Topographic confinement of epithelial clusters induces epithelial-to-mesenchymal transition in compliant matrices

Microchip construction for migration assays: investigating the impact of physical confinement on cell morphology and motility during vaccinia virus infection

Vaccinia virus (VACV)-induced cell migration is thought to be closely related to the rapid transmission of viral infection in the body. The limited studies are mainly based on scratch assay using traditional cell culture techniques, which inevitably ignores the influences of extracellular microenvironment. Physical confinement, inherently presenting in vivo, has proven to be a critical extern cue in modulating migration behaviors of multiple cells, while its impacts on VACV-induced cell motility remain unclear. Herein, we developed a migration assay microchip featuring confined microchannel array to investigate the effect of physical confinement on infected cell morphology and motility during VACV infection. Results showed that different from the random cell migration observed in traditional scratch assay on planar substrate, VACV-infected cells exhibited accelerated directionally persistent migration under confinement microenvironment. Moreover, single-directed elongated dominant lamella appeared to contrast distinctly with multiple protrusions stretched in random directions under unconfined condition. Additionally, the Golgi complex tended to relocate behind the nucleus confined within the microchannel axis compared to the classical reorientation pattern. These differences in characteristic subcellular architecture and organelle reorientation of migrating cells revealed cell biological mechanisms underlying altered migration behavior. Collectively, our study demonstrates that physical confinement acting as a guidance cue has profound impacts on VACV-induced migration behaviors, which provides new insight into cell migration behavior and viral rapid spread during VACV infection.

Matrix obstructions cause multiscale disruption in collective epithelial migration by suppressing leader cell function

During disease and development, physical changes in extracellular matrix cause jamming, unjamming, and scattering in epithelial migration. However, whether disruptions in matrix topology alter collective cell migration speed and cell-cell coordination remains unclear. We microfabricated substrates with stumps of defined geometry, density, and orientation, which create obstructions for migrating epithelial cells. Here, we show that cells lose their speed and directionality when moving through densely spaced obstructions. Although leader cells are stiffer than follower cells on flat substrates, dense obstructions cause overall cell softening. Through a lattice-based model, we identify cellular protrusions, cell-cell adhesions, and leader-follower communication as key mechanisms for obstruction-sensitive collective cell migration. Our modeling predictions and experimental validations show that cells' obstruction sensitivity requires an optimal balance of cell-cell adhesions and protrusions. Both MDCK (more cohesive) and α-catenin-depleted MCF10A cells were less obstruction sensitive than wild-type MCF10A cells. Together, microscale softening, mesoscale disorder, and macroscale multicellular communication enable epithelial cell populations to sense topological obstructions encountered in challenging environments. Thus, obstruction-sensitivity could define "mechanotype" of cells that collectively migrate yet maintain intercellular communication.

Epithelial multicellular clustering enabled by polarized macrophages on soft matrices

Formation of epithelial structures of variegated geometries and sizes is essential for organogenesis, tumor growth, and wound repair. Although epithelial cells are predisposed with potential for multicellular clustering, it remains unclear whether immune cells and mechanical cues from their microenvironment influence this process. To explore this possibility, we cocultured human mammary epithelial cells with prepolarized macrophages on soft or stiff hydrogels. In the presence of M1 (proinflammatory) macrophages on soft matrices, epithelial cells migrated faster and subsequently formed larger multicellular clusters compared to cocultures with M0 (unpolarized) or M2 (anti-inflammatory) macrophages. By contrast, stiff matrices disabled active clustering of epithelial cells due to their enhanced migration and cell-ECM adhesion, regardless of macrophage polarization. We found that the copresence of soft matrices and M1 macrophages reduced focal adhesions, but enhanced fibronectin deposition and nonmuscle myosin-IIA expression, which altogether optimize conditions for epithelial clustering. Upon ROCK inhibition, epithelial clustering was abrogated, indicating a requirement for optimized cellular forces. In these cocultures, TNF-α secretion was the highest with M1 macrophages and TGF-β secretion was exclusively detectable in case of M2 macrophages on soft gels, which indicated potential role of macrophage secreted factors in the observed epithelial clustering. Indeed, exogenous addition of TGF-β promoted epithelial clustering with M1 coculture on soft gels. According to our findings, optimization of both mechanical and immune factors can tune epithelial clustering responses, which could have implications in tumor growth, fibrosis, and would healing.

Reciprocal intra- and extra-cellular polarity enables deep mechanosensing through layered matrices

Adherent cells migrate on layered tissue interfaces to drive morphogenesis, wound healing, and tumor invasion. Although stiffer surfaces are known to enhance cell migration, it remains unclear whether cells sense basal stiff environments buried under softer, fibrous matrix. Using layered collagen-polyacrylamide gel systems, we unveil a migration phenotype driven by cell-matrix polarity. Here, cancer (but not normal) cells with stiff base matrix generate stable protrusions, faster migration, and greater collagen deformation because of "depth mechanosensing" through the top collagen layer. Cancer cell protrusions with front-rear polarity produce polarized collagen stiffening and deformations. Disruption of either extracellular or intracellular polarity via collagen crosslinking, laser ablation, or Arp2/3 inhibition independently abrogates depth-mechanosensitive migration of cancer cells. Our experimental findings, validated by lattice-based energy minimization modeling, present a cell migration mechanism whereby polarized cellular protrusions and contractility are reciprocated by mechanical extracellular polarity, culminating in a cell-type-dependent ability to mechanosense through matrix layers.

