Determining the role of matrix compliance in the differentiation of mammary stem cells

Multipotent stem cells maintain the structure and function of the mammary gland throughout its development and respond to the physiological demands associated with pregnancy and lactation. The ability of mammary stem cells to maintain themselves as well as to give rise to differentiated progeny is not only affected by soluble factors but has increasingly become linked to mechanical cues including the elastic modulus of the extracellular matrix (ECM). Here we describe a protocol for determining how the mechanical properties of the ECM regulate the fate of mammary stem or progenitor cells. This protocol includes detailed methods for the fabrication of substrata with varying stiffness, culture of mammary progenitor cells on synthetic substrata, pharmacological modulation of actomyosin contractility, and analysis of gene expression to define the resulting fate of human mammary stem cells.

Mapping of mechanical strains and stresses around quiescent engineered three-dimensional epithelial tissues

Understanding how physical signals guide biological processes requires qualitative and quantitative knowledge of the mechanical forces generated and sensed by cells in a physiologically realistic three-dimensional (3D) context. Here, we used computational modeling and engineered epithelial tissues of precise geometry to define the experimental parameters that are required to measure directly the mechanical stress profile of 3D tissues embedded within native type I collagen. We found that to calculate the stresses accurately in these settings, we had to account for mechanical heterogeneities within the matrix, which we visualized and quantified using confocal reflectance and atomic force microscopy. Using this technique, we were able to obtain traction forces at the epithelium-matrix interface, and to resolve and quantify patterns of mechanical stress throughout the surrounding matrix. We discovered that whereas single cells generate tension by contracting and pulling on the matrix, the contraction of multicellular tissues can also push against the matrix, causing emergent compression. Furthermore, tissue geometry defines the spatial distribution of mechanical stress across the epithelium, which communicates mechanically over distances spanning hundreds of micrometers. Spatially resolved mechanical maps can provide insight into the types and magnitudes of physical parameters that are sensed and interpreted by multicellular tissues during normal and pathological processes.

New insights into the regulation of epithelial-mesenchymal transition and tissue fibrosis

Tissue fibrosis often presents as the final outcome of chronic disease and is a significant cause of morbidity and mortality worldwide. Fibrosis is driven by continuous expansion of fibroblasts and myofibroblasts. Epithelial-mesenchymal transition (EMT) is a form of cell plasticity in which epithelia acquire mesenchymal phenotypes and is increasingly recognized as an integral aspect of tissue fibrogenesis. In this review, we describe recent insight into the molecular and cellular factors that regulate EMT and its underlying signaling pathways. We also consider how mechanical cues from the microenvironment affect the regulation of EMT. Finally, we discuss the role of EMT in fibrotic diseases and propose approaches for detecting and treating fibrogenesis by targeting EMT.

Dynamics of branched tissue assembly

The assembly of cells into tissues is a complex process controlled by numerous signaling pathways to ensure the fidelity of the final structure. Tissue assembly is also very dynamic, as exemplified by the formation of branched organs. Here we present two examples of tissue assembly in branched systems that highlight this dynamic nature: formation of the tracheal network in Drosophila melanogaster and the ducts of the mammary gland in mice. Extension of the branches during tracheal development is a stereotyped process that produces identical organ geometries across individuals, whereas elongation of the ducts of the pubertal mammary gland is a non-stereotyped process that produces unique patterns. By studying these two organs, we can begin to understand the dynamic nature of development of other stereotyped and non-stereotyped branching systems, including the lung, kidney, and salivary gland.

Engineered tissues to quantify collective cell migration during morphogenesis

Renal development is a complex process involving the dynamic interplay of over 25 different cell types. One distinct step in this process is the formation of the ureteric tree, which develops from the repeated branching of the ureteric bud. During branching of the ureteric bud, cells migrate collectively in unison to form the complex structure of the tree. Here, we present a microlithography-based 3D culture model in which multiple identical kidney epithelial tissues are used to quantify collective cell migration during the process of branching morphogenesis.

