Modeling branching morphogenesis using materials with programmable mechanical instabilities

The architectural features of branching morphogenesis demonstrate exquisite reproducibility among various organs and species despite the unique functionality and biochemical differences of their microenvironment. The regulatory networks that drive branching morphogenesis employ cell-generated and passive mechanical forces, which integrate extracellular signals from the microenvironment into morphogenetic movements. Cell-generated forces function locally to remodel the extracellular matrix (ECM) and control interactions among neighboring cells. Passive mechanical forces are the product of in situ mechanical instabilities that trigger out-of-plane buckling and clefting deformations of adjacent tissues. Many of the molecular and physical signals that underlie buckling and clefting morphogenesis remain unclear and require new experimental strategies to be uncovered. Here, we highlight soft material systems that have been engineered to display programmable buckles and creases. Using synthetic materials to model physicochemical and spatiotemporal features of buckling and clefting morphogenesis might facilitate our understanding of the physical mechanisms that drive branching morphogenesis across different organs and species.

Substratum stiffness tunes proliferation downstream of Wnt3a in part by regulating integrin-linked kinase and frizzled-1

The Wnt/β-catenin pathway controls a variety of cellular behaviors, aberrant activation of which are associated with tumor progression in several types of cancer. The same cellular behaviors are also affected by the mechanical properties of the extracellular matrix (ECM) substratum, which induces signaling through integrins and integrin-linked kinase (ILK). Here, we examined the role of substratum stiffness in the regulation of cell proliferation downstream of Wnt3a. We found that treatment with Wnt3a increased proliferation of cells cultured on stiff substrata, with compliances characteristic of breast tumors, but not of cells on soft substrata, with compliances comparable to that of normal mammary tissue. Depleting ILK rendered cells unresponsive to Wnt3a on both substrata. Ectopic expression of ILK permitted Wnt3a to induce proliferation of cells on both microenvironments, although proliferation on soft substrata remained lower than that on stiff substrata. We further showed that ILK regulates expression of the Wnt receptor frizzled-1 (Fzd1), suggesting the presence of a positive feedback loop between Wnt3a, ILK and Fzd1. These findings suggest that tissue mechanics regulates the cellular response to Wnt under physiological and pathological microenvironmental conditions.This article has an associated First Person interview with the first author of the paper.

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.

A Soft Microenvironment Protects from Failure of Midbody Abscission and Multinucleation Downstream of the EMT-Promoting Transcription Factor Snail

Multinucleation is found in more than one third of tumors and is linked to increased tolerance for mutation, resistance to chemotherapy, and invasive potential. The integrity of the genome depends on proper execution of the cell cycle, which can be altered through mechanotransduction pathways as the tumor microenvironment stiffens during tumorigenesis. Here, we show that signaling downstream of matrix metalloproteinase-3 (MMP3) or TGFβ, known inducers of epithelial-mesenchymal transition (EMT), also promotes multinucleation in stiff microenvironments through Snail-dependent expression of the filament-forming protein septin-6, resulting in midbody persistence, abscission failure, and multinucleation. Consistently, we observed elevated expression of Snail and septin-6 as well as multinucleation in a human patient sample of metaplastic carcinoma of the breast, a rare classification characterized by deposition of collagen fibers and active EMT. In contrast, a soft microenvironment protected mammary epithelial cells from becoming multinucleated by preventing Snail-induced upregulation of septin-6. Our data suggest that tissue stiffening during tumorigenesis synergizes with oncogenic signaling to promote genomic abnormalities that drive cancer progression.Significance: These findings reveal tissue stiffening during tumorigenesis synergizes with oncogenic signaling to promote genomic abnormalities that drive cancer progression. Cancer Res; 78(9); 2277-89. ©2018 AACR.

