Volume Adaptation Controls Stem Cell Mechanotransduction

Hydrogel Elastic Energy: A Stressor Triggering an Adaptive Stress-Mediated Cell Response

The crosstalk between the cells and the extracellular matrix (ECM) is bidirectional and consists of a pushing/pulling stretch exerted by the cells and a mechanical resistance counteracted by the surrounding microenvironment. It is widely recognized that the stiffness of the ECM, its viscoelasticity, and its overall deformation are the most important traits influencing the response of the cells. Here these three parameters are combined into a concept of elastic energy, which in biological terms represents the mechanical feedback that cells perceive when the ECM is deformed. It is shown that elastic energy is a stress factor that influences the response of cells in three-dimensional (3D) cultures. Strikingly, the higher the elastic energy of the matrix and thus the mechanical feedback, the higher the stress state of the cells, which correlates with the formation of G3BP-mediated stress granules. This condition is associated with an increase in alkaline phosphatase (ALP) activity but a decrease in gene expression and is mediated by the nuclear translocation of Yes-associated protein (YAP). This work supports the importance of considering the elastic energy as mechano-controller in regulating cellular stress state in 3D cultures.

Multi-Gradient Bone-Like Nanocomposites Induced by Strain Distribution

The heterogeneity of bones is elegantly adapted to the local strain environment, which is critical for maintaining mechanical functions. Such an adaptation enables the strong correlation between strain distributions and multiple gradients, underlying a promising pathway for creating complex gradient structures. However, this potential remains largely unexplored for the synthesis of functional gradient materials. In this work, heterogeneous bone-like nanocomposites with complex structural and compositional gradients comparable to bones are synthesized by inducing strain distributions within the polymer matrix containing amorphous calcium phosphate (ACP). Uniaxial stretching of composite films exerts the highest strain in the center, which ceases gradually toward the sides, resulting in the gradual decrease of polymer alignment and crystallinity. Simultaneously, the center with high orientation traps most ACP during stretching due to the nanoconfinement effect, which in turn promotes the formation of aligned nanofibrous structures. The sides experiencing the least strain have the smallest amounts of ACP, characteristic of porous architectures. Further crystallization of ACP produces oriented apatite nanorods in the center with a larger crystalline/amorphous ratio than the sides because of template-induced crystallization. The combination of structural and compositional gradients leads to gradient mechanical properties, and the gradient span and magnitude correlate nicely with strain distributions. Accompanying bone-like mechanical gradients, the center is less adhesive and self-healable than the sides, which allows a better recovery after a complete cutting. Our work may represent a general strategy for the synthesis of biomimetic materials with complex gradients thanks to the ubiquitous presence of strain distributions in load-bearing structures.

The Role of Matrix Stiffness And Viscosity on Lipid Phenotype And Fat Lineage Potential

Autologous fat transfer is a common procedure that patients undergo to rejuvenate large soft tissue defects. However, these surgeries are complicated by limited tissue sources, donor-site morbidity, and necrosis. While the biofabrication of fat tissue can serve as a clinical option for reconstructive surgery, the influence of matrix mechanics, specifically stiffness and viscosity, on adipogenesis requires further elucidation. Additionally, the effects of these mechanical parameters on metabolic and thermogenic fat potential have yet to be investigated. In this study, gelatin methacryloyl (GelMA) polymers with varying degrees of methacrylation (DoM) were fabricated to create matrices with different stiffnesses and viscosities. Human adipose-derived mesenchymal stem cells were then encapsulated in mechanically tunable GelMA and underwent adipogenesis to investigate the effects of matrix mechanics on lipid phenotype and fat potential. Mechanical testing confirmed that GelMA stiffness was regulated by DoM and weight composition, whereas viscosity was determined by the latter. Further work revealed that while lipid phenotype became more enriched as matrix stiffness and viscosity declined, the potential toward metabolic and thermogenic fat appeared to be more viscous dependent rather than stiffness dependent. In addition, fatty acid binding protein 4 and uncoupling protein 1 gene expression exhibited viscous-dependent behavior despite comparable levels of peroxisome proliferator-activated receptor gamma. However, despite the superior role of viscosity, lipid quantity and mitochondrial abundance demonstrated stiffness-dependent behavior. Overall, this work revealed that matrix viscosity played a more superior role than stiffness in driving adipogenesis and distinguishing between metabolic and thermogenic fat potential. Ultimately, this differentiation in fat production is important for engineering ideal adipose tissue for large soft tissue defects.

Multimodal mechano-microscopy reveals mechanical phenotypes of breast cancer spheroids in three dimensions

Cancer cell invasion relies on an equilibrium between cell deformability and the biophysical constraints imposed by the extracellular matrix (ECM). However, there is little consensus on the nature of the local biomechanical alterations in cancer cell dissemination in the context of three-dimensional (3D) tumor microenvironments (TMEs). While the shortcomings of two-dimensional (2D) models in replicating in situ cell behavior are well known, 3D TME models remain underutilized because contemporary mechanical quantification tools are limited to surface measurements. Here, we overcome this major challenge by quantifying local mechanics of cancer cell spheroids in 3D TMEs. We achieve this using multimodal mechano-microscopy, integrating optical coherence microscopy-based elasticity imaging with confocal fluorescence microscopy. We observe that non-metastatic cancer spheroids show no invasion while showing increased peripheral cell elasticity in both stiff and soft environments. Metastatic cancer spheroids, however, show ECM-mediated softening in a stiff microenvironment and, in a soft environment, initiate cell invasion with peripheral softening associated with early metastatic dissemination. This exemplar of live-cell 3D mechanotyping supports that invasion increases cell deformability in a 3D context, illustrating the power of multimodal mechano-microscopy for quantitative mechanobiology in situ.

Advances in micropatterning technology for mechanotransduction research

Micropatterning is a sophisticated technique that precisely manipulates the spatial distribution of cell adhesion proteins on various substrates across multiple scales. This precise control over adhesive regions facilitates the manipulation of architectures and physical constraints for single or multiple cells. Furthermore, it allows for an in-depth analysis of how chemical and physical properties influence cellular functionality. In this comprehensive review, we explore the current understanding of the impact of geometrical confinement on cellular functions across various dimensions, emphasizing the benefits of micropatterning in addressing fundamental biological queries. We advocate that utilizing directed self-organization via physical confinement and morphogen gradients on micropatterned surfaces represents an innovative approach to generating functional tissue and controlling morphogenesis in vitro. Integrating this technique with cutting-edge technologies, micropatterning presents a significant potential to bridge a crucial knowledge gap in understanding core biological processes.

Droplet-based microfluidics for engineering shape-controlled hydrogels with stiffness gradient

Current biofabrication strategies are limited in their ability to replicate native shape-to-function relationships, that are dependent on adequate biomimicry of macroscale shape as well as size and microscale spatial heterogeneity, within cell-laden hydrogels. In this study, a novel diffusion-based microfluidics platform is presented that meets these needs in a two-step process. In the first step, a hydrogel-precursor solution is dispersed into a continuous oil phase within the microfluidics tubing. By adjusting the dispersed and oil phase flow rates, the physical architecture of hydrogel-precursor phases can be adjusted to generate spherical and plug-like structures, as well as continuous meter-long hydrogel-precursor phases (up to 1.75 m). The second step involves the controlled introduction a small molecule-containing aqueous phase through a T-shaped tube connector to enable controlled small molecule diffusion across the interface of the aqueous phase and hydrogel-precursor. Application of this system is demonstrated by diffusing co-initiator sodium persulfate (SPS) into hydrogel-precursor solutions, where the controlled SPS diffusion into the hydrogel-precursor and subsequent photo-polymerization allows for the formation of unique radial stiffness patterns across the shape- and size-controlled hydrogels, as well as allowing the formation of hollow hydrogels with controllable internal architectures. Mesenchymal stromal cells are successfully encapsulated within hollow hydrogels and hydrogels containing radial stiffness gradient and found to respond to the heterogeneity in stiffness through the yes-associated protein mechano-regulator. Finally, breast cancer cells are found to phenotypically switch in response to stiffness gradients, causing a shift in their ability to aggregate, which may have implications for metastasis. The diffusion-based microfluidics thus finds application mimicking native shape-to-function relationship in the context of tissue engineering and provides a platform to further study the roles of micro- and macroscale architectural features that exist within native tissues.

