Surface-Driven Collagen Self-Assembly Affects Early Osteogenic Stem Cell Signaling

The surface charge of electroactive materials governs cell behaviour through its effect on protein deposition

The precise mechanisms underlying the cellular response to static electric cues remain unclear, limiting the design and development of biomaterials that utilize this parameter to enhance specific biological behaviours. To gather information on this matter we have explored the interaction of collagen type-I, the most abundant mammalian extracellular protein, with poly(vinylidene fluoride) (PVDF), an electroactive polymer with great potential for tissue engineering applications. Our results reveal significant differences in collagen affinity, conformation, and interaction strength depending on the electric charge of the PVDF surface, which subsequently affects the behaviour of mesenchymal stem cells seeded on them. These findings highlight the importance of surface charge in the establishment of the material-protein interface and ultimately in the biological response to the material. STATEMENT OF SIGNIFICANCE: The development of new tissue engineering strategies relies heavily on the understanding of how biomaterials interact with biological tissues. Although several factors drive this process and their driving principles have been identified, the relevance and mechanism by which the surface potential influences cell behaviour is still unknown. In our study, we investigate the interaction between collagen, the most abundant component of the extracellular matrix, and poly(vinylidene fluoride) with varying surface charges. Our findings reveal substantial variations in the binding forces, structure and adhesion of collagen on the different surfaces, which collectively explain the differential cellular responses. By exposing these differences, our research fills a critical knowledge gap and paves the way for innovations in material design for advanced tissue regeneration strategies.

Impact of changes in collagen construction and molecular state on integrin - binding properties

The interaction between the integrin and collagen is important in cell adhesion and signaling. Collagen, as the main component of extracellular matrix, is a base material for tissue engineering constructs. In tissue engineering, the collagen structure and molecule state may be altered to varying degrees in the process of processing and utilizing, thereby affecting its biological properties. In this work, the impact of changes in collagen structure and molecular state on the binding properties of collagen to integrin α2β1 and integrin specific cell adhesion were explored. The results showed that the molecular structure of collagen is destroyed under the influence of heating, freeze-grinding and irradiation, the triple helix integrity is reduced and molecular breaking degree is increased. The binding ability of collagen to integrin α2β1 is increased with the increase of triple helix integrity and decays exponentially with the increase of molecular breaking degree. The collagen molecular state can also influences the binding ability of collagen to cellular receptor. The collagen fibrils binding to integrin α2β1 and HT1080 cells is stronger than to collagen monomolecule. Meanwhile, the hybrid fibril exhibits a different cellular receptor binding performance from corresponding single species collagen fibril. These findings provide ideas for the design and development of new collagen-based biomaterials and tissue engineering research.

Electric field-mediated adhesive dynamics of cells inside bio-functionalised microchannels offers important cues for active control of cell-substrate adhesion

Adhesive dynamics of cells plays a critical role in determining different biophysical processes orchestrating health and disease in living systems. While the rolling of cells on functionalised substrates having similarity with biophysical pathways appears to be extensively discussed in the literature, the effect of an external stimulus in the form of an electric field on the same remains underemphasized. Here, we bring out the interplay of fluid shear and electric field on the rolling dynamics of adhesive cells in biofunctionalised micro-confinements. Our experimental results portray that an electric field, even restricted to low strengths within the physiologically relevant regimes, can significantly influence the cell adhesion dynamics. We quantify the electric field-mediated adhesive dynamics of the cells in terms of two key parameters, namely, the voltage-altered rolling velocity and the frequency of adhesion. The effect of the directionality of the electric field with respect to the flow direction is also analysed by studying cellular migration with electrical effects acting both along and against the flow. Our experiment, on one hand, demonstrates the importance of collagen functionalisation in the adhesive dynamics of cells through micro channels, while on the other hand, it reveals how the presence of an axial electric field can lead to significant alteration in the kinetic rate of bond breakage, thereby modifying the degree of cell-substrate adhesion and quantifying in terms of the adhesion frequency of the cells. Proceeding further forward, we offer a simple theoretical explanation towards deriving the kinetics of cellular bonding in the presence of an electric field, which corroborates favourably with our experimental outcome. These findings are likely to offer fundamental insights into the possibilities of local control of cellular adhesion via electric field mediated interactions, bearing critical implications in a wide variety of medical conditions ranging from wound healing to cancer metastasis.

