Monitoring developmental force distributions in reconstituted embryonic epithelia

Fibroblast integrin α11β1 is a collagen assembly receptor in mechanoregulated fibrillar adhesions

Solid epithelial cancers with significant desmoplasia are characterized by an excessive deposition of collagen-based matrix, which often supports tumor progression. However, the mechanism of how collagen receptors mediate collagen fibrillogenesis still remains mostly unclear. We show that the collagen-binding integrin α11β1 can co-localize with tensin-1 and deposited collagen I in human pancreatic ductal adenocarcinoma (PDAC) stroma. In addition to the canonical fibrillar adhesion integrin α5β1 expressed by human PDAC cancer-associated fibroblasts (CAFs), tensin-1-positive fibrillar adhesions contained α11β1 but lacked α1β1 and α2β1. CAFs lacking α5β1 expression displayed mechanoregulated and tensin-1 dependent α11β1 fibrillar adhesions, suggesting independent roles of the two integrins with regards to fibrillar adhesions-based de novo fibrillogenesis. Further, we demonstrate that cell surface-associated collagen I assembly necessitated α11β1, but not α5β1 expression. In summary, α11β1 integrin is a novel component of fibrillar adhesions, which is strategically positioned to mediate de novo collagen fibrillogenesis at the cell surface under pro-fibrotic conditions.

An Assessment of the Mechanophysical and Hormonal Impact on Human Endometrial Epithelium Mechanics and Receptivity

The endometrial epithelium and underlying stroma undergo profound changes to support and limit embryo adhesion and invasion, which occur in the secretory phase of the menstrual cycle during the window of implantation. This coincides with a peak in progesterone and estradiol production. We hypothesized that the interplay between hormone-induced changes in the mechanical properties of the endometrial epithelium and stroma supports this process. To study it, we used hormone-responsive endometrial adenocarcinoma-derived Ishikawa cells growing on substrates of different stiffness. We showed that Ishikawa monolayers on soft substrates are more tightly clustered and uniform than on stiff substrates. Probing for mechanical alterations, we found accelerated stress-relaxation after apical nanoindentation in hormone-stimulated monolayers on stiff substrates. Traction force microscopy furthermore revealed an increased number of foci with high traction in the presence of estradiol and progesterone on soft substrates. The detection of single cells and small cell clusters positive for the intermediate filament protein vimentin and the progesterone receptor further underscored monolayer heterogeneity. Finally, adhesion assays with trophoblast-derived AC-1M-88 spheroids were used to examine the effects of substrate stiffness and steroid hormones on endometrial receptivity. We conclude that the extracellular matrix and hormones act together to determine mechanical properties and, ultimately, embryo implantation.

Mechanosensitive hormone signaling promotes mammary progenitor expansion and breast cancer risk

Tissue stem-progenitor cell frequency has been implicated in tumor risk and progression, but tissue-specific factors linking these associations remain ill-defined. We observed that stiff breast tissue from women with high mammographic density, who exhibit increased lifetime risk for breast cancer, associates with abundant stem-progenitor epithelial cells. Using genetically engineered mouse models of elevated integrin mechanosignaling and collagen density, syngeneic manipulations, and spheroid models, we determined that a stiff matrix and high mechanosignaling increase mammary epithelial stem-progenitor cell frequency and enhance tumor initiation in vivo. Augmented tissue mechanics expand stemness by potentiating extracellular signal-related kinase (ERK) activity to foster progesterone receptor-dependent RANK signaling. Consistently, we detected elevated phosphorylated ERK and progesterone receptors and increased levels of RANK signaling in stiff breast tissue from women with high mammographic density. The findings link fibrosis and mechanosignaling to stem-progenitor cell frequency and breast cancer risk and causally implicate epidermal growth factor receptor-ERK-dependent hormone signaling in this phenotype.

