Cell density overrides the effect of substrate stiffness on human mesenchymal stem cells' morphology and proliferation

Influence of Soft and Stiff Matrices on Cytotoxicity in Gingival Fibroblasts: Implications for Soft Tissue Biocompatibility

The biocompatibility of dental materials is critical for ensuring safety in clinical applications. However, standard in vitro cytotoxicity assays often rely on stiff tissue culture plastic (TCP), which does not accurately replicate the biomechanical properties of soft oral tissues. In this study, we compared human gingival fibroblasts (HGFs) cultured on soft, gel-based substrates mimicking gingival tissue stiffness (0.2 kPa) with those cultured on conventional TCP (3 GPa) to assess the influence of substrate stiffness on the cytotoxicity of methyl methacrylate (MMA), as well as other cytotoxic agents, including DMSO and H2O2. The results demonstrated that cells cultured on softer substrates exhibited enhanced resistance to cytotoxic stress, with increased viability and decreased apoptosis and DNA damage following exposure to MMA, DMSO, and H2O2. Notably, HGFs on soft substrates showed significantly greater resilience to MMA-induced cytotoxicity compared to those cultured on TCP. These findings emphasize the critical role of substrate stiffness in modulating cellular responses to toxic agents and highlight the necessity of using physiologically relevant models for cytotoxicity testing of dental materials. This study provides valuable insights for improving biocompatibility assessment protocols in clinical settings.

Control of spatio-temporal patterning via cell growth in a multicellular synthetic gene circuit

A major goal in synthetic development is to build gene regulatory circuits that control patterning. In natural development, an interplay between mechanical and chemical communication shapes the dynamics of multicellular gene regulatory circuits. For synthetic circuits, how non-genetic properties of the growth environment impact circuit behavior remains poorly explored. Here, we first describe an occurrence of mechano-chemical coupling in synthetic Notch (synNotch) patterning circuits: high cell density decreases synNotch-gated gene expression in different cellular systems in vitro. We then construct, both in vitro and in silico, a synNotch-based signal propagation circuit whose outcome can be regulated by cell density. Spatial and temporal patterning outcomes of this circuit can be predicted and controlled via modulation of cell proliferation, initial cell density, and/or spatial distribution of cell density. Our work demonstrates that synthetic patterning circuit outcome can be controlled via cellular growth, providing a means for programming multicellular circuit patterning outcomes.

Geometric constraint of mechanosensing by modification of hydrogel thickness prevents stiffness-induced differentiation in bone marrow stromal cells

Extracellular matrix (ECM) stiffness is fundamental in cell division, movement and differentiation. The stiffness that cells sense is determined not only by the elastic modulus of the ECM material but also by ECM geometry and cell density. We hypothesized that these factors would influence cell traction-induced matrix deformations and cellular differentiation in bone marrow stromal cells (BMSCs). To achieve this, we cultivated BMSCs on polyacrylamide hydrogels that varied in elastic modulus and geometry and measured cell spreading, cell-imparted matrix deformations and differentiation. At low cell density BMSCs spread to a greater extent on stiff compared with soft hydrogels, or on thin compared with thick hydrogels. Cell-imparted matrix deformations were greater on soft compared with stiff hydrogels or thick compared with thin hydrogels. There were no significant differences in osteogenic differentiation relative to hydrogel elastic modulus and thickness. However, increased cell density and/or prolonged culture significantly reduced matrix deformations on soft hydrogels to levels similar to those on stiff substrates. This suggests that at high cell densities cell traction-induced matrix displacements are reduced by both neighbouring cells and the constraint imposed by an underlying stiff support. This may explain observations of the lack of difference in osteogenic differentiation as a function of stiffness.

How to Keep Myofibroblasts under Control: Culture of Mouse Skin Fibroblasts on Soft Substrates

During the physiological healing of skin wounds, fibroblasts recruited from the uninjured adjacent dermis and deeper subcutaneous fascia layers are transiently activated into myofibroblasts to first secrete and then contract collagen-rich extracellular matrix into a mechanically resistant scar. Scar tissue restores skin integrity after damage but comes at the expense of poor esthetics and loss of tissue function. Stiff scar matrix also mechanically activates various precursor cells into myofibroblasts in a positive feedback loop. Persistent myofibroblast activation results in pathologic accumulation of fibrous collagen and hypertrophic scarring, called fibrosis. Consequently, the mechanisms of fibroblast-to-myofibroblast activation and persistence are studied to develop antifibrotic and prohealing treatments. Mechanistic understanding often starts in a plastic cell culture dish. This can be problematic because contact of fibroblasts with tissue culture plastic or glass surfaces invariably generates myofibroblast phenotypes in standard culture. We describe a straight-forward method to produce soft cell culture surfaces for fibroblast isolation and continued culture and highlight key advantages and limitations of the approach. Adding a layer of elastic silicone polymer tunable to the softness of normal skin and the stiffness of pathologic scars allows to control mechanical fibroblast activation while preserving the simplicity of conventional 2-dimensional cell culture.

Matrix stiffness-dependent regulation of immunomodulatory genes in human MSCs is associated with the lncRNA CYTOR

Cell-matrix interactions in 3D environments significantly differ from those in 2D cultures. As such, mechanisms of mechanotransduction in 2D cultures are not necessarily applicable to cell-encapsulating hydrogels that resemble features of tissue architecture. Accordingly, the characterization of molecular pathways in 3D matrices is expected to uncover insights into how cells respond to their mechanical environment in physiological contexts, and potentially also inform hydrogel-based strategies in cell therapies. In this study, a bone marrow-mimetic hydrogel was employed to systematically investigate the stiffness-responsive transcriptome of mesenchymal stromal cells. High matrix rigidity impeded integrin-collagen adhesion, resulting in changes in cell morphology characterized by a contractile network of actin proximal to the cell membrane. This resulted in a suppression of extracellular matrix-regulatory genes involved in the remodeling of collagen fibrils, as well as the upregulation of secreted immunomodulatory factors. Moreover, an investigation of long noncoding RNAs revealed that the cytoskeleton regulator RNA (CYTOR) contributes to these 3D stiffness-driven changes in gene expression. Knockdown of CYTOR using antisense oligonucleotides enhanced the expression of numerous mechanoresponsive cytokines and chemokines to levels exceeding those achievable by modulating matrix stiffness alone. Taken together, our findings further our understanding of mechanisms of mechanotransduction that are distinct from canonical mechanotransductive pathways observed in 2D cultures.

Hydrogel Alginate Considerations for Improved 3D Matrix Stability and Cell Graft Viability and Function in Studying Type 1 Diabetes In Vitro

Biomedical devices such as islet-encapsulating systems are used for treatment of type 1 diabetes (T1D). Despite recent strides in preventing biomaterial fibrosis, challenges remain for biomaterial scaffolds due to limitations on cells contained within. The study demonstrates that proliferation and function of insulinoma (INS-1) cells as well as pancreatic rat islets may be improved in alginate hydrogels with optimized gel%, crosslinking, and stiffness. Quantitative polymerase chain reaction (qPCR)-based graft phenotyping of encapsulated INS-1 cells and pancreatic islets identified a hydrogel stiffness range between 600 and 1000 Pa that improved insulin Ins and Pdx1 gene expression as well as glucose-sensitive insulin-secretion. Barium chloride (BaCl2) crosslinking time is also optimized due to toxicity of extended exposure. Despite possible benefits to cell viability, calcium chloride (CaCl2)-crosslinked hydrogels exhibited a sharp storage modulus loss in vitro. Despite improved stability, BaCl2-crosslinked hydrogels also exhibited stiffness losses over the same timeframe. It is believed that this is due to ion exchange with other species in culture media, as hydrogels incubated in dIH2O exhibited significantly improved stability. To maintain cell viability and function while increasing 3D matrix stability, a range of useful media:dIH2O dilution ratios for use are identified. Such findings have importance to carry out characterization and optimization of cell microphysiological systems with high fidelity in vitro.