Building branched tissue structures: from single cell guidance to coordinated construction

Branched networks are ubiquitous throughout nature, particularly found in tissues that require large surface area within a restricted volume. Many tissues with a branched architecture, such as the vasculature, kidney, mammary gland, lung and nervous system, function to exchange fluids, gases and information throughout the body of an organism. The generation of branched tissues requires regulation of branch site specification, initiation and elongation. Branching events often require the coordination of many cells to build a tissue network for material exchange. Recent evidence has emerged suggesting that cell cooperativity scales with the number of cells actively contributing to branching events. Here, we compare mechanisms that regulate branching, focusing on how cell cohorts behave in a coordinated manner to build branched tissues.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Microfluidic chest cavities reveal that transmural pressure controls the rate of lung development

Mechanical forces are increasingly recognized to regulate morphogenesis, but how this is accomplished in the context of the multiple tissue types present within a developing organ remains unclear. Here, we use bioengineered 'microfluidic chest cavities' to precisely control the mechanical environment of the fetal lung. We show that transmural pressure controls airway branching morphogenesis, the frequency of airway smooth muscle contraction, and the rate of developmental maturation of the lungs, as assessed by transcriptional analyses. Time-lapse imaging reveals that branching events are synchronized across distant locations within the lung, and are preceded by long-duration waves of airway smooth muscle contraction. Higher transmural pressure decreases the interval between systemic smooth muscle contractions and increases the rate of morphogenesis of the airway epithelium. These data reveal that the mechanical properties of the microenvironment instruct crosstalk between different tissues to control the development of the embryonic lung.

Dynamics of Tissue-Induced Alignment of Fibrous Extracellular Matrix

Aligned fibers of extracellular matrix (ECM) affect the direction, efficiency, and persistence of migrating cells. To uncover the mechanisms by which multicellular tissues align their surrounding ECM before migration, we used an engineered three-dimensional culture model to investigate the dynamics of ECM alignment around tissues of defined geometry. Analysis of ECM alignment over time revealed that tissues rapidly reorganize their surrounding matrix, with a characteristic time that depends on the type of cell and the initial tissue geometry. We found that matrix metalloproteinase activity is not required for matrix alignment before cell migration. Instead, alignment is driven by Rho-mediated cytoskeletal contractility and accelerated by propagation of tension through intercellular adhesions. Our data suggest that multicellular tissues align their surrounding matrix by pulling collectively to exert strain, which is primarily a physical process. Consistently, the pattern of matrix alignment depends on tissue geometry and the resulting distribution of mechanical strain, with asymmetric tissues generating a higher degree of matrix alignment along their longest axes. The rapid ability of multicellular tissues to physically remodel their matrix enables their constituent cells to migrate efficiently along aligned fibers and to quickly change their direction according to other microenvironmental cues, which is important for both normal and disease processes.

Abstract 5914: A soft microenvironment protects from failure of midbody abscission and multinucleation downstream of EMT initiators

This study investigates how increased stiffness of the tumor microenvironment can induce cellular multinucleation, an easily observable marker of polyploidy. Up to 37% percent of tumors exhibit whole-genome doubling, which typically precedes other somatic copy number alterations. Additionally, induction of tetraploidy in human cells promotes increased tolerance for mutation, resistance to chemotherapeutic drugs, and transformation in culture.

Tumors are inherently stiffer than normal tissue, and this property has been shown to affect cell growth and proliferation. Similarly, cell cycle errors have long been linked to chromosomal abnormalities. Here, we used engineered two-dimensional substrata that mimic the stiffness of tumor and normal microenvironments to investigate how matrix stiffness regulates multinucleation in mammary epithelial cells. Multinucleation was quantified by staining with Hoescht to visualize the nuclei. Timelapse microscopy enabled visualization of the process by which cells become multinucleated. Changes in gene expression were determined by quantitative RT-PCR.

Cells cultured on “stiff” substrata, representing tumor tissue, showed a nearly 14-fold increase in multinucleation compared to cells cultured on “soft” substrata, representing normal tissue. We found that multinucleation was regulated in part by signaling downstream of matrix metalloproteinase-3 (MMP3), which is commonly upregulated in cancer and known to induce epithelial-mesenchymal transition (EMT). This signaling depended on expression of the Rac1 splice variant, Rac1b, production of ROS, and expression of Snail. Under all conditions, cells cultured on soft substrata maintained a low frequency of multinucleation.

