Expression of Oct4 in human embryonic stem cells is dependent on nanotopographical configuration

High-throughput analysis of topographical cues for the expansion of murine pluripotent stem cells

The expansion of pluripotent stem cells (PSCs)in vitroremains a critical barrier to their use in tissue engineering and regenerative medicine. Biochemical methods for PSC expansion are known to produce heterogeneous cell populations with varying states of pluripotency and are cost-intensive, hindering their clinical translation. Engineering biomaterials to physically control PSC fate offers an alternative approach. Surface or substrate topography is a promising design parameter for engineering biomaterials. Topographical cues have been shown to elicit profound effects on stem cell differentiation and proliferation. Previous reports have shown isotropic substrate topographies to be promising in expanding PSCs. However, the optimal feature to promote PSC proliferation and the pluripotent state has not yet been determined. In this work, the MultiARChitecture (MARC) plate is developed to conduct a high-throughput analysis of topographical cues in a 96-well plate format. The MARC plate is a reproducible and customizable platform for the analysis of multiple topographical patterns and features and is compatible with both microscopic assays and molecular biology techniques. The MARC plate is used to evaluate the expression of pluripotency markersOct4, Nanog, andSox2and the differentiation markerLmnAas well as the proliferation of murine embryonic stem (mES) cells. Our systematic analyses identified three topographical patterns that maintain pluripotency in mES cells after multiple passages: 1µm pillars (1µm spacing, square arrangement), 2µm wells (c-c (x, y) = 4, 4µm), and 5µm pillars (c-c (x, y) = 7.5, 7.5µm). This study represents a step towards developing a biomaterial platform for controlled murine PSC expansion.

Protein ligand and nanotopography separately drive the phenotype of mouse embryonic stem cells

Biochemical and biomechanical signals regulate stem cell function in the niche environments in vivo. Current in vitro culture of mouse embryonic stem cells (mESC) uses laminin (LN-511) to provide mimetic biochemical signaling (LN-521 for human systems) to maintain stemness. Alternative approaches propose topographical cues to provide biomechanical cues, however combined biochemical and topographic cues may better mimic the in vivo environment, but are largely unexplored for in vitro stem cell expansion. In this study, we directly compare in vitro signals from LN-511 and/or topographic cues to maintain stemness, using systematically-varied submicron pillar patterns or flat surfaces with or without preadsorbed LN-511. The adhesion of cells, colony formation, expression of the pluripotency marker,octamer-binding transcription factor 4 (Oct4), and transcriptome profiling were characterized. We observed that either biochemical or topographic signals could maintain stemness of mESCs in feeder-free conditions, indicated by high-level Oct4 and gene profiling by RNAseq. The combination of LN-511 with nanotopography reduced colony growth, while maintaining stemness markers, shifted the cellular phenotype indicating that the integration of biochemical and topographic signals is antagonistic. Overall, significantly faster (up to 2.5 times) colony growth was observed at nanotopographies without LN-511, suggesting for improved ESC expansion.

Design of chemically defined synthetic substrate surfaces for the in vitro maintenance of human pluripotent stem cells: A review

Human pluripotent stem cells (hPSCs) have the potential of long-term self-renewal and differentiation into nearly all cell types in vitro. Prior to the downstream applications, the design of chemically defined synthetic substrates for the large-scale proliferation of quality-controlled hPSCs is critical. Although great achievements have been made, Matrigel and recombinant proteins are still widely used in the fundamental research and clinical applications. Therefore, much effort is still needed to improve the performance of synthetic substrates in the culture of hPSCs, realizing their commercial applications. In this review, we summarized the design of reported synthetic substrates and especially their limitations in terms of cell culture. Moreover, much attention was paid to the development of promising peptide displaying surfaces. Besides, the biophysical regulation of synthetic substrate surfaces as well as the three-dimensional culture systems were described.

The Bumpy Road to Stem Cell Therapies: Rational Design of Surface Topographies to Dictate Stem Cell Mechanotransduction and Fate

Cells sense and respond to a variety of physical cues from their surrounding microenvironment, and these are interpreted through mechanotransductive processes to inform their behavior. These mechanisms have particular relevance to stem cells, where control of stem cell proliferation, potency, and differentiation is key to their successful application in regenerative medicine. It is increasingly recognized that surface micro- and nanotopographies influence stem cell behavior and may represent a powerful tool with which to direct the morphology and fate of stem cells. Current progress toward this goal has been driven by combined advances in fabrication technologies and cell biology. Here, the capacity to generate precisely defined micro- and nanoscale topographies has facilitated the studies that provide knowledge of the mechanotransducive processes that govern the cellular response as well as knowledge of the specific features that can drive cells toward a defined differentiation outcome. However, the path forward is not fully defined, and the "bumpy road" that lays ahead must be crossed before the full potential of these approaches can be fully exploited. This review focuses on the challenges and opportunities in applying micro- and nanotopographies to dictate stem cell fate for regenerative medicine. Here, key techniques used to produce topographic features are reviewed, such as photolithography, block copolymer lithography, electron beam lithography, nanoimprint lithography, soft lithography, scanning probe lithography, colloidal lithography, electrospinning, and surface roughening, alongside their advantages and disadvantages. The biological impacts of surface topographies are then discussed, including the current understanding of the mechanotransductive mechanisms by which these cues are interpreted by the cells, as well as the specific effects of surface topographies on cell differentiation and fate. Finally, considerations in translating these technologies and their future prospects are evaluated.