Impact of Hydroxyapatite on Gelatin/Oxidized Alginate 3D-Printed Cryogel Scaffolds

Fabrication of scaffolds via 3D printing is a promising approach for tissue engineering. In this study, we combined 3D printing with cryogenic crosslinking to create biocompatible gelatin/oxidized alginate (Gel/OxAlg) scaffolds with large pore sizes, beneficial for bone tissue regeneration. To enhance the osteogenic effects and mechanical properties of these scaffolds, we evaluated the impact of hydroxyapatite (HAp) on the rheological characteristics of the 2.86% (1:1) Gel/OxAlg ink. We investigated the morphological and mechanical properties of scaffolds with low, 5%, and high 10% HAp content, as well as the resulting bio- and osteogenic effects. Scanning electron microscopy revealed a reduction in pore sizes from 160 to 180 µm (HAp-free) and from 120 to 140 µm for both HAp-containing scaffolds. Increased stability and higher Young's moduli were measured for 5% and 10% HAp (18 and 21 kPa, respectively) compared to 11 kPa for HAp-free constructs. Biological assessments with mesenchymal stem cells indicated excellent cytocompatibility and osteogenic differentiation in all scaffolds, with high degree of mineralization in HAp-containing constructs. Scaffolds with 5% HAp exhibited improved mechanical characteristics and shape fidelity, demonstrated positive osteogenic impact, and enhanced bone tissue formation. Increasing the HAp content to 10% did not show any advantages in osteogenesis, offering a minor increase in mechanical strength at the cost of significantly compromised shape fidelity.

BMP-2-immobilized PCL 3D printing scaffold with a leaf-stacked structure as a physically and biologically activated bone graft

Although three-dimensional (3D) printing techniques are used to mimic macro- and micro-structures as well as multi-structural human tissues in tissue engineering, efficient target tissue regeneration requires bioactive 3D printing scaffolds. In this study, we developed a bone morphogenetic protein-2 (BMP-2)-immobilized polycaprolactone (PCL) 3D printing scaffold with leaf-stacked structure (LSS) (3D-PLSS-BMP) as a bioactive patient-tailored bone graft. The unique LSS was introduced on the strand surface of the scaffold via heating/cooling in tetraglycol without significant deterioration in physical properties. The BMP-2 adsorbed on3D-PLSS-BMPwas continuously released from LSS over a period of 32 d. The LSS can be a microtopographical cue for improved focal cell adhesion, proliferation, and osteogenic differentiation.In vitrocell culture andin vivoanimal studies demonstrated the biological (bioactive BMP-2) and physical (microrough structure) mechanisms of3D-PLSS-BMPfor accelerated bone regeneration. Thus, bioactive molecule-immobilized 3D printing scaffold with LSS represents a promising physically and biologically activated bone graft as well as an advanced tool for widespread application in clinical and research fields.

High-Throughput Multiparametric Quantification of Mechanics Driven Heterogeneity in Mesenchymal Stromal Cell Population

Mesenchymal stromal or stem cells (MSCs) are one of the most promising candidates for a myriad of cell therapy applications. Despite showing promise in numerous preclinical and clinical studies, MSC-based therapy is not yet a reality for regenerative medicine due to its suboptimal outcome at the clinical endpoint. The mechanical environment is a critical determinant of MSC gene expression and function. This study reports that MSC population becomes phenotypically heterogenous and commits to an unwanted osteoprogenitor pathway when it experiences an abnormal mechanically stiff environment, compared to its native softer environment. A method is developed to measure the heterogeneity using nuclear shape, chromatin state, and CD73 marker. Heterogeneity is shown to be associated with a larger spread in the nuclear shape parameters and a smaller spread in the chromatin openness. Subsequently, intervention strategies are investigated to create a more homogeneous MSC population. Culturing MSCs on soft surfaces or inhibiting actomyosin on stiff surfaces can make them more homogeneous, while inhibiting YAP, Runx2, and actin polymerization helps maintain but does not fully homogenize them. This study offers insights for cell and tissue engineers, aiding in the design of optimal conditions and materials for MSC culture, ultimately enhancing their therapeutic potential.

Tumor-Derived Membrane Vesicles Restrain Migration in Gliomas By Altering Collective Polarization

Mechanobiology is a cornerstone in physiology. However, its role in biomedical applications remains considerably undermined. In this study, we employed cell membrane vesicles (CMVs), which are currently being used as nanodrug carriers, as tactile cues for mechano-regulation of collective cell behaviors. Gliomas, which are among the most resilient brain tumors and have a low patient survival rate, were used as the cell model. We observed that mechanical responses due to the application of glioma- or microglia-derived CMVs resulted in the doubling of the traction stress of glioma cell collectives with a 10-fold increase in the CMV concentration. Glioma-CMVs constrained cell protrusions and hindered their collective migration, with the migration speed of such cells declining by almost 40% compared to the untreated cells. We speculated that the alteration of collective polarization leads to migration speed changes, and this phenomenon was elucidated using the cellular Potts model. In addition to intracellular force modulation and cytoskeletal reorganization, glioma-CMVs altered drug diffusion within glioma spheroids by downregulating the mechano-signaling protein YAP-1 while also marginally enhancing the associated apoptotic events. Our results suggest that glioma-CMVs can be applied as an adjuvant to current treatment approaches to restrict tumor invasion and enhance the penetration of reagents within tumors. Considering the broad impact of mechano-transduction on cell functions, the regulation of cell mechanics through CMVs can provide a foundation for alternative therapeutic strategies.

Morphomigrational description as a new approach connecting cell's migration with its morphology

The examination of morphology and migration of cells plays substantial role in understanding the cellular behaviour, being described by plethora of quantitative parameters and models. These descriptions, however, treat cell migration and morphology as independent properties of temporal cell state, while not taking into account their strong interdependence in adherent cells. Here we present the new and simple mathematical parameter called signed morphomigrational angle (sMM angle) that links cell geometry with translocation of cell centroid, considering them as one morphomigrational behaviour. The sMM angle combined with pre-existing quantitative parameters enabled us to build a new tool called morphomigrational description, used to assign the numerical values to several cellular behaviours. Thus, the cellular activities that until now were characterized using verbal description or by complex mathematical models, are described here by a set of numbers. Our tool can be further used in automatic analysis of cell populations as well as in studies focused on cellular response to environmental directional signals.

Impact of baculoviral transduction of fluorescent actin on cellular forces

Live staining of actin brings valuable information in the field of mechanobiology. Gene transfer of GFP-actin has been reported to disturb cell rheological properties while gene transfer of fluorescent actin binding proteins was not. However the influence of gene transfer on cellular forces in adhered cells has never been investigated. This would provide a more complete picture of mechanical disorders induced by actin live staining for mechanobiology studies. Indeed, most of these techniques were shown to alter cell morphology. Change in cell morphology may in itself be sufficient to perturb cellular forces. Here we focus on quantifying the alterations of cellular stresses that result from baculoviral transduction of GFP-actin in MDCK cell line. We report that GFP-actin transduction increases the proportion of cells with large intracellular or surface stresses, especially in epithelia with low cell density. We show that the enhancement of the mechanical stresses is accompanied by small perturbations of cell shape, but not by a significant change in cell size. We thus conclude that this live staining method enhances the cellular forces but only brings subtle shape alterations.

