hMSCs did not display better bone formation in vivo than control cells. Finally, we observed that TSA treatment strongly enhanced bone formation of ex vivo cultured mouse calvaria, which warrants further exploration of TSA in bone tissue engineering.

Electrospinning (ESP) has lately shown a great potential as a novel scaffold fabrication technique for tissue engineering. Scaffolds are produced by spinning a polymeric solution in fibers through a spinneret connected to a high-voltage electric field. The fibers are then collected on a support, where the scaffold is created. Scaffolds can be of different shapes, depending on the collector geometry, and have high porosity and high surface per volume ratio, since the deposited fibers vary from the microscale to the nanoscale range. Such fibers are quite effective in terms of tissue regeneration, as cells can bridge the scaffold pores and fibers, resulting in a fast and homogeneous tissue growth. Furthermore, fibers can display a nanoporous ultrastructure due to solvent evaporation. The aim of this study was to characterize electrospun scaffolds from poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) copolymers and to unravel the mechanism of pore formation on the fibers. The effect of different fiber diameters and of their surface nanotopology on cell seeding, attachment, and proliferation was studied. Smooth fibers with diameter of 10microm were found to support an optimal cell seeding and attachment within the micrometer range analyzed. Moreover, a nanoporous surface significantly enhanced cell proliferation and cells spreading on the fibers. The fabrication of ESP scaffolds with incorporated dyes with different molecular dimensions is also reported and their release measured. These findings contribute to the field of cell-material interaction and lead to the fabrication of "smart" scaffolds which can direct cells morphology and proliferation, and eventually release biological signals to properly conduct tissue formation.

To engineer tissues with clinically relevant dimensions, one must overcome the challenge of rapidly creating functional blood vessels to supply cells with oxygen and nutrients and to remove waste products. We tested the hypothesis that endothelial cells, cocultured with osteoprogenitor cells, can organize into a prevascular network in vitro. When cultured in a spheroid coculture model with human mesenchymal stem cells, human umbilical vein endothelial cells (HUVECs) form a 3-dimensional prevascular network within 10 days of in vitro culture. The formation of the prevascular network was promoted by seeding 2% or fewer HUVECs. Moreover, the addition of endothelial cells resulted in a 4-fold upregulation of the osteogenic marker alkaline phosphatase. The addition of mouse embryonic fibroblasts did not result in stabilization of the prevascular network. Upon implantation, the prevascular network developed further and structures including lumen could be seen regularly. However, anastomosis with the host vasculature was limited. We conclude that endothelial cells are able to form a 3-dimensional (3D) prevascular network in vitro in a bone tissue engineering setting. This finding is a strong indication that in vitro prevascularization is a promising strategy to improve implant vascularization in bone tissue engineering.

Mesenchymal stem cells are pluripotent cells from bone marrow, which can be differentiated into the osteogenic, chondrogenic, and adipogenic lineages in vitro and are a source of cells in bone and cartilage tissue engineering. An improvement in current tissue-engineering protocols requires more detailed insight into the molecular cues that regulate the distinct steps of osteochondral differentiation. Because Wnt signaling has been widely implicated in mesenchymal differentiation, we analyzed the role of Wnt signaling in human mesenchymal stem cell (hMSC) biology by stimulation of the pathway with lithium chloride and Wnt3A-conditioned medium. We demonstrate a role for low levels of Wnt signaling in proliferation of uncommitted hMSCs and confirm that Wnt signaling controls osteoprogenitor proliferation. On the other hand, at high Wnt levels we observed a block in adipogenic differentiation and an increase in the expression of alkaline phosphatase, suggesting a role in the initiation of osteogenesis. The results of this study suggest that bone tissue engineering could benefit from the activation of critical levels of Wnt signaling at defined stages of differentiation. Moreover, our data suggest that hMSCs provide a valid in vitro model to study the role of Wnt signaling in mesenchymal biology.

Biomaterials can control cell and nuclear morphology. Since the shape of the nucleus influences chromatin architecture, gene expression and cell identity, surface topography can control cell phenotype. This study provides fundamental insights into how surface topography influences nuclear morphology, histone modifications, and expression of histone‐associated proteins through advanced histone mass spectrometry and microarray analysis. The authors find that nuclear confinement is associated with a loss of histone acetylation and nucleoli abundance, while pathway analysis reveals a substantial reduction in gene expression associated with chromosome organization. In light of previous observations where the authors found a decrease in proliferation and metabolism induced by micro‐topographies, they connect these findings with a quiescent phenotype in mesenchymal stem cells, as further shown by a reduction of ribosomal proteins and the maintenance of multipotency on micro‐topographies after long‐term culture conditions. Also, this influence of micro‐topographies on nuclear morphology and proliferation is reversible, as shown by a return of proliferation when re‐cultured on a flat surface. The findings provide novel insights into how biophysical signaling influences the epigenetic landscape and subsequent cellular phenotype. [ABSTRACT FROM AUTHOR]