Transitions in density, pressure, and effective temperature drive collective cell migration into confining environments

Epithelial cell collectives migrate through tissue interfaces and crevices to orchestrate processes of development, tumor invasion, and wound healing. Naturally, traversal of cell collective through confining environments involves crowding due to the narrowing space, which seems tenuous given the conventional inverse relationship between cell density and migration. However, physical transitions required to overcome such epithelial densification for migration across confinements remain unclear. Here, in contiguous microchannels, we show that epithelial (MCF10A) monolayers accumulate higher cell density before entering narrower channels; however, overexpression of breast cancer oncogene +ErbB2 reduced this need for density accumulation across confinement. While wildtype MCF10A cells migrated faster in narrow channels, this confinement sensitivity reduced after +ErbB2 mutation or with constitutively-active RhoA. The migrating collective developed pressure differentials upon encountering microchannels, like fluid flow into narrowing spaces, and this pressure dropped with their continued migration. These transitions of pressure and density altered cell shapes and increased effective temperature, estimated by treating cells as granular thermodynamic system. While +RhoA cells and those in confined regions were effectively warmer, cancer-like +ErbB2 cells remained cooler. Epithelial reinforcement by metformin treatment increased density and temperature differentials across confinement, indicating that higher cell cohesion could reduce unjamming. Our results provide experimental evidence for previously proposed theories of inverse relationship between density and motility-related effective temperature. Indeed, we show across cell lines that confinement increases pressure and effective temperature, which enable migration by reducing density. This physical interpretation of collective cell migration as granular matter could advance our understanding of complex living systems.

Epithelial multicellular clustering enabled by polarized macrophages on soft matrices

Formation of epithelial structures of variegated geometries and sizes is essential for organogenesis, tumor growth, and wound repair. Although epithelial cells are predisposed with potential for multicellular clustering, it remains unclear whether immune cells and mechanical cues from their microenvironment influence this process. To explore this possibility, we co-cultured human mammary epithelial cells with pre-polarized macrophages on soft or stiff hydrogels. In the presence of M1 (proinflammatory) macrophages on soft matrices, epithelial cells migrated faster and subsequently formed larger multicellular clusters, compared to co-cultures with M0 (unpolarized) or M2 (anti-inflammatory) macrophages. By contrast, stiff extracellular matrix (ECM) disabled active clustering of epithelial cells due to their enhanced migration and cell-ECM adhesion, regardless of macrophage polarization. We found that the co-presence of soft matrices and M1 macrophages reduced focal adhesions, but enhanced fibronectin deposition and non-muscle myosin-IIA expression, which altogether optimize conditions for epithelial clustering. Upon Rho-associated kinase (ROCK) inhibition, epithelial clustering was abrogated, indicating a requirement for optimized cellular forces. In these co-cultures, Tumor Necrosis Factor (TNF)-α secretion was the highest with M1 macrophages and Transforming growth factor (TGF)-β secretion was exclusively detectable in case of M2 macrophages on soft gels, which indicated potential role of macrophage secreted factors in the observed epithelial clustering. Indeed, exogenous addition of TGB-β promoted epithelial clustering with M1 co-culture on soft gels. According to our findings, optimization of both mechanical and immune factors can tune epithelial clustering responses, which could have implications in tumor growth, fibrosis, and would healing.

Summary

Authors show proinflammatory macrophages on soft matrices enable epithelial cells to form multicellular clusters. This phenomenon is disabled on stiff matrices due to increased stability of focal adhesions. Inflammatory cytokine secretion is macrophage-dependent, and external addition of cytokines accentuates epithelial clustering on soft matrices.

Impact Statement

Formation of multicellular epithelial structures is critical to tissue homeostasis. However, it has not been shown how the immune system and mechanical environment affect these structures. The present work illustrates how macrophage type affects epithelial clustering in soft and stiff matrix environments.

Nucleoli in epithelial cell collectives respond to tumorigenic, spatial, and mechanical cues

Cancer cells are known to have larger nucleoli, consistent with their higher transcriptional and translational demands. Meanwhile, on stiff extracellular matrix, normal epithelial cells can exhibit genomic and proteomic mechanoactivation toward tumorigenic transformations, such as epithelial-mesenchymal transition and enhanced migration. However, while nucleolar bodies regulate the protein synthesis required for mechanosensation, it remains unknown whether mechanical and spatial extracellular cues can in turn alter nucleoli. Here, we culture mammary epithelial cell sheets on matrices of varying stiffness and show that cancer cells have more nucleoli, with nucleoli occupying larger areas compared with normal cells. By contrast, within normal epithelial sheets, stiffer matrices and leader positioning of cells induce larger nucleolar areas and more nucleolar bodies over time. The observed leader-follower nucleolar differences stem from distinct rates of cell cycle progression. In the nucleoplasm, leader cells on stiffer matrices exhibit higher heterochromatin marker expression and DNA compaction around nucleolar bodies. Overall, our findings advance the emerging framework of cellular mechanobiology in which mechanical cues from the extracellular matrix transmit into the nucleoplasm to alter nucleolar composition, potentially resulting in mechanosensitive ribosomal biogenesis. Ultimately, this proposed mechanosensitivity of nucleoli and associated protein synthesis could have wide implications in disease, development, and regeneration.

Microfluidic Lab-on-a-Chip for Studies of Cell Migration under Spatial Confinement

Understanding cell migration is a key step in unraveling many physiological phenomena and predicting several pathologies, such as cancer metastasis. In particular, confinement has been proven to be a key factor in the cellular migration strategy choice. As our insight in the field improves, new tools are needed in order to empower biologists’ analysis capabilities. In this framework, microfluidic devices have been used to engineer the mechanical and spatial stimuli and to investigate cellular migration response in a more controlled way. In this work, we will review the existing technologies employed in the realization of microfluidic cellular migration assays, namely the soft lithography of PDMS and hydrogels and femtosecond laser micromachining. We will give an overview of the state of the art of these devices, focusing on the different geometrical configurations that have been exploited to study specific aspects of cellular migration. Our scope is to highlight the advantages and possibilities given by each approach and to envisage the future developments in in vitro migration studies under spatial confinement in microfluidic devices.

