Forcing stem cells to behave: a biophysical perspective of the cellular microenvironment

Unravelling the effects of mechanical physiological conditioning on cardiac adipose tissue-derived progenitor cells in vitro and in silico

Mechanical conditioning is incompletely characterized for stimulating therapeutic cells within the physiological range. We sought to unravel the mechanism of action underlying mechanical conditioning of adipose tissue-derived progenitor cells (ATDPCs), both in vitro and in silico. Cardiac ATDPCs, grown on 3 different patterned surfaces, were mechanically stretched for 7 days at 1 Hz. A custom-designed, magnet-based, mechanical stimulator device was developed to apply ~10% mechanical stretching to monolayer cell cultures. Gene and protein analyses were performed for each cell type and condition. Cell supernatants were also collected to analyze secreted proteins and construct an artificial neural network. Gene and protein modulations were different for each surface pattern. After mechanostimulation, cardiac ATDPCs increased the expression of structural genes and there was a rising trend on cardiac transcription factors. Finally, secretome analyses revealed upregulation of proteins associated with both myocardial infarction and cardiac regeneration, such as regulators of the immune response, angiogenesis or cell adhesion. To conclude, mechanical conditioning of cardiac ATDPCs enhanced the expression of early and late cardiac genes in vitro. Additionally, in silico analyses of secreted proteins showed that mechanical stimulation of cardiac ATDPCs was highly associated with myocardial infarction and repair.

Identifying drug resistant cancer cells using microbubble well arrays

Drug resistance is a characteristic of tumor initiating cells that can give rise to metastatic disease. In this work we demonstrate the use of microbubble well arrays as a cell culture platform to enumerate and characterize drug resistant cells in a human derived tumorigenic squamous cell carcinoma cell line. The spherical architecture and compliant hydrophobic composition of the microbubble well favors single cell survival, clonal proliferation and formation of spheres that do not grow on standard tissue culture plastic and are resistant to cisplatin. Spheres form in isolation and in microbubble wells containing proliferating cells and to some degree they stain positive for common stem cell markers CD44 and CD133. Spheres are also observed in cellularized primary human tumors cultured in microbubble arrays. This proof-of-concept study illustrates the potential for microbubble array technology to enumerate cancer cells resistant to standard care drugs with the ability to test alternative drug combinations. This capability can be developed for designing patient specific treatment strategies. Recovery of drug-resistant cells will allow a more full characterization of their gene expression profile thereby expanding our fundamental knowledge and ability to develop new targets to fight metastatic disease.

Physical constraints in cell fate specification. A case in point: Microgravity and phenotypes differentiation

Data obtained by studying mammalian cells in absence of gravity strongly support the notion that cell fate specification cannot be understood according to the current molecular model. A paradigmatic case in point is provided by studying cell populations growing in absence of gravity. When the physical constraint (gravity) is 'experimentally removed', cells spontaneously allocate into two morphologically different phenotypes. Such phenomenon is likely enacted by the intrinsic stochasticity, which, in turn, is successively 'canalized' by a specific gene regulatory network. Both phenotypes are thermodynamically and functionally 'compatibles' with the new, modified environment. However, when the two cell subsets are reseeded into the 1g gravity field the two phenotypes collapse into one. Gravity constraints the system in adopting only one phenotype, not by selecting a pre-existing configuration, but more precisely shaping it de-novo through the modification of the cytoskeleton three-dimensional structure. Overall, those findings highlight how macro-scale features are irreducible to lower-scale explanations. The identification of macroscale control parameters - as those depending on the field (gravity, electromagnetic fields) or emerging from the cooperativity among the field's components (tissue stiffness, cell-to-cell connectivity) - are mandatory for assessing boundary conditions for models at lower scales, thus providing a concrete instantiation of top-down effects.

Bone biomaterials and interactions with stem cells

Bone biomaterials play a vital role in bone repair by providing the necessary substrate for cell adhesion, proliferation, and differentiation and by modulating cell activity and function. In past decades, extensive efforts have been devoted to developing bone biomaterials with a focus on the following issues: (1) developing ideal biomaterials with a combination of suitable biological and mechanical properties; (2) constructing a cell microenvironment with pores ranging in size from nanoscale to submicro- and microscale; and (3) inducing the oriented differentiation of stem cells for artificial-to-biological transformation. Here we present a comprehensive review of the state of the art of bone biomaterials and their interactions with stem cells. Typical bone biomaterials that have been developed, including bioactive ceramics, biodegradable polymers, and biodegradable metals, are reviewed, with an emphasis on their characteristics and applications. The necessary porous structure of bone biomaterials for the cell microenvironment is discussed, along with the corresponding fabrication methods. Additionally, the promising seed stem cells for bone repair are summarized, and their interaction mechanisms with bone biomaterials are discussed in detail. Special attention has been paid to the signaling pathways involved in the focal adhesion and osteogenic differentiation of stem cells on bone biomaterials. Finally, achievements regarding bone biomaterials are summarized, and future research directions are proposed.

Nanotopographic Regulation of Human Mesenchymal Stem Cell Osteogenesis

Mesenchymal stem cell (MSC) differentiation can be manipulated by nanotopographic interface providing a unique strategy to engineering stem cell therapy and circumventing complex cellular reprogramming. However, our understanding of the nanotopographic-mechanosensitive properties of MSCs and the underlying biophysical linkage of the nanotopography-engineered stem cell to directed commitment remains elusive. Here, we show that osteogenic differentiation of human MSCs (hMSCs) can be largely promoted using our nanoengineered topographic glass substrates in the absence of dexamethasone, a key exogenous factor for osteogenesis induction. We demonstrate that hMSCs sense and respond to surface nanotopography, through modulation of adhesion, cytoskeleton tension, and nuclear activation of TAZ (transcriptional coactivator with PDZ-binding motif), a transcriptional modulator of hMSCs. Our findings demonstrate the potential of nanotopographic surfaces as noninvasive tools to advance cell-based therapies for bone engineering and highlight the origin of biophysical response of hMSC to nanotopography.

Modulation of collective cell behaviour by geometrical constraints

Intracellular and extracellular mechanical forces play a crucial role during tissue growth, modulating nuclear shape and function and resulting in complex collective cell behaviour. However, the mechanistic understanding of how the orientation, shape, symmetry and homogeneity of cells are affected by environmental geometry is still lacking. Here we investigate cooperative cell behaviour and patterns under geometric constraints created by topographically patterned substrates. We show how cells cooperatively adopt their geometry, shape, positioning of the nucleus and subsequent proliferation activity. Our findings indicate that geometric constraints induce significant squeezing of cells and nuclei, cytoskeleton reorganization, drastic condensation of chromatin resulting in a change in the cell proliferation rate and the anisotropic growth of cultures. Altogether, this work not only demonstrates complex non-trivial collective cellular responses to geometrical constraints but also provides a tentative explanation of the observed cell culture patterns grown on different topographically patterned substrates. These findings provide important fundamental knowledge, which could serve as a basis for better controlled tissue growth and cell-engineering applications.