A mathematical modelling framework for the regulation of intra-cellular OCT4 in human pluripotent stem cells

Human pluripotent stem cells (hPSCs) have the potential to differentiate into all cell types, a property known as pluripotency. A deeper understanding of how pluripotency is regulated is required to assist in controlling pluripotency and differentiation trajectories experimentally. Mathematical modelling provides a non-invasive tool through which to explore, characterise and replicate the regulation of pluripotency and the consequences on cell fate. Here we use experimental data of the expression of the pluripotency transcription factor OCT4 in a growing hPSC colony to develop and evaluate mathematical models for temporal pluripotency regulation. We consider fractional Brownian motion and the stochastic logistic equation and explore the effects of both additive and multiplicative noise. We illustrate the use of time-dependent carrying capacities and the introduction of Allee effects to the stochastic logistic equation to describe cell differentiation. We conclude both methods adequately capture the decline in OCT4 upon differentiation, but the Allee effect model has the advantage of allowing differentiation to occur stochastically in a sub-set of cells. This mathematical framework for describing intra-cellular OCT4 regulation can be extended to other transcription factors and developed into predictive models.

High-Throughput Routes to Biomaterials Discovery

Many existing clinical treatments are limited in their ability to completely restore decreased or lost tissue and organ function, an unenviable situation only further exacerbated by a globally aging population. As a result, the demand for new medical interventions has increased substantially over the past 20 years, with the burgeoning fields of gene therapy, tissue engineering, and regenerative medicine showing promise to offer solutions for full repair or replacement of damaged or aging tissues. Success in these fields, however, inherently relies on biomaterials that are engendered with the ability to provide the necessary biological cues mimicking native extracellular matrixes that support cell fate. Accelerating the development of such "directive" biomaterials requires a shift in current design practices toward those that enable rapid synthesis and characterization of polymeric materials and the coupling of these processes with techniques that enable similarly rapid quantification and optimization of the interactions between these new material systems and target cells and tissues. This manuscript reviews recent advances in combinatorial and high-throughput (HT) technologies applied to polymeric biomaterial synthesis, fabrication, and chemical, physical, and biological screening with targeted end-point applications in the fields of gene therapy, tissue engineering, and regenerative medicine. Limitations of, and future opportunities for, the further application of these research tools and methodologies are also discussed.

Binary Colloidal Crystal (BCC) Substrates for Controlling the Fate of Mouse Embryonic Stem Cells

Understanding the interactions of stem cells with surface topography can give us an invaluable tool in controlling stemness and fate of stem cells for further use in biomedical applications. In this study, we have fabricated topographical features using a class of cell culture substrates called binary colloidal crystals (BCCs), that are made by self-assembly of mixtures of spherical micron sized silica (Si) and nanometer sized polystyrene (PS) or poly (methyl methacrylate) (PMMA) particles. The substrates formed are arrays of ordered, hexagonally packed large Si particles inter-dispersed with the PS particles that are stabilized by gentle heating, which melts the PS or PMMA forming substrates suitable for cell culture. BCC substrates were used for culture of mouse embryonic stem cells (mESCs). Compared to tissue culture plates, COM1 (Si5-PMMA0.4), COM2 (Si5-PS0.4) and COM4 (Si2-PSC0.22) have shown to provide a better support for mESC proliferation in the presence of the cytokine leukemia inhibitory factor (LIF). The behavior of mESCs with the BCCs in presence and absence of LIF, was further explored and it was found that interaction of mESCs with the culture substrate can be controlled by tuning surface topography and roughness, which is determined by the size and type of particles used in making BCCs. Furthermore, it was shown that limiting cell-surface interactions and controlling colony shape can promote stemness maintenance on COM1 and COM2 substrates as indicated by better proliferation and higher expression of pluripotency genes including Nanog both in presence and in absence of LIF. Together with higher expression of GATA6 gene, it can be stated that these surfaces can be used for endodermic priming of mESCs. Therefore, we believe that these surfaces, especially COM1 and COM2 surfaces can be beneficial as stem cell culture systems for further use in biomedical research.

Tenocyte-imprinted substrate: a topography-based inducer for tenogenic differentiation in adipose tissue-derived mesenchymal stem cells

Tendon tissue engineering based on stem cell differentiation has attracted a great deal of attention in recent years. Previous studies have examined the effect of cell-imprinted polydimethylsiloxane (PDMS) substrate on induction differentiation in stem cells. In this study, we used tenocyte morphology as a positive mold to create a tenocyte-imprinted substrate on PDMS. The morphology and topography of this tenocyte replica on PDMS was evaluated with scanning electron microscopy (SEM) and atomic force microscopy. The tenogenic differentiation induction capacity of the tenocyte replica in adipose tissue-derived mesenchymal stem cells (ADSCs) was then investigated and compared with other groups, including tissue replica (which was produced similarly to the tenocyte replica and was evaluated by SEM), decellularized tendon, and bone morphogenic protein (BMP)-12, as other potential inducers. This comparison gives us an estimate of the ability of tenocyte-imprinted PDMS (called cell replica in the present study) to induce differentiation compared to other inducers. For this reason, ADSCs were divided into five groups, including control, cell replica, tissue replica, decellularized tendon and BMP-12. ADSCs were seeded on each group separately and investigated by the real-time reverse transcription polymerase chain reaction (RT-PCR) technique after seven and 14 days. Our results showed that in spite of the higher effect of the growth factor on tenogenic differentiation, the cell replica can also induce tenocyte marker expression (scleraxis and tenomodulin) in ADSCs. Moreover, the tenogenic differentiation induction capacity of the cell replica was greater than tissue replica. Immunocytochemistry analysis revealed that ADSCs seeding on the cell replica for 14 days led to scleraxis and tenomodulin expression at the protein level. In addition, immunohistochemistry indicated that contrary to the promising results in vitro, there was little difference between ADSCs cultured on tenocyte-imprinted PDMS and untreated ADSCs. The results of such studies could lead to the production of inexpensive cell culture plates or biomaterials that can induce differentiation in stem cells without growth factors or other supplements.