Multinucleation on stiff substrata primarily resulted from midbody abscission failure. A soft microenvironment protected the stability of the genome in epithelial cells by preventing midbody stability, which depended on septin 4, a novel target of Snail.

Importantly, we found that transforming growth factor-β (TGFβ), another EMT-inducer, also caused multinucleation downstream of Snail, which was prevented by culture on soft substrata.

Our data thus suggest that tissue stiffening during tumorigenesis synergizes with oncogenic signaling to promote genomic abnormalities that drive cancer progression. Further, our results suggest that EMT-related signaling pathways are associated with disease progression not necessarily because they induce metastasis, but because they induce genomic instability.

Note: This abstract was not presented at the meeting.

Citation Format: Allison K. Simi, Alisya A. Anlas, Sherry X. Zhang, Tiffaney Hsia, Derek C. Radisky, Celeste M. Nelson. A soft microenvironment protects from failure of midbody abscission and multinucleation downstream of EMT initiators [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5914. doi:10.1158/1538-7445.AM2017-5914

Abstract 4946: Host tissue stiffness regulates chemotherapy-induced cancer cell dormancy

This study investigates how the tumor microenvironment, namely host tissue stiffness, affects the propensity of disseminated tumor cells to become dormant at secondary sites.

Approximately 90% of cancer-related deaths result from metastasis. In breast cancer, disseminated cancer cells cannot be detected at the time of diagnosis of the primary tumor because they form “dormant” micrometastases that are not clinically detectable. Thus, metastases in secondary sites such as the lungs, bone marrow, liver and brain are

usually discovered months or years after the initial diagnosis and/or treatment of the primary tumor.

A 2D polyacrylamide gel-based tissue culture model was used to investigate the effects of host tissue stiffness on the proliferative behavior of breast cancer cells. To assess the dormant phenotype, Ki67 and RNA staining were used to quantify proliferation and RNA

content, respectively. Additionally, changes in levels of cell-cycle regulators were determined by quantitative RT-PCR. Dormant cells have decreased levels of transcription and proliferation.

We found that both tamoxifen and 5-fluorouracil induce dormancy in estrogen receptor-positive breast cancer cells cultured on soft and stiff substrata. After repeated rounds of 5-fluorouracil treatment and recovery, cells cultured on soft substratum, mechanical properties of which represent common metastatic sites, had a higher tendency to recover.

This suggests that cell growth at secondary sites favors a soft microenvironment, characteristic of organs where breast cancer metastases often become clinically overt. Dormant cancer cells, or minimal residual disease, cannot be detected with current

diagnostic tools and cannot be targeted by conventional therapies. Therefore, current clinical practices rely on estimating the probability of recurrence from various prognostic factors including the grade and the stage of the disease. Elucidating the mechanisms that regulate the switch from dormancy to proliferation at metastatic sites will pave the way for new treatments to control this incurable disease.

Citation Format: Alisya Anlas, Celeste M. Nelson. Host tissue stiffness regulates chemotherapy-induced cancer cell dormancy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4946. doi:10.1158/1538-7445.AM2017-4946

Abstract 5943: Role of tissue stiffness and oxygen tension in promoting breast cancer stem cells

Breast tumors are stiff and hypoxic. Nevertheless, it remains unclear how stiff and hypoxic conditions within the tumor microenvironment promote breast cancer progression. Using an innovative engineered culture model to recapitulate these features, we found that, under stiff and hypoxic conditions, breast cancer cells have enhanced integrin-linked kinase (ILK) expression and acquire cancer stem cell (CSC)-like traits, suggesting tissue stiffness and oxygen tension can modulate ILK expression to induce breast CSC formation. Knocking down ILK impairs the mechanosensing of invasive breast cancer cells on stiff microenvironments, inhibits CSC markers and properties. In contrast, ectopic expression of ILK promotes breast CSC formation. In addition to promoting CSC-like phenotype, microarray analysis reveals that stiff and hypoxic microenvironments also regulate genes involved in mRNA processing, splicing and the spliceosome. These data suggest that the non-cellular compartment of the tumor microenvironment, namely tissue mechanics and oxygen tension, can promote breast cancer progression by controlling mechanotransduction and post-transcriptional regulation of breast cancer cells.