Cell-surface contacts determine volume and mechanical properties of human embryonic kidney 293 T cells

Alterations in the organization of the cytoskeleton precede the escape of adherent cells from the framework of cell-cell and cell-matrix interactions into suspension. With cytoskeletal dynamics being linked to cell mechanical properties, many studies elucidated this relationship under either native adherent or suspended conditions. In contrast, tethered cells that mimic the transition between both states have not been the focus of recent research. Using human embryonic kidney 293 T cells we investigated all three conditions in the light of alterations in cellular shape, volume, as well as mechanical properties and relate these findings to the level, structure, and intracellular localization of filamentous actin (F-actin). For cells adhered to a substrate, our data shows that seeding density affects cell size but does not alter their elastic properties. Removing surface contacts leads to cell stiffening that is accompanied by changes in cell shape, and a reduction in cellular volume but no alterations in F-actin density. Instead, we observe changes in the organization of F-actin indicated by the appearance of blebs in the semi-adherent state. In summary, our work reveals an interplay between molecular and mechanical alterations when cells detach from a surface that is mainly dominated by cell morphology.

The influence of entropic crowding in cell monolayers

Cell-cell interaction dictates cell morphology and organization, which play a crucial role in the micro-architecture of tissues that guides their biological and mechanical functioning. Here, we investigate the effect of cell density on the responses of cells seeded on flat substrates using a novel statistical thermodynamics framework. The framework recognizes the existence of nonthermal fluctuations in cellular response and thereby naturally captures entropic interactions between cells in monolayers. In line with observations, the model predicts that cell area and elongation decrease with increasing cell seeding density-both are a direct outcome of the fluctuating nature of the cellular response that gives rise to enhanced cell-cell interactions with increasing cell crowding. The modeling framework also predicts the increase in cell alignment with increasing cell density: this cellular ordering is also due to enhanced entropic interactions and is akin to nematic ordering in liquid crystals. Our simulations provide physical insights that suggest that entropic cell-cell interactions play a crucial role in governing the responses of cell monolayers.

Pushing the Natural Frontier: Progress on the Integration of Biomaterial Cues toward Combinatorial Biofabrication and Tissue Engineering

The engineering of fully functional, biological-like tissues requires biomaterials to direct cellular events to a near-native, 3D niche extent. Natural biomaterials are generally seen as a safe option for cell support, but their biocompatibility and biodegradability can be just as limited as their bioactive/biomimetic performance. Furthermore, integrating different biomaterial cues and their final impact on cellular behavior is a complex equation where the outcome might be very different from the sum of individual parts. This review critically analyses recent progress on biomaterial-induced cellular responses, from simple adhesion to more complex stem cell differentiation, looking at the ever-growing possibilities of natural materials modification. Starting with a discussion on native material formulation and the inclusion of cell-instructive cues, the roles of shape and mechanical stimuli, the susceptibility to cellular remodeling, and the often-overlooked impact of cellular density and cell-cell interactions within constructs, are delved into. Along the way, synergistic and antagonistic combinations reported in vitro and in vivo are singled out, identifying needs and current lessons on the development of natural biomaterial libraries to solve the cell-material puzzle efficiently. This review brings together knowledge from different fields envisioning next-generation, combinatorial biomaterial development toward complex tissue engineering.

Regulation of cell attachment, spreading, and migration by hydrogel substrates with independently tunable mesh size

Hydrogels are widely used as substrates to investigate interactions between cells and their microenvironment as they mimic many attributes of the extracellular matrix. The stiffness of hydrogels is an important property that is known to regulate cell behavior. Beside stiffness, cells also respond to structural cues such as mesh size. However, since the mesh size of hydrogel is intrinsically coupled to its stiffness, its role in regulating cell behavior has never been independently investigated. Here, we report a hydrogel system whose mesh size and stiffness can be independently controlled. Cell behavior, including spreading, migration, and formation of focal adhesions is significantly altered on hydrogels with different mesh sizes but with the same stiffness. At the transcriptional level, hydrogel mesh size affects cellular mechanotransduction by regulating nuclear translocation of yes-associated protein. These findings demonstrate that the mesh size of a hydrogel plays an important role in cell-substrate interactions. STATEMENT OF SIGNIFICANCE: Hydrogels are ideal platforms with which to investigate interactions between cells and their microenvironment as they mimic many physical properties of the extracellular matrix. However, the mesh size of hydrogels is intrinsically coupled to their stiffness, making it challenging to investigate the contribution of mesh size to cell behavior. In this work, we use hydrogel-on-glass substrates with defined thicknesses whose stiffness and mesh size can be independently tuned. We use these substrates to isolate the effects of mesh size on cell behavior, including attachment, spreading, migration, focal adhesion formation and YAP localization in the nucleus. Our results show that mesh size has significant, yet often overlooked, effects, on cell behavior, and contribute to a further understanding of cell-substrate interactions.

Soft Hydrogel Environments that Facilitate Cell Spreading and Aggregation Preferentially Support Chondrogenesis of Adult Stem Cells

Mesenchymal stem/stromal cells (MSCs) represent a promising cell type for treating damaged and diseased synovial joints. The therapeutic potential of MSCs will be facilitated by the engineering of biomaterial environments capable of directing their fate. Here we explored the interplay between matrix elasticity and cell morphology in regulating the chondrogenic differentiation of MSCs when seeded onto or encapsulated within hydrogels made of interpenetrating networks (IPN) of alginate and collagen type I. This IPN system enabled the independent control of substrate stiffness (in 2D and in 3D) and cell morphology (3D only). In a 2D culture environment, the expression of chondrogenic markers SOX9, ACAN and COL2 increased on a soft substrate, which correlated with increased SMAD2/3 nuclear localization, enhanced MSCs condensation and the formation of larger cellular aggregates. The encapsulation of spread MSCs within a soft IPN dramatically increased the expression of cartilage-specific genes, which was linked to higher levels of cellular condensation and nuclear SMAD2/3 localization. Surprisingly, cells forced to adopt a more rounded morphology within the same soft IPNs expressed higher levels of the osteogenic markers RUNX2 and COL1. The insight provided by this study suggests that a mechanobiology informed approach to biomaterial development will be integral to the development of successful cartilage tissue engineering strategies. This article is protected by copyright. All rights reserved.

β-catenin links cell seeding density to global gene expression during mouse embryonic stem cell differentiation

Although cell density is known to affect numerous biological processes including gene expression and cell fate specification, mechanistic understanding of what factors link cell density to global gene regulation is lacking. Here, we reveal that the expression of thousands of genes in mouse embryonic stem cells (mESCs) is affected by cell seeding density and that low cell density enhances the efficiency of differentiation. Mechanistically, β-catenin is localized primarily to adherens junctions during both self-renewal and differentiation at high density. However, when mESCs differentiate at low density, β-catenin translocates to the nucleus and associates with Tcf7l1, inducing co-occupied lineage markers. Meanwhile, Esrrb sustains the expression of pluripotency-associated genes while repressing lineage markers at high density, and its association with DNA decreases at low density. Our results provide new insights into the previously neglected but pervasive phenomenon of density-dependent gene regulation.