Biophysics Role and Biomimetic Culture Systems of ECM Stiffness in Cancer EMT

Oncological diseases have become the second leading cause of death from noncommunicable diseases worldwide and a major threat to human health. With the continuous progress in cancer research, the mechanical cues from the tumor microenvironment environment (TME) have been found to play an irreplaceable role in the progression of many cancers. As the main extracellular mechanical signal carrier, extracellular matrix (ECM) stiffness may influence cancer progression through biomechanical transduction to modify downstream gene expression, promote epithelial-mesenchymal transition (EMT), and regulate the stemness of cancer cells. EMT is an important mechanism that induces cancer cell metastasis and is closely influenced by ECM stiffness, either independently or in conjunction with other molecules. In this review, the unique role of ECM stiffness in EMT in different kinds of cancers is first summarized. By continually examining the significance of ECM stiffness in cancer progression, a biomimetic culture system based on 3D manufacturing and novel material technologies is developed to mimic ECM stiffness. The authors then look back on the novel development of the ECM stiffness biomimetic culture systems and finally provide new insights into ECM stiffness in cancer progression which can broaden the fields' horizons with a view toward developing new cancer diagnosis methods and therapies.

Current hydrogel advances in physicochemical and biological response-driven biomedical application diversity

Hydrogel is a type of versatile platform with various biomedical applications after rational structure and functional design that leverages on material engineering to modulate its physicochemical properties (e.g., stiffness, pore size, viscoelasticity, microarchitecture, degradability, ligand presentation, stimulus-responsive properties, etc.) and influence cell signaling cascades and fate. In the past few decades, a plethora of pioneering studies have been implemented to explore the cell-hydrogel matrix interactions and figure out the underlying mechanisms, paving the way to the lab-to-clinic translation of hydrogel-based therapies. In this review, we first introduced the physicochemical properties of hydrogels and their fabrication approaches concisely. Subsequently, the comprehensive description and deep discussion were elucidated, wherein the influences of different hydrogels properties on cell behaviors and cellular signaling events were highlighted. These behaviors or events included integrin clustering, focal adhesion (FA) complex accumulation and activation, cytoskeleton rearrangement, protein cyto-nuclei shuttling and activation (e.g., Yes-associated protein (YAP), catenin, etc.), cellular compartment reorganization, gene expression, and further cell biology modulation (e.g., spreading, migration, proliferation, lineage commitment, etc.). Based on them, current in vitro and in vivo hydrogel applications that mainly covered diseases models, various cell delivery protocols for tissue regeneration and disease therapy, smart drug carrier, bioimaging, biosensor, and conductive wearable/implantable biodevices, etc. were further summarized and discussed. More significantly, the clinical translation potential and trials of hydrogels were presented, accompanied with which the remaining challenges and future perspectives in this field were emphasized. Collectively, the comprehensive and deep insights in this review will shed light on the design principles of new biomedical hydrogels to understand and modulate cellular processes, which are available for providing significant indications for future hydrogel design and serving for a broad range of biomedical applications.

The effects of surface topography modification on hydrogel properties

Hydrogel has been an attractive biomaterial for tissue engineering, drug delivery, wound healing, and contact lens materials, due to its outstanding properties, including high water content, transparency, biocompatibility, tissue mechanical matching, and low toxicity. As hydrogel commonly possesses high surface hydrophilicity, chemical modifications have been applied to achieve the optimal surface properties to improve the performance of hydrogels for specific applications. Ideally, the effects of surface modifications would be stable, and the modification would not affect the inherent hydrogel properties. In recent years, a new type of surface modification has been discovered to be able to alter hydrogel properties by physically patterning the hydrogel surfaces with topographies. Such physical patterning methods can also affect hydrogel surface chemical properties, such as protein adsorption, microbial adhesion, and cell response. This review will first summarize the works on developing hydrogel surface patterning methods. The influence of surface topography on interfacial energy and the subsequent effects on protein adsorption, microbial, and cell interactions with patterned hydrogel, with specific examples in biomedical applications, will be discussed. Finally, current problems and future challenges on topographical modification of hydrogels will also be discussed.

3D In Vitro Models for Investigating the Role of Stiffness in Cancer Invasion

BACKGROUND

Tumorigenesis is attributed to the interactions of cancer cells with the tumor microenvironment through both biochemical cues and physical stimuli. Increased matrix deposition and realignment of the collagen fibers are detected by cancer cells, inducing epithelial-to-mesenchymal transition, which in turn stimulates cell motility and invasiveness.

METHODS

This review provides an overview of current research on the role of the physical microenvironment in cancer invasion. This was achieved by using a systematic approach and providing meta-analyses. Particular focus was placed on in vitro three-dimensional models of epithelial cancers. We investigated questions such as the effect of matrix stiffening, activation of stromal cells, and identified potential advances in mechano-based therapies.

RESULTS

Meta-analysis revealed that 64% of studies report cancer invasion promotion as stiffness increases, while 36% report the opposite. Experimental approaches and data interpretations were varied, each affecting the invasion of cancer differently. Examples are the experimental timeframes used (24 h to 21 days), the type of polymer used (24 types), and choice of cell line (33 cell lines). The stiffness of the 3D matrices varied from 0.5 to 300 kPa and 19% of these matrices' stiffness were outside commonly accepted physiological range. 100% of the studies outside biological stiffness range (above 20 kPa) report that stiffness does not promote cancer invasion.

CONCLUSIONS

Taking this analysis into account, we inform on the type of experimental approaches that could be the most relevant and provide what would be a standardized protocol and reporting strategy.

Nanoscale Topographies for Corneal Endothelial Regeneration

The corneal endothelium is the innermost layer of the cornea that selectively pumps ions and metabolites and regulates the hydration level of the cornea, ensuring its transparency. Trauma or disease affecting human corneal endothelial cells (hCECs) can result in major imbalances of such transport activity with consequent deterioration or loss of vision. Since tissue transplantation from deceased donors is only available to a fraction of patients worldwide, alternative solutions are urgently needed. Cell therapy approaches, in particular by attempting to expand primary culture of hCECs in vitro, aim to tackle this issue. However, existing cell culture protocols result in limited expansion of this cell type. Recent studies in this field have shown that topographical features with specific dimensions and shapes could improve the efficacy of hCEC expansion. Therefore, potential solutions to overcome the limitation of the conventional culture of hCECs may include recreating nanometer scale topographies (nanotopographies) that mimic essential biophysical cues present in their native environment. In this review, we summarize the current knowledge and understanding of the effect of substrate topographies on the response of hCECs. Moreover, we also review the latest developments for the nanofabrication of such bio-instructive cell substrates.