Screening as a strategy to drive regenerative medicine research

In the field of regenerative medicine, optimization of the parameters leading to a desirable outcome remains a huge challenge. Examples include protocols for the guided differentiation of pluripotent cells towards specialized and functional cell types, phenotypic maintenance of primary cells in cell culture, or engineering of materials for improved tissue interaction with medical implants. This challenge originates from the enormous design space for biomaterials, chemical and biochemical compounds, and incomplete knowledge of the guiding biological principles. To tackle this challenge, high-throughput platforms allow screening of multiple perturbations in one experimental setup. In this review, we provide an overview of screening platforms that are used in regenerative medicine. We discuss their fabrication techniques, and in silico tools to analyze the extensive data sets typically generated by these platforms.

High-Aspect-Ratio Nanostructured Surfaces as Biological Metamaterials

Materials patterned with high-aspect-ratio nanostructures have features on similar length scales to cellular components. These surfaces are an extreme topography on the cellular level and have become useful tools for perturbing and sensing the cellular environment. Motivation comes from the ability of high-aspect-ratio nanostructures to deliver cargoes into cells and tissues, access the intracellular environment, and control cell behavior. These structures directly perturb cells' ability to sense and respond to external forces, influencing cell fate, and enabling new mechanistic studies. Through careful design of their nanoscale structure, these systems act as biological metamaterials, eliciting unusual biological responses. While predominantly used to interface eukaryotic cells, there is growing interest in nonanimal and prokaryotic cell interfacing. Both experimental and theoretical studies have attempted to develop a mechanistic understanding for the observed behaviors, predominantly focusing on the cell-nanostructure interface. This review considers how high-aspect-ratio nanostructured surfaces are used to both stimulate and sense biological systems.

Emerging Methods for Enhancing Pluripotent Stem Cell Expansion

Pluripotent stem cells (PSCs) have great potential to revolutionize the fields of tissue engineering and regenerative medicine as well as stem cell therapeutics. However, the end goal of using PSCs for therapeutic use remains distant due to limitations in current PSC production. Conventional methods for PSC expansion have limited potential to be scaled up to produce the number of cells required for the end-goal of therapeutic use due to xenogenic components, high cost or low efficiency. In this mini review, we explore novel methods and emerging technologies of improving PSC expansion: the use of the two-dimensional mechanobiological strategies of topography and stiffness and the use of three-dimensional (3D) expansion methods including encapsulation, microcarrier-based culture, and suspension culture. Additionally, we discuss the limitations of conventional PSC expansion methods as well as the challenges in implementing non-conventional methods.

Application of amphiphilic fluorophore-derived nanoparticles to provide contrast to human embryonic stem cells without affecting their pluripotency and to monitor their differentiation into neuron-like cells

Fluorogenic labeling is a potential technique in biology that allows for direct detection and tracking of cells undergoing various biological processes. Compared to traditional genetic modification approaches, labeling cells with nanoparticles has advantages, especially for the additional safety they provide by avoiding genomic integration. However, it remains a challenge to determine whether nanoparticles interfere with cell traits and provide long-lasting signals in living cells. We employed an amphiphilic fluorophore-derived nanoparticle (denoted by TPE-11) bearing a tetraphenylethene (TPE) moiety and two ionic heads; this nanoparticle has an aggregation-induced emission (AIE) effect and the ability to self-assemble. TPE-11 exhibited the property of higher or longer fluorescence intensities in cell imaging than the other two nanomaterials under the same conditions. We used this nanomaterial to label human embryonic stem (hES) cells and monitor their differentiation. Treatment with low concentrations of TPE-11 (8.0 μg/mL) resulted in high-intensity labeling of hES cells, and immunostaining analysis and teratoma formation assays showed that at this concentration, their pluripotency remained unaltered. TPE-11 nanoparticles allowed for long-term monitoring of hES cell differentiation into neuron-like cells; remarkably, strong nanoparticle signals were detected throughout the nearly 40-day differentiation process. Thus, these results demonstrate that the TPE-11 nanoparticle has excellent biocompatibility for hES cells and is a potential fluorogen for labeling and tracking the differentiation of human pluripotent stem cells.

STATEMENT OF SIGNIFICANCE

This study uses a nanoparticle-based approach to label human embryonic stem (hES) cells and monitor their differentiation. hES cells are distinguished by two distinctive properties: the state of their pluripotency and the potential to differentiate into various cell types. Thus, these cells will be useful as a source of cells for transplantation or tissue engineering applications. We noticed the effect of aggregation-induced emission, and the ability to self-assemble could enhance the persistence of signals. Treatment with low concentrations of TPE-11 nanoparticles showed high-intensity labeling of hES cells, and immunostaining analysis and teratoma formation assays showed that at this concentration, their pluripotency remained unaltered. Additionally, these nanoparticles allowed for long-term monitoring of hES cell differentiation into neuron-like cells lasting for 40 days.