Intrinsic Mechanical Cues and Their Impact on Stem Cells and Embryogenesis

Although understanding how soluble cues direct cellular processes revolutionised the study of cell biology in the second half of the 20th century, over the last two decades, new insights into how mechanical cues similarly impact cell fate decisions has gained momentum. During development, extrinsic cues such as fluid flow, shear stress and compressive forces are essential for normal embryogenesis to proceed. Indeed, both adult and embryonic stem cells can respond to applied forces, but they can also detect intrinsic mechanical cues from their surrounding environment, such as the stiffness of the extracellular matrix, which impacts differentiation and morphogenesis. Cells can detect changes in their mechanical environment using cell surface receptors such as integrins and focal adhesions. Moreover, dynamic rearrangements of the cytoskeleton have been identified as a key means by which forces are transmitted from the extracellular matrix to the cell and vice versa. Although we have some understanding of the downstream mechanisms whereby mechanical cues are translated into changes in cell behaviour, many of the signalling pathways remain to be defined. This review discusses the importance of intrinsic mechanical cues on adult cell fate decisions, the emerging roles of cell surface mechano-sensors and the cytoskeleton in enabling cells to sense its microenvironment, and the role of intracellular signalling in translating mechanical cues into transcriptional outputs. In addition, the contribution of mechanical cues to fundamental processes during embryogenesis such as apical constriction and convergent extension is discussed. The continued development of tools to measure the biomechanical properties of soft tissues in vivo is likely to uncover currently underestimated contributions of these cues to adult stem cell fate decisions and embryogenesis, and may inform on regenerative strategies for tissue repair.

Wavelength-Selective Softening of Hydrogel Networks

Photoresponsive hydrogels hold key potential in advanced biomedical applications including tissue engineering, regenerative medicine, and drug delivery, as well as intricately engineered functions such as biosensing, soft robotics, and bioelectronics. Herein, the wavelength-dependent degradation of bio-orthogonal poly(ethylene glycol) hydrogels is reported, using three selective activation levels. Specifically, three chromophores are exploited, that is, ortho-nitrobenzene, dimethyl aminobenzene, and bimane, each absorbing light at different wavelengths. By examining their photochemical action plots, the wavelength-dependent reactivity of the photocleavable moieties is determined. The wavelength-selective addressability of individual photoreactive units is subsequently translated into hydrogel design, enabling wavelength-dependent cleavage of the hydrogel networks on-demand. Critically, this platform technology allows for the fabrication of various hydrogels, whose mechanical properties can be fine-tuned using different colors of light to reach a predefined value, according to the chromophore ratios used. The softening is shown to influence the spreading of pre-osteoblastic cells adhering to the gels as a demonstration of their potential utility. Furthermore, the materials and photodegradation processes are non-toxic to cells, making this platform attractive for biomaterials engineering.

Causal contributors to tissue stiffness and clinical relevance in urology

Mechanomedicine is an emerging field focused on characterizing mechanical changes in cells and tissues coupled with a specific disease. Understanding the mechanical cues that drive disease progression, and whether tissue stiffening can precede disease development, is crucial in order to define new mechanical biomarkers to improve and develop diagnostic and prognostic tools. Classically known stromal regulators, such as fibroblasts, and more recently acknowledged factors such as the microbiome and extracellular vesicles, play a crucial role in modifications to the stroma and extracellular matrix (ECM). These modifications ultimately lead to an alteration of the mechanical properties (stiffness) of the tissue, contributing to disease onset and progression. We describe here classic and emerging mediators of ECM remodeling, and discuss state-of-the-art studies characterizing mechanical fingerprints of urological diseases, showing a general trend between increased tissue stiffness and severity of disease. Finally, we point to the clinical potential of tissue stiffness as a diagnostic and prognostic factor in the urological field, as well as a possible target for new innovative drugs.

Mesenchymal Stem Cells Resist Mechanical Confinement through the Activation of the Cortex during Cell Division

The mechanical properties of the natural extracellular matrix (ECM) change extensively, but these specific properties provide a relatively stable environment for resident cells. Although the effect of matrix stiffness on cell functions has been widely studied, the molecular mechanism was still not fully understood. Matrix stiffening is a common phenomenon in tissue damaging processes. To explore the effect of the increase in local matrix stiffness on cell behaviors, a three-dimensional (3D) cell culture system with a tunable modulus but constant other physical parameters was constructed by the alginate hydrogel with different molecular weights and cross-linking degrees. By using this culture system, the transcriptome response of mesenchymal stem cells (MSCs) to matrix stiffness was explored. Furthermore, a finite element model was developed to simulate the interaction between cells and the matrix. Results revealed that the increased matrix stiffness promoted the proliferation-related signaling of MSCs, and this process depended on the increased cortex tension caused by the activation of RAS and myosin II.

Compounded topographical and physicochemical cueing by micro-engineered chitosan substrates on rat dorsal root ganglion neurons and human mesenchymal stem cells

Given the intertwined physicochemical effects exerted in vivo by both natural and synthetic (e.g., biomaterial) interfaces on adhering cells, the evaluation of structure-function relationships governing cellular response to micro-engineered surfaces for applications in neuronal tissue engineering requires the use of in vitro testing platforms which consist of a clinically translatable material with tunable physiochemical properties. In this work, we micro-engineered chitosan substrates with arrays of parallel channels with variable width (20 and 60 μm). A citric acid (CA)-based crosslinking approach was used to provide an additional level of synergistic cueing on adhering cells by regulating the chitosan substrate's stiffness. Morphological and physicochemical characterization was conducted to unveil the structure-function relationships which govern the activity of rat dorsal root ganglion neurons (DRGs) and human mesenchymal stem cells (hMSCs), ultimately singling out the key role of microtopography, roughness and substrate's stiffness. While substrate's stiffness predominantly affected hMSC spreading, the modulation of the channels' design affected the neuronal architecture's complexity and guided the morphological transition of hMSCs. Finally, the combined analysis of tubulin expression and cell morphology allowed us to cast new light on the predominant role of the microtopography over substrate's stiffness in the process of hMSCs neurogenic differentiation.

Hyperthermia induced disruption of mechanical balance leads to G1 arrest and senescence in cells

Human body temperature limits below 40°C during heat stroke or fever. The implications of prolonged exposure to the physiologically relevant temperature (40°C) on cellular mechanobiology is poorly understood. Here, we have examined the effects of heat stress (40°C for 72 h incubation) in human lung adenocarcinoma (A549), mouse melanoma (B16F10), and non-cancerous mouse origin adipose tissue cells (L929). Hyperthermia increased the level of ROS, γ-H2AX and HSP70 and decreased mitochondrial membrane potential in the cells. Heat stress impaired cell division, caused G1 arrest, induced cellular senescence, and apoptosis in all the tested cell lines. The cells incubated at 40°C for 72 h displayed a significant decrease in the f-actin level and cellular traction as compared with cells incubated at 37°C. Also, the cells showed a larger focal adhesion area and stronger adhesion at 40°C than at 37°C. The mitotic cells at 40°C were unable to round up properly and displayed retracting actin stress fibers. Hyperthermia down-regulated HDAC6, increased the acetylation level of microtubules, and perturbed the chromosome alignment in the mitotic cells at 40°C. Overexpression of HDAC6 rescued the cells from the G1 arrest and reduced the delay in cell rounding at 40°C suggesting a crucial role of HDAC6 in hyperthermia mediated responses. This study elucidates the significant role of cellular traction, focal adhesions, and cytoskeletal networks in mitotic cell rounding and chromosomal misalignment. It also highlights the significance of HDAC6 in heat-evoked senile cellular responses.