Matrix stiffness aggravates osteoarthritis progression through H3K27me3 demethylation induced by mitochondrial damage

Abnormal epigenetics is the initial factor of the occurrence and development of osteoarthritis (OA), and abnormal mechanical load is a key pathogenic factor of OA. However, how abnormal mechanical load affects chondrocyte epigenetics is unclear. Chondrocytes reportedly respond to mechanics through the extracellular matrix (ECM), which has a role in regulating epigenetics in various diseases, and mitochondria are potential mediators of communication between mechanics and epigenetics. Therefore, it is hypothesized that the matrix mechanics of cartilage regulates their epigenetics through mitochondria and leads to OA. The matrix stiffness of OA cartilage on the stress-concentrated side increases, mitochondrial damage of chondrocyte is severe, and the chondrocyte H3K27me3 is demethylated. Moreover, mitochondrial permeability transition pore (mPTP) opens to increase the translocation of plant homeodomain finger protein 8 (Phf8) into the nucleus to catalyze H3K27me3 demethylation. This provides a new perspective for us to understand the mechanism of OA based on mechanobiology.

Integrated Cross-Scale Manipulation and Modulable Encapsulation of Cell-Laden Hydrogel for Constructing Tissue-Mimicking Microstructures

Engineered microstructures that mimic in vivo tissues have demonstrated great potential for applications in regenerative medicine, drug screening, and cell behavior exploration. However, current methods for engineering microstructures that mimic the multi-extracellular matrix and multicellular features of natural tissues to realize tissue-mimicking microstructures in vitro remain insufficient. Here, we propose a versatile method for constructing tissue-mimicking heterogeneous microstructures by orderly integration of macroscopic hydrogel exchange, microscopic cell manipulation, and encapsulation modulation. First, various cell-laden hydrogel droplets are manipulated at the millimeter scale using electrowetting on dielectric to achieve efficient hydrogel exchange. Second, the cells are manipulated at the micrometer scale using dielectrophoresis to adjust their density and arrangement within the hydrogel droplets. Third, the photopolymerization of these hydrogel droplets is triggered in designated regions by dynamically modulating the shape and position of the excitation ultraviolet beam. Thus, heterogeneous microstructures with different extracellular matrix geometries and components were constructed, including specific cell densities and patterns. The resulting heterogeneous microstructure supported long-term culture of hepatocytes and fibroblasts with high cell viability (over 90%). Moreover, the density and distribution of the 2 cell types had significant effects on the cell proliferation and urea secretion. We propose that our method can lead to the construction of additional biomimetic heterogeneous microstructures with unprecedented potential for use in future tissue engineering applications.

Unveiling the Intricate Connection: Cell Volume as a Key Regulator of Mechanotransduction

The volumes of living cells undergo dynamic changes to maintain the cells' structural and functional integrity in many physiological processes. Minor fluctuations in cell volume can serve as intrinsic signals that play a crucial role in cell fate determination during mechanotransduction. In this review, we discuss the variability of cell volume and its role in vivo, along with an overview of the mechanisms governing cell volume regulation. Additionally, we provide insights into the current approaches used to control cell volume in vitro. Furthermore, we summarize the biological implications of cell volume regulation and discuss recent advances in understanding the fundamental relationship between cell volume and mechanotransduction. Finally, we delve into the potential underlying mechanisms, including intracellular macromolecular crowding and cellular mechanics, that govern the global regulation of cell fate in response to changes in cell volume. By exploring the intricate interplay between cell volume and mechanotransduction, we underscore the importance of considering cell volume as a fundamental signaling cue to unravel the basic principles of mechanotransduction. Additionally, we propose future research directions that can extend our current understanding of cell volume in mechanotransduction. Overall, this review highlights the significance of considering cell volume as a fundamental signal in understanding the basic principles in mechanotransduction and points out the possibility of controlling cell volume to control cell fate, mitigate disease-related damage, and facilitate the healing of damaged tissues.

Stiffness Mediated-Mechanosensation of Airway Smooth Muscle Cells on Linear Stiffness Gradient Hydrogels

In obstructive airway diseases such as asthma and chronic obstructive pulmonary disease (COPD), the extracellular matrix (ECM) protein amount and composition of the airway smooth muscle (ASM) is often remodelled, likely altering tissue stiffness. The underlying mechanism of how human ASM cell (hASMC) mechanosenses the aberrant microenvironment is not well understood. Physiological stiffnesses of the ASM were measured by uniaxial compression tester using porcine ASM layers under 0, 5 and 10% longitudinal stretch above in situ length. Linear stiffness gradient hydrogels (230 kPa range) were fabricated and functionalized with ECM proteins, collagen I (ColI), fibronectin (Fn) and laminin (Ln), to recapitulate the above-measured range of stiffnesses. Overall, hASMC mechanosensation exhibited a clear correlation with the underlying hydrogel stiffness. Cell size, nuclear size and contractile marker alpha-smooth muscle actin (αSMA) expression showed a strong correlation to substrate stiffness. Mechanosensation, assessed by Lamin-A intensity and nuc/cyto YAP, exhibited stiffness-mediated behaviour only on ColI and Fn-coated hydrogels. Inhibition studies using blebbistatin or Y27632 attenuated most mechanotransduction-derived cell morphological responses, αSMA and Lamin-A expression and nuc/cyto YAP (blebbistatin only). This study highlights the interplay and complexities between stiffness and ECM protein type on hASMC mechanosensation, relevant to airway remodelling in obstructive airway diseases.

In situ stress estimation in quantitative micro-elastography

In quantitative micro-elastography (QME), a pre-characterized compliant layer with a known stress-strain curve is utilized to map stress at the sample surface. However, differences in the boundary conditions of the compliant layer when it is mechanically characterized and when it is used in QME experiments lead to inconsistent stress estimation and consequently, inaccurate elasticity measurements. Here, we propose a novel in situ stress estimation method using an optical coherence tomography (OCT)-based uniaxial compression testing system integrated with the QME experimental setup. By combining OCT-measured axial strain with axial stress determined using a load cell in the QME experiments, we can estimate in situ stress for the compliant layer, more accurately considering its boundary conditions. Our proposed method shows improved accuracy, with an error below 10%, compared to 85% using the existing QME technique with no lubrication. Furthermore, demonstrations on hydrogels and cells indicate the potential of this approach for improving the characterization of the micro-scale mechanical properties of cells and their interactions with the surrounding biomaterial, which has potential for application in cell mechanobiology.

Noninvasive and Continuous Monitoring of On-Chip Stem Cell Osteogenesis Using a Reusable Electrochemical Immunobiosensor

Noninvasive monitoring of biofabricated tissues during the biomanufacturing process is needed to obtain reproducible, healthy, and functional tissues. Measuring the levels of biomarkers secreted from tissues is a promising strategy to understand the status of tissues during biofabrication. Continuous and real-time information from cultivated tissues enables users to achieve scalable manufacturing. Label-free biosensors are promising candidates for detecting cell secretomes since they can be noninvasive and do not require labor-intensive processes such as cell lysing. Moreover, most conventional monitoring techniques are single-use, conducted at the end of the fabrication process, and, challengingly, are not permissive to in-line and continual detection. To address these challenges, we developed a noninvasive and continual monitoring platform to evaluate the status of cells during the biofabrication process, with a particular focus on monitoring the transient processes that stem cells go through during in vitro differentiation over extended periods. We designed and evaluated a reusable electrochemical immunosensor with the capacity for detecting trace amounts of secreted osteogenic markers, such as osteopontin (OPN). The sensor has a low limit of detection (LOD), high sensitivity, and outstanding selectivity in complex biological media. We used this OPN immunosensor to continuously monitor on-chip osteogenesis of human mesenchymal stem cells (hMSCs) cultured 2D and 3D hydrogel constructs inside a microfluidic bioreactor for more than a month and were able to observe changing levels of OPN secretion during culture. The proposed platform can potentially be adopted for monitoring a variety of biological applications and further developed into a fully automated system for applications in advanced cellular biomanufacturing.