Epithelial-Mesenchymal Transition (EMT) as a Therapeutic Target

Metastasis is the spread of cancer cells from the primary tumour to distant sites and organs throughout the body. It is the primary cause of cancer morbidity and mortality, and is estimated to account for 90% of cancer-related deaths. During the initial steps of the metastatic cascade, epithelial cancer cells undergo an epithelial-mesenchymal transition (EMT), and as a result become migratory and invasive mesenchymal-like cells while acquiring cancer stem cell properties and therapy resistance. As EMT is involved in such a broad range of processes associated with malignant transformation, it has become an increasingly interesting target for the development of novel therapeutic strategies. Anti-EMT therapeutic strategies could potentially not only prevent the invasion and dissemination of cancer cells, and as such prevent the formation of metastatic lesions, but also attenuate cancer stemness and increase the effectiveness of more classical chemotherapeutics. In this review, we give an overview about the pros and cons of therapies targeting EMT and discuss some already existing candidate drug targets and high-throughput screening tools to identify novel anti-EMT compounds.

Kelly M, M. Buchowski J, P. Zebala L, Luhmann S, C. Gupta M, A. Setton L. Verteporfin treatment controls morphology, phenotype, and global gene expression for cells of the human nucleus pulposus

Cells of the nucleus pulposus (NP) are essential contributors to extracellular matrix synthesis and function of the intervertebral disc. With age and degeneration, the NP becomes stiffer and more dehydrated, which is associated with a loss of phenotype and biosynthetic function for its resident NP cells. Also, with aging, the NP cell undergoes substantial morphological changes from a rounded shape with pronounced vacuoles in the neonate and juvenile, to one that is more flattened and spread with a loss of vacuoles. Here, we make use of the clinically relevant pharmacological treatment verteporfin (VP), previously identified as a disruptor of yes-associated protein-TEA domain family member-binding domain (TEAD) signaling, to promote morphological changes in adult human NP cells in order to study variations in gene expression related to differences in cell shape. Treatment of adult, degenerative human NP cells with VP caused a shift in morphology from a spread, fibroblastic-like shape to a rounded, clustered morphology with decreased transcriptional activity of TEAD and serum-response factor. These changes were accompanied by an increased expression of vacuoles, NP-specific gene markers, and biosynthetic activity. The contemporaneous observation of VP-induced changes in cell shape and prominent, time-dependent changes within the transcriptome of NP cells occurred over all timepoints in culture. Enriched gene sets with the transition to VP-induced cell rounding suggest a major role for cell adhesion, cytoskeletal remodeling, vacuolar lumen, and MAPK activity in the NP phenotypic and functional response to changes in cell shape.

Mechanosensing of Mechanical Confinement by Mesenchymal-Like Cells

Mesenchymal stem cells (MSCs) and tumor cells have the unique capability to migrate out of their native environment and either home or metastasize, respectively, through extremely heterogeneous environments to a distant location. Once there, they can either aid in tissue regrowth or impart an immunomodulatory effect in the case of MSCs, or form secondary tumors in the case of tumor cells. During these journeys, cells experience physically confining forces that impinge on the cell body and the nucleus, ultimately causing a multitude of cellular changes. Most drastically, confining individual MSCs within hydrogels or confining monolayers of MSCs within agarose wells can sway MSC lineage commitment, while applying a confining compressive stress to metastatic tumor cells can increase their invasiveness. In this review, we seek to understand the signaling cascades that occur as cells sense confining forces and how that translates to behavioral changes, including elongated and multinucleated cell morphologies, novel migrational mechanisms, and altered gene expression, leading to a unique MSC secretome that could hold great promise for anti-inflammatory treatments. Through comparison of these altered behaviors, we aim to discern how MSCs alter their lineage selection, while tumor cells may become more aggressive and invasive. Synthesizing this information can be useful for employing MSCs for therapeutic approaches through systemic injections or tissue engineered grafts, and developing improved strategies for metastatic cancer therapies.

Microfabrication of poly(acrylamide) hydrogels with independently controlled topography and stiffness

The stiffness and topography of a cell's extracellular matrix (ECM) are physical cues that play a key role in regulating processes that determine cellular fate and function. While substrate stiffness can dictate cell differentiation lineage, migration, and self-organization, topographical features can change the cell's differentiation profile or migration ability. Although both physical cues are present and intrinsic to the native tissues in vivo, in vitro studies have been hampered by the lack of technological set-ups that would be compatible with cell culture and characterization. In vitro studies therefore either focused on screening stiffness effects in cells cultured on flat substrates or on determining topography effects in cells cultured onto hard materials. Here, we present a reliable, microfabrication method to obtain well defined topographical structures of micrometer size (5-10 μm) on soft polyacrylamide hydrogels with tunable mechanical stiffness (3-145 kPa) that closely mimic the in vivo situation. Topographically microstructured polyacrylamide hydrogels are polymerized by capillary force lithography using flexible materials as molds. The topographical microstructures are resistant to swelling, can be conformally functionalized by ECM proteins and sustain the growth of cell lines (fibroblasts and myoblasts) and primary cells (mouse intestinal epithelial cells). Our method can independently control stiffness and topography, which allows to individually assess the contribution of each physical cue to cell response or to explore potential synergistic effects. We anticipate that our fabrication method will be of great utility in tissue engineering and biophysics, especially for applications where the use of complex in vivo-like environments is of paramount importance.