Note: This abstract was not presented at the meeting.

Citation Format: Mei-Fong Pang, Derek C. Radisky, Celeste M. Nelson. Role of tissue stiffness and oxygen tension in promoting breast cancer stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5943. doi:10.1158/1538-7445.AM2017-5943

Microfabricated tissues for investigating traction forces involved in cell migration and tissue morphogenesis

Cell-generated forces drive an array of biological processes ranging from wound healing to tumor metastasis. Whereas experimental techniques such as traction force microscopy are capable of quantifying traction forces in multidimensional systems, the physical mechanisms by which these forces induce changes in tissue form remain to be elucidated. Understanding these mechanisms will ultimately require techniques that are capable of quantifying traction forces with high precision and accuracy in vivo or in systems that recapitulate in vivo conditions, such as microfabricated tissues and engineered substrata. To that end, here we review the fundamentals of traction forces, their quantification, and the use of microfabricated tissues designed to study these forces during cell migration and tissue morphogenesis. We emphasize the differences between traction forces in two- and three-dimensional systems, and highlight recently developed techniques for quantifying traction forces.

Generating tissue topology through remodeling of cell-cell adhesions

During tissue morphogenesis, cellular rearrangements give rise to a large variety of three-dimensional structures. Final tissue architecture varies greatly across organs, and many develop to include combinations of folds, tubes, and branched networks. To achieve these different tissue geometries, constituent cells must follow different programs that dictate changes in shape and/or migratory behavior. One essential component of these changes is the remodeling of cell-cell adhesions. Invasive migratory behavior and separation between tissues require localized breakdown of cadherin-mediated adhesions. Conversely, tissue folding and fusion require the formation and reinforcement of cell-cell adhesions. Cell-cell adhesion plays a critical role in tissue morphogenesis; its manipulation may therefore prove to be invaluable in generating complex topologies ex vivo. Recapitulating these shapes in engineered tissues would enable a better understanding of how these processes occur in vivo, and may lead to improved design of organs for clinical applications. In this review, we discuss work investigating the formation of folds, tubes, and branched networks with an emphasis on known or possible roles for cell-cell adhesion. We then examine recently developed tools that could be adapted to manipulate cell-cell adhesion in engineered tissues.

A 3D Culture Model to Study How Fluid Pressure and Flow Affect the Behavior of Aggregates of Epithelial Cells

Cells are surrounded by mechanical stimuli in their microenvironment. It is important to determine how cells respond to the mechanical information that surrounds them in order to understand both development and disease progression, as well as to be able to predict cell behavior in response to physical stimuli. Here we describe a protocol to determine the effects of interstitial fluid flow on the migratory behavior of an aggregate of epithelial cells in a three-dimensional (3D) culture model. This protocol includes detailed methods for the fabrication of a 3D cell culture chamber with hydrostatic pressure control, the culture of epithelial cells as an aggregate in a collagen gel, and the analysis of collective cell behavior in response to pressure-driven flow.

From static to animated: Measuring mechanical forces in tissues

Cells are physical objects that exert mechanical forces on their surroundings as they migrate and take their places within tissues. New techniques are now poised to enable the measurement of cell-generated mechanical forces in intact tissues in vivo, which will illuminate the secret dynamic lives of cells and change our current perception of cell biology.