Understanding the extracellular forces that determine cell fate and maintenance

Stem cells interpret signals from their microenvironment while simultaneously modifying the niche through secreting factors and exerting mechanical forces. Many soluble stem cell cues have been determined over the past century, but in the past decade, our molecular understanding of mechanobiology has advanced to explain how passive and active forces induce similar signaling cascades that drive self-renewal, migration, differentiation or a combination of these outcomes. Improvements in stem cell culture methods, materials and biophysical tools that assess function have improved our understanding of these cascades. Here, we summarize these advances and offer perspective on ongoing challenges.

Developing a biomimetic tooth bud model

A long-term goal is to bioengineer, fully functional, living teeth for regenerative medicine and dentistry applications. Biologically based replacement teeth would avoid insufficiencies of the currently used dental implants. Using natural tooth development as a guide, a model was fabricated using post-natal porcine dental epithelial (pDE), porcine dental mesenchymal (pDM) progenitor cells, and human umbilical vein endothelial cells (HUVEC) encapsulated within gelatin methacrylate (GelMA) hydrogels. Previous publications have shown that post-natal DE and DM cells seeded onto synthetic scaffolds exhibited mineralized tooth crowns composed of dentin and enamel. However, these tooth structures were small and formed within the pores of the scaffolds. The present study shows that dental cell-encapsulated GelMA constructs can support mineralized dental tissue formation of predictable size and shape. Individually encapsulated pDE or pDM cell GelMA constructs were analysed to identify formulas that supported pDE and pDM cell attachment, spreading, metabolic activity, and neo-vasculature formation with co-seeded endothelial cells (HUVECs). GelMa constructs consisting of pDE-HUVECS in 3% GelMA and pDM-HUVECs within 5% GelMA supported dental cell differentiation and vascular mineralized dental tissue formation in vivo. These studies are the first to demonstrate the use of GelMA hydrogels to support the formation of post-natal dental progenitor cell-derived mineralized and functionally vascularized tissues of specified size and shape. These results introduce a novel three-dimensional biomimetic tooth bud model for eventual bioengineered tooth replacement teeth in humans. Copyright © 2017 John Wiley & Sons, Ltd.

Why we need mechanics to understand animal regeneration

Mechanical forces are an important contributor to cell fate specification and cell migration during embryonic development in animals. Similarities between embryogenesis and regeneration, particularly with regards to pattern formation and large-scale tissue movements, suggest similarly important roles for physical forces during regeneration. While the influence of the mechanical environment on stem cell differentiation in vitro is being actively exploited in the fields of tissue engineering and regenerative medicine, comparatively little is known about the role of stresses and strains acting during animal regeneration. In this review, we summarize published work on the role of physical principles and mechanical forces in animal regeneration. Novel experimental techniques aimed at addressing the role of mechanics in embryogenesis have greatly enhanced our understanding at scales from the subcellular to the macroscopic - we believe the time is ripe for the field of regeneration to similarly leverage the tools of the mechanobiological research community.

Gravity Constraints Drive Biological Systems Toward Specific Organization Patterns: Commitment of cell specification is constrained by physical cues

Different cell lineages growing in microgravity undergo a spontaneous transition leading to the emergence of two distinct phenotypes. By returning these populations in a normal gravitational field, the two phenotypes collapse, recovering their original configuration. In this review, we hypothesize that, once the gravitational constraint is removed, the system freely explores its phenotypic space, while, when in a gravitational field, cells are "constrained" to adopt only one favored configuration. We suggest that the genome allows for a wide range of "possibilities" but it is unable per se to choose among them: the emergence of a specific phenotype is enabled by physical constraints that drive the system toward a preferred solution. These findings may help in understanding how cells and tissues behave in both development and cancer.

Functional and Biomimetic Materials for Engineering of the Three-Dimensional Cell Microenvironment

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology. The extracellular matrix (ECM) within the cell microenvironment serves not only as a structural foundation for cells but also as a source of three-dimensional (3D) biochemical and biophysical cues that trigger and regulate cell behaviors. Increasing evidence suggests that the 3D character of the microenvironment is required for development of many critical cell responses observed in vivo, fueling a surge in the development of functional and biomimetic materials for engineering the 3D cell microenvironment. Progress in the design of such materials has improved control of cell behaviors in 3D and advanced the fields of tissue regeneration, in vitro tissue models, large-scale cell differentiation, immunotherapy, and gene therapy. However, the field is still in its infancy, and discoveries about the nature of cell-microenvironment interactions continue to overturn much early progress in the field. Key challenges continue to be dissecting the roles of chemistry, structure, mechanics, and electrophysiology in the cell microenvironment, and understanding and harnessing the roles of periodicity and drift in these factors. This review encapsulates where recent advances appear to leave the ever-shifting state of the art, and it highlights areas in which substantial potential and uncertainty remain.

Native extracellular matrix-derived semipermeable, optically transparent, and inexpensive membrane inserts for microfluidic cell culture

Semipermeable cell culture membranes are commonly used in multilayered microfluidic devices to mimic the basement membrane in vivo and to create compartmentalized microenvironments for physiological cell growth and differentiation. However, existing membranes are predominantly made up of synthetic polymers, providing limited capacity to replicate cellular interactions with native extracellular matrices that play a crucial role in the induction of physiological phenotypes. Here we describe a new type of cell culture membranes engineered from native extracellular matrix (ECM) materials that are thin, semipermeable, optically transparent, and amenable to integration into microfluidic cell culture devices. Facile and cost-effective fabrication of these membranes was achieved by controlled sequential steps of vitrification that transformed three-dimensional (3D) ECM hydrogels into structurally stable thin films. By modulating the composition of the ECM, our technique provided a means to tune key membrane properties such as optical transparency, stiffness, and porosity. For microfluidic cell culture, we constructed a multilayered microdevice consisting of two parallel chambers separated by a thin membrane insert derived from different types of ECM. This study showed that our ECM membranes supported attachment and growth of various types of cells (epithelial, endothelial, and mesenchymal cells) under perfusion culture conditions. Our data also revealed the promotive effects of the membranes on adhesion-associated intracellular signaling that mediates cell-ECM interactions. Moreover, we demonstrated the use of these membranes for constructing compartmentalized microfluidic cell culture systems to induce physiological tissue differentiation or to replicate interfaces between different tissue types. Our approach provides a robust platform to produce and engineer biologically active cell culture substrates that serve as promising alternatives to conventional synthetic membrane inserts. This strategy may contribute to the development of physiologically relevant in vitro cell culture models for a wide range of applications.