Shaping Cell Fate: Influence of Topographical Substratum Properties on Embryonic Stem Cells

Development of multicellular organisms is a highly orchestrated process, with cells responding to factors and features present in the extracellular milieu. Changes in the surrounding environment help decide the fate of cells at various stages of development. This review highlights recent research that details the effects of mechanical properties of the surrounding environment and extracellular matrix and the underlying molecular mechanisms that regulate the behavior of embryonic stem cells (ESCs). In this study, we review the role of mechanical properties during embryogenesis and discuss the effect of engineered microtopographies on ESC pluripotency.

Insoluble Microenvironment Facilitating the Generation and Maintenance of Pluripotency

Induced pluripotent stem cells (iPSCs) hold enormous potential as a tool to generate cells for tissue engineering and regenerative medicine. Since the initial report of iPSCs in 2006, many different methods have been developed to enhance the safety and efficiency of this technology. Recent studies indicate that the extracellular signals can promote the production of iPSCs, and even replace the Yamanaka factors. Noticeably, abundant evidences suggest that the insoluble microenvironment, including the culture substrate and neighboring cells, directly regulates the expression of core pluripotency genes and the epigenetic modification of the chromatins, hence, impacts the reprogramming dynamics. These studies provide new strategies for developing safer and more efficient method for iPSC generation. In this review, we examine the publications addressing the insoluble extracellular microenvironment that boosts iPSC generation and self-renewal. We also discuss cell adhesion-mediated molecular mechanisms, through which the insoluble extracellular cues interplay with reprogramming.

Quasi-Stem Cells Derived from Human Somatic Cells by Chemically Modified Carbon Nanotubes

Surface modification of micro- and nanotopography was employed to alter the surface properties of scaffolds for controlling cell attachment, proliferation, and differentiation. This study reports a method for generating multinucleated colonies as evidenced by spherical colony formation through nanotopography-induced expression of reprogramming factors in human dermal fibroblasts. Colony formation was achieved by subjecting the cells to specific environments such as culturing with single-walled carbon nanotubes and poly-l-lysine (PLL-SWCNTs). We obtained encouraging results showing that PLL-SWCNT treatment transformed fibroblast cells, and the transformed cells expressed the pluripotency-associated factors OCT4, NANOG, and SOX2 in addition to TRA-1-60 and SSEA-4, which are characteristic stem cell markers. Downregulation of lamin A/C, HDAC1, HDAC6, Bcl2, cytochrome c, p-FAK, p-ERK, and p-JNK and upregulation of H3K4me3 and p-p38 were confirmed in the generated colonies, indicating reprogramming of cells. This protocol increases the possibility of successfully reprogramming somatic cells into induced pluripotent stem cells (iPSCs), thereby overcoming the difficulties in iPSC generation such as genetic mutations, carcinogenesis, and undetermined risk factors.

Breath figures in tissue engineering and drug delivery: State-of-the-art and future perspectives

The breath figure (BF) method is an easy, low-cost method to prepare films with a highly organized honeycomb-like porous surface. The particular surface topography and porous nature of these materials makes them valuable substrates for studying the complex effects of topography on cell fate, and to produce biomimetic materials with high performance in tissue engineering. Numerous researchers over the last two decades have studied the effects of the honeycomb topography on a variety of primary and immortalized cell lines, and drew important conclusions that can be translated to the construction of optimal biomaterials for cell culture. The literature also encouragingly shows the potential of honeycomb films to induce differentiation of stem cells down a specific lineage without the need for biochemical stimuli. Here, we review the main studies where BF honeycomb films are used as substrates for tissue engineering applications. Furthermore, we highlight the numerous advantages of the porous nature of the films, such as the enhanced, spatially controlled adsorption of proteins, the topographical cues influencing cellular behavior, and the enhanced permeability which is essential both in vitro and in vivo. Finally, this review highlights the elegant use of honeycomb films as drug-eluting biomaterials or as reservoirs for distinct drug delivery systems.

STATEMENT OF SIGNIFICANCE

Combining biocompatible surfaces and 3D nano/microscale topographies, such as pores or grooves, is an effective strategy for manufacturing tissue engineering scaffolds. The breath figure (BF) method is an easy technique to prepare cell culture substrates with an organized, honeycomb-like porous surface. These surface features make these scaffolds valuable for studying how the cells interact with the biomaterials. Their unique surface topography can also resemble the natural environment of the tissues in the human body. For that reason, numerous studies, using different cell types, have shown that honeycomb films can constitute high performance substrates for cell culture. Here, we review those studies, we highlight the advantages of honeycomb films in tissue engineering and we discuss their potential as unique drug-eluting systems.

Cell density-dependent differential proliferation of neural stem cells on omnidirectional nanopore-arrayed surface

Recently, the importance of surface nanotopography in the determination of stem cell fate and behavior has been revealed. In the current study, we generated polystyrene cell-culture dishes with an omnidirectional nanopore arrayed surface (ONAS) (diameter: 200 nm, depth: 500 nm, center-to-center distance: 500 nm) and investigated the effects of nanotopography on rat neural stem cells (NSCs). NSCs cultured on ONAS proliferated better than those on the flat surface when cell density was low and showed less spontaneous differentiation during proliferation in the presence of mitogens. Interestingly, NSCs cultured on ONAS at clonal density demonstrated a propensity to generate neurospheres, whereas those on the flat surface migrated out, proliferated as individuals, and spread out to attach to the surface. However, the differential patterns of proliferation were cell density-dependent since the distinct phenomena were lost when cell density was increased. ONAS modulated cytoskeletal reorganization and inhibited formation of focal adhesion, which is generally observed in NSCs grown on flat surfaces. ONAS appeared to reinforce NSC-NSC interaction, restricted individual cell migration and prohibited NSC attachment to the nanopore surface. These data demonstrate that ONAS maintains NSCs as undifferentiated while retaining multipotency and is a better topography for culturing low density NSCs.