Bone-matrix mineralization dampens integrin-mediated mechanosignalling and metastatic progression in breast cancer

In patients with breast cancer, lower bone mineral density increases the risk of bone metastasis. Although the relationship between bone-matrix mineralization and tumour-cell phenotype in breast cancer is not well understood, mineralization-induced rigidity is thought to drive metastatic progression via increased cell-adhesion forces. Here, by using collagen-based matrices with adjustable intrafibrillar mineralization, we show that, unexpectedly, matrix mineralization dampens integrin-mediated mechanosignalling and induces a less proliferative stem-cell-like phenotype in breast cancer cells. In mice with xenografted decellularized physiological bone matrices seeded with human breast tumour cells, the presence of bone mineral reduced tumour growth and upregulated a gene-expression signature that is associated with longer metastasis-free survival in patients with breast cancer. Our findings suggest that bone-matrix changes in osteogenic niches regulate metastatic progression in breast cancer and that in vitro models of bone metastasis should integrate organic and inorganic matrix components to mimic physiological and pathologic mineralization.

Tissue tension permits β-catenin phosphorylation to drive mesoderm specification in human embryonic stem cells

The role of morphogenetic forces in cell fate specification is an area of intense interest. Our prior studies suggested that the development of high cell-cell tension in human embryonic stem cells (hESC) colonies permits the Src-mediated phosphorylation of junctional β-catenin that accelerates its release to potentiate Wnt-dependent signaling critical for initiating mesoderm specification. Using an ectopically expressed nonphosphorylatable mutant of β-catenin (Y654F), we now provide direct evidence that impeding tension-dependent Src-mediated β-catenin phosphorylation impedes the expression of Brachyury (T) and the epithelial-to-mesenchymal transition (EMT) necessary for mesoderm specification. Addition of exogenous Wnt3a or inhibiting GSK3β activity rescued mesoderm expression, emphasizing the importance of force dependent Wnt signaling in regulating mechanomorphogenesis. Our work provides a framework for understanding tension-dependent β-catenin/Wnt signaling in the self-organization of tissues during developmental processes including gastrulation.

A convolutional neural network STIFMap reveals associations between stromal stiffness and EMT in breast cancer

Intratumor heterogeneity associates with poor patient outcome. Stromal stiffening also accompanies cancer. Whether cancers demonstrate stiffness heterogeneity, and if this is linked to tumor cell heterogeneity remains unclear. We developed a method to measure the stiffness heterogeneity in human breast tumors that quantifies the stromal stiffness each cell experiences and permits visual registration with biomarkers of tumor progression. We present Spatially Transformed Inferential Force Map (STIFMap) which exploits computer vision to precisely automate atomic force microscopy (AFM) indentation combined with a trained convolutional neural network to predict stromal elasticity with micron-resolution using collagen morphological features and ground truth AFM data. We registered high-elasticity regions within human breast tumors colocalizing with markers of mechanical activation and an epithelial-to-mesenchymal transition (EMT). The findings highlight the utility of STIFMap to assess mechanical heterogeneity of human tumors across length scales from single cells to whole tissues and implicates stromal stiffness in tumor cell heterogeneity.

Novel cryo-tomography workflow reveals nanometer-scale responses of epithelial cells to matrix stiffness and dimensionality

Matrix stiffness and dimensionality have been shown to be major determinants of cell behavior. However, a workflow for examining nanometer-scale responses of the associated molecular machinery is not available. Here, we describe a comprehensive, quantitative workflow that permits the analysis of cells responding to mechanical and dimensionality cues in their native state at nanometer scale by cryogenic electron tomography. Using this approach, we quantified distinct cytoskeletal nanoarchitectures and vesicle phenotypes induced in human mammary epithelial cells in response to stiffness and dimensionality of reconstituted basement membrane. Our workflow closely recapitulates the microenvironment associated with acinar morphogenesis and identified distinct differences in situ at nanometer scale. Using drug treatment, we showed that molecular events and nanometer-scale rearrangements triggered by engagement of apical cell receptors with reconstituted basement membrane correspond to changes induced by reduction of cortical tension. Our approach is fully adaptable to any kind of stiffness regime, extracellular matrix composition, and drug treatment.