A comparison study between electrospun polycaprolactone and piezoelectric poly(3-hydroxybutyrate-co-3-hydroxyvalerate) scaffolds for bone tissue engineering

In this study, bone scaffolds composed of polycaprolactone (PCL), piezoelectric poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and a combination of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and silicate containing hydroxyapatite (PHBV-SiHA) were successfully fabricated by a conventional electrospinning process. The morphological, chemical, wetting and biological properties of the scaffolds were examined. All fabricated scaffolds are composed of randomly oriented fibres with diameters from 800nm to 12μm. Fibre size increased with the addition of SiHA to PHBV scaffolds. Moreover, fibre surface roughness in the case of hybrid scaffolds was also increased. XRD, FTIR and Raman spectroscopy were used to analyse the chemical composition of the scaffolds, and contact angle measurements were performed to reveal the wetting behaviour of the synthesized materials. To determine the influence of the piezoelectric nature of PHBV in combination with SiHA nanoparticles on cell attachment and proliferation, PCL (non-piezoelectric), pure PHBV, and PHBV-SiHA scaffolds were seeded with human mesenchymal stem cells (hMSCs). In vitro study on hMSC adhesion, viability, spreading and osteogenic differentiation showed that the PHBV-SiHA scaffolds had the largest adhesion and differentiation abilities compared with other scaffolds. Moreover, the piezoelectric PHBV scaffolds have demonstrated better calcium deposition potential compared with non-piezoelectric PCL. The results of the study revealed pronounced advantages of hybrid PHBV-SiHA scaffolds to be used in bone tissue engineering.

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.

Traction Force Measurement Using Deformable Microposts

Recent findings suggest that mechanical forces strongly influence wound repair and fibrosis across multiple organ systems. Traction force is vital to the characterization of cellular responses to mechanical stimuli. Using hydrogel-based traction force microscopy, a FRET-based tension sensor, or microengineered cantilevers, the magnitude of traction forces can be measured. Here, we describe a traction force measurement methodology using a dense array of elastomeric microposts. This platform can be used to measure the traction force of a single cell or a colony of cells with or without geometric confinement.

Progress in Integrative Biomaterial Systems to Approach Three-Dimensional Cell Mechanotransduction

Mechanotransduction between cells and the extracellular matrix regulates major cellular functions in physiological and pathological situations. The effect of mechanical cues on biochemical signaling triggered by cell-matrix and cell-cell interactions on model biomimetic surfaces has been extensively investigated by a combination of fabrication, biophysical, and biological methods. To simulate the in vivo physiological microenvironment in vitro, three dimensional (3D) microstructures with tailored bio-functionality have been fabricated on substrates of various materials. However, less attention has been paid to the design of 3D biomaterial systems with geometric variances, such as the possession of precise micro-features and/or bio-sensing elements for probing the mechanical responses of cells to the external microenvironment. Such precisely engineered 3D model experimental platforms pave the way for studying the mechanotransduction of multicellular aggregates under controlled geometric and mechanical parameters. Concurrently with the progress in 3D biomaterial fabrication, cell traction force microscopy (CTFM) developed in the field of cell biophysics has emerged as a highly sensitive technique for probing the mechanical stresses exerted by cells onto the opposing deformable surface. In the current work, we first review the recent advances in the fabrication of 3D micropatterned biomaterials which enable the seamless integration with experimental cell mechanics in a controlled 3D microenvironment. Then, we discuss the role of collective cell-cell interactions in the mechanotransduction of engineered tissue equivalents determined by such integrative biomaterial systems under simulated physiological conditions.

Substrate stiffness governs the initiation of B cell activation by the concerted signaling of PKCβ and focal adhesion kinase

The mechanosensing ability of lymphocytes regulates their activation in response to antigen stimulation, but the underlying mechanism remains unexplored. Here, we report that B cell mechanosensing-governed activation requires BCR signaling molecules. PMA-induced activation of PKCβ can bypass the Btk and PLC-γ2 signaling molecules that are usually required for B cells to discriminate substrate stiffness. Instead, PKCβ-dependent activation of FAK is required, leading to FAK-mediated potentiation of B cell spreading and adhesion responses. FAK inactivation or deficiency impaired B cell discrimination of substrate stiffness. Conversely, adhesion molecules greatly enhanced this capability of B cells. Lastly, B cells derived from rheumatoid arthritis (RA) patients exhibited an altered BCR response to substrate stiffness in comparison with healthy controls. These results provide a molecular explanation of how initiation of B cell activation discriminates substrate stiffness through a PKCβ-mediated FAK activation dependent manner.

Engineering the geometrical shape of mesenchymal stromal cells through defined cyclic stretch regimens

Stem cells have been predicted to improve disease outcomes and patient lives. Steering stem cell fate - through controlling cell shape - may substantially accelerate progress towards this goal. As mesenchymal stromal cells (MSCs) are continuously exposed in vivo to a dynamically changing biomechanical environment, we hypothesized that exogenous forces can be applied for engineering a variety of significantly different MSC shapes. We applied specific cyclic stretch regimens to human MSCs and quantitatively measured the resulting cell shape, alignment, and expression of smooth muscle (SMC) differentiation markers, as those have been associated with elongated morphology. As proof of principle, a range of different shapes, alignments, and correlating SMC marker levels were generated by varying strain, length, and repetition of stretch. However, the major determinant of biomechanically engineering cellular shape was the repetition of a chosen stretch regimen, indicating that the engineered shape and associated differentiation were complex non-linear processes relying on sustained biomechanical stimulation. Thus, forces are key regulators of stem cell shape and the targeted engineering of specific MSC shapes through biomechanical forces represents a novel mechanobiology concept that could exploit naturally occurring in vivo forces for improving stem cell fate in clinical regenerative therapies.