To lead or to herd: optimal strategies for 3D collective migration of cell clusters

Cells migrating in clusters play a significant role in a number of biological processes such as embryogenesis, wound healing, and tumor metastasis during cancer progression. A variety of environmental and biochemical factors can influence the collective migration of cells with differing degrees of cell autonomy and inter-cellular coupling strength. For example, weakly coupled cells can move collectively under the influence of contact guidance from neighboring cells or the environment. Alternatively strongly coupled cells might follow one or more leader cells to move as a single cohesive unit. Additionally, chemical and mechanical signaling between these cells may alter the degree of coupling and determine effective cluster sizes. Being able to understand this collective cell migration process is critical in the prediction and manipulation of outcomes of key biological processes. Here we focus on understanding how various environmental and cellular factors influence small clusters of cells migrating collectively within a 3D fibrous matrix. We combine existing knowledge of single-cell migration in 2D and 3D environments, prior experimental observations of cell-cell interactions and collective migration, and a newly developed stochastic model of cell migration in 3D matrices, to simulate the migration of cell clusters in different physiologically relevant environments. Our results show that based on the extracellular environment and the strength of cell-cell mechanical coupling, two distinct optimal approaches to driving collective cell migration emerge. The ability to effectively employ these two distinct migration strategies might be critical for cells to collectively migrate through the heterogeneous tissue environments within the body.

Stereolithography 3D Bioprinting

Stereolithography (SLA) 3D bioprinting has emerged as a prominent bioprinting method addressing the requirements of complex tissue fabrication. This chapter addresses the advancement in SLA 3D bioprinting in concurrent with the development of novel photocrosslinkable biomaterials with enhanced physical and chemical properties. We discuss the cytocompatible photoinitiators operating in the wide spectrum of the ultraviolet (UV) and the visible light and high-resolution dynamic mask projection systems with a suitable illumination source. The potential of SLA 3D bioprinting has been explored in various themes, like bone and neural tissue engineering and in the development of controlled microenvironments to study cell behavior. The flexible design and versatility of SLA bioprinting makes it an attractive bioprinting process with myriad possibilities and clinical applications.

Longer collagen fibers trigger multicellular streaming on soft substrates via enhanced forces and cell-cell cooperation

Grouped cells often leave large cell colonies in the form of narrow multicellular streams. However, it remains unknown how collective cell streaming exploits specific matrix properties, like stiffness and fiber length. It is also unclear how cellular forces, cell-cell adhesion and velocities are coordinated within streams. To independently tune stiffness and collagen fiber length, we developed new hydrogels and discovered invasion-like streaming of normal epithelial cells on soft substrates coated with long collagen fibers. Here, streams arise owing to a surge in cell velocities, forces, YAP activity and expression of mesenchymal marker proteins in regions of high-stress anisotropy. Coordinated velocities and symmetric distribution of tensile and compressive stresses support persistent stream growth. Stiff matrices diminish cell-cell adhesions, disrupt front-rear velocity coordination and do not promote sustained fiber-dependent streaming. Rac inhibition reduces cell elongation and cell-cell cooperation, resulting in a complete loss of streaming in all matrix conditions. Our results reveal a stiffness-modulated effect of collagen fiber length on collective cell streaming and unveil a biophysical mechanism of streaming governed by a delicate balance of enhanced forces, monolayer cohesion and cell-cell cooperation.This article has an associated First Person interview with the first authors of the paper.

Biomechanical regulation of EMT and epithelial morphogenesis in amniote epiblast

Epiblast is composed of pluripotent cells which will give rise to all cell lineages in a human body. It forms a single-cell layered epithelium conserved among all amniotic vertebrates (birds, reptiles and mammals) and undergoes complex morphogenesis both before and during gastrulation. Our knowledge of the amniote epiblast is based on data acquired through cellular and molecular analyses of early chick and mouse embryos in vivo and mammalian pluripotent stem cells (PSCs) in vitro. Very few studies have been published on biomechanical characteristics of the amniote epiblast, largely due to lack of experimental tools for measuring and perturbing biomechanical properties. Also missing is a conceptual framework that can integrate both biomechanical and molecular parameters of the epiblast. This review is aimed at providing a background based on which epiblast morphogenesis, including its transition between the epithelial and mesenchymal states, can be understood from a biomechanical perspective. This simple developmental biology system is suitable for testing a multitude of theoretical models in biomechanics, leading to a better understanding of biomechanical logics and constraints governing multicellular organization.

Physical defects in basement membrane-mimicking collagen-IV matrices trigger cellular EMT and invasion

In fibrosis and cancer, degradation of basement membrane (BM) and cell invasion are considered as key outcomes of a cellular transformation called epithelial-mesenchymal transition (EMT). Here, we pose a converse question - can preexisting physical defects in the BM matrix cause EMT in normal epithelial cells? On a BM-mimicking matrix of collagen-IV-coated polyacrylamide (PA) gel, we have discovered a reverse phenomenon in which preexisting defects trigger EMT in normal epithelial cells. Through spatiotemporal measurements and simulations in silico, we demonstrate that the EMT precedes cellular mechanoactivation on defective matrices, but they occur concurrently on stiff matrices. The defect-dependent EMT caused cell invasion though a stroma-mimicking collagen-I layer, which could be disabled through MMP9 inhibition. Our findings reveal that the known BM degradation caused by cellular EMT and invasion is not a one-way process. Instead, normal epithelial cells can exploit physical defects in the BM matrix to undergo disease-like cellular transformations.

Controlled dissolution of freeform 3D printed carbohydrate glass scaffolds in hydrogels using a hydrophobic spray coating

Freeform 3D printing combined with sacrificial molding promises to lead advances in production of highly complex tubular systems for biomedical applications. Here we leverage a purpose-built isomalt 3D printer to generate complex channel geometries in hydrogels which would be inaccessible with other techniques. To control the dissolution of the scaffold, we propose an enabling technology consisting of an automated nebulizer coating system which applies octadecane to isomalt scaffolds. Octadecane, a saturated hydrocarbon, protects the rigid mold from dissolution and provides ample time for gels to set around the sacrificial structure. With a simplified model of the nebulizer system, the robotic motion was optimized for uniform coating. Using a combination of stimulated Raman scattering (SRS) microscopy and X-ray computed tomography, the coating was characterized to assess surface roughness and consistency. Colorimetric measurements of dissolution rates allowed optimization of sprayer parameters, yielding a decrease in dissolution rates by at least 4 orders of magnitude. High fidelity channels are ensured by surfactant treatment of the coating, which prevents bubbles from clinging to the surface. Spontaneous Raman scattering microspectroscopy and white light microscopy indicate cleared channels are free of octadecane following gentle flushing. The capabilities of the workflow are highlighted with several complex channel architectures including helices, blind channels, and multiple independent channels within polyacrylamide hydrogels of varying stiffnesses.