Nanotopographic Influence on the In Vitro Behavior of Induced Pluripotent Stem Cells

While the influence of nanotopography on stem cell behavior has been extensively investigated on adult stem cells, far fewer studies have investigated the interaction of induced pluripotent stem cells (iPSCs) with various nanotopographical patterns. The purpose of this study was to identify nanopatterns that can influence the stemness and proliferation, as well as the adhesive properties in iPSCs, and thereby explore the feasibility of applying these nano-features for regenerative medicine. Three kinds of nanopatterns were fabricated from polydimethylsiloxane membranes, irregular patterned membrane (IPM), groove patterned membrane (GPM), and postpatterned membrane (PPM), in addition to flat patterned membrane (FPM) which did not have any nanotopographic features and was used as the control pattern. On the surfaces of GPM or PPM, iPSCs showed tendency for aggregation and did not spread out well at passage 1. However, with continued passaging (P6, P10), the tendency to form aggregates was greatly reduced. While iPSCs cultured on GPM and PPM had low population doubling time values compared with FPM and IPM at P1, the differences disappeared in later passages. The expression of the cell proliferation marker Ki67 in iPSCs gradually decreased with continued passaging in cells cultured on FPM and IPM, but not in those cultured on GPM and PPM. The expression of Oct3/4 and Nanog, marker of stemness, was significantly higher on GPM and PPM than on FPM at P6 and P10. At P5, numerous filopodia were demonstrated in the peripheral attachments of iPSC colonies on FPM and IPM, while GPM and PPM generally had globular appearance. The expression of the focal adhesion (FA) molecules α-actinin, vinculin, phalloidin, or FA kinase was significantly greater on GPM and PPM than on FPM and IPM at P6 or P10. In conclusion, continued passaging on regular nanopatterns, including groove- and post-forms, was effective in maintaining an undifferentiated state and proliferation of iPSCs and also in increasing the expression of FA molecules.

Current approaches for modulation of the nanoscale interface in the regulation of cell behavior

Regulation of cell behavior in response to nanoscale features has been the focus of much research in recent years and the successful generation of nanoscale features capable of mimicking the natural nanoscale interface has been of great interest in the field of biomaterials research. In this review, we discuss relevant nanofabrication techniques and how they are combined with bioengineering applications to mimic the natural extracellular matrix (ECM) and create valuable nanoscale interfaces.

The Horizon of Materiobiology: A Perspective on Material-Guided Cell Behaviors and Tissue Engineering

Although the biological functions of cell and tissue can be regulated by biochemical factors (e.g., growth factors, hormones), the biophysical effects of materials on the regulation of biological activity are receiving more attention. In this Review, we systematically summarize the recent progress on how biomaterials with controllable properties (e.g., compositional/degradable dynamics, mechanical properties, 2D topography, and 3D geometry) can regulate cell behaviors (e.g., cell adhesion, spreading, proliferation, cell alignment, and the differentiation or self-maintenance of stem cells) and tissue/organ functions. How the biophysical features of materials influence tissue/organ regeneration have been elucidated. Current challenges and a perspective on the development of novel materials that can modulate specific biological functions are discussed. The interdependent relationship between biomaterials and biology leads us to propose the concept of "materiobiology", which is a scientific discipline that studies the biological effects of the properties of biomaterials on biological functions at cell, tissue, organ, and the whole organism levels. This Review highlights that it is more important to develop ECM-mimicking biomaterials having a self-regenerative capacity to stimulate tissue regeneration, instead of attempting to recreate the complexity of living tissues or tissue constructs ex vivo. The principles of materiobiology may benefit the development of novel biomaterials providing combinative bioactive cues to activate the migration of stem cells from endogenous reservoirs (i.e., cell niches), stimulate robust and scalable self-healing mechanisms, and unlock the body's innate powers of regeneration.

Correlation of focal adhesion assembly and disassembly with cell migration on nanotopography

Selective cell adhesion is desirable to control cell growth and migration on biomedical implants. Mesenchymal cell migration is regulated through focal adhesions (FAs) and can be modulated by their microenvironment, including changes in surface topography. We use the Number and Molecular Brightness (N&B) imaging analysis to provide a unique perspective on FA assembly and disassembly. This imaging analysis generates a map of real-time fluctuations of protein monomers, dimers, and higher order aggregates of FA proteins, such as paxillin during assembly and disassembly. We show a dynamic view of how nanostructured surfaces (nanoline gratings or nanopillars) regulate single molecular dynamics. In particular, we report that the smallest nanopillars (100 nm spacing) gave rise to a low population of disassembling adhesion clusters of ∼2 paxillin proteins whereas the larger nanopillars (380 nm spacing) gave rise to a much larger population of larger disassembling clusters of ∼3-5 paxillin proteins. Cells were more motile on the smaller nanopillars (spaced 100-130 nm apart) compared to all other surfaces studied. Thus, physical nanotopography influences cell motility, adhesion size, and adhesion assembly and disassembly. We report for the first time, with single molecular detection, how nanotopography influences cell motility and protein reorganization in adhesions.

Two-photon polymerized "nichoid" substrates maintain function of pluripotent stem cells when expanded under feeder-free conditions

Background

The use of pluripotent cells in stem cell therapy has major limitations, mainly related to the high costs and risks of exogenous conditioning and the use of feeder layers during cell expansion passages.