Stem cell bioprinting for applications in regenerative medicine

Many regenerative medicine applications seek to harness the biologic power of stem cells in architecturally complex scaffolds or microenvironments. Traditional tissue engineering methods cannot create such intricate structures, nor can they precisely control cellular position or spatial distribution. These limitations have spurred advances in the field of bioprinting, aimed to satisfy these structural and compositional demands. Bioprinting can be defined as the programmed deposition of cells or other biologics, often with accompanying biomaterials. In this concise review, we focus on recent advances in stem cell bioprinting, including performance, utility, and applications in regenerative medicine. More specifically, this review explores the capability of bioprinting to direct stem cell fate, engineer tissue(s), and create functional vascular networks. Furthermore, the unique challenges and concerns related to bioprinting living stem cells, such as viability and maintaining multi- or pluripotency, are discussed. The regenerative capacity of stem cells, when combined with the structural/compositional control afforded by bioprinting, provides a unique and powerful tool to address the complex demands of tissue engineering and regenerative medicine applications.

Focal adhesion kinase signaling regulates anti-inflammatory function of bone marrow mesenchymal stromal cells induced by biomechanical force

Mesenchymal stromal cells (MSCs) have tremendous potential for use in regenerative medicine due to their multipotency and immune cell regulatory functions. Biomimetic physical forces have been shown to direct differentiation and maturation of MSCs in tissue engineering applications; however, the effect of force on immunomodulatory activity of MSCs has been largely overlooked. Here we show in human bone marrow-derived MSCs that wall shear stress (WSS) equivalent to the fluid frictional force present in the adult arterial vasculature significantly enhances expression of four genes that mediate MSC immune regulatory function, PTGS2, HMOX1, IL1RN, and TNFAIP6. Several mechanotransduction pathways are stimulated by WSS, including calcium ion (Ca²⁺) flux and activation of Akt, MAPK, and focal adhesion kinase (FAK). Inhibition of PI3K-Akt by LY294002 or Ca²⁺ signaling with chelators, ion channel inhibitors, or Ca²⁺ free culture conditions failed to attenuate WSS-induced COX2 expression. In contrast, the FAK inhibitor PF-562271 blocked COX2 induction, implicating focal adhesions as critical sensory components upstream of this key immunomodulatory factor. In co-culture assays, WSS preconditioning stimulates MSC anti-inflammatory activity to more potently suppress TNF-α production by activated immune cells, and this improved potency depended upon the ability of FAK to stimulate COX2 induction. Taken together, our data demonstrate that biomechanical force potentiates the reparative and regenerative properties of MSCs through a FAK signaling cascade and highlights the potential for innovative force-based approaches for enhancement in MSC therapeutic efficacy.

Bioinspired 3D Multilayered Shape Memory Scaffold with a Hierarchically Changeable Micropatterned Surface for Efficient Vascularization

How to achieve three-dimensional (3D) cell alignment and subsequent prompt tissue regeneration remains a great challenge. Here, inspired by the interior 3D architecture of native arteries, we develop a new 3D multilayered shape memory vascular scaffold with a hierarchically changeable micropatterned surface for vascularization. The shape memory function renders the implantation of the scaffold safe and convenient via minimally invasive surgery. By co-culturing endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) on the 3D multilayered structure, the inner monolayer, which has a square micropatterned surface, can promote EC adhesion and migration, resulting in a rapid endothelialization, and the outer multilayers, which have rectangular micropatterned surfaces, can induce a circumferential alignment of VSMCs. After implantation in the cervical artery of a New Zealand rabbit for 120 days, the graft developed a high capacity for modulating cellular 3D alignment, to generate a neonatal functional blood vessel with an endothelium layer in the inner layer and multilevel VSMC circumferential alignments in the outer layers.

Antimicrobial and bone-forming activity of a copper coated implant in a rabbit model

Current strategies in implant technology are directed to generate bioactive implants that are capable to activate the regenerative potential of the surrounding tissue. On the other hand, implant-related infections are a common problem in orthopaedic trauma patients. To meet both challenges, i.e. to generate a bone implant with regenerative and antimicrobial characteristics, we tested the use of copper coated nails for surgical fixation in a rabbit model. Copper acetate was galvanically deposited with a copper load of 1 µg/mm² onto a porous oxide layer of Ti6Al4V nails, which were used for the fixation of a tibia fracture, inoculated with bacteria. After implantation of the nail the concentration of copper ions did not increase in blood which indicates that copper released from the implant was locally restricted to the fracture site. After four weeks, analyses of the extracted implants revealed a distinct antimicrobial effect of copper, because copper completely prevented both a weak adhesion and firm attachment of biofilm-forming bacteria on the titanium implant. To evaluate fracture healing, radiographic examination demonstrated an increased callus index in animals with copper coated nails. This result indicates a stimulated bone formation by releasing copper ions. We conclude that the use of implants with a defined load of copper ions enables both prevention of bacterial infection and the stimulation of regenerative processes.

Nonmonotonic Self-Deformation of Cell Nuclei on Topological Surfaces with Micropillar Array

Cells respond to the mechanical signals from their surroundings and integrate physiochemical signals to initiate intricate mechanochemical processes. While many studies indicate that topological features of biomaterials impact cellular behaviors profoundly, little research has focused on the nuclear response to a mechanical force generated by a topological surface. Here, we fabricated a polymeric micropillar array with an appropriate dimension to induce a severe self-deformation of cell nuclei and investigated how the nuclear shape changed over time. Intriguingly, the nuclei of mesenchymal stem cells (MSCs) on the poly(lactide-co-glycolide) (PLGA) micropillars exhibited a significant initial deformation followed by a partial recovery, which led to an "overshoot" phenomenon. The treatment of cytochalasin D suppressed the recovery of nuclei, which indicated the involvement of actin cytoskeleton in regulating the recovery at the second stage of nuclear deformation. Additionally, we found that MSCs exhibited different overshoot extents from their differentiated lineage, osteoblasts. These findings enrich the understanding of the role of the cell nucleus in mechanotransduction. As the first quantitative report on nonmonotonic kinetic process of self-deformation of a cell organelle on biomaterials with unique topological surfaces, this study sheds new insight into cell-biomaterial interactions.

Combined influence of biophysical and biochemical cues on maintenance and proliferation of hematopoietic stem cells

Homeostasis of hematopoietic stem and progenitor cells (HSPC) is controlled by a combination of biochemical and biophysical environmental cues in the bone marrow (BM) niche, where a tight balance of quiescence and proliferation of HSPC is maintained. Specifically, alongside soluble factors and extracellular matrix (ECM) proteins, spatial confinement and ECM stiffness have been recognized to be critical for regulation of HSPC fate. Here we employ a modular, glycosaminoglycan (GAG)-based biohybrid hydrogel system to balance proliferation of human HSPC and maintenance of quiescent hematopoietic stem cells (HSC) through simultaneous regulation of exogenous biochemical and biophysical cues. Our results demonstrate that HSPC respond to increased spatial confinement with lowered proliferation and cell cycling, which results in higher frequency of quiescent LTC-IC (long-term culture initiating cells), while GAG-rich 3D environments further support maintenance of the cells.