Endothelial Cell Mechanotransduction in the Dynamic Vascular Environment

The vascular endothelial cells (ECs) that line the inner layer of blood vessels are responsible for maintaining vascular homeostasis under physiological conditions. In the presence of disease or injury, ECs can become dysfunctional and contribute to a progressive decline in vascular health. ECs are constantly exposed to a variety of dynamic mechanical stimuli, including hemodynamic shear stress, pulsatile stretch, and passive signaling cues derived from the extracellular matrix. This review describes the molecular mechanisms by which ECs perceive and interpret these mechanical signals. The translational applications of mechanosensing are then discussed in the context of endothelial-to-mesenchymal transition and engineering of vascular grafts.

Predicting Collective Migration of Cell Populations Defined by Varying Repolarization Dynamics

Collective migration of heterogeneous cell populations is an essential aspect of fundamental biological processes, including morphogenesis, wound healing, and tumor invasion. Through experiments and modeling, it has been shown that cells attain front-rear polarity, generate forces, and form adhesions to migrate. However, it remains unclear how the ability of individual cells in a population to dynamically repolarize themselves into new directions could regulate the collective response. We present a vertex-based model in which each deformable cell randomly chooses a new polarization direction after every defined time interval, elongates, proportionally generates forces, and causes collective migration. Our simulations predict that cell types that repolarize at longer time intervals attain more elongated shapes, migrate faster, deform the cell sheet, and roughen the leading edge. By imaging collectively migrating epithelial cell monolayers at high temporal resolution, we found longer repolarization intervals and elongated shapes of cells at the leading edge compared to those within the monolayer. Based on these experimental measurements and simulations, we defined aggressive mutant leader cells by long repolarization interval and minimal intercellular contact. The cells with frequent and random repolarization were defined as normal cells. In simulations with uniformly dispersed leader cells in a normal cell population at a 1:10 ratio, the resulting migration and deformation of the heterogeneous cell sheet remained low. However, when the 10% mutant leaders were placed only at the leading edge, we predicted a rise in the migration of an otherwise normal cell sheet. Our model predicts that a repolarization-based definition of leader cells and their placement within a healthy population can generate myriad modes of collective cell migration, which can enhance our understanding of collective cell migration in disease and development.

The TRPV4-TAZ mechanotransduction signaling axis in matrix stiffness- and TGFβ1-induced epithelial-mesenchymal transition

Introduction

The implantation of biomaterials into soft tissue leads to the development of foreign body response, a non-specific inflammatory condition that is characterized by the presence of fibrotic tissue. Epithelial-mesenchymal transition (EMT) is a key event in development, fibrosis, and oncogenesis. Emerging data support a role for both a mechanical signal and a biochemical signal in EMT. We hypothesized that transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive channel, is a mediator of EMT.

Methods

Normal human primary epidermal keratinocytes (NHEKs) were seeded on collagen-coated plastic plates or varied stiffness polyacrylamide gels in the presence or absence of TGFβ1, Immunofluorescence, immunoblot, and polymerase chain reaction analysis were performed to determine expression level of EMT markers and signaling proteins. Knock-down of TRPV4 function was achieved by siRNA transfection or by GSK2193874 treatment.

Results

We found that knock-down of TRPV4 blocked both matrix stiffness- and TGFβ1-induced EMT in NHEKs. In a murine skin fibrosis model, TRPV4 deletion resulted in decreased expression of the mesenchymal marker, α-SMA, and increased expression of epithelial marker, E-cadherin. Mechanistically, our data showed that: i) TRPV4 was essential for the nuclear translocation of TAZ in response to matrix stiffness and TGFβ1; ii) Antagonism of TRPV4 inhibited both matrix stiffness-induced and TGFβ1-induced expression of TAZ proteins; and iii) TRPV4 antagonism suppressed both matrix stiffness-induced and TGFβ1-induced activation of Smad2/3, but not of AKT.

Conclusions

These data identify a novel role for TRPV4-TAZ mechanotransduction signaling axis in regulating EMT in NHEKs in response to both matrix stiffness and TGFβ1.

TRPV4 regulates matrix stiffness and TGFβ1-induced epithelial-mesenchymal transition

Substrate stiffness (or rigidity) of the extracellular matrix has important functions in numerous pathophysiological processes including fibrosis. Emerging data support a role for both a mechanical signal, for example, matrix stiffness, and a biochemical signal, for example, transforming growth factor β1 (TGFβ1), in epithelial‐mesenchymal transition (EMT), a process critically involved in fibrosis. Here, we report evidence showing that transient receptor potential vanilloid 4 (TRPV4), a mechanosensitive channel, is the likely mediator of EMT in response to both TGFβ1 and matrix stiffness. Specifically, we found that: (a) genetic ablation or pharmacological inhibition of TRPV4 blocked matrix stiffness and TGFβ1‐induced EMT in normal mouse primary epidermal keratinocytes (NMEKs) as determined by changes in morphology, adhesion, migration and alterations of expression of EMT markers including E‐cadherin, N‐cadherin (NCAD) and α‐smooth muscle actin (α‐SMA), and (b) TRPV4 deficiency prevented matrix stiffness‐induced EMT in NMEKs over a pathophysiological range. Intriguingly, TRPV4 deletion in mice suppressed expression of mesenchymal markers, NCAD and α‐SMA, in a bleomycin‐induced murine skin fibrosis model. Mechanistically, we found that: (a) TRPV4 was essential for the nuclear translocation of YAP/TAZ (yes‐associated protein/transcriptional coactivator with PDZ‐binding motif) in response to matrix stiffness and TGFβ1, (b) TRPV4 deletion inhibited both matrix stiffness‐ and TGFβ1‐induced expression of YAP/TAZ proteins and (c) TRPV4 deletion abrogated both matrix stiffness‐ and TGFβ1‐induced activation of AKT, but not Smad2/3, suggesting a mechanism by which TRPV4 activity regulates EMT in NMEKs. Altogether, these data identify a novel role for TRPV4 in regulating EMT.