Methods

We developed an innovative three-dimensional culture substrate made of “nichoid” microstructures, nanoengineered via two-photon laser polymerization. The nichoids limit the dimension of the adhering embryoid bodies during expansion, by counteracting cell migration between adjacent units of the substrate by its microarchitecture. We expanded mouse embryonic stem cells on the nichoid for 2 weeks. We compared the expression of pluripotency and differentiation markers induced in cells with that induced by flat substrates and by a culture layer made of kidney-derived extracellular matrix.

Results

The nichoid was found to be the only substrate, among those tested, that maintained the expression of the OCT4 pluripotency marker switched on and, simultaneously, the expression of the differentiation markers GATA4 and α-SMA switched off. The nichoid promotes pluripotency maintenance of embryonic stem cells during expansion, in the absence of a feeder layer and exogenous conditioning factors, such as the leukocyte inhibitory factor.

Conclusions

We hypothesized that the nichoid microstructures induce a genetic reprogramming of cells by controlling their cytoskeletal tension. Further studies are necessary to understand the exact mechanism by which the physical constraint provided by the nichoid architecture is responsible for cell reprogramming. The nichoid may help elucidate mechanisms of pluripotency maintenance, while potentially cutting the costs and risks of both feed-conditioning and exogenous conditioning for industrial-scale expansion of stem cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0387-z) contains supplementary material, which is available to authorized users.

Mesenchymal Stem Cell Fate: Applying Biomaterials for Control of Stem Cell Behavior

The materials pipeline for biomaterials and tissue engineering applications is under continuous development. Specifically, there is great interest in the use of designed materials in the stem cell arena as materials can be used to manipulate the cells providing control of behavior. This is important as the ability to "engineer" complexity and subsequent in vitro growth of tissues and organs is a key objective for tissue engineers. This review will describe the nature of the materials strategies, both static and dynamic, and their influence specifically on mesenchymal stem cell fate.

Substrate topography: A valuable in vitro tool, but a clinical red herring for in vivo tenogenesis

Controlling the cell-substrate interactions at the bio-interface is becoming an inherent element in the design of implantable devices. Modulation of cellular adhesion in vitro, through topographical cues, is a well-documented process that offers control over subsequent cellular functions. However, it is still unclear whether surface topography can be translated into a clinically functional response in vivo at the tissue/device interface. Herein, we demonstrated that anisotropic substrates with a groove depth of ∼317nm and ∼1988nm promoted human tenocyte alignment parallel to the underlying topography in vitro. However, the rigid poly(lactic-co-glycolic acid) substrates used in this study upregulated the expression of chondrogenic and osteogenic genes, indicating possible tenocyte trans-differentiation. Of significant importance is that none of the topographies assessed (∼37nm, ∼317nm and ∼1988nm groove depth) induced extracellular matrix orientation parallel to the substrate orientation in a rat patellar tendon model. These data indicate that two-dimensional imprinting technologies are useful tools for in vitro cell phenotype maintenance, rather than for organised neotissue formation in vivo, should multifactorial approaches that consider both surface topography and substrate rigidity be established.

STATEMENT OF SIGNIFICANCE

Herein, we ventured to assess the influence of parallel groves, ranging from nano- to micro-level, on tenocytes response in vitro and on host response using a tendon and a subcutaneous model. In vitro analysis indicates that anisotropically ordered micro-scale grooves, as opposed to nano-scale grooves, maintain physiological cell morphology. The rather rigid PLGA substrates appeared to induce trans-differentiation towards chondrogenic and/or steogenic lineage, as evidence by TILDA gene analysis. In vivo data in both tendon and subcutaneous models indicate that none of the substrates induced bidirectional host cell and tissue growth. Collective, these observations indicate that two-dimensional imprinting technologies are useful tools for in vitro cell phenotype maintenance, rather than for directional neotissue formation, should multifactorial approaches that consider both surface topography and substrate rigidity be established.

Hierarchical Micro-Nano Surface Topography Promotes Long-Term Maintenance of Undifferentiated Mouse Embryonic Stem Cells

Understanding of stem cell-surface interactions and, in particular, long-term maintenance of stem cell pluripotency on well-defined synthetic surfaces is crucial for fundamental research and biomedical applications of stem cells. Here, we show that synthetic surfaces possessing hierarchical micro-nano roughness (MN-surfaces) promote long-term self-renewal (>3 weeks) of mouse embryonic stem cells (mESCs) as monitored by the expression levels of the pluripotency markers octamer-binding transcription factor 4 (Oct4), Nanog, and alkaline phosphatase. On the contrary, culturing of mESCs on either smooth (S-) or nanorough polymer surfaces (N-surfaces) leads to their fast differentiation. Moreover, we show that regular passaging of mESCs on the hierarchical MN-polymer surface leads to an increased homogeneity and percentage of Oct4-positive stem cell colonies as compared to mESCs grown on fibroblast feeder cells. Immunostaining revealed the absence of focal adhesion markers on all polymer substrates studied. However, only the MN-surfaces elicited the formation of actin-positive cell protrusions, indicating an alternative anchorage mechanism involved in the maintenance of mESC stemness.

Bilayered vascular graft derived from human induced pluripotent stem cells with biomimetic structure and function

BACKGROUND

We developed an aligned bi-layered vascular graft derived from human induced pluripotent stem cells (iPSCs) that recapitulates the cellular composition, orientation, and anti-inflammatory function of blood vessels.

MATERIALS & METHODS

The luminal layer consisted of longitudinal-aligned nanofibrillar collagen containing primary endothelial cells (ECs) or iPSC-derived ECs (iPSC-ECs). The outer layer contained circumferentially oriented nanofibrillar collagen with primary smooth muscle cells (SMCs) or iPSC-derived SMCs(iPSC-SMCs).