Impact of fluidic agitation on human pluripotent stem cells in stirred suspension culture

The success of human pluripotent stem cells (hPSCs) as a source of future cell therapies hinges, in part, on the availability of a robust and scalable culture system that can readily produce a clinically relevant number of cells and their derivatives. Stirred suspension culture has been identified as one such promising platform due to its ease of use, scalability, and widespread use in the pharmaceutical industry (e.g., CHO cell-based production of therapeutic proteins) among others. However, culture of undifferentiated hPSCs in stirred suspension is a relatively new development within the past several years, and little is known beyond empirically optimized culture parameters. In particular, detailed characterizations of different agitation rates and their influence on the propagation of hPSCs are often not reported in the literature. In the current study, we systematically investigated various agitation rates to characterize their impact on cell yield, viability, and the maintenance of pluripotency. Additionally, we closely examined the distribution of cell aggregates and how the observed culture outcomes are attributed to their size distribution. Overall, our results showed that moderate agitation maximized the propagation of hPSCs to approximately 38-fold over 7 days by keeping the cell aggregates below the critical size, beyond which the cells are impacted by the diffusion limit, while limiting cell death caused by excessive fluidic forces. Furthermore, we observed that fluidic agitation could regulate not only cell aggregation, but also expression of some key signaling proteins in hPSCs. This indicates a new possibility to guide stem cell fate determination by fluidic agitation in stirred suspension cultures. Biotechnol. Bioeng. 2017;114: 2109-2120. © 2017 Wiley Periodicals, Inc.

Mechanical design in embryos: mechanical signalling, robustness and developmental defects

Embryos are shaped by the precise application of force against the resistant structures of multicellular tissues. Forces may be generated, guided and resisted by cells, extracellular matrix, interstitial fluids, and how they are organized and bound within the tissue's architecture. In this review, we summarize our current thoughts on the multiple roles of mechanics in direct shaping, mechanical signalling and robustness of development. Genetic programmes of development interact with environmental cues to direct the composition of the early embryo and endow cells with active force production. Biophysical advances now provide experimental tools to measure mechanical resistance and collective forces during morphogenesis and are allowing integration of this field with studies of signalling and patterning during development. We focus this review on concepts that highlight this integration, and how the unique contributions of mechanical cues and gradients might be tested side by side with conventional signalling systems. We conclude with speculation on the integration of large-scale programmes of development, and how mechanical responses may ensure robust development and serve as constraints on programmes of tissue self-assembly.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

A mathematical model of mechanotransduction reveals how mechanical memory regulates mesenchymal stem cell fate decisions

BACKGROUND

Mechanical and biophysical properties of the cellular microenvironment regulate cell fate decisions. Mesenchymal stem cell (MSC) fate is influenced by past mechanical dosing (memory), but the mechanisms underlying this process have not yet been well defined. We have yet to understand how memory affects specific cell fate decisions, such as the differentiation of MSCs into neurons, adipocytes, myocytes, and osteoblasts.

RESULTS

We study a minimal gene regulatory network permissive of multi-lineage MSC differentiation into four cell fates. We present a continuous model that is able to describe the cell fate transitions that occur during differentiation, and analyze its dynamics with tools from multistability, bifurcation, and cell fate landscape analysis, and via stochastic simulation. Whereas experimentally, memory has only been observed during osteogenic differentiation, this model predicts that memory regions can exist for each of the four MSC-derived cell lineages. We can predict the substrate stiffness ranges over which memory drives differentiation; these are directly testable in an experimental setting. Furthermore, we quantitatively predict how substrate stiffness and culture duration co-regulate the fate of a stem cell, and we find that the feedbacks from the differentiating MSC onto its substrate are critical to preserve mechanical memory. Strikingly, we show that re-seeding MSCs onto a sufficiently soft substrate increases the number of cell fates accessible.

CONCLUSIONS

Control of MSC differentiation is crucial for the success of much-lauded regenerative therapies based on MSCs. We have predicted new memory regions that will directly impact this control, and have quantified the size of the memory region for osteoblasts, as well as the co-regulatory effects on cell fates of substrate stiffness and culture duration. Taken together, these results can be used to develop novel strategies to better control the fates of MSCs in vitro and following transplantation.

Biomechanical Forces Promote Immune Regulatory Function of Bone Marrow Mesenchymal Stromal Cells

Mesenchymal stromal cells (MSCs) are believed to mobilize from the bone marrow in response to inflammation and injury, yet the effects of egress into the vasculature on MSC function are largely unknown. Here we show that wall shear stress (WSS) typical of fluid frictional forces present on the vascular lumen stimulates antioxidant and anti-inflammatory mediators, as well as chemokines capable of immune cell recruitment. WSS specifically promotes signaling through NFκB-COX2-prostaglandin E2 (PGE2 ) to suppress tumor necrosis factor-α (TNF-α) production by activated immune cells. Ex vivo conditioning of MSCs by WSS improved therapeutic efficacy in a rat model of traumatic brain injury, as evidenced by decreased apoptotic and M1-type activated microglia in the hippocampus. These results demonstrate that force provides critical cues to MSCs residing at the vascular interface which influence immunomodulatory and paracrine activity, and suggest the potential therapeutic use of force for MSC functional enhancement. Stem Cells 2017;35:1259-1272.

Acoustic tweezing cytometry enhances osteogenesis of human mesenchymal stem cells through cytoskeletal contractility and YAP activation

Human mesenchymal stem cells (hMSCs) have great potential for cell-based therapies for treating degenerative bone diseases. It is known that mechanical cues in the cell microenvironment play an important role in regulating osteogenic (bone) differentiation of hMSCs. However, mechanoregulation of lineage commitment of hMSCs in conventional two-dimensional (2D) monocultures or bioengineered three-dimensional (3D) tissue constructs remains suboptimal due to complex biomaterial design criteria for hMSC culture. In this study, we demonstrate the feasibility of a novel cell mechanics and mechanobiology tool, acoustic tweezing cytometry (ATC), for mechanical stimulation of hMSCs. ATC utilizes ultrasound (US) pulses to actuate functionalized lipid microbubbles (MBs) which are covalently attached to hMSCs via integrin binding to exert forces to the cells. ATC stimulation increases cytoskeletal contractility of hMSCs regardless of the cell area. Furthermore, ATC application rescues osteogenic differentiation of hMSCs in culture conditions that are intrinsically repressive for hMSC osteogenesis (e.g., soft cell culture surfaces). ATC application activates transcriptional regulator YAP to enhance hMSC osteogenesis. Our data further show that F-actin, myosin II, and RhoA/ROCK signaling functions upstream of YAP activity in mediating ATC-stimulated hMSC osteogenesis. With the capability of applying controlled dynamic mechanical stimuli to cells, ATC provides a powerful tool for mechanoregulation of stem cell behaviors in tissue engineering and regenerative medicine applications.