Force tuning through regulation of clathrin-dependent integrin endocytosis

Integrin endocytosis is essential for many fundamental cellular processes. Whether and how the internalization impacts cellular mechanics remains elusive. Whereas previous studies reported the contribution of the integrin activator, talin, in force development, the involvement of inhibitors is less documented. We identified ICAP-1 as an integrin inhibitor involved in mechanotransduction by co-working with NME2 to control clathrin-mediated endocytosis of integrins at the edge of focal adhesions (FA). Loss of ICAP-1 enables β3-integrin-mediated force generation independently of β1 integrin. β3-integrin-mediated forces were associated with a decrease in β3 integrin dynamics stemming from their reduced diffusion within adhesion sites and slow turnover of FA. The decrease in β3 integrin dynamics correlated with a defect in integrin endocytosis. ICAP-1 acts as an adaptor for clathrin-dependent endocytosis of integrins. ICAP-1 controls integrin endocytosis by interacting with NME2, a key regulator of dynamin-dependent clathrin-coated pits fission. Control of clathrin-mediated integrin endocytosis by an inhibitor is an unprecedented mechanism to tune forces at FA.

ECM dimensionality tunes actin tension to modulate endoplasmic reticulum function and spheroid phenotypes of mammary epithelial cells

Patient-derived organoids and cellular spheroids recapitulate tissue physiology with remarkable fidelity. We investigated how engagement with a reconstituted basement membrane in three dimensions (3D) supports the polarized, stress resilient tissue phenotype of mammary epithelial spheroids. Cells interacting with reconstituted basement membrane in 3D had reduced levels of total and actin-associated filamin and decreased cortical actin tension that increased plasma membrane protrusions to promote negative plasma membrane curvature and plasma membrane protein associations linked to protein secretion. By contrast, cells engaging a reconstituted basement membrane in 2D had high cortical actin tension that forced filamin unfolding and endoplasmic reticulum (ER) associations. Enhanced filamin-ER interactions increased levels of PKR-like ER kinase effectors and ER-plasma membrane contact sites that compromised calcium homeostasis and diminished cell viability. Consequently, cells with decreased cortical actin tension had reduced ER stress and survived better. Consistently, cortical actin tension in cellular spheroids regulated polarized basement membrane membrane deposition and sensitivity to exogenous stress. The findings implicate cortical actin tension-mediated filamin unfolding in ER function and underscore the importance of tissue mechanics in organoid homeostasis.

The nature of cell division forces in epithelial monolayers

Epithelial cells undergo striking morphological changes during division to ensure proper segregation of genetic and cytoplasmic materials. These morphological changes occur despite dividing cells being mechanically restricted by neighboring cells, indicating the need for extracellular force generation. Beyond driving cell division itself, forces associated with division have been implicated in tissue-scale processes, including development, tissue growth, migration, and epidermal stratification. While forces generated by mitotic rounding are well understood, forces generated after rounding remain unknown. Here, we identify two distinct stages of division force generation that follow rounding: (1) Protrusive forces along the division axis that drive division elongation, and (2) outward forces that facilitate postdivision spreading. Cytokinetic ring contraction of the dividing cell, but not activity of neighboring cells, generates extracellular forces that propel division elongation and contribute to chromosome segregation. Forces from division elongation are observed in epithelia across many model organisms. Thus, division elongation forces represent a universal mechanism that powers cell division in confining epithelia.