Modulation of invasive phenotype by interstitial pressure-driven convection in aggregates of human breast cancer cells

This paper reports the effect of elevated pressure on the invasive phenotype of patterned three-dimensional (3D) aggregates of MDA-MB-231 human breast cancer cells. We found that the directionality of the interstitial pressure profile altered the frequency of invasion by cells located at the surface of an aggregate. In particular, application of pressure at one end of an aggregate suppressed invasion at the opposite end. Experimental alteration of the configuration of cell aggregates and computational modeling of the resulting flow and solute concentration profiles revealed that elevated pressure inhibited invasion by altering the chemical composition of the interstitial fluid near the surface of the aggregate. Our data reveal a link between hydrostatic pressure, interstitial convection, and invasion.

Matrix compliance regulates Rac1b localization, NADPH oxidase assembly, and epithelial-mesenchymal transition

Substratum stiffness controls the subcellular localization of Rac1b, a highly activated splice variant of the small GTPase Rac1. On stiff substrata, Rac1b localizes to the plasma membrane, forming a complex with NADPH oxidase and generating ROS, thus inducing the expression of the transcription factor Snail and downstream signaling to EMT.

Epithelial–mesenchymal transition (EMT) is a form of epithelial plasticity implicated in fibrosis and tumor metastasis. Here we show that the mechanical rigidity of the microenvironment plays a pivotal role in the promotion of EMT by controlling the subcellular localization and downstream signaling of Rac GTPases. Soft substrata, with compliances comparable to that of normal mammary tissue, are protective against EMT, whereas stiffer substrata, with compliances characteristic of breast tumors, promote EMT. Rac1b, a highly activated splice variant of Rac1 found in tumors, localizes to the plasma membrane in cells cultured on stiff substrata or in collagen-rich regions of human breast tumors. At the membrane, Rac1b forms a complex with NADPH oxidase and promotes the production of reactive oxygen species, expression of Snail, and activation of the EMT program. In contrast, soft microenvironments inhibit the membrane localization of Rac1b and subsequent redox changes. These results reveal a novel mechanotransduction pathway in the regulation of epithelial plasticity via EMT.

Photoresponsive coumarin-stabilized polymeric nanoparticles as a detectable drug carrier

The ability to create aqueous suspended stable nanoparticles of the hydrophobic homopolymer poly(ϵ-caprolactone) end-functionalized with coumarin moieties (CPCL) is demonstrated. Nanoparticles of CPCL are prepared in a continuous manner using nanoprecipitation. The resulting nanoparticles are spherical in morphology, about 40 nm in diameter, and possess a narrow size distribution and excellent stability over 4 months by repulsive surface charge. Nanoparticle size can be easily controlled by manipulating the concentration of CPCL in the solution. The interparticle assembly between the nanoparticles can be reversibly adjusted with photoirradiation due to photoinduced [2 + 2] cyclodimerization and cleavage between the coumarin molecules. In addition, the CPCL nanoparticles show significant cellular uptake without cytotoxicity, and the intrinsic fluorescence of the coumarin functional group permits the direct detection of cellular internalization.

PI3K signaling in the regulation of branching morphogenesis

Branching morphogenesis drives the formation of epithelial organs including the mammary gland, lung, kidney, salivary gland and prostate. Branching at the cellular level also drives development of the nervous and vascular systems. A variety of signaling pathways are orchestrated together to establish the pattern of these branched organs. The phosphoinositide 3-kinase (PI3K) signaling network is of particular interest because of the diverse outcomes it generates, including proliferation, motility, growth, survival and cell death. Here, we focus on the role of the PI3K pathway in the development of branched tissues. Cultured cells, explants and transgenic mice have revealed that the PI3K pathway is critical for the regulation of cell proliferation, apoptosis and motility during branching of tissues.

Sculpting organs: mechanical regulation of tissue development

The ramified architectures of organs such as the mammary gland and lung are generated via branching morphogenesis, a developmental process through which individual cells bud and pinch off of pre-existing epithelial sheets. Although specified by signaling programs, organ development requires integration of all aspects of the microenvironment. We describe the essential role of endogenous cellular contractility in the formation of branching tubes. We also highlight the role of exogenous forces in normal and aberrant branching.