RESULTS

On the aligned scaffolds, cells organized F-actin assembly within 8º from the direction of nanofibrils. When compared to randomly-oriented scaffolds, EC-seeded aligned scaffolds had significant reduced inflammatory response, based on adhesivity to monocytes.

CONCLUSION

This study highlights the importance of anisotropic scaffolds in directing cell form and function, and has therapeutic significance as physiologically relevant blood vessels.

Engineering nanoscale stem cell niche: direct stem cell behavior at cell-matrix interface

Biophysical cues on the extracellular matrix (ECM) have proven to be significant regulators of stem cell behavior and evolution. Understanding the interplay of these cells and their extracellular microenvironment is critical to future tissue engineering and regenerative medicine, both of which require a means of controlled differentiation. Research suggests that nanotopography, which mimics the local, nanoscale, topographic cues within the stem cell niche, could be a way to achieve large-scale proliferation and control of stem cells in vitro. This Progress Report reviews the history and contemporary advancements of this technology, and pays special attention to nanotopographic fabrication methods and the effect of different nanoscale patterns on stem cell response. Finally, it outlines potential intracellular mechanisms behind this response.

Regulation of stem cell fate by nanomaterial substrates

Stem cells are increasingly studied because of their potential to underpin a range of novel therapies, including regenerative strategies, cell type-specific therapy and tissue repair, among others. Bionanomaterials can mimic the stem cell environment and modulate stem cell differentiation and proliferation. New advances in these fields are presented in this review. This work highlights the importance of topography and elasticity of the nano-/micro-environment, or niche, for the initiation and induction of stem cell differentiation and proliferation.

Nanotopographical Surfaces for Stem Cell Fate Control: Engineering Mechanobiology from the Bottom

During embryogenesis and tissue maintenance and repair in an adult organism, a myriad of stem cells are regulated by their surrounding extracellular matrix (ECM) enriched with tissue/organ-specific nanoscale topographical cues to adopt different fates and functions. Attributed to their capability of self-renewal and differentiation into most types of somatic cells, stem cells also hold tremendous promise for regenerative medicine and drug screening. However, a major challenge remains as to achieve fate control of stem cells in vitro with high specificity and yield. Recent exciting advances in nanotechnology and materials science have enabled versatile, robust, and large-scale stem cell engineering in vitro through developments of synthetic nanotopographical surfaces mimicking topological features of stem cell niches. In addition to generating new insights for stem cell biology and embryonic development, this effort opens up unlimited opportunities for innovations in stem cell-based applications. This review is therefore to provide a summary of recent progress along this research direction, with perspectives focusing on emerging methods for generating nanotopographical surfaces and their applications in stem cell research. Furthermore, we provide a review of classical as well as emerging cellular mechano-sensing and -transduction mechanisms underlying stem cell nanotopography sensitivity and also give some hypotheses in regard to how a multitude of signaling events in cellular mechanotransduction may converge and be integrated into core pathways controlling stem cell fate in response to extracellular nanotopography.

Topography design concept of a tissue engineering scaffold for controlling cell function and fate through actin cytoskeletal modulation

The physiological role of the actin cytoskeleton is well known: it provides mechanical support and endogenous force generation for formation of a cell shape and for migration. Furthermore, a growing number of studies have demonstrated another significant role of the actin cytoskeleton: it offers dynamic epigenetic memory for guiding cell fate, in particular, proliferation and differentiation. Because instantaneous imbalance in the mechanical homeostasis is adjusted through actin remodeling, a synthetic extracellular matrix (ECM) niche as a source of topographical and mechanical cues is expected to be effective at modulation of the actin cytoskeleton. In this context, the synthetic ECM niche determines cell migration, proliferation, and differentiation, all of which have to be controlled in functional tissue engineering scaffolds to ensure proper regulation of tissue/organ formation, maintenance of tissue integrity and repair, and regeneration. Here, with an emphasis on the epigenetic role of the actin cytoskeletal system, we propose a design concept of micro/nanotopography of a tissue engineering scaffold for control of cell migration, proliferation, and differentiation in a stable and well-defined manner, both in vitro and in vivo.

Human pluripotent stem cells on artificial microenvironments: a high content perspective

Self-renewing stem cell populations are increasingly considered as resources for cell therapy and tools for drug discovery. Human pluripotent stem (hPS) cells in particular offer a virtually unlimited reservoir of homogeneous cells and can be differentiated toward diverse lineages. Many diseases show impairment in self-renewal or differentiation, abnormal lineage choice or other aberrant cell behavior in response to chemical or physical cues. To investigate these responses, there is a growing interest in the development of specific assays using hPS cells, artificial microenvironments and high content analysis. Several hurdles need to be overcome that can be grouped into three areas: (i) availability of robust, homogeneous, and consistent cell populations as a starting point; (ii) appropriate understanding and use of chemical and physical microenvironments; (iii) development of assays that dissect the complexity of cell populations in tissues while mirroring specific aspects of their behavior. Here we review recent progress in the culture of hPS cells and we detail the importance of the environment surrounding the cells with a focus on synthetic material and suitable high content analysis approaches. The technologies described, if properly combined, have the potential to create a paradigm shift in the way diseases are modeled and drug discovery is performed.