The Role of Mechanotransduction in Contact Inhibition of Locomotion and Proliferation

Contact inhibition (CI) represents a crucial tumor-suppressive mechanism responsible for controlling the unbridled growth of cells, thus preventing the formation of cancerous tissues. CI can be further categorized into two distinct yet interrelated components: CI of locomotion (CIL) and CI of proliferation (CIP). These two components of CI have historically been viewed as separate processes, but emerging research suggests that they may be regulated by both distinct and shared pathways. Specifically, recent studies have indicated that both CIP and CIL utilize mechanotransduction pathways, a process that involves cells sensing and responding to mechanical forces. This review article describes the role of mechanotransduction in CI, shedding light on how mechanical forces regulate CIL and CIP. Emphasis is placed on filamin A (FLNA)-mediated mechanotransduction, elucidating how FLNA senses mechanical forces and translates them into crucial biochemical signals that regulate cell locomotion and proliferation. In addition to FLNA, trans-acting factors (TAFs), which are proteins or regulatory RNAs capable of directly or indirectly binding to specific DNA sequences in distant genes to regulate gene expression, emerge as sensitive players in both the mechanotransduction and signaling pathways of CI. This article presents methods for identifying these TAF proteins and profiling the associated changes in chromatin structure, offering valuable insights into CI and other biological functions mediated by mechanotransduction. Finally, it addresses unanswered research questions in these fields and delineates their possible future directions.

A Perspective on Developing Modeling and Image Analysis Tools to Investigate Mechanosensing Proteins

The shift of funding organizations to prioritize interdisciplinary work points to the need for workflow models that better accommodate interdisciplinary studies. Most scientists are trained in a specific field and are often unaware of the kind of insights that other disciplines could contribute to solving various problems. In this paper, we present a perspective on how we developed an experimental pipeline between a microscopy and image analysis/bioengineering lab. Specifically, we connected microscopy observations about a putative mechanosensing protein, obscurin, to image analysis techniques that quantify cell changes. While the individual methods used are well established (fluorescence microscopy; ImageJ WEKA and mTrack2 programs; MATLAB), there are no existing best practices for how to integrate these techniques into a cohesive, interdisciplinary narrative. Here, we describe a broadly applicable workflow of how microscopists can more easily quantify cell properties (e.g., perimeter, velocity) from microscopy videos of eukaryotic (MDCK) adherent cells. Additionally, we give examples of how these foundational measurements can create more complex, customizable cell mechanics tools and models.

Dendronized chitosan hydrogel with GIT1 to accelerate bone defect repair through increasing local neovascular amount

Bone defects have long been a major healthcare issue because of the difficulties in regenerating bone mass volume and the high cost of treatment. G protein-coupled receptor kinase 2 interacting protein 1 (GIT1) has been proven to play an important role both in vascular development and in bone fracture healing. In this study, a type of thermoresponsive injectable hydrogel from oligoethylene glycol-based dendronized chitosan (G1-CS) was loaded with GIT1-plasmids (G1-CS/GIT1), and used to fill unicortical bone defects. RT-PCR analysis confirmed that G1-CS/GIT1 enhanced DNA transfection in MSCs both in vitro and in vivo. From the results of micro-CT, RT-PCR and histological analysis, it can be concluded that G1-CS/GIT1 accelerated the bone healing rate and increased the amount of neovascularization around the bone defects. In addition, an adeno-associated virus (AAV)-GIT1 was constructed to transfect mesenchymal stem cells. The results of capillary tube formation assay, immunofluorescence staining and western blot analysis proved that high expression of GIT1 induces mesenchymal stem cells to differentiate into endothelial cells. RT-PCR analysis and capillary tube formation assay confirmed that the Notch signaling pathway was activated in the differentiation process. Overall, we developed an efficient strategy through combination of injectable hydrogel and G1T1 for bone tissue engineering.

3D Volumetric Mechanosensation of MCF7 Breast Cancer Spheroids in a Linear Stiffness Gradient GelAGE

The tumor microenvironment presents spatiotemporal shifts in biomechanical properties with cancer progression. Hydrogel biomaterials like GelAGE offer the stiffness tuneability to recapitulate dynamic changes in tumor tissues by altering photo-energy exposures. Here, a tuneable hydrogel with spatiotemporal control of stiffness and mesh-network is developed. The volume of MCF7 spheroids encapsulated in a linear stiffness gradient demonstrates an inverse relationship with stiffness (p < 0.0001). As spheroids are exposed to increased crosslinking (stiffer) and greater mechanical confinement, spheroid stiffness increases. Protein expression (TRPV4, β1 integrin, E-cadherin, and F-actin) decreases with increasing stiffness while showing strong correlations to spheroid volume (r2  > 0.9). To further investigate the role of volume, MCF7 spheroids are grown in a soft matrix for 5 days prior to a second polymerisation which presents a stiffness gradient to equally expanded spheroids. Despite being exposed to variable stiffness, these spheroids show even protein expression, confirming volume as a key regulator. Overall, this work showcases the versatility of GelAGE and demonstrates volume expansion as a key regulator of 3D mechanosensation in MCF7 breast cancer spheroids. This platform has the potential to further investigation into the role of stiffness and dimensionality in 3D spheroid culture for other types of cancers and diseases.

Dynamic Stiffening Hydrogel with Instructive Stiffening Timing Modulates Stem Cell Fate In Vitro and Enhances Bone Remodeling In Vivo

Biomechanical stimuli derived from the extracellular matrix (ECM) extremely tune stem cell fate through 3D and spatiotemporal changes in vivo. The matrix stiffness is a crucial factor during bone tissue development. However, most in vitro models to study the osteogenesis of mesenchymal stem cells (MSCs) are static or stiffening in a 2D environment. Here, a dynamic and controllable stiffening 3D biomimetic model is created to regulate the osteogenic differentiation of MSCs with a dual-functional gelatin macromer that can generate a double-network hydrogel by sequential enzymatic and light-triggered crosslinking reactions. The findings show that these dynamic hydrogels allowed cells to spread and expand prior to the secondary crosslinking and to sense high stiffness after stiffening. The MSCs in the dynamic hydrogels, especially the hydrogel stiffened at the late period, present significantly elevated osteogenic ECM secretion, gene expression, and nuclear localization of Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). In vivo evaluation of animal experiments further indicates that the enhancement of dynamic stiffening on osteogenesis of MSCs substantially promotes bone remodeling. Consequently, this work reveals that the 3D dynamic stiffening microenvironment as a critical biophysical cue not only mediates the stem cell fate in vitro, but also augments bone restoration in vivo.

Elastic modulus of hydrogel regulates osteogenic differentiation via liquid-liquid phase separation of YAP

Craniofacial bone defects induced by congenital malformations, trauma, or diseases frequently challenge the orthodontic or restorative treatment. Stem cell-based bone regenerative approaches emerged as a promising method to resolve bone defects. Microenvironment physical cues, such as the matrix elastic modulus or matrix topography, regulate stem cell differentiation via multiple genes. We constructed gelatin methacryloyl (GelMA), a well-known scaffold, to investigate the impact of elastic modulus on osteogenic differentiation in a three-dimensional environment. Confocal microscope was used to observe and assess the condensates fission and fusion. New bone formation was evaluated by micro-computed tomography at 6 weeks in calvarial defect rat. We found that the light curing increased elastic modulus of GelMA, and the pore size of GelMA decreased. The expression of osteogenic markers was inhibited in hBMSCs cultured in the low-elastic-modulus GelMA. In contrast, the expression of YAP, TAZ and TEAD was increased in the hBMSCs in the low-elastic-modulus GelMA. Furthermore, YAP assembled via liquid-liquid phase separation (LLPS) into condensates that were sensitive to 1'6-hexanediol. YAP recruit TAZ and TEAD4, but not RUNX2 into the condensates. In vivo, we also found that hBMSCs in high-elastic-modulus GelMA was more apt to form new bone. This study provides new insight into the mechanism of osteogenic differentiation. Reagents that can regulate the elastic modulus of substrate or LLPS may be applied to promote bone regeneration.

Nuclear Import and Export of YAP and TAZ

Yes-associated Protein (YAP) and its paralog Transcriptional Coactivator with PDZ-binding Motif (TAZ) are major regulators of gene transcription/expression, primarily controlled by the Hippo pathway and the cytoskeleton. Integrating an array of chemical and mechanical signals, they impact growth, differentiation, and regeneration. Accordingly, they also play key roles in tumorigenesis and metastasis formation. Their activity is primarily regulated by their localization, that is, Hippo pathway- and/or cytoskeleton-controlled cytosolic or nuclear sequestration. While many details of such prevailing retention models have been elucidated, much less is known about their actual nuclear traffic: import and export. Although their size is not far from the cutoff for passive diffusion through the nuclear pore complex (NPC), and they do not contain any classic nuclear localization (NLS) or nuclear export signal (NES), evidence has been accumulating that their shuttling involves mediated and thus regulatable/targetable processes. The aim of this review is to summarize emerging information/concepts about their nucleocytoplasmic shuttling, encompassing the relevant structural requirements (NLS, NES), nuclear transport receptors (NTRs, karyophererins), and NPC components, along with the potential transport mechanisms and their regulation. While dissecting retention vs. transport is often challenging, the emerging picture suggests that YAP/TAZ shuttles across the NPC via multiple, non-exclusive, mediated mechanisms, constituting a novel and intriguing facet of YAP/TAZ biology.