The Role of Integrin Receptor's α and β Subunits of Mouse Mesenchymal Stem Cells on the Interaction of Marine-Derived Blacktip Reef Shark (Carcharhinus melanopterus) Skin Collagen

Marine collagen (MC) has recently attracted more attention in tissue engineering as a biomaterial substitute due to its significant role in cellular signaling mechanisms, especially in mesenchymal stem cells (MSCs). However, the actual signaling mechanism of MC in MSC growth, which is highly influenced by their molecular pattern, is poorly understood. Hence, we investigated the integrin receptors (α₁β₁, α₂β₁, α₁₀β₁, and α₁₁β₁) binding mechanism and proliferation of MCs (blacktip reef shark collagen (BSC) and blue shark collagen (SC)) compared to bovine collagen (BC) on MSCs behavior through functionalized collagen molecule probing for the first time. The results showed that BSC and SC had higher proliferation rates and accelerated scratch wound healing by increasing migratory rates of MSCs. Cell adhesion and spreading results demonstrated that MC had a better capacity to anchor MSCs and maintain cell morphology than controls. Living cell observations showed that BSC was gradually assembled by cells into the ECM network within 24 h. Interestingly, qRT-PCR and ELISA revealed that the proliferative effect of MC was triggered by interacting with specific integrin receptors such as α₂β₁, α₁₀β₁, and α₁₁β₁ of MSCs. Accordingly, BSC accelerated MSCs' growth, adhesion, shape, and spreading by interacting with specific integrin subunits (α₂ and β₁) and thereby triggering further signaling cascade mechanisms.

Junctional epithelium and hemidesmosomes: Tape and rivets for solving the "percutaneous device dilemma" in dental and other permanent implants

The percutaneous device dilemma describes etiological factors, centered around the disrupted epithelial tissue surrounding non-remodelable devices, that contribute to rampant percutaneous device infection. Natural percutaneous organs, in particular their extracellular matrix mediating the "device"/epithelium interface, serve as exquisite examples to inspire longer lasting long-term percutaneous device design. For example, the tooth's imperviousness to infection is mediated by the epithelium directly surrounding it, the junctional epithelium (JE). The hallmark feature of JE is formation of hemidesmosomes, cell/matrix adhesive structures that attach surrounding oral gingiva to the tooth's enamel through a basement membrane. Here, the authors survey the multifaceted functions of the JE, emphasizing the role of the matrix, with a particular focus on hemidesmosomes and their five main components. The authors highlight the known (and unknown) effects dental implant - as a model percutaneous device - placement has on JE regeneration and synthesize this information for application to other percutaneous devices. The authors conclude with a summary of bioengineering strategies aimed at solving the percutaneous device dilemma and invigorating greater collaboration between clinicians, bioengineers, and matrix biologists.

Block copolymer nanopatterns affect cell spreading: Stem versus cancer bone cells

Bone healing after a tumor removal can be promoted by biomaterials that enhance the bone regeneration and prevent the tumor relapse. Herein, we obtained several nanopatterns by self-assembly of polystyrene-block-poly-(2-vinylpyridine) (PS-b-P2VP) with different molecular weights and investigated the adhesion and morphology of human bone marrow mesenchymal stem cells (BMMSC) and osteosarcoma cell line (SaOS-2) on these patterns aiming to identify topography and chemistry that promote bone healing. We analyzed > 2000 cells per experimental condition using imaging software and different morphometric descriptors, namely area, perimeter, aspect ratio, circularity, surface/area, and fractal dimension of cellular contour (FDC). The obtained data were used as inputs for principal component analysis, which showed distinct response of BMMSC and SaOS-2 to the surface topography and chemistry. Among the studied substrates, micellar nanopatterns assembled from the copolymer with high molecular weight promote the adhesion and spreading of BMMSC and have an opposite effect on SaOS-2. This nanopattern is thus beneficial for bone regeneration after injury or pathology, e.g. bone fracture or tumor removal.