Matrix compliance permits NF-κB activation to drive therapy resistance in breast cancer

Triple-negative breast cancers (TNBCs) are associated with poor survival mediated by treatment resistance. TNBCs are fibrotic, yet little is known regarding how the extracellular matrix (ECM) evolves following therapy and whether it impacts treatment response. Analysis revealed that while primary untreated TNBCs are surrounded by a rigid stromal microenvironment, chemotherapy-resistant residual tumors inhabit a softer niche. TNBC organoid cultures and xenograft studies showed that organoids interacting with soft ECM exhibit striking resistance to chemotherapy, ionizing radiation, and death receptor ligand TRAIL. A stiff ECM enhanced proapoptotic JNK activity to sensitize cells to treatment, whereas a soft ECM promoted treatment resistance by elevating NF-κB activity and compromising JNK activity. Treatment-resistant residual TNBCs residing within soft stroma had elevated activated NF-κB levels, and disengaging NF-κB activity sensitized tumors in a soft matrix to therapy. Thus, the biophysical properties of the ECM modify treatment response, and agents that modulate stiffness-dependent NF-κB or JNK activity could enhance therapeutic efficacy in patients with TNBC.

Stiff stroma increases breast cancer risk by inducing the oncogene ZNF217

Women with dense breasts have an increased lifetime risk of malignancy that has been attributed to a higher epithelial density. Quantitative proteomics, collagen analysis, and mechanical measurements in normal tissue revealed that stroma in the high-density breast contains more oriented, fibrillar collagen that is stiffer and correlates with higher epithelial cell density. microRNA (miR) profiling of breast tissue identified miR-203 as a matrix stiffness-repressed transcript that is downregulated by collagen density and reduced in the breast epithelium of women with high mammographic density. Culture studies demonstrated that ZNF217 mediates a matrix stiffness- and collagen density-induced increase in Akt activity and mammary epithelial cell proliferation. Manipulation of the epithelium in a mouse model of mammographic density supported a causal relationship between stromal stiffness, reduced miR-203, higher levels of the murine homolog Zfp217, and increased Akt activity and mammary epithelial proliferation. ZNF217 was also increased in the normal breast epithelium of women with high mammographic density, correlated positively with epithelial proliferation and density, and inversely with miR-203. The findings identify ZNF217 as a potential target toward which preexisting therapies, such as the Akt inhibitor triciribine, could be used as a chemopreventive agent to reduce cancer risk in women with high mammographic density.

Mechanical Tension Promotes Formation of Gastrulation-like Nodes and Patterns Mesoderm Specification in Human Embryonic Stem Cells

Embryogenesis is directed by morphogens that induce differentiation within a defined tissue geometry. Tissue organization is mediated by cell-cell and cell-extracellular matrix (ECM) adhesions and is modulated by cell tension and tissue-level forces. Whether cell tension regulates development by modifying morphogen signaling is less clear. Human embryonic stem cells (hESCs) exhibit an intrinsic capacity for self-organization, which motivates their use as a tractable model of early human embryogenesis. We engineered patterned substrates that recapitulate the biophysical properties of the early embryo and mediate the self-organization of "gastrulation-like" nodes in cultured hESCs. Tissue geometries that generated local nodes of high cell-adhesion tension directed the spatial patterning of the BMP4-dependent "gastrulation-like" phenotype by enhancing phosphorylation and junctional release of β-catenin to promote Wnt signaling and mesoderm specification. Furthermore, direct force application via mechanical stretching promoted BMP-dependent mesoderm specification, confirming that tissue-level forces can directly regulate cell fate specification in early human development.

Characterizing liver sinusoidal endothelial cell fenestrae on soft substrates upon AFM imaging and deep learning

BACKGROUND

Liver sinusoidal endothelial cells (LSECs) display unique fenestrated morphology. Alterations in the size and number of fenestrae play a crucial role in the progression of various liver diseases. While their features have been visualized using atomic force microscopy (AFM), the in situ imaging methods and off-line analyses are further required for fenestra quantification.

METHODS

Primary mouse LSECs were cultured on a collagen-I-coated culture dish, or a polydimethylsiloxane (PDMS) or polyacrylamide (PA) hydrogel substrate. An AFM contact mode was applied to visualize fenestrae on individual fixed LSECs. Collected images were analyzed using an in-house developed image recognition program based on fully convolutional networks (FCN).