Desuppression of TGF-β signaling via nuclear c-Abl-mediated phosphorylation of TIF1γ/TRIM33 at Tyr-524, -610, and -1048

Protein-tyrosine kinases regulate a broad range of intracellular processes occurring primarily just beneath the plasma membrane. With the greatest care to prevent dephosphorylation, we have shown that nuclear tyrosine phosphorylation regulates global chromatin structural states. However, the roles for tyrosine phosphorylation in the nucleus are poorly understood. Here we identify transcriptional intermediary factor 1-γ (TIF1γ/TRIM33/Ectodermin), which suppresses transforming growth factor-β (TGF-β) signaling through the association with Smad2/3 transcription factor, as a new nuclear substrate of c-Abl tyrosine kinase. Replacement of the three tyrosine residues Tyr-524, -610, and -1048 with phenylalanine (3YF) inhibits c-Abl-mediated phosphorylation of TIF1γ and enhances TIF1γ's association with Smad3. Importantly, knockdown-rescue experiments show that 3YF strengthens TIF1γ's ability to suppress TGF-β signaling. Intriguingly, activation of c-Abl by epidermal growth factor (EGF) induces desuppression of TGF-β signaling via enhancing the tyrosine phosphorylation level of TIF1γ. TGF-β together with EGF synergistically provokes desuppressive responses of epithelial-to-mesenchymal transition through tyrosine phosphorylation of TIF1γ. These results suggest that nuclear c-Abl-mediated tyrosine phosphorylation of TIF1γ has a desuppressive role in TGF-β-Smad2/3 signaling.

The Bioelectric Code: Reprogramming Cancer and Aging From the Interface of Mechanical and Chemical Microenvironments

Cancer is a complex, heterogeneous group of diseases that can develop through many routes. Broad treatments such as chemotherapy destroy healthy cells in addition to cancerous ones, but more refined strategies that target specific pathways are usually only effective for a limited number of cancer types. This is largely due to the multitude of physiological variables that differ between cells and their surroundings. It is therefore important to understand how nature coordinates these variables into concerted regulation of growth at the tissue scale. The cellular microenvironment might then be manipulated to drive cells toward a desired outcome at the tissue level. One unexpected parameter, cellular membrane voltage (Vm), has been documented to exert control over cellular behavior both in culture and in vivo. Manipulating this fundamental cellular property influences a remarkable array of organism-wide patterning events, producing striking outcomes in both tumorigenesis as well as regeneration. These studies suggest that Vm is not only a key intrinsic cellular property, but also an integral part of the microenvironment that acts in both space and time to guide cellular behavior. As a result, there is considerable interest in manipulating Vm both to treat cancer as well as to regenerate organs damaged or deteriorated during aging. However, such manipulations have produced conflicting outcomes experimentally, which poses a substantial barrier to understanding the fundamentals of bioelectrical reprogramming. Here, we summarize these inconsistencies and discuss how the mechanical microenvironment may impact bioelectric regulation.

Three-Dimensional Hepatocellular Carcinoma/Fibroblast Model on a Nanofibrous Membrane Mimics Tumor Cell Phenotypic Changes and Anticancer Drug Resistance

Three-dimensional (3D) in vitro tissue or organ models can effectively mimic the complex microenvironment of many types of human tissues for medical applications. Unfortunately, development of 3D cancer models, which involve cancer/stromal cells in a 3D environment, has remained elusive due to the extreme complexity of the tumor microenvironment (TME) and the stepwise progression of human cancer. Here, we developed hepatocellular carcinoma (HCC) models, which consist of fibroblasts as stromal cells, HCC cells, and a nanofibrous membrane to mimic the complex TME. The 3D HCC models were fabricated using three distinct culture methods: cancer cells grown directly on the nanofibrous membrane (mono model), fibroblasts covering the nanofibrous membrane (layer model), and both cancer cells and fibroblasts grown on the nanofibrous membrane (mixed model). Interestingly, the mono model and layer model showed similar tissue structures, whereas the mixed model resulted in phenotypic changes to the cancer cells. Further analysis demonstrated that the mixed models promoted the expression of fibronectin and vimentin, and showed higher resistance to anticancer drugs compared with the other models. Thus, our 3D HCC model could be utilized for testing efficient anticancer therapies at various stages of cancer, with potential application to different tumor types.

Past matrix stiffness primes epithelial cells and regulates their future collective migration through a mechanical memory

During morphogenesis and cancer metastasis, grouped cells migrate through tissues of dissimilar stiffness. Although the influence of matrix stiffness on cellular mechanosensitivity and motility are well-recognized, it remains unknown whether these matrix-dependent cellular features persist after cells move to a new microenvironment. Here, we interrogate whether priming of epithelial cells by a given matrix stiffness influences their future collective migration on a different matrix - a property we refer to as the 'mechanical memory' of migratory cells. To prime cells on a defined matrix and track their collective migration onto an adjoining secondary matrix of dissimilar stiffness, we develop a modular polyacrylamide substrate through step-by-step polymerization of different PA compositions. We report that epithelial cells primed on a stiff matrix migrate faster, display higher actomyosin expression, form larger focal adhesions, and retain nuclear YAP even after arriving onto a soft secondary matrix, as compared to their control behavior on a homogeneously soft matrix. Priming on a soft ECM causes a reverse effect. The depletion of YAP dramatically reduces this memory-dependent migration. Our results present a previously unidentified regulation of mechanosensitive collective cell migration by past matrix stiffness, in which mechanical memory depends on YAP activity.