Development and characterisation of cytocompatible polyester substrates with tunable mechanical properties and degradation rate

Although it has been repeatedly indicated the importance to develop implantable devices and cell culture substrates with tissue-specific rigidity, current commercially available products, in particular cell culture substrates, have rigidity values well above most tissues in the body. Herein, six resorbable polyester films were fabricated using compression moulding with a thermal presser into films with tailored stiffness by appropriately selecting the ratio of their building up monomers (e.g. lactide, glycolide, trimethylene carbonate, dioxanone, ε-caprolactone). Typical NMR and FTIR spectra were obtained, suggesting that the fabrication process did not have a negative effect on the conformation of the polymers. Surface roughness analysis revealed no apparent differences between the films as a function of polymer composition. Subject to polymer composition, polymeric films were obtained with glass transition temperatures from -52 °C to 61 °C; contact angles in water from 81 ° to 94 °; storage modulus from 108 MPa to 2,756 MPa and loss modulus from 8 MPa to 507 MPa (both in wet state, at 1 Hz frequency and at 37 °C); ultimate tensile strength from 8 MPa to 62 MPa, toughness from 23 MJ/m³ to 287 MJ/m³, strain at break from 3 % to 278 %, macro-scale Young's modulus from 110 MPa to 2,184 MPa (all in wet state); and nano-scale Young's modulus from 6 kPa to 15,019 kPa (in wet state). With respect to in vitro degradation in phosphate buffered saline at 37 °C, some polymeric films [e.g. poly(glycolide-lactide) 30 / 70] started degrading from day 7 (shortest timepoint assessed), whilst others [e.g. poly(glycolide-co-ε-caprolactone) 10 / 90] were more resilient to degradation up to day 21 (longest timepoint assessed). In vitro biological analysis using human dermal fibroblasts and a human monocyte cell line (THP-1) showed the potential of the polymeric films to support cell growth and controlled immune response. Evidently, the selected polymers exhibited properties suitable for a range of clinical indications.

Steering cell behavior through mechanobiology in 3D: A regenerative medicine perspective

Mechanobiology, translating mechanical signals into biological ones, greatly affects cellular behavior. Steering cellular behavior for cell-based regenerative medicine approaches requires a thorough understanding of the orchestrating molecular mechanisms, among which mechanotransducive ones are being more and more elucidated. Because of their wide use and highly mechanotransduction dependent differentiation, this review focuses on mesenchymal stromal cells (MSCs), while also briefly relating the discussed results to other cell types. While the mechanotransduction pathways are relatively well-studied in 2D, much remains unknown of the role and regulation of these pathways in 3D. Ultimately, cells need to be cultured in a 3D environment to create functional de novo tissue. In this review, we explore the literature on the roles of different material properties on cellular behavior and mechanobiology in 2D and 3D. For example, while stiffness plays a dominant role in 2D MSCs differentiation, it seems to be of subordinate importance in 3D MSCs differentiation, where matrix remodeling seems to be key. Also, the role and regulation of some of the main mechanotransduction players are discussed, focusing on MSCs. We have only just begun to fundamentally understand MSCs and other stem cells behavior in 3D and more fundamental research is required to advance biomaterials able to replicate the stem cell niche and control cell activity. This better understanding will contribute to smarter tissue engineering scaffold design and the advancement of regenerative medicine.

Maintaining resting cardiac fibroblasts in vitro by disrupting mechanotransduction

Mechanical cues activate cardiac fibroblasts and induce differentiation into myofibroblasts, which are key steps for development of cardiac fibrosis. In vitro, the high stiffness of plastic culturing conditions will also induce these changes. It is therefore challenging to study resting cardiac fibroblasts and their activation in vitro. Here we investigate the extent to which disrupting mechanotransduction by culturing cardiac fibroblasts on soft hydrogels or in the presence of biochemical inhibitors can be used to maintain resting cardiac fibroblasts in vitro. Primary cardiac fibroblasts were isolated from adult mice and cultured on plastic or soft (4.5 kPa) polyacrylamide hydrogels. Myofibroblast marker gene expression and smooth muscle α-actin (SMA) fibers were quantified by real-time PCR and immunostaining, respectively. Myofibroblast differentiation was prevented on soft hydrogels for 9 days, but had occurred after 15 days on hydrogels. Transferring myofibroblasts to soft hydrogels reduced expression of myofibroblast-associated genes, albeit SMA fibers remained present. Inhibitors of transforming growth factor β receptor I (TGFβRI) and Rho-associated protein kinase (ROCK) were effective in preventing and reversing myofibroblast gene expression. SMA fibers were also reduced by blocker treatment although cell morphology did not change. Reversed cardiac fibroblasts maintained the ability to re-differentiate after the removal of blockers, suggesting that these are functionally similar to resting cardiac fibroblasts. However, actin alpha 2 smooth muscle (Acta2), lysyl oxidase (Lox) and periostin (Postn) were no longer sensitive to substrate stiffness, suggesting that transient treatment with mechanotransduction inhibitors changes the mechanosensitivity of some fibrosis-related genes. In summary, our results bring novel insight regarding the relative importance of specific mechanical signaling pathways in regulating different myofibroblast-associated genes. Furthermore, combining blocker treatment with the use of soft hydrogels has not been tested previously and revealed that only some genes remain mechano-sensitive after phenotypic reversion. This is important information for researchers using inhibitors to maintain a "resting" cardiac fibroblast phenotype in vitro as well as for our current understanding of mechanosensitive gene regulation.

Sensing Traction Force on the Matrix Induces Cell-Cell Distant Mechanical Communications for Self-Assembly

The long-range biomechanical force propagating across a large scale may reserve the capability to trigger coordinative responses within cell population such as during angiogenesis, epithelial tubulogenesis, and cancer metastasis. How cells communicate in a distant manner within the group for self-assembly remains largely unknown. Here, we found that airway smooth muscle cells (ASMCs) rapidly self-assembled into a well-constructed network on 3D Matrigel containing type I collagen (COL), which relied on long-range biomechanical force across the matrix to direct cell-cell distant interactions. Similar results happened by HUVEC cells to mimic angiogenesis. Interestingly, single ASMCs initiated multiple extended protrusions precisely pointing to neighboring cells in distance (100-300 μm away or 5-10 folds of the diameter of a round single cell), depending on traction force sensing. Individual ASMCs mechanosensed each other to move directionally on both nonfibrous Matrigel only and Matrigel containing fibrous COL but lost mutual sensing on the cross-linked gel or coated glass due to no long-range force transmission. The bead tracking assay demonstrated distant transmission of traction force (up to 400 μm) during the matrix deformation, and finite element method modeling confirmed the consistency between maximum strain distribution on the matrix and cell directional movements in experiments. Furthermore, ASMCs recruited COL from the hydrogel to build a fibrous network to mechanically stabilize the cell network. Our results revealed principally that cells can sense traction force transmitted through the matrix to initiate cell-cell distant mechanical communications, resulting in cell directional migration and coordinated cell and COL self-assembly with active matrix remodeling. As an interesting phenomenon, cells seem to be able to "make a phone call" via long-range biomechanics, which implicates physiological importance such as for tissue pattern formation.

PCL-Blended Chitosan Substrates for Patterning the Heterotypic Cell Distribution in an Epithelial and Mesenchymal Coculture System

Cell-cell and cell-substrate interactions in coculture systems are very important to the context of biomaterial scaffolds for tissue engineering applications. Understanding the cellular interactions and distribution of epithelial-mesenchymal microtissues on the controllable biomaterial surfaces is useful to study the organoid applications. The aim of the present study is to investigate the effects of chitosan/poly(ε-caprolactone) (PCL)-blended biomaterials on the distribution and spheroid formation of HaCaT and Hs68 cells in a coculture system. In this study, we demonstrated that the cocultured cells gradually changed their pattern from core/shell spheroid to monolayered morphology as the PCL content increased in the blended substrates. This indicates that the chitosan/PCL-blended substrates are able to regulate cell-substrate and cell-cell interactions to modify the distribution of HaCaT and Hs68 cells similar to various mesenchymal-epithelial organizations in biological tissues. Moreover, we also developed a two-dimension lattice model to elaborate the dependence of cell spheroid development on complex cell-cell interactions. This information may be helpful to develop appropriate biomaterials with appropriate properties to the applications of engineered epithelial-mesenchymal organoids.