Advanced Mechanical Testing Technologies at the Cellular Level: The Mechanisms and Application in Tissue Engineering

Mechanics, as a key physical factor which affects cell function and tissue regeneration, is attracting the attention of researchers in the fields of biomaterials, biomechanics, and tissue engineering. The macroscopic mechanical properties of tissue engineering scaffolds have been studied and optimized based on different applications. However, the mechanical properties of the overall scaffold materials are not enough to reveal the mechanical mechanism of the cell-matrix interaction. Hence, the mechanical detection of cell mechanics and cellular-scale microenvironments has become crucial for unraveling the mechanisms which underly cell activities and which are affected by physical factors. This review mainly focuses on the advanced technologies and applications of cell-scale mechanical detection. It summarizes the techniques used in micromechanical performance analysis, including atomic force microscope (AFM), optical tweezer (OT), magnetic tweezer (MT), and traction force microscope (TFM), and analyzes their testing mechanisms. In addition, the application of mechanical testing techniques to cell mechanics and tissue engineering scaffolds, such as hydrogels and porous scaffolds, is summarized and discussed. Finally, it highlights the challenges and prospects of this field. This review is believed to provide valuable insights into micromechanics in tissue engineering.

Volume adaptation of neonatal cardiomyocyte spheroids in 3D stiffness gradient GelMA

Present understandings of cardiomyocyte mechanobiology have primarily been developed using 2-dimensional, monocellular cell cultures, however the emergence of 3-dimensional (3D) multicellular cardiac constructs has enabled us to develop more sophisticated recapitulations of the cardiac microenvironment. Several of these strategies have illustrated that incorporating elements of the extracellular matrix (ECM) can promote greater maturation and enhance desirable cardiac functions, such as contractility, but the responses of these cardiac constructs to biophysically aberrant conditions, such as in the post-infarct heart, has remained relatively unexplored. In our study, we employ a stiffness gradient gelatin methacryloyl (GelMA) hydrogel platform to unpack the mechanobiology of cardiac spheroids. We encapsulated neonatal rat cardiac cell spheroids in a 4.4-18.7 kPa linear stiffness gradient up to 120 h. We found the proportion of viable cells within the spheroids increased over time, but the cell number per spheroid decreased. Spheroids expand more in softer matrices while stiffer matrices promote larger nuclei without changing nuclei shape. Volume expansion came primarily from cells expressing vimentin. We did not observe any correlations between stiffness and mechanomarker expression, however we found that after 120 h post-encapsulation, the localization of YAP, the localization of MRTF-A and the expression of Lamin-A was correlated with spheroid morphology. The same trends were not observed 24 h post-encapsulation, indicating that volume adaptation can take a relatively long time. Our data demonstrates that cardiac spheroids are mechanosensitive and that their capacity to respond to ECM-based cues depends on their capacity to adapt their volume with a 3D microenvironment.

Protease-degradable hydrogels with multifunctional biomimetic peptides for bone tissue engineering

Mimicking bone extracellular matrix (ECM) is paramount to develop novel biomaterials for bone tissue engineering. In this regard, the combination of integrin-binding ligands together with osteogenic peptides represents a powerful approach to recapitulate the healing microenvironment of bone. In the present work, we designed polyethylene glycol (PEG)-based hydrogels functionalized with cell instructive multifunctional biomimetic peptides (either with cyclic RGD-DWIVA or cyclic RGD-cyclic DWIVA) and cross-linked with matrix metalloproteinases (MMPs)-degradable sequences to enable dynamic enzymatic biodegradation and cell spreading and differentiation. The analysis of the intrinsic properties of the hydrogel revealed relevant mechanical properties, porosity, swelling and degradability to engineer hydrogels for bone tissue engineering. Moreover, the engineered hydrogels were able to promote human mesenchymal stem cells (MSCs) spreading and significantly improve their osteogenic differentiation. Thus, these novel hydrogels could be a promising candidate for applications in bone tissue engineering, such as acellular systems to be implanted and regenerate bone or in stem cells therapy.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

A maladaptive feedback mechanism between the extracellular matrix and cytoskeleton contributes to hypertrophic cardiomyopathy pathophysiology

Hypertrophic cardiomyopathy is an inherited disorder due to mutations in contractile proteins that results in a stiff, hypercontractile myocardium. To understand the role of cardiac stiffness in disease progression, here we create an in vitro model of hypertrophic cardiomyopathy utilizing hydrogel technology. Culturing wild-type cardiac myocytes on hydrogels with a Young's Moduli (stiffness) mimicking hypertrophic cardiomyopathy myocardium is sufficient to induce a hypermetabolic mitochondrial state versus myocytes plated on hydrogels simulating healthy myocardium. Significantly, these data mirror that of myocytes isolated from a murine model of human hypertrophic cardiomyopathy (cTnI-G203S). Conversely, cTnI-G203S myocyte mitochondrial function is completely restored when plated on hydrogels mimicking healthy myocardium. We identify a mechanosensing feedback mechanism between the extracellular matrix and cytoskeletal network that regulates mitochondrial function under healthy conditions, but participates in the progression of hypertrophic cardiomyopathy pathophysiology resulting from sarcomeric gene mutations. Importantly, we pinpoint key 'linker' sites in this schema that may represent potential therapeutic targets.

Rational Design of Soft-Hard Interfaces through Bioinspired Engineering

Soft-hard tissue interfaces in nature present a diversity of hierarchical transitions in composition and structure to address the challenge of stress concentrations that would otherwise arise at their interface. The translation of these into engineered materials holds promise for improved function of biomedical interfaces. Here, soft-hard tissue interfaces found in the body in health and disease, and the application of the diverse, functionally graded, and hierarchical structures that they present to bioinspired engineering materials are reviewed. A range of such bioinspired engineering materials and associated manufacturing technologies that are on the horizon in interfacial tissue engineering, hydrogel bioadhesion at the interfaces, and healthcare and medical devices are described.

From mesenchymal niches to engineered in vitro model systems: Exploring and exploiting biomechanical regulation of vertebrate hedgehog signalling

Tissue patterning is the result of complex interactions between transcriptional programs and various mechanical cues that modulate cell behaviour and drive morphogenesis. Vertebrate Hedgehog signalling plays key roles in embryogenesis and adult tissue homeostasis, and is central to skeletal development and the osteogenic differentiation of mesenchymal stem cells. The expression of several components of the Hedgehog signalling pathway have been reported to be mechanically regulated in mesodermal tissue patterning and osteogenic differentiation in response to external stimulation. Since a number of bone developmental defects and skeletal diseases, such as osteoporosis, are directly linked to aberrant Hedgehog signalling, a better knowledge of the regulation of Hedgehog signalling in the mechanosensitive bone marrow-residing mesenchymal stromal cells will present novel avenues for modelling these diseases and uncover novel opportunities for extracellular matrix-targeted therapies. In this review, we present a brief overview of the key molecular players involved in Hedgehog signalling and the basic concepts of mechanobiology, with a focus on bone development and regeneration. We also highlight the correlation between the activation of the Hedgehog signalling pathway in response to mechanical cues and osteogenesis in bone marrow-derived mesenchymal stromal cells. Finally, we propose different tissue engineering strategies to apply the expanding knowledge of 3D material-cell interactions in the modulation of Hedgehog signalling in vitro for fundamental and translational research applications.