RESULTS

Key scanning parameters were first optimized for visualizing the fenestrae on LSECs on culture dish, which was also applicable for the LSECs cultured on various hydrogels. The intermediate-magnification morphology images of LSECs were used for developing the FCN-based, fenestra recognition program. This program enabled us to recognize the vast majority of fenestrae from AFM images after twice trainings at a typical accuracy of 81.6% on soft substrate and also quantify the statistics of porosity, number of fenestrae and distribution of fenestra diameter.

CONCLUSIONS

Combining AFM imaging with FCN training is able to quantify the morphological distributions of LSEC fenestrae on various substrates.

SIGNIFICANCE

AFM images acquired and analyzed here provided the global information of surface ultramicroscopic structures over an entire cell, which is fundamental in understanding their regulatory mechanisms and pathophysiological relevance in fenestra-like evolution of individual cells on stiffness-varied substrates.

The Protein Mat(ters)-Revealing the Biologically Relevant Mechanical Contribution of Collagen- and Fibronectin-Coated Micropatterns

Understanding cell-material interactions requires accurate characterization of the substrate mechanics, which are generally measured by indentation-type atomic force microscopy. To facilitate cell-substrate interaction, model extracellular matrix coatings are used although their tensile mechanical properties are generally unknown. In this study, beyond standard compressive stiffness estimation, we performed a novel tensile mechanical characterization of collagen- and fibronectin-micropatterned polyacrylamide hydrogels. We further demonstrate the impact of the protein mat on the tensile stiffness characterization of adherent cells. To our knowledge, our study is the first to uncover direction-dependent mechanical behavior of the protein coatings and to demonstrate that it affects cellular response relative to substrate mechanics.

Versatile and High-throughput Force Measurement Platform for Dorsal Cell Mechanics

We present a high-throughput microfluidics technique facilitating in situ measurements of cell mechanics parameters at the dorsal side of the cell, including molecular binding strengths, local traction forces, and viscoelastic properties. By adjusting the flow rate, the force magnitude exerted on the cell can be modulated ranging from ~14 pN to 2 nN to perturb various force-dependent processees in cells. Time-lapse images were acquired to record events due to such perturbation. The values of various mechanical parameters are subsequently obtained by single particle tracking. Up to 50 events can be measured simultaneously in a single experiment. Integrating the microfluidic techniques with the analytic framework established in computational fluid dynamics, our method is physiologically relevant, reliable, economic and efficient.

Fine tuning the extracellular environment accelerates the derivation of kidney organoids from human pluripotent stem cells

The generation of organoids is one of the biggest scientific advances in regenerative medicine. Here, by lengthening the time that human pluripotent stem cells (hPSCs) were exposed to a three-dimensional microenvironment, and by applying defined renal inductive signals, we generated kidney organoids that transcriptomically matched second-trimester human fetal kidneys. We validated these results using ex vivo and in vitro assays that model renal development. Furthermore, we developed a transplantation method that utilizes the chick chorioallantoic membrane. This approach created a soft in vivo microenvironment that promoted the growth and differentiation of implanted kidney organoids, as well as providing a vascular component. The stiffness of the in ovo chorioallantoic membrane microenvironment was recapitulated in vitro by fabricating compliant hydrogels. These biomaterials promoted the efficient generation of renal vesicles and nephron structures, demonstrating that a soft environment accelerates the differentiation of hPSC-derived kidney organoids.

Cell-cell adhesion interface: orthogonal and parallel forces from contraction, protrusion, and retraction

The epithelial lateral membrane plays a central role in the integration of intercellular signals and, by doing so, is a principal determinant in the emerging properties of epithelial tissues. Mechanical force, when applied to the lateral cell-cell interface, can modulate the strength of adhesion and influence intercellular dynamics. Yet the relationship between mechanical force and epithelial cell behavior is complex and not completely understood. This commentary aims to provide an investigative look at the usage of cellular forces at the epithelial cell-cell adhesion interface.