Gelatin-based 3D conduits for transdifferentiation of mesenchymal stem cells into Schwann cell-like phenotypes

In this study, gelatin-based 3D conduits with three different microstructures (nanofibrous, macroporous and ladder-like) were fabricated for the first time via combined molding and thermally induced phase separation (TIPS) technique for peripheral nerve regeneration. The effects of conduit microstructure and mechanical properties on the transdifferentiation of bone marrow-derived mesenchymal stem cells (MSCs) into Schwann cell (SC) like phenotypes were examined to help facilitate neuroregeneration and understand material-cell interfaces. Results indicated that 3D macroporous and ladder-like structures enhanced MSC attachment, proliferation and spreading, creating interconnected cellular networks with large numbers of viable cells compared to nanofibrous and 2D-tissue culture plate counterparts. 3D-ladder-like conduit structure with complex modulus of ∼0.4×10⁶Pa and pore size of ∼150μm provided the most favorable microenvironment for MSC transdifferentiation leading to ∼85% immunolabeling of all SC markers. On the other hand, the macroporous conduits with complex modulus of ∼4×10⁶Pa and pore size of ∼100μm showed slightly lower (∼65% for p75, ∼75% for S100 and ∼85% for S100β markers) immunolabeling. Transdifferentiated MSCs within 3D-ladder-like conduits secreted significant amounts (∼2.5pg/mL NGF and ∼0.7pg/mL GDNF per cell) of neurotrophic factors, while MSCs in macroporous conduits released slightly lower (∼1.5pg/mL NGF and 0.7pg/mL GDNF per cell) levels. PC12 cells displayed enhanced neurite outgrowth in media conditioned by conduits with transdifferentiated MSCs. Overall, conduits with macroporous and ladder-like 3D structures are promising platforms in transdifferentiation of MSCs for neuroregeneration and should be further tested in vivo.

STATEMENT OF SIGNIFICANCE

This manuscript focuses on the effect of microstructure and mechanical properties of gelatin-based 3D conduits on the transdifferentiation of mesenchymal stem cells to Schwann cell-like phenotypes. This work builds on our recently accepted manuscript in Acta Biomaterialia focused on multifunctional 2D films, and focuses on 3D microstructured conduits designed to overcome limitations of current strategies to facilitate peripheral nerve regeneration. The comparison between conduits fabricated with nanofibrous, macroporous and ladder-like microstructures showed that the ladder-like conduits showed the most favorable environment for MSC transdifferentiation to Schwann-cell like phenotypes, as seen by both immunolabeling as well as secretion of neurotrophic factors. This work demonstrates the importance of controlling the 3D microstructure to facilitate tissue engineering strategies involving stem cells that can serve as promising approaches for peripheral nerve regeneration.

In vitro biomimetic platforms featuring a perfusion system and 3D spheroid culture promote the construction of tissue-engineered corneal endothelial layers

Corneal endothelial cells (CECs) are very important for the maintenance of corneal transparency. However, in vitro, CECs display limited proliferation and loss of phenotype via endothelial to mesenchymal transformation (EMT) and cellular senescence. In this study, we demonstrate that continuous supplementary nutrition using a perfusion culture bioreactor and three-dimensional (3D) spheroid culture can be used to improve CEC expansion in culture and to construct a tissue-engineered CEC layer. Compared with static culture, perfusion-derived CECs exhibited an increased proliferative ability as well as formed close cell-cell contact junctions and numerous surface microvilli. We also demonstrated that the CEC spheroid culture significantly down-regulated gene expression of the proliferation marker Ki67 and EMT-related markers Vimentin and α-SMA, whereas the gene expression level of the CEC marker ATP1A1 was significantly up-regulated. Furthermore, use of the perfusion system in conjunction with a spheroid culture on decellularized corneal scaffolds and collagen sheets promoted the generation of CEC monolayers as well as neo-synthesized ECM formation. This study also confirmed that a CEC spheroid culture on a curved collagen sheet with controlled physiological intraocular pressure could generate a CEC monolayer. Thus, our results show that the use of a perfusion system and 3D spheroid culture can promote CEC expansion and the construction of tissue-engineered corneal endothelial layers in vitro.

Electrical Differentiation of Mesenchymal Stem Cells into Schwann-Cell-Like Phenotypes Using Inkjet-Printed Graphene Circuits

Graphene-based materials (GBMs) have displayed tremendous promise for use as neurointerfacial substrates as they enable favorable adhesion, growth, proliferation, spreading, and migration of immobilized cells. This study reports the first case of the differentiation of mesenchymal stem cells (MSCs) into Schwann cell (SC)-like phenotypes through the application of electrical stimuli from a graphene-based electrode. Electrical differentiation of MSCs into SC-like phenotypes is carried out on a flexible, inkjet-printed graphene interdigitated electrode (IDE) circuit that is made highly conductive (sheet resistance < 1 kΩ/sq) via a postprint pulse-laser annealing process. MSCs immobilized on the graphene printed IDEs and electrically stimulated/treated (etMSCs) display significant enhanced cellular differentiation and paracrine activity above conventional chemical treatment strategies [≈85% of the etMSCs differentiated into SC-like phenotypes with ≈80 ng mL^-1 of nerve growth factor (NGF) secretion vs. 75% and ≈55 ng mL^-1 for chemically treated MSCs (ctMSCs)]. These results help pave the way for in vivo peripheral nerve regeneration where the flexible graphene electrodes could conform to the injury site and provide intimate electrical simulation for nerve cell regrowth.