Mechanosensation mediates volume adaptation of cardiac cells and spheroids in 3D

With the adoption of 3-dimensional (3D) cell culture for in vitro modelling of cardiac function and regenerative medicine applications, there is an increased need to understand cardiomyocyte mechanosensation in 3D. With existing studies of cardiomyocyte mechanosensation primarily focussed on the behaviour of individual cells in a 2-Dimensional context, it is unclear whether mechanosensation is the same in a 3D, multicellular context. In this study, H9C2 cardiac-derived myoblasts were encapsulated as individual cells and as cell spheroids within stiffness gradient gelatin methacryloyl (GelMA) hydrogels to investigate individual and collective cardiac cell mechanosensation in 3D. Over a 3.68–17.52 ​kPa stiffness range, it was found that H9C2 cells have a limited capacity to adapt their volume to increasing substrate stiffness, demonstrated by the lack of changes in cell volume and shape across the stiffness gradient. Morphological trends were reflected by the expression of the mechanomarkers YAP, MRTF-A and Lamin-A, which were better correlated with cell and nuclear volume than with substrate stiffness. The localisation of YAP and MRTF-A were dependent on the relative volumes of the cytoplasm and nucleus while Lamin-A expression was elevated with increasing cytoplasmic and nuclear volumes. When cultured as spheroids rather than as individual cells, H9C2 cells adopted a distinct morphology with comparably smaller nuclei than individually cultured cells, while retaining the same overall cell volume. As spheroids, H9C2 cells were sensitive to stiffness cues, shown by decreasing YAP and MRTF-A nuclear localisation, increasing Lamin-A expression, and increasing vinculin expression with increasing substrate stiffness. Like the individually cultured H9C2 cells, mechanomarker expression was correlated to volume adaptation. With increasing cytoplasmic volume, YAP and MRTF-A became less nuclear localised, vinculin expression was increased, and with increasing nuclear volume, the Lamin-A expression fincreased. Together, these data suggest that cardiac cell volume adaptation may be enhanced by cell-cell interactions.

Three-dimensional mechanical characterization of murine skeletal muscle using quantitative micro-elastography

Skeletal muscle function is governed by both the mechanical and structural properties of its constituent tissues, which are both modified by disease. Characterizing the mechanical properties of skeletal muscle tissue at an intermediate scale, i.e., between that of cells and organs, can provide insight into diseases such as muscular dystrophies. In this study, we use quantitative micro-elastography (QME) to characterize the micro-scale elasticity of ex vivo murine skeletal muscle in three-dimensions in whole muscles. To address the challenge of achieving high QME image quality with samples featuring uneven surfaces and geometry, we encapsulate the muscles in transparent hydrogels with flat surfaces. Using this method, we study aging and disease in quadriceps tissue by comparing normal wild-type (C57BL/6J) mice with dysferlin-deficient BLAJ mice, a model for the muscular dystrophy dysferlinopathy, at 3, 10, and 24 months of age (sample size of three per group). We observe a 77% decrease in elasticity at 24 months in dysferlin-deficient quadriceps compared to wild-type quadriceps.

Circular Patterns of Dynamic Covalent Hydrogels with Gradient Stiffness for Screening of the Stem Cell Microenvironment

As extracellular matrix (ECM) mimetic materials, hydrogels have been widely used for broad biomedical applications. However, with so many physical or chemical cues in the matrix that regulate cell behaviors or functions, it remains challenging to design a customizable hydrogel with the desired properties on demand. In the current study, we aim to establish a circular-patterned hydrogel model with gradient stiffness for screening the most favorable ECM environment for specific cells or certain application purposes. First, six types of hydrogels with a wide stiffness range of 1.2-28.9 kPa were prepared by dynamic covalent cross-linking between gelatin derivatives and oxidized hyaluronic acid. Taking advantage of their instantaneous self-healing property from dynamic chemistry, the hydrogels were further spliced into one whole piece of circular-patterned hydrogel. When rabbit bone marrow mesenchymal stem cells were seeded in the center, the influences of matrix stiffness on the regulation of stem cell adhesion, migration, and differentiation were directly observed and compared under one visual field. In addition, these hydrogels all possessed good biocompatibility, degradability, and injectability, showing great potential for tissue-engineering-related applications.

Visible-Light Stiffness Patterning of GelMA Hydrogels Towards In Vitro Scar Tissue Models

Variations in mechanical properties of the extracellular matrix occurs in various processes, such as tissue fibrosis. The impact of changes in tissue stiffness on cell behaviour are studied in vitro using various types of biomaterials and methods. Stiffness patterning of hydrogel scaffolds, through the use of stiffness gradients for instance, allows the modelling and studying of cellular responses to fibrotic mechanisms. Gelatine methacryloyl (GelMA) has been used extensively in tissue engineering for its inherent biocompatibility and the ability to precisely tune its mechanical properties. Visible light is now increasingly employed for crosslinking GelMA hydrogels as it enables improved cell survival when performing cell encapsulation. We report here, the photopatterning of mechanical properties of GelMA hydrogels with visible light and eosin Y as the photoinitiator using physical photomasks and projection with a digital micromirror device. Using both methods, binary hydrogels with areas of different stiffnesses and hydrogels with stiffness gradients were fabricated. Their mechanical properties were characterised using force indentation with atomic force microscopy, which showed the efficiency of both methods to spatially pattern the elastic modulus of GelMA according to the photomask or the projected pattern. Crosslinking through projection was also used to build constructs with complex shapes. Overall, this work shows the feasibility of patterning the stiffness of GelMA scaffolds, in the range from healthy to pathological stiffness, with visible light. Consequently, this method could be used to build in vitro models of healthy and fibrotic tissue and study the cellular behaviours involved at the interface between the two.

Modelling the Tumor Microenvironment: Recapitulating Nano- and Micro-Scale Properties that Regulate Tumor Progression

Breast cancer remains a significant burden with 1 in 8 women affected and metastasis posing a significant challenge for patient survival. Disease progression involves remodeling of the extracellular matrix (ECM). In breast cancer, tissue stiffness increases owing to an increase in collagen production by recruited cancer-associated fibroblasts (CAFs). These stromal modifications are notable during primary tumor growth and have a dualistic action by creating a hard capsule to prevent penetration of anti-cancer therapies and forming a favorable environment for tumor progression. Remodeling of the tumor microenvironment immediately presented to cells can include changes in protein composition, concentration and structural arrangement and provides the first mechanical stimuli in the metastatic cascade. Not surprisingly, metastatic cancer cells possess the ability to mechanically adapt, and their adaptability ensures not only survival but successful invasion within altered environments. In the past decade, the importance of the microenvironment and its regulatory role in diseases have gained traction and this is evident in the shift from plastic culture to the development of novel biomaterials that mimic in vivo tissue. With these advances, elucidations can be made into how ECM remodeling and more specifically, altered cell-ECM adhesions, regulate tumor growth and cancer cell plasticity. Such enabling tools in mechanobiology will identify fundamental mechanisms in cancer progression that eventually help develop preventative and therapeutic treatment from a clinical perspective. This review will focus on current platforms engineered to mimic the micro and nano-properties of the tumor microenvironment and subsequent understanding of mechanically regulated pathways in cancer.

Analysis of strain estimation methods in phase-sensitive compression optical coherence elastography

In compression optical coherence elastography (OCE), deformation is quantified as the local strain at each pixel in the OCT field-of-view. A range of strain estimation methods have been demonstrated, yet it is unclear which method provides the best performance. Here, we analyze the two most prevalent strain estimation methods used in phase-sensitive compression OCE, i.e., weighted least squares (WLS) and the vector method. We introduce a framework to compare strain imaging metrics, incorporating strain sensitivity, strain signal-to-noise ratio (SNR), strain resolution, and strain accuracy. In addition, we propose a new phase unwrapping algorithm in OCE, fast phase unwrapping (FPU), and combine it with WLS, termed WLSFPU. Using the framework, we compare this new strain estimation method with both a current implementation of WLS that incorporates weighted phase unwrapping (WPU), termed WLSWPU, and the vector method. Our analysis reveals that the three methods provide similar strain sensitivity, strain SNR, and strain resolution, but that WLSFPU extends the dynamic range of accurate, measurable local strain, e.g., measuring a strain of 2.5 mɛ with ∼4% error, that is ×11 and ×15 smaller than the error measured using WLSWPU and the vector method, respectively. We also demonstrate, for the first time, the capability to detect sub-resolution contrast in compression OCE, i.e., changes in strain occurring within the strain axial resolution, and how this contrast varies between the different strain estimation methods. Lastly, we compare the performance of the three strain estimation methods on mouse skeletal muscle and human breast tissue and demonstrate that WLSFPU avoids strain imaging artifacts resulting from phase unwrapping errors in WLSWPU and provides improved contrast over the other two methods.