Fabrication of a stepwise degradable hybrid bioscaffold based on the natural and partially denatured collagen

As a major component of extracellular matrixes (ECMs), collagen is an attractive biomaterial to fabricate porous scaffold for tissue engineering due to their similarity to the in vivo static microenvironment. However, the collagen-based porous scaffolds were difficult to mimic the dynamically remolded porous structure of ECM during the cell proliferation and tissue development, and always have poor mechanical property and not easy to handle. Here, natural collagen and partially denatured collagen was used to prepare the stepwise degradable hybrid bioscaffold with suitable mechanical property and dynamically remolded inner porous structure, which is desirable for the applications of tissue engineering. The collagen-based microporous scaffold was first prepared and used as physical support, then, the mechanical strength of which was reinforced by the import of the partially denatured collagen to give the hybrid bioscaffold. The fabrication conditions of the hybrid scaffolds were optimized, of which the thermal stability, mechanical property, and swelling property was explored. The stepwise enzymatic degradation process and the corresponding porous structure variation of the hybrid scaffold was confirmed by SEM and cell culture assays.

Real-Time and In Situ Monitoring of the Synthesis of Silica Nanoparticles

The real-time and in situ monitoring of the synthesis of nanomaterials (NMs) remains a challenging task, which is of pivotal importance by assisting fundamental studies (e.g., synthesis kinetics and colloidal phenomena) and providing optimized quality control. In fact, the lack of reproducibility in the synthesis of NMs is a bottleneck against the translation of nanotechnologies into the market toward daily practice. Here, we address an impedimetric millifluidic sensor with data processing by machine learning (ML) as a sensing platform to monitor silica nanoparticles (SiO₂NPs) over a 24 h synthesis from a single measurement. The SiO₂NPs were selected as a model NM because of their extensive applications. Impressively, simple ML-fitted descriptors were capable of overcoming interferences derived from SiO₂NP adsorption over the signals of polarizable Au interdigitate electrodes to assure the determination of the size and concentration of nanoparticles over synthesis while meeting the trade-off between accuracy and speed/simplicity of computation. The root-mean-square errors were calculated as ∼2.0 nm (size) and 2.6 × 10¹⁰ nanoparticles mL^-1 (concentration). Further, the robustness of the ML size descriptor was successfully challenged in data obtained along independent syntheses using different devices, with the global average accuracy being 103.7 ± 1.9%. Our work advances the developments required to transform a closed flow system basically encompassing the reactional flask and an impedimetric sensor into a scalable and user-friendly platform to assess the in situ synthesis of SiO₂NPs. Since the sensor presents a universal response principle, the method is expected to enable the monitoring of other NMs. Such a platform may help to pave the way for translating "sense-act" systems into practice use in nanotechnology.

In Vitro Matured Human Pluripotent Stem Cell-derived Cardiomyocytes Form Grafts With Enhanced Structure and Function in Injured Hearts

BACKGROUND

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have tremendous promise for application in cardiac regeneration, but their translational potential is limited by an immature phenotype. We hypothesized that large-scale manufacturing of mature hPSC-CMs could be achieved via culture on polydimethylsiloxane (PDMS) lined roller bottles and that the transplantation of these cells would mediate better structural and functional outcomes than with conventional immature hPSC-CM populations.

METHODS

We comprehensively phenotyped hPSC-CMs after in vitro maturation for 20 and 40 days on either PDMS or standard tissue culture plastic (TCP) substrates. All hPSC-CMs were generated using a transgenic hPSC line that stably expressed a voltage-sensitive fluorescent reporter to facilitate in vitro and in vivo electrophysiological studies, and cardiomyocyte populations were also analyzed in vitro by immunocytochemistry, ultrastructure and fluorescent calcium imaging, as well as bulk and single-cell transcriptomics. We next compared outcomes after the transplantation of these populations into a guinea pig model of myocardial infarction (MI) using endpoints including histology, optical mapping of graft- and host-derived action potentials, echocardiography, and telemetric electrocardiographic (ECG) monitoring.