Interstitial fluid pressure regulates collective invasion in engineered human breast tumors via Snail, vimentin, and E-cadherin

Many solid tumors exhibit elevated interstitial fluid pressure (IFP). This elevated pressure within the core of the tumor results in outward flow of interstitial fluid to the tumor periphery. We previously found that the directionality of IFP gradients modulates collective invasion from the surface of patterned three-dimensional (3D) aggregates of MDA-MB-231 human breast cancer cells. Here, we used this 3D engineered tumor model to investigate the molecular mechanisms underlying IFP-induced changes in invasive phenotype. We found that IFP alters the expression of genes associated with epithelial-mesenchymal transition (EMT). Specifically, the levels of Snail, vimentin, and E-cadherin were increased under pressure conditions that promoted collective invasion. These changes in gene expression were sufficient to direct collective invasion in response to IFP. Furthermore, we found that IFP modulates the motility and persistence of individual cells within the aggregates, which are also influenced by the expression levels of EMT markers. Together, these data provide insight into the molecular mechanisms that guide collective invasion from primary tumors in response to IFP.

On Buckling Morphogenesis

Cell-generated mechanical forces drive many of the tissue movements and rearrangements that are required to transform simple populations of cells into the complex three-dimensional geometries of mature organs. However, mechanical forces do not need to arise from active cellular movements. Recent studies have illuminated the roles of passive forces that result from mechanical instabilities between epithelial tissues and their surroundings. These mechanical instabilities cause essentially one-dimensional epithelial tubes and two-dimensional epithelial sheets to buckle or wrinkle into complex topologies containing loops, folds, and undulations in organs as diverse as the brain, the intestine, and the lung. Here, I highlight examples of buckling and wrinkling morphogenesis, and suggest that this morphogenetic mechanism may be broadly responsible for sculpting organ form.

Tissue Stiffness and Hypoxia Modulate the Integrin-Linked Kinase ILK to Control Breast Cancer Stem-like Cells

Breast tumors are stiffer and more hypoxic than nonmalignant breast tissue. Here we report that stiff and hypoxic microenvironments promote the development of breast cancer stem-like cells (CSC) through modulation of the integrin-linked kinase ILK. Depleting ILK blocked stiffness and hypoxia-dependent acquisition of CSC marker expression and behavior, whereas ectopic expression of ILK stimulated CSC development under softer or normoxic conditions. Stiff microenvironments also promoted tumor formation and metastasis in ovo, where depleting ILK significantly abrogated the tumorigenic and metastatic potential of invasive breast cancer cells. We further found that the ILK-mediated phenotypes induced by stiff and hypoxic microenvironments are regulated by PI3K/Akt. Analysis of human breast cancer specimens revealed an association between substratum stiffness, ILK, and CSC markers, insofar as ILK and CD44 were expressed in cancer cells located in tumor regions predicted to be stiff. Our results define ILK as a key mechanotransducer in modulating breast CSC development in response to tissue mechanics and oxygen tension. Cancer Res; 76(18); 5277-87. ©2016 AACR.

Abstract 4395: Tissue stiffness and hypoxia regulate breast cancer stem cells through ILK

The mechanical and chemical properties of the cellular microenvironment can influence the phenotype of cancer cells. The hypoxic conditions within solid breast tumors can contribute to invasiveness and the formation of breast cancer stem cells (CSCs). Integrin-linked kinase (ILK) is a critical mediator for mechanotransduction, a process that converts exogenous mechanical stimuli into biochemical signals. Yet, it is unclear how the physical properties of the tumor microenvironment, including matrix stiffness and hypoxia, integrate to direct the formation of the breast CSC niche.

Using an innovative engineered culture model, we found that stiff and hypoxic microenvironments activate integrin signaling and a stem-like gene signature in breast cancer cells, suggesting that integrin signaling, matrix mechanics and oxygen tension cooperatively promote the formation of breast CSCs. Stiff and hypoxic microenvironments promote integrin signaling and the expression of CSC markers including CD44 and Nanog. Knocking down ILK reverts the CSC phenotype of invasive breast cancer cells on stiff matrix, and abrogates their ability to form mammospheres and colonies in soft agar. In contrast, ectopic expression of ILK enhances CSC properties. ILK promotes integrin signaling and the CSC phenotype through the PI3K/Akt pathway. Stiff microenvironments promote tumor formation and metastasis in ovo. ILK depletion significantly abrogates the tumorigenenic and metastatic potential of invasive breast cancer cells in ovo. Importantly, we found that breast cancer cells expressing ILK and the CSC marker CD44 were only present in the regions of tumors predicted to be stiff in breast cancer patient samples.