Extracellular matrix and cytoskeletal dynamics during branching morphogenesis

Branching morphogenesis is a fundamental developmental process which results in amplification of epithelial surface area for exchanging molecules in organs including the lung, kidney, mammary gland and salivary gland. These complex tree-like structures are built by iterative rounds of simple routines of epithelial morphogenesis, including bud formation, extension, and bifurcation, that require constant remodeling of the extracellular matrix (ECM) and the cytoskeleton. In this review, we highlight the current understanding of the role of the ECM and cytoskeletal dynamics in branching morphogenesis across these different organs. The cellular and molecular mechanisms shared during this morphogenetic process provide insight into the development of other branching organs.

Inhibitory morphogens and monopodial branching of the embryonic chicken lung

BACKGROUND

Branching morphogenesis generates a diverse array of epithelial patterns, including dichotomous and monopodial geometries. Dichotomous branching can be instructed by concentration gradients of epithelial-derived inhibitory morphogens, including transforming growth factor-β (TGFβ), which is responsible for ramification of the pubertal mammary gland. Here, we investigated the role of autocrine inhibitory morphogens in monopodial branching morphogenesis of the embryonic chicken lung.

RESULTS

Computational modeling and experiments using cultured organ explants each separately revealed that monopodial branching patterns cannot be specified by a single epithelial-derived autocrine morphogen gradient. Instead, signaling by means of TGFβ1 and bone morphogenetic protein-4 (BMP4) differentially affect the rates of branching and growth of the airways. Allometric analysis revealed that development of the epithelial tree obeys power-law dynamics; TGFβ1 and BMP4 have distinct but reversible effects on the scaling coefficient of the power law.

CONCLUSIONS

These data suggest that although autocrine inhibition cannot specify monopodial branching, inhibitory morphogens define the dynamics of lung morphogenesis.

Regulation of Epithelial-Mesenchymal Transition by Transmission of Mechanical Stress through Epithelial Tissues

Epithelial-mesenchymal transition (EMT) is a phenotypic shift wherein epithelial cells lose or loosen attachments to their neighbors and assume a mesenchymal-like morphology. EMT drives a variety of developmental processes, but may also be adopted by tumor cells during neoplastic progression. EMT is regulated by both biochemical and physical signals from the microenvironment, including mechanical stress, which is increasingly recognized to play a major role in development and disease progression. Biological systems generate, transmit and concentrate mechanical stress into spatial patterns; these gradients in mechanical stress may serve to spatially pattern developmental and pathologic EMTs. Here we review how epithelial tissues generate and respond to mechanical stress gradients, and highlight the mechanisms by which mechanical stress regulates and patterns EMT.

Snail1, Snail2, and E47 promote mammary epithelial branching morphogenesis

Several E-box-binding transcription factors regulate individual and collective cell migration and enhance the motility of epithelial cells by promoting epithelial-mesenchymal transition (EMT). Here, we characterized the role of a subset of these transcription factors and the EMT proteome in branching morphogenesis of mammary epithelial tissues using a three-dimensional organotypic culture model of the mammary duct. We found that the transcription factors Snail1, Snail2, and E47 were transiently upregulated at branch sites; decreasing the expression of these transcription factors inhibited branching. Conversely, ectopic expression of Snail1, Snail2, and E47 induced branching in the absence of exogenous stimuli. These changes correlated with the expression of mesenchymal markers and repression of E-cadherin, which was essential for branching. Snail1 and Snail2 also promoted cell survival at branch sites, but this was not sufficient to induce branching. These findings indicate that Snail1, Snail2, and E47 can promote collective migration during branching morphogenesis of mammary epithelial tissues through key regulators of EMT.