RESULTS

We demonstrated the economic generation of >1x10⁸ mature hPSC-CMs per PDMS-lined roller bottle. Compared to their counterparts generated on TCP substrates, PDMS-matured hPSC-CMs exhibited increased cardiac gene expression and more mature structural and functional properties in vitro. More importantly, intra-cardiac grafts formed with PDMS-matured myocytes showed greatly enhanced structure and alignment, better host-graft electromechanical integration, less pro-arrhythmic behavior, and greater beneficial effects on contractile function.

CONCLUSIONS

In summary, we describe practical methods for the scaled generation of mature hPSC-CMs and provide the first evidence that the transplantation of more mature cardiomyocytes yields better outcomes in vivo.

Shear-stress sensing by PIEZO1 regulates tendon stiffness in rodents and influences jumping performance in humans

Athletic performance relies on tendons, which enable movement by transferring forces from muscles to the skeleton. Yet, how load-bearing structures in tendons sense and adapt to physical demands is not understood. Here, by performing calcium (Ca²⁺) imaging in mechanically loaded tendon explants from rats and in primary tendon cells from rats and humans, we show that tenocytes detect mechanical forces through the mechanosensitive ion channel PIEZO1, which senses shear stresses induced by collagen-fibre sliding. Through tenocyte-targeted loss-of-function and gain-of-function experiments in rodents, we show that reduced PIEZO1 activity decreased tendon stiffness and that elevated PIEZO1 mechanosignalling increased tendon stiffness and strength, seemingly through upregulated collagen cross-linking. We also show that humans carrying the PIEZO1 E756del gain-of-function mutation display a 13.2% average increase in normalized jumping height, presumably due to a higher rate of force generation or to the release of a larger amount of stored elastic energy. Further understanding of the PIEZO1-mediated mechanoregulation of tendon stiffness should aid research on musculoskeletal medicine and on sports performance.

Frugal Approach toward Developing a Biomimetic, Microfluidic Network-on-a-Chip for In Vitro Analysis of Microvascular Physiology

Several disease conditions, such as cancer metastasis and atherosclerosis, are deeply connected with the complex biophysical phenomena taking place in the complicated architecture of the tiny blood vessels in human circulatory systems. Traditionally, these diseases have been probed by devising various animal models, which are otherwise constrained by ethical considerations as well as limited predictive capabilities. Development of an engineered network-on-a-chip, which replicates not only the functional aspects of the blood-carrying microvessels of human bodies, but also its geometrical complexity and hierarchical microstructure, is therefore central to the evaluation of organ-assist devices and disease models for therapeutic assessment. Overcoming the constraints of reported resource-intensive fabrication techniques, here, we report a facile, simple yet niche combination of surface engineering and microfabrication strategy to devise a highly ordered hierarchical microtubular network embedded within a polydimethylsiloxane (PDMS) slab for dynamic cell culture on a chip, with a vision of addressing the exclusive aspects of the vascular transport processes under medically relevant paradigms. The design consists of hierarchical complexity ranging from capillaries (∼80 μm) to large arteries (∼390 μm) and a simultaneous tuning of the interfacial material chemistry. The fluid flow behavior is characterized numerically within the hierarchical network, and a confluent endothelial layer is realized on the inner wall of microfluidic device. We further explore the efficacy of the device as a vascular deposition assay of circulatory tumor cells (MG-63 osteosarcoma cells) present in whole blood. The proposed paradigm of mimicking an in vitro vascular network in a low-cost paradigm holds further potential for probing cellular dynamics as well as offering critical insights into various vascular transport processes.