An in vitro hyaluronic acid hydrogel based platform to model dormancy in brain metastatic breast cancer cells

Breast cancer cells (BCCs) can remain dormant at the metastatic site, which when revoked leads to formation of metastasis several years after the treatment of primary tumor. Particularly, awakening of dormant BCCs in the brain results in breast cancer brain metastasis (BCBrM) which marks the most advanced stage of the disease with a median survival period of ~4-16 months. However, our understanding of dormancy associated with BCBrM remains obscure, in part, due to the lack of relevant in vitro platforms to model dormancy associated with BCBrM. To address this need, we developed an in vitro hyaluronic acid (HA) hydrogel platform to model dormancy in brain metastatic BCCs via exploiting the bio-physical cues provided by HA hydrogels while bracketing the normal brain and metastatic brain malignancy relevant stiffness range. In this system, we observed that MDA-MB-231Br and BT474Br3 brain metastatic BCCs exhibited a dormant phenotype when cultured on soft (0.4 kPa) HA hydrogel compared to stiff (4.5 kPa) HA hydrogel as characterized by significantly lower EdU and Ki67 positivity. Further, we demonstrated the nuclear localization of p21 and p27 (markers associated with dormancy) in dormant MDA-MB-231Br cells contrary to their cytoplasmic localization in the proliferative population. We also demonstrated that the stiffness-based dormancy in MDA-MB-231Br cells was reversible and was, in part, mediated by focal adhesion kinases and the initial cell seeding density. Finally, RNA sequencing confirmed the dormant phenotype in MDA-MB-231Br cells. This platform could further our understanding of dormancy in BCBrM and could be adapted for anti-metastatic drug screening. STATEMENT OF SIGNIFICANCE: Our understanding of dormancy associated with BCBrM remains obscure, in part, due to the lack of relevant in vitro platforms to model dormancy associated with BCBrM. Herein, we present a HA hydrogel-based platform to model dormancy in brain metastatic BCCs while recapitulating key aspects of brain microenvironment. We demonstrated that the biophysical cues provided the HA hydrogel mediates dormancy in brain metastatic BCCs by assessing both proliferation and cell cycle arrest markers. We also established the role of focal adhesion kinases and initial cell seeding density in the stiffness-mediated dormancy in brain metastatic BCCs. Further, RNA-seq. confirmed the dormant phenotype in brain metastatic BCCs. This platform could be utilized to further our understanding of microenvironmental regulation of dormancy in BCBrM.

Surface modifications to polydimethylsiloxane substrate for stabilizing prolonged bone marrow stromal cell culture

Polydimethylsiloxane (PDMS) has been extensively used as a supporting material for studies of cell mechanobiology, cell micropatterning and microscale-cell analysis in microfluidic chips due to its numerous advantages, such as low cytotoxicity, ease of modification, inexpensive costs and biocompatibility. However, the innate hydrophobicity of PDMS often poses a problem for stable cell adhesion, seriously limiting its applicability for prolonged cell culture. UV exposure and protein coating are suboptimal solutions, while chemical surface functionalization is often associated with laborious procedures and producing environmental toxics. Plasma treatment can render a hydrophilic substrate by altering the surface chemistry, but such effect is often short-lived due to its tendency to hydrophobic recovery. Variation of physical properties of the substratum are known to influence cell behaviour. Nevertheless, the combination of varying PDMS substratum properties via base:curing agent ratio and plasma treatment to stabilize the long-term culture of bone marrow derived stromal cells (BMSCs) still remain poorly understood. In this study, we developed a protocol to maintain the hydrophilicity of the plasma-treated PDMS over a range of substratum properties. This study demonstrated that varying the substratum properties of PDMS can enhance the stability of BMSC culture for at least three weeks, while plasma treatment with or without additional collagen coating further enhanced such effect. The changes in the physical properties of PDMS have rendered difference in BMSCs adhesion, proliferation and in-vitro plasticity, thereby offering a simple and effective strategy for PDMS surface modification to enable long term cell analysis in PDMS-based culture platform.

Experimental study on the mechanical properties of biological hydrogels of different concentrations

BACKGROUND

Biological hydrogels provide a conducive three-dimensional extracellular matrix environment for encapsulating and cultivating living cells. Microenvironmental modulus of hydrogels dictates several characteristics of cell functions such as proliferation, adhesion, self-renewal, differentiation, migration, cell morphology and fate. Precise measurement of the mechanical properties of gels is necessary for investigating cellular mechanobiology in a variety of applications in tissue engineering. Elastic properties of gels are strongly influenced by the amount of crosslinking density.

OBJECTIVE

The main purpose of the present study was to determine the elastic modulus of two types of well-known biological hydrogels: Agarose and Gelatin Methacryloyl.

METHODS

Mechanical properties such as Young's modulus, fracture stress and failure strain of the prescribed gels with a wide range of concentrations were determined using tension and compression tests.

RESULTS

The elastic modulus, failure stress and strain were found to be strongly influenced when the amount of concentration in the hydrogels was changed. The elastic modulus for a lower level of concentration, not considered in this study, was also predicted using statistical analysis.

CONCLUSIONS

Closed matching of the mechanical properties of the gels revealed that the bulk tension and compression tests could be confidently used for assessing mechanical properties of delicate biological hydrogels.

Single-Cell Response to the Rigidity of Semiconductor Nanomembranes on Compliant Substrates

Single-crystalline semiconductor nanomembranes (NMs) bonded to compliant substrates are increasingly used for biomedical research and in health care. Nevertheless, there is a limited understanding of how individual cells sense the unique mechanical properties of these substrates and adjust their behavior in response to them. In this work, we performed proliferation assays, cytoskeleton analysis, and focal adhesion (FA) studies for NIH-3T3 fibroblasts on 220 and 20 nm single-crystalline Si on polydimethylsiloxane (PDMS) substrates with an elastic modulus of ∼31 kPa. We also characterized cell response on bulk Si as a reference. Our in vitro studies show that varying the thickness of the NM between 20 and 220 nm affects the proliferation rate of the cells, their cytoskeleton, fiber organization, spread area, and degree of FA. For example, cultured cells on 220 nm Si/PMDS exhibit the same response as on bulk Si, that is, they are well-spread with a pentagonal (or dendritic) shape and show a good organization of stress fibers and FAs. On the other hand, the cells on 20 nm Si/PDMS are spherical, with fiber organization and FAs in undetectable levels. We explained the results of our in vitro studies through a shear-lag mechanical model. The calculated FA-substrate contact stiffnesses for fibroblasts on bulk Si and 220 nm Si/PDMS closely match, and they are significantly higher than the stiffness of the integrin clutches and the plaque. Conversely, focal contacts with 20 nm Si/PDMS have comparable lateral compliance to adhesion-mediating intracellular organisms. In conclusion, our work relies on recent advances in NM technology to fill a critical knowledge gap about how individual cells sense and react to the mechanical properties of NM-based substrates. Our findings will have a major impact on the design of flexible electronic materials for applications in biomedical science and health care.