Our data suggest that ILK act as an essential mechanosensor that regulates breast CSC phenotype in response to matrix mechanics and oxygen tension to regulate the formation of breast CSCs.

Citation Format: Mei-Fong Pang, Melody Stallings-Mann, Michael J. Siedlik, Victor D. Varner, Siyang Han, Derek C. Radisky, Celeste M. Nelson. Tissue stiffness and hypoxia regulate breast cancer stem cells through ILK. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4395.

Abstract 4269: Interstitial fluid pressure alters cell motility and collective invasion via EMT marker expression in an engineered model of a human breast tumor

We developed a three-dimensional (3D) engineered model of a solid human breast tumor to study the effects of interstitial fluid pressure (IFP) on collective invasion and the expression levels of epithelial-mesenchymal transition (EMT) markers. Many solid tumors exhibit elevated IFP; as these tumors grow, intra-tumoral vascular and lymphatic vessels collapse. The non-functioning lymphatic system impairs drainage, and immature hyperpermeable blood vessels cause fluid to accumulate within the interstitial space. As a result, IFP rises steeply beyond the tumor periphery and plateaus at pressures as high as 50 mm Hg above normal at the tumor core. This pressure profile, in turn, leads to outward fluid flow from the core of the tumor. IFP has been shown to affect the migratory behavior of individual cells in 3D cell culture models, though its role in collective cancer invasion remains unknown. Moreover, the underlying molecular mechanisms linking IFP to changes in cell motility remain unclear. We sought to address these questions using our engineered model.

Our 3D culture model consists of an aggregate of MDA-MB-231 breast cancer cells (mimicking a solid tumor) embedded within a 3D collagen gel that is flanked by two media reservoirs. The IFP profile experienced by the cancer cells is established by altering the heights of the media reservoirs on either side of the collagen, creating a hydrostatic pressure gradient. Transcript levels of EMT markers in the aggregates subjected to a variety of pressure profiles were determined using quantitative real-time PCR. Expression of these markers was also manipulated ectopically through the creation of stable cell lines. Time-lapse imaging and cell tracking were used to determine the persistence and motility of individual cells within the aggregates.

We found that the direction of IFP-induced flow determines the invasive phenotype of tumor cells. Additionally, high expression levels of both mesenchymal (Snail1, vimentin) and epithelial (E-cadherin, keratin-8) markers were characteristic of collectively invading aggregates, suggesting that partial EMT is important for collective invasion. Ectopic expression and knockdown of EMT markers revealed that they are necessary and sufficient for collective invasion in response to IFP. Time-lapse imaging analysis demonstrated that IFP and EMT marker expression also affect the motility and persistence of individual cells within the aggregates, further confirming that IFP is an important regulator of collective invasion. In conclusion, we used a robust culture model of a human breast tumor to gain insight into the mechanisms guiding collective invasion from primary tumors in response to IFP; IFP alters expression levels of EMT markers, thereby regulating collective invasion.

Citation Format: Alexandra S. Piotrowski-Daspit, Joe Tien, Celeste M. Nelson. Interstitial fluid pressure alters cell motility and collective invasion via EMT marker expression in an engineered model of a human breast tumor. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4269.