A Review of Single-Cell Adhesion Force Kinetics and Applications

Cells exert, sense, and respond to the different physical forces through diverse mechanisms and translating them into biochemical signals. The adhesion of cells is crucial in various developmental functions, such as to maintain tissue morphogenesis and homeostasis and activate critical signaling pathways regulating survival, migration, gene expression, and differentiation. More importantly, any mutations of adhesion receptors can lead to developmental disorders and diseases. Thus, it is essential to understand the regulation of cell adhesion during development and its contribution to various conditions with the help of quantitative methods. The techniques involved in offering different functionalities such as surface imaging to detect forces present at the cell-matrix and deliver quantitative parameters will help characterize the changes for various diseases. Here, we have briefly reviewed single-cell mechanical properties for mechanotransduction studies using standard and recently developed techniques. This is used to functionalize from the measurement of cellular deformability to the quantification of the interaction forces generated by a cell and exerted on its surroundings at single-cell with attachment and detachment events. The adhesive force measurement for single-cell microorganisms and single-molecules is emphasized as well. This focused review should be useful in laying out experiments which would bring the method to a broader range of research in the future.

Effect of molecular chirality on the collagen self-assembly

The function of molecular chirality in collagen self-assembly was presented.

Fibrillogenesis of acrylic acid-grafted-collagen without self-assembly property inspired by the hybrid fibrils of xenogeneic collagen

Along with advancements in both protein and chemistry science, the chemical modification of proteins is attracting more and more attention. More specifically, the attachment of polymers or reactive moieties into collagen offers a method to add novel functions to this protein. However, the fibrillogenesis of the modified collagen with high grafting density cannot always be achieved. Here, inspired by the hybrid fibrils of xenogeneic collagen, fibrillogenesis of acrylic acid-grafted-collagen (AAc-g-Col) without self-assembly property was achieved by the induction of natural collagen (Col). The step-by-step co-assembly process of AAc-g-Col and Col was confirmed by turbidity assay. The formation of Col/AAc-g-Col hybrid fibrils was verified by TEM since the acryloyl groups of the hybrid fibrils were labelled using HS-AuNPs based on the Michael addition. Moreover, rheology, SEM, and MTT assays revealed that the fibrillary structures and biocompatibility of the Col/AAc-g-Col hydrogel were comparable to that of the Col hydrogel, although they presented a lower viscoelasticity.

Endothelialization of PDMS-based microfluidic devices under high shear stress conditions

Microfluidic systems made out of polydimethylsiloxane (PDMS) offer a platform to mimic vascular flow conditions in model systems at well-defined shear stresses. However, extracellular matrix (ECM) proteins that are physisorbed on the PDMS are not reliably attached under high shear stress conditions, which makes long-term experiments difficult. To overcome this limitation, we functionalized PDMS surfaces with 3-aminopropyltriethoxysilane (APTES) by using different surface activation methods to develop a stable linkage between the PDMS surface and collagen, which served as a model ECM protein. The stability of the protein coating inside the microfluidic devices was evaluated in perfusion experiments with phosphate-buffered saline (PBS) at 10-40 dynes/cm² wall shear stress. To assess the stability of cell adhesion, endothelial cells were grown in a multi-shear device over a shear stress range of 20-150 dynes/cm². Cells on the APTES-mediated collagen coating were stable over the entire shear stress range in PBS (pH 9) for 48 h. The results suggest that at high pH values, the electrostatic interaction between APTES-coated surfaces and collagen molecules offer a very promising tool to modify PDMS-based microfluidic devices for long-term endothelialization under high shear stress conditions.

A facile surface modification of poly(dimethylsiloxane) with amino acid conjugated self-assembled monolayers for enhanced osteoblast cell behavior