Stiffness and topography of biomaterials dictate cell-matrix interaction in musculoskeletal cells at the bio-interface: A concise progress review

Mutually interacted musculoskeletal tissues work together within the physiological environment full of varieties of external stimulus. Consistent with the locomotive function of the tissues, musculoskeletal cells are remarkably mechanosensitive to the physical cues. Signals like extracellular matrix (ECM) stiffness, topography, and geometry can be sensed and transduced into intracellular signaling cascades to trigger a series of cell responses, including cell adhesion, cell phenotype maintenance, cytoskeletal reconstruction, and stem cell differentiation (Du et al., 2011; Murphy et al., 2014; Lv et al., 2015; Kim et al., 2016; Kumar et al., 2017). With the development of tissue engineering and regenerative medicine, the potent effects of ECM physical properties on cell behaviors at the cell-matrix interface are drawing much attention. To mimic the interaction between cell and its ECM physical properties, developing advanced biomaterials with desired characteristics which could achieve the biointerface between cells and the surrounded matrix close to the physiological conditions becomes a great hotspot. In this review, based on the current publications in the field of biointerfaces, we systematically summarized the significant roles of stiffness and topography on musculoskeletal cell behaviors. We hope to shed light on the importance of physical cues in musculoskeletal tissue engineering and provide up to date strategies towards the natural or artificial replication of physiological microenvironment.

Bioengineering Considerations for a Nurturing Way to Enhance Scalable Expansion of Human Pluripotent Stem Cells

Understanding how defects in mechanotransduction affect cell-to-cell variability will add to the fundamental knowledge of human pluripotent stem cell (hPSC) culture, and may suggest new approaches for achieving a robust, reproducible, and scalable process that result in consistent product quality and yields. Here, the current state of the understanding of the fundamental mechanisms that govern the growth kinetics of hPSCs between static and dynamic cultures is reviewed, the factors causing fluctuations are identified, and culture strategies that might eliminate or minimize the occurrence of cell-to-cell variability arising from these fluctuations are discussed. The existing challenges in the development of hPSC expansion methods for enabling the transition from process development to large-scale production are addressed, a mandatory step for industrial and clinical applications of hPSCs.

Silk fibroin for skin injury repair: Where do things stand?

Several synthetic and natural materials are used in soft tissue engineering and regenerative medicine with varying degrees of success. Among them, silkworm silk protein fibroin, a naturally occurring protein-based biomaterial, exhibits many promising characteristics such as biocompatibility, controllable biodegradability, tunable mechanical properties, aqueous preparation, minimal inflammation in host tissue, low cost and ease of use. Silk fibroin is often used alone or in combination with other materials in various formats and is also a promising delivery system for bioactive compounds as part of such repair scenarios. These properties make silk fibroin an excellent biomaterial for skin tissue engineering and repair applications. This review focuses on the promising characteristics and recent advances in the use of silk fibroin for skin wound healing and/or soft-tissue repair applications. The benefits and limitations of silk fibroin as a scaffolding biomaterial in this context are also discussed. STATEMENT OF SIGNIFICANCE: Silk protein fibroin is a natural biomaterial with important biological and mechanical properties for soft tissue engineering applications. Silk fibroin is obtained from silkworms and can be purified using alkali or enzyme based degumming (removal of glue protein sericin) procedures. Fibroin is used alone or in combination with other materials in different scaffold forms, such as nanofibrous mats, hydrogels, sponges or films tailored for specific applications. The investigations carried out using silk fibroin or its blends in skin tissue engineering have increased dramatically in recent years due to the advantages of this unique biomaterial. This review focuses on the promising characteristics of silk fibroin for skin wound healing and/or soft-tissue repair applications.

Subendothelial stiffness alters endothelial cell traction force generation while exerting a minimal effect on the transcriptome

Endothelial cells respond to changes in subendothelial stiffness by altering their migration and mechanics, but whether those responses are due to transcriptional reprogramming remains largely unknown. We measured traction force generation and also performed gene expression profiling for two endothelial cell types grown in monolayers on soft or stiff matrices: primary human umbilical vein endothelial cells (HUVEC) and immortalized human microvascular endothelial cells (HMEC-1). Both cell types respond to changes in subendothelial stiffness by increasing the traction stresses they exert on stiffer as compared to softer matrices, and exhibit a range of altered protein phosphorylation or protein conformational changes previously implicated in mechanotransduction. However, the transcriptome has only a minimal role in this conserved biomechanical response. Only few genes were differentially expressed in each cell type in a stiffness-dependent manner, and none were shared between them. In contrast, thousands of genes were differentially regulated in HUVEC as compared to HMEC-1. HUVEC (but not HMEC-1) upregulate expression of TGF-β2 on stiffer matrices, and also respond to application of exogenous TGF-β2 by enhancing their endogenous TGF-β2 expression and their cell-matrix traction stresses. Altogether, these findings provide insights into the relationship between subendothelial stiffness, endothelial mechanics and variation of the endothelial cell transcriptome, and reveal that subendothelial stiffness, while critically altering endothelial cells’ mechanical behavior, minimally affects their transcriptome.

Nanofibrillar cellulose wound dressing supports the growth and characteristics of human mesenchymal stem/stromal cells without cell adhesion coatings

BACKGROUND

In the field of regenerative medicine, delivery of human adipose-derived mesenchymal stem/stromal cells (hASCs) has shown great promise to promote wound healing. However, a hostile environment of the injured tissue has shown considerably to limit the survival rate of the transplanted cells, and thus, to improve the cell survival and retention towards successful cell transplantation, an optimal cell scaffold is required. The objective of this study was to evaluate the potential use of wood-derived nanofibrillar cellulose (NFC) wound dressing as a cell scaffold material for hASCs in order to develop a cell transplantation method free from animal-derived components for wound treatment.

METHODS

Patient-derived hASCs were cultured on NFC wound dressing without cell adhesion coatings. Cell characteristics, including cell viability, morphology, cytoskeletal structure, proliferation potency, and mesenchymal cell and differentiation marker expression, were analyzed using cell viability assays, electron microscopy, immunocytochemistry, and quantitative or reverse transcriptase PCR. Student's t test and one-way ANOVA followed by a Tukey honestly significant difference post hoc test were used to determine statistical significance.

RESULTS

hASCs were able to adhere to NFC dressing and maintained high cell survival without cell adhesion coatings with a cell density-dependent manner for the studied period of 2 weeks. In addition, NFC dressing did not induce any remarkable cytotoxicity towards hASCs or alter the morphology, proliferation potency, filamentous actin structure, the expression of mesenchymal vimentin and extracellular matrix (ECM) proteins collagen I and fibronectin, or the undifferentiated state of hASCs.

CONCLUSIONS

As a result, NFC wound dressing offers a functional cell culture platform for hASCs to be used further for in vivo wound healing studies in the future.

Cell Type Classification and Unsupervised Morphological Phenotyping From Low-Resolution Images Using Deep Learning

Convolutional neural networks (ConvNets) have proven to be successful in both the classification and semantic segmentation of cell images. Here we establish a method for cell type classification utilizing images taken with a benchtop microscope directly from cell culture flasks, eliminating the need for a dedicated imaging platform. Significant flask-to-flask morphological heterogeneity was discovered and overcome to support network generalization to novel data. Cell density was found to be a prominent source of heterogeneity even when cells are not in contact. For the same cell types, expert classification was poor for single-cell images and better for multi-cell images, suggesting experts rely on the identification of characteristic phenotypes within subsets of each population. We also introduce Self-Label Clustering (SLC), an unsupervised clustering method relying on feature extraction from the hidden layers of a ConvNet, capable of cellular morphological phenotyping. This clustering approach is able to identify distinct morphological phenotypes within a cell type, some of which are observed to be cell density dependent. Finally, our cell classification algorithm was able to accurately identify cells in mixed populations, showing that ConvNet cell type classification can be a label-free alternative to traditional cell sorting and identification.