A tension-mediated glycocalyx-integrin feedback loop promotes mesenchymal-like glioblastoma

Glioblastoma multiforme (GBMs) are recurrent lethal brain tumours. Recurrent GBMs often exhibit mesenchymal, stem-like phenotypes that could explain their resistance to therapy. Analyses revealed that recurrent GBMs have increased tension and express high levels of glycoproteins that increase the bulkiness of the glycocalyx. Studies showed that a bulky glycocalyx potentiates integrin mechanosignalling and tissue tension and promotes a mesenchymal, stem-like phenotype in GBMs. Gain- and loss-of-function studies implicated integrin mechanosignalling as an inducer of GBM growth, survival, invasion and treatment resistance, and a mesenchymal, stem-like phenotype. Mesenchymal-like GBMs were highly contractile and expressed elevated levels of glycoproteins that expanded their glycocalyx, and they were surrounded by a stiff extracellular matrix that potentiated integrin mechanosignalling. Our findings suggest that there is a dynamic and reciprocal link between integrin mechanosignalling and a bulky glycocalyx, implying a causal link towards a mesenchymal, stem-like phenotype in GBMs. Strategies to ameliorate GBM tissue tension offer a therapeutic approach to reduce mortality due to GBM.

Volumetric optical coherence microscopy with a high space-bandwidth-time product enabled by hybrid adaptive optics

Optical coherence microscopy (OCM) is a promising modality for high resolution imaging, but has limited ability to capture large-scale volumetric information about dynamic biological processes with cellular resolution. To enhance the throughput of OCM, we implemented a hybrid adaptive optics (hyAO) approach that combines computational adaptive optics with an intentionally aberrated imaging beam generated via hardware adaptive optics. Using hyAO, we demonstrate the depth-equalized illumination and collection ability of an astigmatic beam compared to a Gaussian beam for cellular-resolution imaging. With this advantage, we achieved volumetric OCM with a higher space-bandwidth-time product compared to Gaussian-beam acquisition that employed focus-scanning across depth. HyAO was also used to perform volumetric time-lapse OCM imaging of cellular dynamics over a 1mm × 1mm × 1mm field-of-view with 2 μm isotropic spatial resolution and 3-minute temporal resolution. As hyAO is compatible with both spectral-domain and swept-source beam-scanning OCM systems, significant further improvements in absolute volumetric throughput are possible by use of ultrahigh-speed swept sources.

Traction Force Microscopy for Noninvasive Imaging of Cell Forces

The forces exerted by cells on their surroundings play an integral role in both physiological processes and disease progression. Traction force microscopy is a noninvasive technique that enables the in vitro imaging and quantification of cell forces. Utilizing expertise from a variety of disciplines, recent developments in traction force microscopy are enhancing the study of cell forces in physiologically relevant model systems, and hold promise for further advancing knowledge in mechanobiology. In this chapter, we discuss the methods, capabilities, and limitations of modern approaches for traction force microscopy, and highlight ongoing efforts and challenges underlying future innovations.

α5β1-Integrin promotes tension-dependent mammary epithelial cell invasion by engaging the fibronectin synergy site

Fibronectin-ligated α5β1 integrin promotes malignancy by inducing tissue tension.