Polydimethylsiloxane (PDMS) is a biocompatible synthetic polymer and used in various applications due to its low toxicity and tunable surface properties. However, PDMS does not have any chemical cues for cell binding. Plasma treatment, protein coating or surface modification with various molecules have been used to improve its surface characteristics. Still, these techniques are either last for a very limited time or have very complicated experimental procedures. In the present study, simple and one-step surface modification of PDMS is successfully accomplished by the preparation of hydrophilic and hydrophobic amino acid conjugated self-assembled monolayers (SAMs) for enhanced interactions at the cell-substrate interface. Synthesis of histidine and leucine conjugated (3-aminopropyl)-triethoxysilane (His-APTES and Leu-APTES) were confirmed with proton nuclear magnetic resonance spectroscopy (¹H NMR) and optimum conditions for the modification of PDMS with SAMs were investigated by X-ray photoelectron spectroscopy (XPS) analysis, combined with water contact angle (WCA) measurements. Results indicated that both SAMs enhanced cellular behavior in vitro. Furthermore, hydrophilic His-APTES modification provides a superior environment for the osteoblast maturation with higher alkaline phosphatase activity and mineralization. As histidine, leucine, and functional groups of these SAMs are naturally found in biological systems, modification of PDMS with them increases its cell-substrate surface biomimetic properties. This study establishes a successful modification of PDMS for in vitro cell studies, offering a biomimetic and easy procedure for potential applications in microfluidics, cell-based therapies, or drug investigations.

Biomaterial Stiffness Guides Cross-talk between Chondrocytes: Implications for a Novel Cellular Response in Cartilage Tissue Engineering

The exquisite cartilage architecture maintains an orderly dynamic equilibrium as a result of the interplay between chondrocyte functions and the unique extracellular matrix (ECM) microenvironment. Numerous studies have demonstrated that extracellular cues, including topological, mechanical, and biochemical properties of the underlying substrates, dictate the chondrocyte behaviors. Consequently, developing advanced biomaterials with the desired characteristics which could achieve the biointerface between cells and the surrounded matrix close to the physiological conditions becomes a great hotspot in bioengineering. However, how the substrate stiffness influences the intercellular communication among chondrocytes is still poorly reported. We used polydimethylsiloxane with varied stiffnesses as a cell culture substrate to elucidate a novel cell-to-cell communication in a collective of chondrocytes. First, morphological images collected using scanning electron microscopy revealed that the tunable substrate stiffnesses directed the changes in intercellular links among chondrocytes. Next, fibronectin, which played a vital role in the connection of ECM components or linkage of ECM to chondrocytes, was shown to be gathered along cell-cell contact areas and was changed with the tunable substrate stiffnesses. Furthermore, transmembrane junctional proteins including connexin 43 (Cx43) and pannexin 1 (Panx1), which are responsible for gap junction formation in cell-to-cell communication, were mediated by the tunable substrate stiffnesses. Finally, through a scrape loading/dye transfer assay, we revealed cell-to-cell communication changes in a living chondrocyte population in response to the tunable substrate stiffnesses via cell-to-cell fluorescent molecule transport. Taken together, this novel cell-to-cell communication regulated by biomaterial stiffness could help us to increase the understanding of cell behaviors under biomechanical control and may ultimately lead to refining cell-based cartilage tissue engineering.

Traction force microscopy for understanding cellular mechanotransduction

Under physiological and pathological conditions, mechanical forces generated from cells themselves or transmitted from extracellular matrix (ECM) through focal adhesions (FAs) and adherens junctions (AJs) are known to play a significant role in regulating various cell behaviors. Substantial progresses have been made in the field of mechanobiology towards novel methods to understand how cells are able to sense and adapt to these mechanical forces over the years. To address these issues, this review will discuss recent advancements of traction force microscopy (TFM), intracellular force microscopy (IFM), and monolayer stress microscopy (MSM) to measure multiple aspects of cellular forces exerted by cells at cell-ECM and cell-cell junctional intracellular interfaces. We will also highlight how these methods can elucidate the roles of mechanical forces at interfaces of cell-cell/cell-ECM in regulating various cellular functions. [BMB Reports 2020; 53(2): 74-81].