Mammary branch initiation and extension are inhibited by separate pathways downstream of TGFβ in culture

During the branching morphogenesis process that builds epithelial trees, signaling from stimulatory and inhibitory growth factors is integrated to control branch initiation and extension into the surrounding stroma. Here, we examined the relative roles played by these stimulatory and inhibitory signals in the patterning of branch initiation and extension of model mammary epithelial tubules in culture. We found that although several growth factors could stimulate branching, they did not determine the sites at which new branches formed or the lengths to which branches extended. Instead, branch initiation and extension were defined by two separate signals downstream of the inhibitory morphogen, transforming growth factor (TGF)-β. Branch initiation was controlled by signaling through p38 mitogen-activated protein kinase, whereas branch extension was controlled by Smad-mediated induction of a second diffusible inhibitor, Wnt5a. These data suggest that mammary epithelial branching is patterned predominately by repulsive signaling, and that TGFβ activates multiple inhibitory pathways to refine the architecture of the tree.

Branch formation during organ development

Invertebrates and vertebrates use branching morphogenesis to build epithelial trees to maximize the surface area of organs within a given volume. Several molecular regulators of branching have recently been discovered, a number of which are conserved across different organs and species. Signals that control branching at the cellular and tissue levels are also starting to emerge, and are rapidly unveiling the physical nature of branch development. Here we discuss the molecular, cellular, and physical processes that govern branch formation, and highlight the major outstanding questions in the field.

Lithographically defined two- and three-dimensional tissue microarrays

Traditional methods to study normal and pathological development of tissues have been limited by -difficulties in controlling experimental conditions and quantifying biological processes of interest. Here we describe methods to create microarrays of engineered tissues that enable controlled and quantitative investigations. Using soft lithography-based techniques, extracellular matrix proteins can be microcontact printed or micromolded to make two- and three-dimensional micropatterned scaffolds. The ultimate form and resulting properties of the tissue construct are dictated by the geometry of the patterned extracellular matrix components. This chapter describes elastomeric stamp fabrication, microcontact printing and micromolding of extracellular matrix proteins, cell culture in micropatterned substrata, and quantitative immunofluorescence analysis of micropatterned tissues.

The mechanics of development: Models and methods for tissue morphogenesis

Embryonic development is a physical process during which groups of cells are sculpted into functional organs. The mechanical properties of tissues and the forces exerted on them serve as epigenetic regulators of morphogenesis. Understanding these mechanobiological effects in the embryo requires new experimental approaches. Here we focus on branching of the lung airways and bending of the heart tube to describe examples of mechanical and physical cues that guide cell fate decisions and organogenesis. We highlight recent technological advances to measure tissue elasticity and endogenous mechanical stresses in real time during organ development. We also discuss recent progress in manipulating forces in intact embryos.

Geometrically controlled endothelial tubulogenesis in micropatterned gels

We present a novel approach to control endothelial tubulogenesis by spatially patterning cells within micromolded collagen gels. Endothelial cells cultured within microscale channels that were filled with collagen gel organized into tubes with lumens within 24-48 h of seeding. These tubes extended up to 1 cm in length, and exhibited cell-cell junction formation characteristic of early stage capillary vessels. Tube diameter could be controlled by varying collagen concentrations or channel width. The geometry of the microfabricated template also could be used to guide the development of branches during tube formation, allowing for the generation of more complex capillary architectures. Time-lapse imaging of tube formation revealed a highly dynamic process involving coalescence of endothelial cells, reorganization and alignment of collagen fibers into a central core, and arrangement of cells into cords. This platform may be of use to generate geometrically defined vascular networks for tissue engineering applications as well as a means to better understand the process of endothelial tubulogenesis.

Adipose stroma induces branching morphogenesis of engineered epithelial tubules

The mammary gland and other treelike organs develop their characteristic fractal geometries through branching morphogenesis, a process in which the epithelium bifurcates and invades into the surrounding stroma. Controlling the pattern of branching is critical for engineering these organs. In vivo, the branching process is instructed by stromal-epithelial interactions and adipocytes form the largest component of the fatty stroma that surrounds the mammary epithelium. Here, we used microlithographic approaches to engineer a three-dimensional culture model that enables analysis of the effect of adipocytes on the pattern of branching morphogenesis of mammary epithelial cells. We found that adipocyte-rich stroma induces branching through paracrine signals, including hepatocyte growth factor, but does not affect the branching pattern per se. This tissue engineering approach can be expanded to other organs, and should enable piecemeal analysis of the cellular populations that control patterning during normal development.