Appreciating the role of cell shape changes in the mechanobiology of epithelial tissues

The wide range of epithelial cell shapes reveals the complexity and diversity of the intracellular mechanisms that serve to construct their morphology and regulate their functions. Using mechanosensitive steps, epithelial cells can sense a variety of different mechanochemical stimuli and adapt their behavior by reshaping their morphology. These changes of cell shape rely on a structural reorganization in space and time that generates modifications of the tensional state and activates biochemical cascades. Recent studies have started to unveil how the cell shape maintenance is involved in mechanical homeostatic tasks to sustain epithelial tissue folding, identity, and self-renewal. Here, we review relevant works that integrated mechanobiology to elucidate some of the core principles of how cell shape may be conveyed into spatial information to guide collective processes such as epithelial morphogenesis. Among many other parameters, we show that the regulation of the cell shape can be understood as the result of the interplay between two counteracting mechanisms: actomyosin contractility and intercellular adhesions, and that both do not act independently but are functionally integrated to operate on molecular, cellular, and tissue scales. We highlight the role of cadherin-based adhesions in force-sensing and mechanotransduction, and we report recent developments that exploit physics of liquid crystals to connect cell shape changes to orientational order in cell aggregates. Finally, we emphasize that the further intermingling of different disciplines to develop new mechanobiology assays will lead the way toward a unified picture of the contribution of cell shape to the pathophysiological behavior of epithelial tissues.

YAP/TAZ in Bone and Cartilage Biology

YAP and TAZ were initially described as the main regulators of organ growth during development and more recently implicated in bone biology. YAP and TAZ are regulated by mechanical and cytoskeletal cues that lead to the control of cell fate in response to the cellular microenvironment. The mechanical component represents a major signal for bone tissue adaptation and remodelling, so YAP/TAZ contributes significantly in bone and cartilage homeostasis. Recently, mice and cellular models have been developed to investigate the precise roles of YAP/TAZ in bone and cartilage cells, and which appear to be crucial. This review provides an overview of YAP/TAZ regulation and function, notably providing new insights into the role of YAP/TAZ in bone biology.

Cell Adhesion and Migration on Thickness Gradient Bilayer Polymer Brush Surfaces: Effects of Properties of Polymeric Materials of the Underlayer

In the field of tissue engineering and biomaterials, controlling the surface properties and mechanical properties of scaffold materials is crucial and has attracted much attention. Here, two types of bilayer polymer brushes composed of a hydrophilic underlying layer and a cationic surface layer [made of poly(2-aminoethyl methacrylate)] with a thickness gradient were prepared by surface-initiated atom-transfer radical polymerization. To investigate the influence of the stiffness as a mechanical property of the polymer brush on cell behavior, the underlayer was prepared from either 2-methacryloyloxyethyl phosphorylcholine or oligo(ethylene glycol) methyl ether methacrylate, with the bilayers designated as gradient poly(2-methacryloyloxyethyl phosphorylcholine)-block-poly(2-aminoethyl methacrylate) [grad-pMbA] and gradient poly(oligo[ethylene glycol] methyl ether methacrylate)-block-poly(2-aminoethyl methacrylate) [grad-pEGbA], respectively. Characterization of these surfaces was performed by spectroscopic ellipsometry, X-ray reflectivity, and determination of the zeta potential, static contact angle, and force curve. These diblock copolymer brushes with a thickness gradient helped to distinguish the effects of the mechanical and surface properties of the brushes on cell behavior. The attachment and motility of L929 fibroblasts and epithelial MCF 10A cells on the fabricated brushes were then assessed. L929 cells had a round shape on the thin surface layer of grad-pMbA and spread well on thicker areas. In contrast, MCF 10A cells spread well in areas of any thickness of either grad-pMbA or grad-pEGbA. Single MCF 10A cells migrated randomly on grad-pMbA, whereas grouped cells started to climb up along the thickness gradient of grad-pMbA. In contrast, both single and grouped MCF 10A cells migrated randomly on grad-pEGbA. These thickness gradient diblock copolymer brushes are simple, reproducible, and reasonable platforms that can facilitate practical applications of biomaterials, for example, in tissue engineering and biomaterials.

Scaffold geometry modulation of mechanotransduction and its influence on epigenetics

The dynamics of cell mechanics and epigenetic signatures direct cell behaviour and fate, thus influencing regenerative outcomes. In recent years, the utilisation of 2D geometric (i.e. square, circle, hexagon, triangle or round-shaped) substrates for investigating cell mechanics in response to the extracellular microenvironment have gained increasing interest in regenerative medicine due to their tunable physicochemical properties. In contrast, there is relatively limited knowledge of cell mechanobiology and epigenetics in the context of 3D biomaterial matrices, i.e., hydrogels and scaffolds. Scaffold geometry provides biophysical signals that trigger a nucleus response (regulation of gene expression) and modulates cell behaviour and function. In this review, we explore the potential of additive manufacturing to incorporate multi length-scale geometry features on a scaffold. Then, we discuss how scaffold geometry direct cell and nuclear mechanosensing. We further discuss how cell epigenetics, particularly DNA/histone methylation and histone acetylation, are modulated by scaffold features that lead to specific gene expression and ultimately influence the outcome of tissue regeneration. Overall, we highlight that geometry of different magnitude scales can facilitate the assembly of cells and multicellular tissues into desired functional architectures through the mechanotransduction pathway. Moving forward, the challenge confronting biomedical engineers is the distillation of the vast knowledge to incorporate multiscaled geometrical features that would collectively elicit a favourable tissue regeneration response by harnessing the design flexibility of additive manufacturing. STATEMENT OF SIGNIFICANCE: It is well-established that cells sense and respond to their 2D geometric microenvironment by transmitting extracellular physiochemical forces through the cytoskeleton and biochemical signalling to the nucleus, facilitating epigenetic changes such as DNA methylation, histone acetylation, and microRNA expression. In this context, the current review presents a unique perspective and highlights the importance of 3D architectures (dimensionality and geometries) on cell and nuclear mechanics and epigenetics. Insight into current challenges around the study of mechanobiology and epigenetics utilising additively manufactured 3D scaffold geometries will progress biomaterials research in this space.

Microsphere incorporation as a strategy to tune the biological performance of bioinks

Although alginate is widely used as a matrix in the formulation of cell-laden inks, this polymer often requires laborious processing strategies due to its lack of cell adhesion moieties. The main objective of the present work was to explore the incorporation of microspheres into alginate-based bioinks as a simple and tuneable way to solve the cell adhesion problems, while adding extra biological functionality and improving their mechanical properties. To this end, three types of microspheres with different mineral contents (i.e. gelatine with 0% of hydroxyapatite, gelatine with 25 wt% of hydroxyapatite nanoparticles and 100 wt% of calcium -deficient hydroxyapatite) were synthesised and incorporated into the formulation of cell-laden inks. The results showed that the addition of microspheres generally improved the rheological properties of the ink, favoured cell proliferation and positively affected osteogenic cell differentiation. Furthermore, this differentiation was found to be influenced by the type of microsphere and the ability of the cells to migrate towards them, which was highly dependent on the stiffness of the bioink. In this regard, Ca²⁺ supplementation in the cell culture medium had a pronounced effect on the relaxation of the stiffness of these cell-loaded inks, influencing the overall cell performance. In conclusion, we have developed a powerful and tuneable strategy for the fabrication of alginate-based bioinks with enhanced biological characteristics by incorporating microspheres into the initial ink formulation.

Strategies for improving adipose-derived stem cells for tissue regeneration

Adipose-derived stem cells (ADSCs) have promising applications in tissue regeneration. Currently, there are only a few ADSC products that have been approved for clinical use. The clinical application of ADSCs still faces many challenges. Here, we review emerging strategies to improve the therapeutic efficacy of ADSCs in tissue regeneration. First, a great quantity of cells is often needed for the stem cell therapies, which requires the advanced cell expansion technologies. In addition cell-derived products are also required for the development of ‘cell-free’ therapies to overcome the drawbacks of cell-based therapies. Second, it is necessary to strengthen the regenerative functions of ADSCs, including viability, differentiation and paracrine ability, for the tissue repair and regeneration required for different physiological and pathophysiological conditions. Third, poor delivery efficiency also restricts the therapeutic effect of ADSCs. Effective methods to improve cell delivery include alleviating harsh microenvironments, enhancing targeting ability and prolonging cell retention. Moreover, we also point out some critical issues about the sources, effectiveness and safety of ADSCs. With these advanced strategies to improve the therapeutic efficacy of ADSCs, ADSC-based treatment holds great promise for clinical applications in tissue regeneration.