Quasi-3D morphology and modulation of focal adhesions of human adult stem cells through combinatorial concave elastomeric surfaces with varied stiffness

In vivo cell niches are complex architectures that provide a wide range of biochemical and mechanical stimuli to control cell behavior and fate. With the aim to provide in vitro microenvironments mimicking physiological niches, microstructured substrates have been exploited to support cell adhesion and to control cell shape as well as three dimensional morphology. At variance with previous methods, we propose a simple and rapid protein subtractive soft lithographic method to obtain microstructured polydimethylsiloxane substrates for studying stem cell adhesion and growth. The shape of adult renal stem cells and nuclei is found to depend predominantly on micropatterning of elastomeric surfaces and only weakly on the substrate mechanical properties. Differently, focal adhesions in their shape and density but not in their alignment mainly depend on the elastomer stiffness almost regardless of microscale topography. Local surface topography with concave microgeometry enhancing adhesion drives stem cells in a quasi-three dimensional configuration where stiffness might significantly steer mechanosensing as highlighted by focal adhesion properties.

Telopeptide-dependent xenogeneic collagen co-assembly

The function of telopeptide in xenogeneic collagen co-assembly was shown.

Substrate elasticity regulates adipose-derived stromal cell differentiation towards osteogenesis and adipogenesis through β-catenin transduction

It is generally recognised that mesenchymal stem cells (MSCs) can differentiate into multiple lineages through guidance from the biophysical properties of the substrates. However, the precise biophysical mechanism that enables MSCs to respond to substrate properties remains unclear. In the current study, polydimethylsiloxane (PDMS) substrates with different stiffnesses were fabricated and the way in which the elastic modulus of the substrate regulated differentiation towards osteogenesis and adipogenesis in adipose-derived stromal cells (ASCs) was explored. Initially, a cell morphology change by SEM was observed between the stiff and soft substrates. The cytoskeleton stains including microfilament by F-actin and microtubule by α- and β-tubulin further showed a larger cell spreading area on the stiff substrate. Then the expression of vinculin, in charge for the linkage of adhesion molecules to the actin cytoskeleton, was enhanced on the stiff substrate. This change in focal adhesion plaque further triggered intracellular β-catenin signaling and promoted its nuclear translocation especially on the stiff substrate. The influence of β-catenin signaling on direct differentiation to osteogenic lineages was through direct binding between its downstream protein, Lef-1, and the osteogenic transcriptional factors, Runx2 and Osx, while on differentiation to adipogenic lineages was through modulating the expression of PPARγ. The imbalance of stiffness-induced β-catenin signaling finally induced a stronger osteogenesis and a weaker adipogenesis on the stiff substrate relative to those on the soft substrate. This study indicates the importance of stiffness on ASC differentiation and could help to increase understanding of the mechanism underlying molecular signal transduction from mechanosensing, mechanotransducing to stem cell differentiation.

STATEMENT OF SIGNIFICANCE

Mesenchymal stem cells can differentiate into multiple lineages, such as adipogenesis, myogenesis, neurogenesis, angiogenesis and osteogenesis, through influence of biophysical properties of the extracellular matrix. However, the precise bio-mechanism that triggers stem cell differentiation in response to matrix biophysical properties remains unclear. In the current study, we provide a series of experiments involving the characterization of cell morphology, microfilament, microtubule and adhesion capacity of adipose-derived stromal cells (ASCs) in response to substrate stiffness, and further elucidation of cytoplasmic β-catenin-dependent signal transduction, nuclear translocation and resultant promoter activation of transcriptional factors for osteogenesis and adipogenesis. This study provides an explanation on deeper understanding of bio-mechanism underlying substrate stiffness-triggered β-catenin signal transduction from active mechanosensing, mechanotransducing to stem cell differentiation.