The geometrical shape of mesenchymal stromal cells measured by quantitative shape descriptors is determined by the stiffness of the biomaterial and by cyclic tensile forces

Controlling mesenchymal stromal cell (MSC) shape is a novel method for investigating and directing MSC behaviour in vitro. it was hypothesized that specifigc MSC shapes can be generated by using stiffness-defined biomaterial surfaces and by applying cyclic tensile forces. Biomaterials used were thin and thick silicone sheets, fibronectin coating, and compacted collagen type I sheets. The MSC morphology was quantified by shape descriptors describing dimensions and membrane protrusions. Nanoscale stiffness was measured by atomic force microscopy and the expression of smooth muscle cell (SMC) marker genes (ACTA2, TAGLN, CNN1) by quantitative reverse-transcription polymerase chain reaction. Cyclic stretch was applied with 2.5% or 5% amplitudes. Attachment to biomaterials with a higher stiffness yielded more elongated MSCs with fewer membrane protrusions compared with biomaterials with a lower stiffness. For cyclic stretch, compacted collagen sheets were selected, which were associated with the most elongated MSC shape across all investigated biomaterials. As expected, cyclic stretch elongated MSCs during stretch. One hour after cessation of stretch, however, MSC shape was rounder again, suggesting loss of stretch-induced shape. Different shape descriptor values obtained by different stretch regimes correlated significantly with the expression levels of SMC marker genes. Values of approximately 0.4 for roundness and 3.4 for aspect ratio were critical for the highest expression levels of ACTA2 and CNN1. Thus, specific shape descriptor values, which can be generated using biomaterial-associated stiffness and tensile forces, can serve as a template for the induction of specific gene expression levels in MSC. Copyright © 2017 John Wiley & Sons, Ltd.

In Situ Activation of Penile Progenitor Cells With Low-Intensity Extracorporeal Shockwave Therapy

BACKGROUND

We previously reported that progenitor cells, or stem cells, exist within penile tissue. We hypothesized that acoustic wave stimulation by low-intensity extracorporeal shockwave therapy (Li-ESWT) would activate local stem or progenitor cells within the penis, producing regenerative effects.

AIMS

To study the feasibility of in situ penile progenitor cell activation by Li-ESWT.

METHODS

We performed a cohort analysis of young and middle-age male Sprague-Dawley rats treated with 5-ethynyl-2'-deoxyuridine (EdU) pulse followed by Li-ESWT. In addition, Li-ESWT was applied to cultured Schwann cells and endothelial cells to study the molecular mechanism involved in cell proliferation. Thirty minutes before Li-ESWT, each rat received an intraperitoneal injection of EdU. Li-ESWT was applied to the penis at very low (0.02 mJ/mm² at 3 Hz for 300 pulses) or low (0.057 mJ/mm² at 3 Hz for 500 pulses) energy levels. The endothelial and Schwann cells were treated with very low energy (0.02 mJ/mm² at 3 Hz for 300 pulses) in vitro.

OUTCOMES

At 48 hours or 1 week after Li-ESWT, penile tissues were harvested for histologic study to assess EdU⁺ and Ki-67⁺ cells, and cell proliferation, Ki-67 expression, Erk1/2 phosphorylation, translocation, and angiogenesis were examined in cultured Schwann and endothelial cells after Li-ESWT.

RESULTS

Li-ESWT significantly increased EdU⁺ cells within penile erectile tissues (P < .01) at 48 hours and 1 week. There were more cells activated in young animals than in middle-age animals, and the effect depended on dosage. Most activated cells were localized within subtunical spaces. In vitro studies indicated that Li-ESWT stimulated cell proliferation through increased phosphorylation of Erk1/2.

CLINICAL TRANSLATION

The present results provide a possible explanation for the clinical benefits seen with Li-ESWT.

STRENGTHS AND LIMITATIONS

The main limitation of the present project was the short period of study and the animal model used. Li-ESWT could be less effective in improving erectile function in old animals because of the decreased number and quality of penile stem or progenitor cells associated with aging.

CONCLUSION

Li-ESWT activation of local penile progenitor cells might be one of the mechanisms that contribute to the beneficial effects of shockwave treatment for erectile dysfunction, which represents a non-invasive alternative to exogenous stem cell therapy. Lin G, Reed-Maldonado AB, Wang B, et al. In Situ Activation of Penile Progenitor Cells With Low-Intensity Extracorporeal Shockwave Therapy. J Sex Med 2017;14:493-501.

EGF-mediated EGFR/ERK signaling pathway promotes germinative cell proliferation in Echinococcus multilocularis that contributes to larval growth and development

Background

Larvae of the tapeworm E. multilocularis cause alveolar echinococcosis (AE), one of the most lethal helminthic infections in humans. A population of stem cell-like cells, the germinative cells, is considered to drive the larval growth and development within the host. The molecular mechanisms controlling the behavior of germinative cells are largely unknown.

Methodology/Principal findings

Using in vitro cultivation systems we show here that the EGFR/ERK signaling in the parasite can promote germinative cell proliferation in response to addition of human EGF, resulting in stimulated growth and development of the metacestode larvae. Inhibition of the signaling by either the EGFR inhibitors CI-1033 and BIBW2992 or the MEK/ERK inhibitor U0126 impairs germinative cell proliferation and larval growth.

Conclusions/Significance

These data demonstrate the contribution of EGF-mediated EGFR/ERK signaling to the regulation of germinative cells in E. multilocularis, and suggest the EGFR/ERK signaling as a potential therapeutic target for AE and perhaps other human cestodiasis.

E. multilocularis is considered as one of the most lethal parasitic helminth in humans. It grows like tumors mainly in human liver and infiltrates other tissues, and even metastasizes. It is believed that the parasite possesses a population of stem cell-like cells, the germinative cells. These cells are totipotent, have the ability for extensive self-renewal, and drive the parasite’s development and growth in the host. However, mechanisms controlling the behavior of germinative cells are poorly understood. Here, we show that the highly conserved EGFR/ERK signaling pathway in the parasite promoted germinative cell proliferation upon addition of human EGF (epidermal growth factor) in vitro, resulting in stimulated growth and development of the parasite. Our study provides information important for understanding this mechanism regulating germinative cells and the complex host-parasite interaction, and we hope it will help in developing new therapeutic strategies for the treatment of human helminthic infections.

Impact of matrix stiffness on fibroblast function

Chronic non-healing wounds, caused by impaired production of growth factors and reduced vascularization, represent a significant burden to patients, health care professionals, and health care system. While several wound dressing biomaterials have been developed, the impact of the mechanical properties of the dressings on the residing cells and consequently on the healing of the wounds is largely overlooked. The primary focus of this study is to explore whether manipulation of the substrate mechanics can regulate the function of fibroblasts, particularly in the context of their angiogenic activity. A photocrosslinkable hydrogel platform with orthogonal control over gel modulus and cell adhesive sites was developed to explore the quantitative relationship between ECM compliance and fibroblast function. Increase in matrix stiffness resulted in enhanced fibroblast proliferation and stress fiber formation. However, the angiogenic activity of fibroblasts was found to be optimum when the cells were seeded on compliant matrices. Thus, the observations suggest that the stiffness of the wound dressing material may play an important role in the progression of wound healing.