Abstract 1926: Hippo pathway effectors YAP/TAZ integrate tissue mechanics and Wnt signaling to regulate microRNA biogenesis

Although mechanical cues including extracellular matrix (ECM) stiffness are known to regulate cell and tissue behavior, it remains unclear how these mechanical cues integrate with biochemical signals to control these processes. The Hippo pathway and its transcriptional coactivators YAP and TAZ, which have emerged as major regulators of organ size, proliferation and cancer development, are thought to be involved in this integration. In addition to being regulated by the canonical Hippo pathway, YAP/TAZ have been found to mediate responses to both biochemical and mechanical signals including ECM stiffness and Wnt signaling. Moreover, recent studies have shown that microRNAs (miRNAs) are sensitive to mechanical cues and YAP/TAZ may regulate miRNA biogenesis through mechanotransduction. Although misregulation of YAP/TAZ and miRNA expression are frequently observed in multiple cancers, which appears to be associated with the altered mechanics of the tumor microenvironment, the signaling mechanisms controlling these processes remain poorly understood and controversial. Here we examine how YAP/TAZ integrate biochemical and mechanical signals to regulate miRNA biogenesis. We found that Wnt3A and stiff ECM synergistically lead to nuclear accumulation and activation of YAP/TAZ. Surprisingly, ECM stiffness induces miRNA-18a expression while Wnt3A signaling has an opposite effect on miRNA-18a expression depending on the stiffness of the cellular microenvironment. Thus, YAP/TAZ activity may define a mechanism by which cells respond to ECM stiffness and Wnt signaling to control miRNA biogenesis.

Citation Format: Siyang Han, Celeste M. Nelson. Hippo pathway effectors YAP/TAZ integrate tissue mechanics and Wnt signaling to regulate microRNA biogenesis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1926.

Engineering Three-dimensional Epithelial Tissues Embedded within Extracellular Matrix

The architecture of branched organs such as the lungs, kidneys, and mammary glands arises through the developmental process of branching morphogenesis, which is regulated by a variety of soluble and physical signals in the microenvironment. Described here is a method created to study the process of branching morphogenesis by forming engineered three-dimensional (3D) epithelial tissues of defined shape and size that are completely embedded within an extracellular matrix (ECM). This method enables the formation of arrays of identical tissues and enables the control of a variety of environmental factors, including tissue geometry, spacing, and ECM composition. This method can also be combined with widely used techniques such as traction force microscopy (TFM) to gain more information about the interactions between cells and their surrounding ECM. The protocol can be used to investigate a variety of cell and tissue processes beyond branching morphogenesis, including cancer invasion.

Pulling together: Tissue-generated forces that drive lumen morphogenesis

Mechanical interactions are essential for bending and shaping tissues during morphogenesis. A common feature of nearly all internal organs is the formation of a tubular network consisting of an epithelium that surrounds a central lumen. Lumen formation during organogenesis requires precisely coordinated mechanical and biochemical interactions. Whereas many genetic regulators of lumen formation have been identified, relatively little is known about the mechanical cues that drive lumen morphogenesis. Lumens can be shaped by a variety of physical behaviors including wrapping a sheet of cells around a hollow core, rearranging cells to expose a lumenal cavity, or elongating a tube via cell migration, though many of the details underlying these movements remain poorly understood. It is essential to define how forces generated by individual cells cooperate to produce the tissue-level forces that drive organogenesis. Transduction of mechanical forces relies on several conserved processes including the contraction of cytoskeletal networks or expansion of lumens through increased fluid pressure. The morphogenetic events that drive lumen formation serve as a model for similar mechanical processes occurring throughout development. To understand how lumenal networks arise, it will be essential to investigate how biochemical and mechanical processes integrate to generate complex structures from comparatively simple interactions.

Computational models of airway branching morphogenesis

The bronchial network of the mammalian lung consists of millions of dichotomous branches arranged in a highly complex, space-filling tree. Recent computational models of branching morphogenesis in the lung have helped uncover the biological mechanisms that construct this ramified architecture. In this review, we focus on three different theoretical approaches - geometric modeling, reaction-diffusion modeling, and continuum mechanical modeling - and discuss how, taken together, these models have identified the geometric principles necessary to build an efficient bronchial network, as well as the patterning mechanisms that specify airway geometry in the developing embryo. We emphasize models that are integrated with biological experiments and suggest how recent progress in computational modeling has advanced our understanding of airway branching morphogenesis.