Graphene-Oxide-Based FRET Platform for Sensing Xenogeneic Collagen Coassembly

Xenogeneic collagen coassembly (XCCA) offers a new view for the design and performance regulation of novel collagen-based biomaterials. But there is still a lack of accurate and sensitive method for monitoring XCCA. In this study, a simple and efficient graphene-oxide (GO)-based fluorescence resonance energy transfer (FRET) platform has been developed to sense XCCA. We first designed a fluorescein isothiocyanate (FITC)-labeled porcine skin collagen (PSC) that adsorbed on the GO surface and effectively quenched its fluorescence. Upon the addition of grass carp skin collagen (GCSC), the XCCA between PSC and GCSC resulted in desorption of FITC-PSC from GO surface and thus caused an increase in fluorescence signal. Under the optimal conditions, the fluorescence signal linearly increased with the increase in the GCSC concentration in the range of 50-1000 μg/mL, with a sensitivity of 22 μg/mL (S/N = 3). Furthermore, the developed strategy also exhibited excellent specificity and anti-interference ability. More interestingly, the thermal stability of collagen fibrils formed by XCCA is linearly related to the GCSC concentration. These results open a facile, effective, and sensitive approach for sensing XCCA and provide a new strategy for arbitrarily regulating the thermal stability of collagen fibrils.

Biomaterial surface energy-driven ligand assembly strongly regulates stem cell mechanosensitivity and fate on very soft substrates

Cell instructive biomaterial cues are a major topic of interest in both basic and applied research. In this work, we clarify how surface energy of soft biomaterials can dramatically affect mesenchymal stem cell receptor recruitment and downstream signaling related to cell fate. We elucidate how surface protein self-assembly and the resulting surface topology can act to steer mechanotransduction and related biological response of attached cells. These findings fill a critical gap in our basic understanding of cell–biomaterial interaction and highlight soft biomaterial surface energy as a dominant design factor that should not be neglected.

Although mechanisms of cell–material interaction and cellular mechanotransduction are increasingly understood, the mechanical insensitivity of mesenchymal cells to certain soft amorphous biomaterial substrates has remained largely unexplained. We reveal that surface energy-driven supramolecular ligand assembly can regulate mesenchymal stem cell (MSC) sensing of substrate mechanical compliance and subsequent cell fate. Human MSCs were cultured on collagen-coated hydrophobic polydimethylsiloxane (PDMS) and hydrophilic polyethylene-oxide-PDMS (PEO-PDMS) of a range of stiffnesses. Although cell contractility was similarly diminished on soft substrates of both types, cell spreading and osteogenic differentiation occurred only on soft PDMS and not hydrophilic PEO-PDMS (elastic modulus <1 kPa). Substrate surface energy yields distinct ligand topologies with accordingly distinct profiles of recruited transmembrane cell receptors and related focal adhesion signaling. These differences did not differentially regulate Rho-associated kinase activity, but nonetheless regulated both cell spreading and downstream differentiation.

Self-assembly of collagen-based biomaterials: preparation, characterizations and biomedical applications

By combining regulatory parameters with characterization methods, researchers can selectively fabricate collagenous biomaterials with various functional responses for biomedical applications.

Biophysical Regulation of Cell Behavior-Cross Talk between Substrate Stiffness and Nanotopography

The stiffness and nanotopographical characteristics of the extracellular matrix (ECM) influence numerous developmental, physiological, and pathological processes in vivo. These biophysical cues have therefore been applied to modulate almost all aspects of cell behavior, from cell adhesion and spreading to proliferation and differentiation. Delineation of the biophysical modulation of cell behavior is critical to the rational design of new biomaterials, implants, and medical devices. The effects of stiffness and topographical cues on cell behavior have previously been reviewed, respectively; however, the interwoven effects of stiffness and nanotopographical cues on cell behavior have not been well described, despite similarities in phenotypic manifestations. Herein, we first review the effects of substrate stiffness and nanotopography on cell behavior, and then focus on intracellular transmission of the biophysical signals from integrins to nucleus. Attempts are made to connect extracellular regulation of cell behavior with the biophysical cues. We then discuss the challenges in dissecting the biophysical regulation of cell behavior and in translating the mechanistic understanding of these cues to tissue engineering and regenerative medicine.

Collagen nanofibril self-assembly on a natural polymeric material for the osteoinduction of stem cells in vitro and biocompatibility in vivo

This manuscript reports the characterization of molecularly self-assembled collagen nanofibers on a natural polymeric microporous structure and their ability to support stem cell differentiation in vitro and host tissue response in vivo.