Extracellular matrix elasticity and topography: material-based cues that affect cell function via conserved mechanisms

Chemical, mechanical, and topographic extracellular matrix (ECM) cues have been extensively studied for their influence on cell behavior. These ECM cues alter cell adhesion, cell shape, and cell migration and activate signal transduction pathways to influence gene expression, proliferation, and differentiation. ECM elasticity and topography, in particular, have emerged as material properties of intense focus based on strong evidence these physical cues can partially dictate stem cell differentiation. Cells generate forces to pull on their adhesive contacts, and these tractional forces appear to be a common element of cells' responses to both elasticity and topography. This review focuses on recently published work that links ECM topography and mechanics and their influence on differentiation and other cell behaviors. We also highlight signaling pathways typically implicated in mechanotransduction that are (or may be) shared by cells subjected to topographic cues. Finally, we conclude with a brief discussion of the potential implications of these commonalities for cell based therapies and biomaterial design.

The importance of three-dimensional scaffold structure on stemness maintenance of mouse embryonic stem cells

Revealing the mechanisms of cell fate regulation is important for scientific research and stem cell-based therapy. The traditional two-dimensional (2D) cultured mES cells are in a very different 2D niche from the in vivo equivalent-inner cell mass (ICM). Because the cell fate decision could be regulated by many cues which could be impacted by geometry, the traditional 2D culture system would hamper us from understanding the in vivo situations correctly. Three-dimensional (3D) scaffold was believed to provide a 3D environment closed to the in vivo one. In this work, three different scaffolds were prepared for cell culture. Several characters of mES cells were changed under 3D scaffolds culture compared to 2D, and these changes were mainly due to the alteration in geometry but not the matrix. The self-renewal of mES cells was promoted by the introducing of dimensionality. The stemness maintenance of mES was supported by all three 3D scaffolds without feeder cells in the long-time culture. Our findings demonstrated that the stemness maintenance of mES cells was promoted by the 3D geometry of scaffolds and this would provide a promising platform for ES cell research.

Materials as stem cell regulators

The stem cell/material interface is a complex, dynamic microenvironment in which the cell and the material cooperatively dictate one another's fate: the cell by remodelling its surroundings, and the material through its inherent properties (such as adhesivity, stiffness, nanostructure or degradability). Stem cells in contact with materials are able to sense their properties, integrate cues via signal propagation and ultimately translate parallel signalling information into cell fate decisions. However, discovering the mechanisms by which stem cells respond to inherent material characteristics is challenging because of the highly complex, multicomponent signalling milieu present in the stem cell environment. In this Review, we discuss recent evidence that shows that inherent material properties may be engineered to dictate stem cell fate decisions, and overview a subset of the operative signal transduction mechanisms that have begun to emerge. Further developments in stem cell engineering and mechanotransduction are poised to have substantial implications for stem cell biology and regenerative medicine.

Harnessing nanotopography and integrin-matrix interactions to influence stem cell fate

Stem cells respond to nanoscale surface features, with changes in cell growth and differentiation mediated by alterations in cell adhesion. The interaction of nanotopographical features with integrin receptors in the cells' focal adhesions alters how the cells adhere to materials surfaces, and defines cell fate through changes in both cell biochemistry and cell morphology. In this Review, we discuss how cell adhesions interact with nanotopography, and we provide insight as to how materials scientists can exploit these interactions to direct stem cell fate and to understand how the behaviour of stem cells in their niche can be controlled. We expect knowledge gained from the study of cell-nanotopography interactions to accelerate the development of next-generation stem cell culture materials and implant interfaces, and to fuel discovery of stem cell therapeutics to support regenerative therapies.

The effect of physical and chemical cues on hepatocellular function and morphology

Physical topographical features and/or chemical stimuli to the extracellular matrix (ECM) provide essential cues that manipulate cell functions. From the physical point of view, contoured nanostructures are very important for cell behavior in general, and for cellular functions. From the chemical point of view, ECM proteins containing an RGD sequence are known to alter cell functions. In this study, the influence of integrated physical and chemical cues on a liver cell line (HepG2) was investigated. To mimic the physical cues provided by the ECM, amorphous TiO2 nanogratings with specific dimensional and geometrical characteristics (nanogratings 90 nm wide and 150 nm apart) were fabricated. To mimic the chemical cues provided by the ECM, the TiO2 inorganic film was modified by immobilization of the RGD motif. The hepatic cell line morphological and functional changes induced by simultaneously combining these diversified cues were investigated, including cellular alignment and the expression of different functional proteins. The combination of nanopatterns and surface modification with RGD induced cellular alignment and expression of functional proteins, indicating that physical and chemical cues are important factors for optimizing hepatocyte function.

Surface engineering of synthetic polymer materials for tissue engineering and regenerative medicine applications

When using polymer materials as scaffolds for tissue engineering or regenerative medicine applications the initial, and often lasting, interaction between cells and the material areviasurfaces.

Engineered micromechanical cues affecting human pluripotent stem cell regulations and fate

The survival, growth, self-renewal, and differentiation of human pluripotent stem cells (hPSCs) are influenced by their microenvironment, or so-called "niche," consisting of particular chemical and physical cues. Previous studies on mesenchymal stem cells and other stem cells have collectively uncovered the importance of physical cues and have begun to shed light on how stem cells sense and process such cues. In an attempt to support similar progress in mechanobiology of hPSCs, we review mechanosensory machinery, which plays an important role in cell-extracellular matrix interactions, cell-cell interactions, and subsequent intracellular responses. In addition, we review recent studies on the mechanobiology of hPSCs, in which engineered micromechanical environments were used to investigate effects of specific physical cues. Identifying key physical cues and understanding their mechanism will ultimately help in harnessing the full potential of hPSCs for clinical applications.