Tumors are fibrotic and characterized by abundant, remodeled, and cross-linked collagen that stiffens the extracellular matrix stroma. The stiffened collagenous stroma fosters malignant transformation of the tissue by increasing tumor cell tension to promote focal adhesion formation and potentiate growth factor receptor signaling through kinase. Importantly, collagen cross-linking requires fibronectin (FN). Fibrotic tumors contain abundant FN, and tumor cells frequently up-regulate the FN receptor α5β1 integrin. Using transgenic and xenograft models and tunable two- and three-dimensional substrates, we show that FN-bound α5β1 integrin promotes tension-dependent malignant transformation through engagement of the synergy site that enhances integrin adhesion force. We determined that ligation of the synergy site of FN permits tumor cells to engage a zyxin-stabilized, vinculin-linked scaffold that facilitates nucleation of phosphatidylinositol (3,4,5)-triphosphate at the plasma membrane to enhance phosphoinositide 3-kinase (PI3K)-dependent tumor cell invasion. The data explain why rigid collagen fibrils potentiate PI3K activation to promote malignancy and offer a perspective regarding the consistent up-regulation of α5β1 integrin and FN in many tumors and their correlation with cancer aggression.

A Strong Contractile Actin Fence and Large Adhesions Direct Human Pluripotent Colony Morphology and Adhesion

Cell-type-specific functions and identity are tightly regulated by interactions between the cell cytoskeleton and the extracellular matrix (ECM). Human pluripotent stem cells (hPSCs) have ultimate differentiation capacity and exceptionally low-strength ECM contact, yet the organization and function of adhesion sites and associated actin cytoskeleton remain poorly defined. We imaged hPSCs at the cell-ECM interface with total internal reflection fluorescence microscopy and discovered that adhesions at the colony edge were exceptionally large and connected by thick ventral stress fibers. The actin fence encircling the colony was found to exert extensive Rho-ROCK-myosin-dependent mechanical stress to enforce colony morphology, compaction, and pluripotency and to define mitotic spindle orientation. Remarkably, differentiation altered adhesion organization and signaling characterized by a switch from ventral to dorsal stress fibers, reduced mechanical stress, and increased integrin activity and cell-ECM adhesion strength. Thus, pluripotency appears to be linked to unique colony organization and adhesion structure.

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Human pluripotent colonies have exceptional actin structure and focal adhesions

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Contraction-dependent tight colony compaction enforces pluripotency

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Colony morphology is maintained by edge-oriented cell divisions

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Differentiation alters actin orientation, integrin activity, and adhesion strength

A semi-interpenetrating network of polyacrylamide and recombinant basement membrane allows pluripotent cell culture in a soft, ligand-rich microenvironment

The physical properties of the extracellular matrix play an essential role in guiding stem cell differentiation and tissue morphogenesis both in vivo and in vitro. Existing work to investigate the role of matrix mechanics in directing stem cell proliferation, self-renewal, and differentiation has been limited by the poor attachment and survival of human pluripotent cells cultured on soft matrices (Young's modulus E ≲ 1000 Pa). To address this limitation we developed a protocol for generating semi-interpenetrating networks of polyacrylamide and recombinant basement membrane. Using these materials, we found that human embryonic stem cells (hESCs) remained proliferative and pluripotent even when grown in small colonies and on surfaces ranging in stiffness from 150 to 12000 Pa, spanning the range of tissue stiffnesses likely to be encountered in the embryo. Considerable recent attention has focused on the role of the transcriptional coactivator and Hippo effector YAP in regulating differentiation and cell proliferation both in the early embryo and in vitro. We found that while YAP localized to the nucleus on substrates of E ≳ 1000 Pa, its localization was heterogeneous on substrates of moduli ≲ 450 Pa, with predominantly nuclear localization at the colony periphery and mixed cytoplasmic and nuclear localization for cells in the colony interior, a pattern reminiscent of YAP subcellular localization in the inner cell mass (ICM) of the early embryo. In addition, hESC colony dynamics were highly responsive to substrate stiffness, with cells assembling into monolayers, multilayer structures, and transient, hollow rosettes in response to decreasing substrate stiffnesses in the range of 12000 to 150 Pa. We suggest that soft, ligand-rich substrates such as are described here provide a promising means of recapitulating aspects of early mammalian development that are otherwise inaccessible, and more broadly may be useful in the derivation of complex tissues from pluripotent cells in an in vitro setting.