Methyl-β-cyclodextrin, an actin depolymerizer augments the antiproliferative potential of microtubule-targeting agents

Methyl-β-cyclodextrin (MCD), an established pharmacological excipient, depolymerizes the actin cytoskeleton. In this work, we investigated the effect of MCD-mediated actin depolymerization on various cellular phenotypes including traction force, cell stiffness, focal adhesions, and intracellular drug accumulation. In addition to a reduction in the contractile cellular traction, MCD acutely inhibits the maturation of focal adhesions. Alteration of contractile forces and focal adhesions affects the trypsin-mediated detachment kinetics of cells. Moreover, MCD-mediated actin depolymerization increases the intracellular accumulation of microtubule-targeting agents (MTAs) by ~50% with respect to the untreated cells. As MCD treatment enhances the intracellular concentration of drugs, we hypothesized that the MCD-sensitized cancer cells could be effectively killed by low doses of MTAs. Our results in cervical, breast, hepatocellular, prostate cancer and multidrug-resistant breast cancer cells confirmed the above hypothesis. Further, the combined use of MCD and MTAs synergistically inhibits the proliferation of tumor cells. These results indicate the potential use of MCD in combination with MTAs for cancer chemotherapy and suggest that targeting both actin and microtubules simultaneously may be useful for cancer therapy. Importantly, the results provide significant insight into the crosstalk between actin and microtubules in regulating the traction force and dynamics of cell deadhesion.

Soft substrate maintains proliferative and adipogenic differentiation potential of human mesenchymal stem cells on long-term expansion by delaying senescence

Human mesenchymal stem cells (hMSCs), during in vitro expansion, gradually lose their distinct spindle morphology, self-renewal ability, multi-lineage differentiation potential and enter replicative senescence. This loss of cellular function is a major roadblock for clinical applications which demand cells in large numbers. Here, we demonstrate a novel role of substrate stiffness in the maintenance of hMSCs over long-term expansion. When serially passaged for 45 days from passage 3 to passage 18 on polyacrylamide gel of Young's modulus E=5 kPa, hMSCs maintained their proliferation rate and showed nine times higher population doubling in comparison to their counterparts cultured on plastic Petri-plates. They did not express markers of senescence, maintained their morphology and other mechanical properties such as cell stiffness and cellular traction, and were significantly superior in adipogenic differentiation potential. These results were demonstrated in hMSCs from two different sources, umbilical cord and bone marrow. In summary, our result shows that a soft gel is a suitable substrate to maintain the stemness of mesenchymal stem cells. As preparation of polyacrylamide gel is a well-established, and well-standardized protocol, we propose that this novel system of cell expansion will be useful in therapeutic and research applications of hMSCs.

Hydrogel scaffold with substrate elasticity mimicking physiological-niche promotes proliferation of functional keratinocytes

High numbers of autologous human primary keratinocytes (HPKs) are required for patients with burns, wounds and for gene therapy of skin disorders. Although freshly isolated HPKs exhibit a robust regenerative capacity, traditional methodology fails to provide a sufficient number of cells. Here we demonstrated a well characterized, non-cytotoxic and inert hydrogel as a substrate that mimics skin elasticity, which can accelerate proliferation and generate higher numbers of HPKs compared to existing tissue culture plastic (TCP) dishes. More importantly, this novel method was independent of feeder layer or any exogenous pharmaceutical drug. The HPKs from the hydrogel-substrate were functional as demonstrated by wound-healing assay, and the expression of IFN-γ-responsive genes (CXCL10, HLADR). Importantly, gene delivery efficiency by a lentiviral based delivery system was significantly higher in HPKs cultured on hydrogels compared with TCP. In conclusion, our study provides the first evidence that cell-material mechanical interaction is enough to provide a rapid expansion of functional keratinocytes that might be used as autologous grafts for skin disorders.

High numbers of autologous human primary keratinocytes (HPKs) are required for patients with burns, wounds and for gene therapy of skin disorders.

Phytochrome-Based Extracellular Matrix with Reversibly Tunable Mechanical Properties

Interrogation and control of cellular fate and function using optogenetics is providing revolutionary insights into biology. Optogenetic control of cells is achieved by coupling genetically encoded photoreceptors to cellular effectors and enables unprecedented spatiotemporal control of signaling processes. Here, a fast and reversibly switchable photoreceptor is used to tune the mechanical properties of polymer materials in a fully reversible, wavelength-specific, and dose- and space-controlled manner. By integrating engineered cyanobacterial phytochrome 1 into a poly(ethylene glycol) matrix, hydrogel materials responsive to light in the cell-compatible red/far-red spectrum are synthesized. These materials are applied to study in human mesenchymal stem cells how different mechanosignaling pathways respond to changing mechanical environments and to control the migration of primary immune cells in 3D. This optogenetics-inspired matrix allows fundamental questions of how cells react to dynamic mechanical environments to be addressed. Further, remote control of such matrices can create new opportunities for tissue engineering or provide a basis for optically stimulated drug depots.

Early hMSC morphology and proliferation on model polyelectrolyte multilayers

Polyelectrolyte multilayers (PEMs) are a category of materials commonly used as coatings on surfaces that interact with cells. The properties of PEMs have been established to be controlled by not only polyelectrolyte choice, but by the identity of the initially applied (bottom) layer. In this work, 5-bilayer PEMs consisting of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS) were coated on gold-sputtered quartz substrates with different first layer materials. A final poly-l-lysine (PLL) layer was added to all PEMs to provide identical top layers conducive to cell growth. As in previous work, initial layer selection affected PEM roughness. All coated surfaces, including the PLL-only control, showed increased dispersive surface energy but decreased polar surface energy, as compared to uncoated surfaces. When human mesenchymal stem cells (hMSCs) were cultured on these surfaces, analysis through lateral cell imaging for the first 90 min and fluorescent staining after 1 day showed improved attachment on surfaces with a PDADMAC bottom layer. However, after 4 days, a higher cell density was observed on the PLL-only and uncoated control surfaces, indicating that the PEMs negatively affected hMSC proliferation. Both the long and short time period results did not correlate to any of the roughness and surface energy trends, indicating more complex interactions between the cells and the surface relating to charge distribution and functional group density.

Proteomic analysis of the effects of cell culture density on the metastasis of breast cancer cells

Cancer cell progression and proliferation increase cell density, resulting in changes to the tumour site, including the microenvironment. What is not known is if increased cell density influences the aggressiveness of cancer cells, especially their proliferation, migration, and invasion capabilities. In this study, we found that dense cell culture enhances the aggressiveness of the metastatic cancer cell lines, 4T1 and ZR-75-30, by increasing their proliferation, migration, and invasion capabilities. However, a less metastatic cell line, MCF-7, did not show an increase in aggressiveness, following dense cell culture conditions. We conducted a differential proteomic analysis on 4T1 cells cultured under dense or sparse conditions and identified an increase in expression for proteins involved in migration, including focal adhesion, cytoskeletal reorganization, and transendothelial migration. In contrast, 4T1 cells grown under sparse conditions had higher expression levels for proteins involved in metabolism, including lipid and phospholipid binding, lipid and cholesterol transporter activity, and protein binding. These results suggest that the high-density tumour microenvironment can cause a change in cellular behaviour, leading towards more aggressive cancers. SIGNIFICANCE OF THE STUDY: Metastasis of cancer cells is an obstacle to the clinical treatment of cancer. We found that dense cultures made metastatic cancer cells more potent in terms of proliferation, migration, and invasion. The proteomic and bioinformatic analyses provided some valuable clues for further intensive studies about the effects of cell density on cancer cell aggressiveness, which were associated with events such as pre-mRNA splicing and RNA transport, focal adhesion and cytoskeleton reorganization, ribosome biogenesis, and transendothelial migration, or associated with proteins, such as JAM-1 and S100A11. This investigation gives us new perspectives to investigate the metastasis mechanisms related to the microenvironment of tumour sites.