Increased Tissue Stiffness in Tumors from Mice with Neurofibromatosis-1 Optic Glioma

Children with neurofibromatosis type 1 (NF1) cancer predisposition syndrome are prone to the development of low-grade brain tumors (gliomas) within the optic pathway (optic gliomas). One of the key obstacles to developing successful therapeutic strategies for these tumors is the striking lack of information about the mechanical properties that characterize these tumors relative to non-neoplastic optic nerve tissue. To study the physical changes that may occur when an optic nerve glioma is present, we employed atomic force microscopy to measure the stiffness of healthy versus tumor-bearing optic nerve tissue. We found that the average elastic moduli of non-neoplastic and tumor-bearing optic nerves were ∼3 and ∼6 kPa, respectively. Based on previous studies implicating changes in extracellular matrix remodeling in other, related optic nerve pathological states, we found decreased expression of one major metalloproteinase protein (MMP-2) and unchanged expression of lysyl oxidase and a second metalloproteinase, MMP-9, in murine optic gliomas relative to normal non-neoplastic optic nerve. Collectively, these observations suggest a productive interplay between physical properties of mouse optic nerve gliomas and the extracellular matrix.

Scattering of Cell Clusters in Confinement

Epithelial-to-mesenchymal transition (EMT) enables scattering of cell clusters and disseminates motile cells to distant locations in vivo during embryonic development and cancer metastasis. Both stiffness and topography of the extracellular matrix (ECM) have been shown to influence EMT. In this work, we examine how the integrity of epithelial cell clusters is regulated by subcellular forces, protrusions, and adhesions for varying ECM inputs, such as stiffness, topography, and dimensionality. Our model simulates multicell networks of defined sizes and shapes in ECMs of varied stiffness and geometry. The integrity of cell clusters is dictated by cell-cell junctions, which depend on subcellular forces and adhesion dynamics within each cell of the cluster. Our simulations demonstrate an enhanced dissociation of cell-cell junctions in stiffer and more confined three-dimensional (3D) environments, consistent with experimental findings. In narrow channels, the cell edges parallel to the axis of channels lose their cell-cell junctions more readily than those oriented in the perpendicular direction. The inhibition of protrusive activity and cell polarity disables confinement-dependent cell scattering. Here, cell adhesion and spreading along channel walls is found to be essential for scattering. The model also predicts that two-dimensional (2D) confinement of clusters restricts cell spreading and simultaneously blunts the confinement-sensitive cell scattering. This new, to our knowledge, multiscale model integrates molecular adhesion dynamics, subcellular forces, cellular deformation, and macroscale mechanical properties of the ECM to predict the state of cell clusters of defined shapes and sizes. The predictions made by our model not only match experimental findings from a number of experimental setups, but also provide a new conceptual framework for understanding mechanosensitive cell scattering and EMT.

Material Strategies for Modulating Epithelial to Mesenchymal Transitions

Epithelial to mesenchymal transitions (EMT) involve the phenotypic change of epithelial cells into fibroblast-like cells. This process is accompanied by the loss of cell-cell contacts, increased extracellular matrix (ECM) production, stress fiber alignment, and an increase in cell mobility. While essential for development and wound repair, EMT has also been recognized as a contributing factor to fibrotic diseases and cancer. Both chemical and mechanical cues, such as tumor necrosis factor alpha, NF-κB, Wnt, Notch, interleukin-8, metalloproteinase-3, ECM proteins, and ECM stiffness can determine the degree and duration of EMT events. Additionally, transforming growth factor beta is a primary driver of EMT and, interestingly, can be activated through cell-mediated mechanoactivation. In this review, we highlight recent findings demonstrating the contribution of mechanical stimuli, such as tissue and material stiffness, in driving EMT. We then highlight material strategies for controlling EMT events. Finally, we discuss drivers of the similar process of endothelial to mesenchymal transition (EndoMT) and corresponding material strategies for controlling EndoMT.

Comprehensive study on cellular morphologies, proliferation, motility, and epithelial-mesenchymal transition of breast cancer cells incubated on electrospun polymeric fiber substrates

The progress of microenvironment-mediated tumor progression in an artificial extracellular matrix explores the design criteria to understand the cancer progression mechanism and metastatic potential. This study was aimed at examining the combination of both surface topographies (fiber alignments) and different stiffness of polymeric substrates (PLLA and PCL) to evaluate the effects on the cellular morphologies, proliferation, motility, and gene expression regarding epithelial to mesenchymal transition (EMT) of two different types of breast cancer cells (MDA-MB-231 and MCF-7). The cellular morphologies (roundness and nuclear elongation factor), E-cadherin and vimentin expression, and cellular motility in terms of cellular migration speed, persistent time, and diffusivity have been comprehensively discussed. We demonstrated that the microenvironment of cell culture substrates influences cancer progression and metastatic potential.

Comprehensive study on cellular morphologies, proliferation, motility, and epithelial–mesenchymal transition of breast cancer cells incubated on electrospun polymeric fiber substrates

Aligned fibers substrates caused elongation and alignment of the MDA-MB-231 cells along the fiber directionsviareducing the cell roundness and E-cadherin expression.

Mechanotransduction in cancer

Tissue stiffness is tightly controlled under normal conditions, but changes with disease. In cancer, tumors often tend to be stiffer than the surrounding uninvolved tissue, yet the cells themselves soften. Within the past decade, and particularly in the last few years, there is increasing evidence that the stiffness of the extracellular matrix modulates cancer and stromal cell mechanics and function, influencing such disease hallmarks as angiogenesis, migration, and metastasis. This review briefly summarizes recent studies that investigate how cancer cells and fibrosis-relevant stromal cells respond to ECM stiffness, the possible sensing appendages and signaling mechanisms involved, and the emergence of novel substrates - including substrates with scar-like fractal heterogeneity - that mimic the in vivo mechanical environment of the cancer cell.