Pluripotent stem cell expansion and neural differentiation in 3-D scaffolds of tunable Poisson's ratio

Biophysical properties of the scaffolds such as the elastic modulus, have been recently shown to impact stem cell lineage commitment. On the other hand, the contribution of the Poisson's ratio, another important biophysical property, to the stem cell fate decision, has not been studied. Scaffolds with tunable Poisson's ratio (ν) (termed as auxetic scaffolds when Poisson's ratio is zero or negative) are anticipated to provide a spectrum of unique biophysical 3-D microenvironments to influence stem cell fate. To test this hypothesis, in the present work we fabricated auxetic polyurethane scaffolds (ν=0 to -0.45) and evaluated their effects on neural differentiation of mouse embryonic stem cells (ESCs) and human induced pluripotent stem cells (hiPSCs). Compared to the regular scaffolds (ν=+0.30) before auxetic conversion, the auxetic scaffolds supported smaller aggregate formation and higher expression of β-tubulin III upon neural differentiation. The influences of pore structure, Poisson's ratio, and elastic modulus on neural lineage commitment were further evaluated using a series of auxetic scaffolds. The results indicate that Poisson's ratio may confound the effects of elastic modulus, and auxetic scaffolds with proper pore structure and Poisson's ratio enhance neural differentiation. This study demonstrates that tuning the Poisson's ratio of the scaffolds together with elastic modulus and microstructure would enhance the capability to generate broader, more diversified ranges of biophysical 3-D microenvironments for the modulation of cellular differentiation.

STATEMENT OF SIGNIFICANCE

Biophysical signaling from the substrates and scaffolds plays a critical role in neural lineage commitment of pluripotent stem cells. While the contribution of elastic modulus has been well studied, the influence of Poisson's ratio along with microstructure of the scaffolds remains unknown largely due to the lack of technology to produce materials with tailorable Poisson's ratio. This study fabricated auxetic polyurethane scaffolds with different elastic modulus, Poisson's ratio and microstructure and evaluated neural differentiation of pluripotent stem cells. The findings add a novel angle to understand the impact of biophysical microenvironment on stem cell fate decisions.

Fluid shear stress activates YAP1 to promote cancer cell motility

Mechanical stress is pervasive in egress routes of malignancy, yet the intrinsic effects of force on tumour cells remain poorly understood. Here, we demonstrate that frictional force characteristic of flow in the lymphatics stimulates YAP1 to drive cancer cell migration; whereas intensities of fluid wall shear stress (WSS) typical of venous or arterial flow inhibit taxis. YAP1, but not TAZ, is strictly required for WSS-enhanced cell movement, as blockade of YAP1, TEAD1-4 or the YAP1–TEAD interaction reduces cellular velocity to levels observed without flow. Silencing of TEAD phenocopies loss of YAP1, implicating transcriptional transactivation function in mediating force-enhanced cell migration. WSS dictates expression of a network of YAP1 effectors with executive roles in invasion, chemotaxis and adhesion downstream of the ROCK–LIMK–cofilin signalling axis. Altogether, these data implicate YAP1 as a fluid mechanosensor that functions to regulate genes that promote metastasis.

Fluid frictional forces around cancer cells influence chemokine production and delivery of chemotherapeutic drugs but it is unclear if they directly impact tumour biology through biomechanical effects. Here, the authors show that wall shear stress stimulates cancer cell migration through a ROCK–LIMK–YAP axis.

Tensile stress stimulates the expression of osteogenic cytokines/growth factors and matricellular proteins in the mouse cranial suture at the site of osteoblast differentiation

Mechanical stress promotes osteoblast proliferation and differentiation from mesenchymal stem cells (MSCs). Although numerous growth factors and cytokines are known to regulate this process, information regarding the differentiation of mechanically stimulated osteoblasts from MSCs in in vivo microenvironment is limited. To determine the significant factors involved in this process, we performed a global analysis of differentially expressed genes, in response to tensile stress, in the mouse cranial suture wherein osteoblasts differentiate from MSCs. We found that the gene expression levels of several components involved in bone morphogenetic protein, Wnt, and epithelial growth factor signalings were elevated with tensile stress. Moreover gene expression of some extracellular matrices (ECMs), such as cysteine rich protein 61 (Cyr61)/CCN1 and galectin-9, were upregulated. These ECMs have the ability to modulate the activities of cytokines and are known as matricellular proteins. Cyr61/CCN1 expression was prominently increased in the fibroblastic cells and preosteoblasts in the suture. Thus, for the first time we demonstrated the mechanical stimulation of Cyr61/CCN1 expression in osteogenic cells in an ex vivo system. These results suggest the importance of matricellular proteins along with the cytokine-mediated signaling for the mechanical regulation of MSC proliferation and differentiation into osteoblastic cell lineage in vivo.

Proliferation and osteogenic differentiation of amniotic fluid-derived stem cells

The osteogenic differentiation of stem cells.

Nanostructured scaffold as a determinant of stem cell fate

The functionality of stem cells is tightly regulated by cues from the niche, comprising both intrinsic and extrinsic cell signals. Besides chemical and growth factors, biophysical signals are important components of extrinsic signals that dictate the stem cell properties. The materials used in the fabrication of scaffolds provide the chemical cues whereas the shape of the scaffolds provides the biophysical cues. The effect of the chemical composition of the scaffolds on stem cell fate is well researched. Biophysical signals such as nanotopography, mechanical forces, stiffness of the matrix, and roughness of the biomaterial influence the fate of stem cells. However, not much is known about their role in signaling crosstalk, stem cell maintenance, and directed differentiation. Among the various techniques for scaffold design, nanotechnology has special significance. The role of nanoscale topography in scaffold design for the regulation of stem cell behavior has gained importance in regenerative medicine. Nanotechnology allows manipulation of highly advanced surfaces/scaffolds for optimal regulation of cellular behavior. Techniques such as electrospinning, soft lithography, microfluidics, carbon nanotubes, and nanostructured hydrogel are described in this review, along with their potential usage in regenerative medicine. We have also provided a brief insight into the potential signaling crosstalk that is triggered by nanomaterials that dictate a specific outcome of stem cells. This concise review compiles recent developments in nanoscale architecture and its importance in directing stem cell differentiation for prospective therapeutic applications.