Tissue Mechanics Orchestrate Wnt-Dependent Human Embryonic Stem Cell Differentiation

Regenerative medicine is predicated on understanding the mechanisms regulating development and applying these conditions to direct stem cell fate. Embryogenesis is guided by cell-cell and cell-matrix interactions, but it is unclear how these physical cues influence stem cells in culture. We used human embryonic stem cells (hESCs) to examine whether mechanical features of the extracellular microenvironment could differentially modulate mesoderm specification. We found that, on a hydrogel-based compliant matrix, hESCs accumulate β-catenin at cell-cell adhesions and show enhanced Wnt-dependent mesoderm differentiation. Mechanistically, Src-driven ubiquitination of E-cadherin by Cbl-like ubiquitin ligase releases P120-catenin to facilitate transcriptional activity of β-catenin, which initiates and reinforces mesoderm differentiation. By contrast, on a stiff hydrogel matrix, hESCs show elevated integrin-dependent GSK3 and Src activity that promotes β-catenin degradation and inhibits differentiation. Thus, we found that mechanical features of the microenvironmental matrix influence tissue-specific differentiation of hESCs by altering the cellular response to morphogens.

Physiological ranges of matrix rigidity modulate primary mouse hepatocyte function in part through hepatocyte nuclear factor 4 alpha

Matrix rigidity has important effects on cell behavior and is increased during liver fibrosis; however, its effect on primary hepatocyte function is unknown. We hypothesized that increased matrix rigidity in fibrotic livers would activate mechanotransduction in hepatocytes and lead to inhibition of liver-specific functions. To determine the physiologically relevant ranges of matrix stiffness at the cellular level, we performed detailed atomic force microscopy analysis across liver lobules from normal and fibrotic livers. We determined that normal liver matrix stiffness was around 150 Pa and increased to 1-6 kPa in areas near fibrillar collagen deposition in fibrotic livers. In vitro culture of primary hepatocytes on collagen matrix of tunable rigidity demonstrated that fibrotic levels of matrix stiffness had profound effects on cytoskeletal tension and significantly inhibited hepatocyte-specific functions. Normal liver stiffness maintained functional gene regulation by hepatocyte nuclear factor 4 alpha (HNF4α), whereas fibrotic matrix stiffness inhibited the HNF4α transcriptional network. Fibrotic levels of matrix stiffness activated mechanotransduction in primary hepatocytes through focal adhesion kinase. In addition, blockade of the Rho/Rho-associated protein kinase pathway rescued HNF4α expression from hepatocytes cultured on stiff matrix.

CONCLUSION

Fibrotic levels of matrix stiffness significantly inhibit hepatocyte-specific functions in part by inhibiting the HNF4α transcriptional network mediated through the Rho/Rho-associated protein kinase pathway. Increased appreciation of the role of matrix rigidity in modulating hepatocyte function will advance our understanding of the mechanisms of hepatocyte dysfunction in liver cirrhosis and spur development of novel treatments for chronic liver disease. (Hepatology 2016;64:261-275).

Liquid drops attract or repel by the inverted Cheerios effect

Solid particles floating at a liquid interface exhibit a long-ranged attraction mediated by surface tension. In the absence of bulk elasticity, this is the dominant lateral interaction of mechanical origin. Here, we show that an analogous long-range interaction occurs between adjacent droplets on solid substrates, which crucially relies on a combination of capillarity and bulk elasticity. We experimentally observe the interaction between droplets on soft gels and provide a theoretical framework that quantitatively predicts the interaction force between the droplets. Remarkably, we find that, although on thick substrates the interaction is purely attractive and leads to drop-drop coalescence, for relatively thin substrates a short-range repulsion occurs, which prevents the two drops from coming into direct contact. This versatile interaction is the liquid-on-solid analog of the "Cheerios effect." The effect will strongly influence the condensation and coarsening of drops on soft polymer films, and has potential implications for colloidal assembly and mechanobiology.