Insight Into Rho Kinase Isoforms in Obesity and Energy Homeostasis

Obesity and associated complications increasingly jeopardize global health and contribute to the rapidly rising prevalence of type 2 diabetes mellitus and obesity-related diseases. Developing novel methods for the prevention and treatment of excess body adipose tissue expansion can make a significant contribution to public health. Rho kinase is a Rho-associated coiled-coil-containing protein kinase (Rho kinase or ROCK). The ROCK family including ROCK1 and ROCK2 has recently emerged as a potential therapeutic target for the treatment of metabolic disorders. Up-regulated ROCK activity has been involved in the pathogenesis of all aspects of metabolic syndrome including obesity, insulin resistance, dyslipidemia and hypertension. The RhoA/ROCK-mediated actin cytoskeleton dynamics have been implicated in both white and beige adipogenesis. Studies using ROCK pan-inhibitors in animal models of obesity, diabetes, and associated complications have demonstrated beneficial outcomes. Studies via genetically modified animal models further established isoform-specific roles of ROCK in the pathogenesis of metabolic disorders including obesity. However, most reported studies have been focused on ROCK1 activity during the past decade. Due to the progress in developing ROCK2-selective inhibitors in recent years, a growing body of evidence indicates more attention should be devoted towards understanding ROCK2 isoform function in metabolism. Hence, studying individual ROCK isoforms to reveal their specific roles and principal mechanisms in white and beige adipogenesis, insulin sensitivity, energy balancing regulation, and obesity development will facilitate significant breakthroughs for systemic treatment with isoform-selective inhibitors. In this review, we give an overview of ROCK functions in the pathogenesis of obesity and insulin resistance with a particular focus on the current understanding of ROCK isoform signaling in white and beige adipogenesis, obesity and thermogenesis in adipose tissue and other major metabolic organs involved in energy homeostasis regulation.

Interrogating cardiac muscle cell mechanobiology on stiffness gradient hydrogels

Extracellular matrix (ECM) remodeling is a major facet of cardiac development and disease, yet our understanding of cardiomyocyte mechanotransduction remains limited. To enhance our understanding of cardiomyocyte mechanosensation, we studied stiffness-driven changes to cell morphology and mechanomarker expression in H9C2 cells and neonatal rat cardiomyocytes (NRCMs). Linear stiffness gradient polyacrylamide hydrogels (2-33 kPa) coated with ECM proteins including Collagen I (Col), Fibronectin (Fn) or Laminin (Ln) were used to represent necrotic, healthy, and infarcted cardiac tissue on a continuous stiffness gradient. Cell size, cell shape and nuclear size were found to be mechanosensitive in H9C2 cells, as was the expression or nuclear translocalization of the mechanomarkers Lamin-A, YAP, and MRTF-A. Minor differences were observed between the different ECM coatings, with the same overarching stiffness-dependent trends being observed across Col, Fn and Ln coated hydrogels. Inhibition of mechanotransduction in H9C2 cells using blebbistatin or Y27632 resulted in disruptions to cell shape, nuclear shape, and nuclear size, however, trends in cell size and mechanomarker expression were not significantly attenuated. Mechanosensation in NRCMs was much less marked, with no significant changes in cell morphology being detected, although YAP did become increasingly nuclear localized with increasing stiffness. In α-actinin positive cells, striations formed with regular structure and frequency at all stiffnesses for Col and Fn coated hydrogels, but not Ln coated gels. In this study, we used our stiffness gradient hydrogels to comprehensively map the relationship between ECM stiffness and cardiac cell phenotype and found that less mature H9C2 cardiac cells are more sensitive to ECM changes than the more developed neonatal cardiomyocytes.

Tethering Cells via Enzymatic Oxidative Crosslinking Enables Mechanotransduction in Non-Cell-Adhesive Materials

Cell-matrix interactions govern cell behavior and tissue function by facilitating transduction of biomechanical cues. Engineered tissues often incorporate these interactions by employing cell-adhesive materials. However, using constitutively active cell-adhesive materials impedes control over cell fate and elicits inflammatory responses upon implantation. Here, an alternative cell-material interaction strategy that provides mechanotransducive properties via discrete inducible on-cell crosslinking (DOCKING) of materials, including those that are inherently non-cell-adhesive, is introduced. Specifically, tyramine-functionalized materials are tethered to tyrosines that are naturally present in extracellular protein domains via enzyme-mediated oxidative crosslinking. Temporal control over the stiffness of on-cell tethered 3D microniches reveals that DOCKING uniquely enables lineage programming of stem cells by targeting adhesome-related mechanotransduction pathways acting independently of cell volume changes and spreading. In short, DOCKING represents a bioinspired and cytocompatible cell-tethering strategy that offers new routes to study and engineer cell-material interactions, thereby advancing applications ranging from drug delivery, to cell-based therapy, and cultured meat.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Characterising and engineering biomimetic materials for viscoelastic mechanotransduction studies

The mechanical behavior of soft tissue extracellular matrix is time dependent. Moreover, it evolves over time due to physiological processes as well as aging and disease. Measuring and quantifying the time-dependent mechanical behavior of soft tissues and materials pose a challenge, not only because of their labile and hydrated nature but also because of the lack of a common definition of terms and understanding of models for characterizing viscoelasticity. Here, we review the most important measurement techniques and models used to determine the viscoelastic properties of soft hydrated materials—or hydrogels—underlining the difference between viscoelastic behavior and the properties and descriptors used to quantify viscoelasticity. We then discuss the principal factors, which determine tissue viscoelasticity in vivo and summarize what we currently know about cell response to time-dependent materials, outlining fundamental factors that have to be considered when interpreting results. Particular attention is given to the relationship between the different time scales involved (mechanical, cellular and observation time scales), as well as scaling principles, all of which must be considered when designing viscoelastic materials and performing experiments for biomechanics or mechanobiology applications. From this overview, key considerations and directions for furthering insights and applications in the emergent field of cell viscoelastic mechanotransduction are provided.

Mechanics of 3D Cell-Hydrogel Interactions: Experiments, Models, and Mechanisms

Hydrogels are highly water-swollen molecular networks that are ideal platforms to create tissue mimetics owing to their vast and tunable properties. As such, hydrogels are promising cell-delivery vehicles for applications in tissue engineering and have also emerged as an important base for ex vivo models to study healthy and pathophysiological events in a carefully controlled three-dimensional environment. Cells are readily encapsulated in hydrogels resulting in a plethora of biochemical and mechanical communication mechanisms, which recapitulates the natural cell and extracellular matrix interaction in tissues. These interactions are complex, with multiple events that are invariably coupled and spanning multiple length and time scales. To study and identify the underlying mechanisms involved, an integrated experimental and computational approach is ideally needed. This review discusses the state of our knowledge on cell-hydrogel interactions, with a focus on mechanics and transport, and in this context, highlights recent advancements in experiments, mathematical and computational modeling. The review begins with a background on the thermodynamics and physics fundamentals that govern hydrogel mechanics and transport. The review focuses on two main classes of hydrogels, described as semiflexible polymer networks that represent physically cross-linked fibrous hydrogels and flexible polymer networks representing the chemically cross-linked synthetic and natural hydrogels. In this review, we highlight five main cell-hydrogel interactions that involve key cellular functions related to communication, mechanosensing, migration, growth, and tissue deposition and elaboration. For each of these cellular functions, recent experiments and the most up to date modeling strategies are discussed and then followed by a summary of how to tune hydrogel properties to achieve a desired functional cellular outcome. We conclude with a summary linking these advancements and make the case for the need to integrate experiments and modeling to advance our fundamental understanding of cell-matrix interactions that will ultimately help identify new therapeutic approaches and enable successful tissue engineering.

Comparison of three different acidic solutions in tendon decellularized extracellular matrix bio-ink fabrication for 3D cell printing

Decellularized extracellular matrix (dECM) hydrogels are being increasingly investigated for use in bio-inks for three-dimensional cell printing given their good cytocompatibility and biomimetic properties. The osmotic pressure and stiffness of bio-ink are important factors affecting the biological functions of printed cells. However, little attention has been given to the osmotic pressure and stiffness of the dECM bio-inks. Here, we compared three types of commonly used acidic solutions in the bio-fabrication of a tendon derived dECM bio-ink for 3D cell printing (0.5 M acetic acid, 0.1 M hydrochloric acid and 0.02 M hydrochloric acid). We found that low pH value of 0.1 M hydrochloric acid could accelerate the digestion process for dECM powders. This could lead to a much softer dECM hydrogel with storage modulus less than 100 Pa. This soft dECM hydrogel facilitated the spreading and proliferation of stem cells encapsulated within it. It also showed better tendon-inducing ability compared with two others much stiffer dECM hydrogels. However, this over-digested dECM hydrogel was more unstable as it could shrink with the culture time going on. For 0.5 M acetic acid made dECM bio-ink, the hyperosmotic state of the bio-ink led to much lower cellular viability rates. Postprocess (Dilution or dialysis) to tailor the osmotic pressure of hydrogels could be a necessary step before mixed with cells. Thus, kindly choosing the type and concentration of acidic solution is necessary for dECM bio-ink preparation. And a balance should be made between the digestion period, strength of acidic solution, as well as the size and concentration of the dECM powders. STATEMENT OF SIGNIFICANCE: The dECM bio-ink has been widely used in 3D cell printing for tissue engineering and organ modelling. In this study, we found that different types of acid have different digestion and dissolution status for the dECM materials. A much softer tendon derived dECM hydrogel with lower stiffness could facilitate the cellular spreading, proliferation and tendon differentiation. We also demonstrated that the osmotic pressure should be taken care of in the preparation of dECM bio-ink with 0.5 M acetic acid. Thus, kindly choosing the type and concentration of acidic solution is necessary for dECM bio-ink preparation.

Engineering the Cellular Microenvironment of Post-infarct Myocardium on a Chip

Myocardial infarctions are one of the most common forms of cardiac injury and death worldwide. Infarctions cause immediate necrosis in a localized region of the myocardium, which is followed by a repair process with inflammatory, proliferative, and maturation phases. This repair process culminates in the formation of scar tissue, which often leads to heart failure in the months or years after the initial injury. In each reparative phase, the infarct microenvironment is characterized by distinct biochemical, physical, and mechanical features, such as inflammatory cytokine production, localized hypoxia, and tissue stiffening, which likely each contribute to physiological and pathological tissue remodeling by mechanisms that are incompletely understood. Traditionally, simplified two-dimensional cell culture systems or animal models have been implemented to elucidate basic pathophysiological mechanisms or predict drug responses following myocardial infarction. However, these conventional approaches offer limited spatiotemporal control over relevant features of the post-infarct cellular microenvironment. To address these gaps, Organ on a Chip models of post-infarct myocardium have recently emerged as new paradigms for dissecting the highly complex, heterogeneous, and dynamic post-infarct microenvironment. In this review, we describe recent Organ on a Chip models of post-infarct myocardium, including their limitations and future opportunities in disease modeling and drug screening.

Caveolin-1 mediates soft scaffold-enhanced adipogenesis of human mesenchymal stem cells

BACKGROUND

Human bone marrow-derived mesenchymal stem cells (hBMSCs) can differentiate into adipocytes upon stimulation and are considered an appropriate cell source for adipose tissue engineering. In addition to biochemical cues, the stiffness of a substrate that cells attach to has also been shown to affect hBMSC differentiation potential. Of note, most current studies are conducted on monolayer cultures which do not directly inform adipose tissue engineering, where 3-dimensional (3D) scaffolds are often used to create proper tissue architecture. In this study, we aim to examine the adipogenic differentiation of hBMSCs within soft or stiff scaffolds and investigate the molecular mechanism mediating the response of hBMSCs to substrate stiffness in 3D culture, specifically the involvement of the integral membrane protein, caveolin-1 (CAV1), known to regulate signaling in MSCs via compartmentalizing and concentrating signaling molecules.

METHODS

By adjusting the photo-illumination time, photocrosslinkable gelatin scaffolds with the same polymer concentration but different stiffnesses were created. hBMSCs were seeded within soft and stiff scaffolds, and their response to adipogenic induction under different substrate mechanical conditions was characterized. The functional involvement of CAV1 was assessed by suppressing its expression level using CAV1-specific siRNA.

RESULTS

The soft and stiff scaffolds used in this study had a compressive modulus of ~0.5 kPa and ~23.5 kPa, respectively. hBMSCs showed high viability in both scaffold types, but only spread out in the soft scaffolds. hBMSCs cultured in soft scaffolds displayed significantly higher adipogenesis, as revealed by histology, qRT-PCR, and immunostaining. Interestingly, a lower CAV1 level was observed in hBMSCs in the soft scaffolds, concomitantly accompanied by increased levels of Yes-associated protein (YAP) and decreased YAP phosphorylation, when compared to cells seeded in the stiff scaffolds. Interestingly, reducing CAV1 expression with siRNA was shown to further enhance hBMSC adipogenesis, which may function through activation of the YAP signaling pathway.

CONCLUSIONS

Soft biomaterials support superior adipogenesis of encapsulated hBMSCs in 3D culture, which is partially mediated by the CAV1-YAP axis. Suppressing CAV1 expression levels represents a robust method in the promotion of hBMSC adipogenesis.

Advances in Extracellular Matrix-Mimetic Hydrogels to Guide Stem Cell Fate

In the fields of regenerative medicine and tissue engineering, stem cells offer vast potential for treating or replacing diseased and damaged tissue. Much progress has been made in understanding stem cell biology, yielding protocols for directing stem cell differentiation toward the cell type of interest for a specific application. One particularly interesting and powerful signaling cue is the extracellular matrix (ECM) surrounding stem cells, a network of biopolymers that, along with cells, makes up what we define as a tissue. The composition, structure, biochemical features, and mechanical properties of the ECM are varied in different tissues and developmental stages, and serve to instruct stem cells toward a specific lineage. By understanding and recapitulating some of these ECM signaling cues through engineered ECM-mimicking hydrogels, stem cell fate can be directed in vitro. In this review, we will summarize recent advances in material systems to guide stem cell fate, highlighting innovative methods to capture ECM functionalities and how these material systems can be used to provide basic insight into stem cell biology or make progress toward therapeutic objectives.

Speckle-dependent accuracy in phase-sensitive optical coherence tomography

Phase-sensitive optical coherence tomography (OCT) is used to measure motion in a range of techniques, such as Doppler OCT and optical coherence elastography (OCE). In phase-sensitive OCT, motion is typically estimated using a model of the OCT signal derived from a single reflector. However, this approach is not representative of turbid samples, such as tissue, which exhibit speckle. In this study, for the first time, we demonstrate, through theory and experiment that speckle significantly lowers the accuracy of phase-sensitive OCT in a manner not accounted for by the OCT signal-to-noise ratio (SNR). We describe how the inaccuracy in speckle reduces phase difference sensitivity and introduce a new metric, speckle brightness, to quantify the amount of constructive interference at a given location in an OCT image. Experimental measurements show an almost three-fold degradation in sensitivity between regions of high and low speckle brightness at a constant OCT SNR. Finally, we apply these new results in compression OCE to demonstrate a ten-fold improvement in strain sensitivity, and a five-fold improvement in contrast-to-noise by incorporating independent speckle realizations. Our results show that speckle introduces a limit to the accuracy of phase-sensitive OCT and that speckle brightness should be considered to avoid erroneous interpretation of experimental data.

Analysis of sensitivity in quantitative micro-elastography

Quantitative micro-elastography (QME), a variant of compression optical coherence elastography (OCE), is a technique to image tissue elasticity on the microscale. QME has been proposed for a range of applications, most notably tumor margin assessment in breast-conserving surgery. However, QME sensitivity, a key imaging metric, has yet to be systematically analyzed. Consequently, it is difficult to optimize imaging performance and to assess the potential of QME in new application areas. To address this, we present a framework for analyzing sensitivity that incorporates the three main steps in QME image formation: mechanical deformation, its detection using optical coherence tomography (OCT), and signal processing used to estimate elasticity. Firstly, we present an analytical model of QME sensitivity, validated by experimental data, and demonstrate that sub-kPa elasticity sensitivity can be achieved in QME. Using silicone phantoms, we demonstrate that sensitivity is dependent on friction, OCT focus depth, and averaging methods in signal processing. For the first time, we show that whilst lubrication of layer improves accuracy by reducing surface friction, it reduces sensitivity due to the time-dependent effect of lubricant exudation from the layer boundaries resulting in increased friction. Furthermore, we demonstrate how signal processing in QME provides a trade-off between sensitivity and resolution that can be used to optimize imaging performance. We believe that our framework to analyze sensitivity can help to sustain the development of QME and, also, that it can be readily adapted to other OCE techniques.