Tissue geometry determines sites of mammary branching morphogenesis in organotypic cultures

Engineered extracellular matrices: emerging strategies for decoupling structural and molecular signals that regulate epithelial branching morphogenesis

The extracellular matrix (ECM) is a heterogeneous mixture of proteoglycans and fibrous proteins that form the non-cellular component of tissues and organs. During normal development, homeostasis, and disease progression, the ECM provides dynamic structural and molecular signals that influence the form and function of individual cells and multicellular tissues. Here, we review recent developments in the design and fabrication of engineered ECMs and the application of these systems to study the morphogenesis of epithelial tissues. We emphasize emerging techniques for reproducing the structural and molecular complexity of native ECM, and we highlight how these techniques may be used to decouple the different signals that drive epithelial morphogenesis. Engineered models of native ECM will enable further investigation of the dynamic mechanisms by which the microenvironment influences tissue morphogenesis.

PhysiBoSS: a multi-scale agent-based modelling framework integrating physical dimension and cell signalling

MOTIVATION

Due to the complexity and heterogeneity of multicellular biological systems, mathematical models that take into account cell signalling, cell population behaviour and the extracellular environment are particularly helpful. We present PhysiBoSS, an open source software which combines intracellular signalling using Boolean modelling (MaBoSS) and multicellular behaviour using agent-based modelling (PhysiCell).

RESULTS

PhysiBoSS provides a flexible and computationally efficient framework to explore the effect of environmental and genetic alterations of individual cells at the population level, bridging the critical gap from single-cell genotype to single-cell phenotype and emergent multicellular behaviour. PhysiBoSS thus becomes very useful when studying heterogeneous population response to treatment, mutation effects, different modes of invasion or isomorphic morphogenesis events. To concretely illustrate a potential use of PhysiBoSS, we studied heterogeneous cell fate decisions in response to TNF treatment. We explored the effect of different treatments and the behaviour of several resistant mutants. We highlighted the importance of spatial information on the population dynamics by considering the effect of competition for resources like oxygen.

AVAILABILITY AND IMPLEMENTATION

PhysiBoSS is freely available on GitHub (https://github.com/sysbio-curie/PhysiBoSS), with a Docker image (https://hub.docker.com/r/gletort/physiboss/). It is distributed as open source under the BSD 3-clause license.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Current strategies and opportunities to manufacture cells for modeling human lungs

Chronic lung diseases remain major healthcare burdens, for which the only curative treatment is lung transplantation. In vitro human models are promising platforms for identifying and testing novel compounds to potentially decrease this burden. Directed differentiation of pluripotent stem cells is an important strategy to generate lung cells to create such models. Current lung directed differentiation protocols are limited as they do not 1) recapitulate the diversity of respiratory epithelium, 2) generate consistent or sufficient cell numbers for drug discovery platforms, and 3) establish the histologic tissue-level organization critical for modeling lung function. In this review, we describe how lung development has formed the basis for directed differentiation protocols, and discuss the utility of available protocols for lung epithelial cell generation and drug development. We further highlight tissue engineering strategies for manipulating biophysical signals during directed differentiation such that future protocols can recapitulate both chemical and physical cues present during lung development.

Substrate curvature as a cue to guide spatiotemporal cell and tissue organization

Recent evidence clearly shows that cells respond to various physical cues in their environments, guiding many cellular processes and tissue morphogenesis, pathology, and repair. One aspect that is gaining significant traction is the role of local geometry as an extracellular cue. Elucidating how geometry affects cell and tissue behavior is, indeed, crucial to design artificial scaffolds and understand tissue growth and remodeling. Perhaps the most fundamental descriptor of local geometry is surface curvature, and a growing body of evidence confirms that surface curvature affects the spatiotemporal organization of cells and tissues. While well-defined in differential geometry, curvature remains somewhat ambiguously treated in biological studies. Here, we provide a more formal curvature framework, based on the notions of mean and Gaussian curvature, and summarize the available evidence on curvature guidance at the cell and tissue levels. We discuss the involved mechanisms, highlighting the interplay between tensile forces and substrate curvature that forms the foundation of curvature guidance. Moreover, we show that relatively simple computational models, based on some application of curvature flow, are able to capture experimental tissue growth remarkably well. Since curvature guidance principles could be leveraged for tissue regeneration, the implications for geometrical scaffold design are also discussed. Finally, perspectives on future research opportunities are provided.

The role of extracellular matrix in biomechanics and its impact on bioengineering of cells and 3D tissues

The cells and tissues of the human body are constantly exposed to exogenous and endogenous forces that are referred to as biomechanical cues. They guide and impact cellular processes and cell fate decisions on the nano-, micro- and macro-scale, and are therefore critical for normal tissue development and maintaining tissue homeostasis. Alterations in the extracellular matrix composition of a tissue combined with abnormal mechanosensing and mechanotransduction can aberrantly activate signaling pathways that promote disease development. Such processes are therefore highly relevant for disease modelling or when aiming for the development of novel therapies. In this mini review, we describe the main biomechanical cues that impact cellular fates. We highlight their role during development, homeostasis and in disease. We also discuss current techniques and tools that allow us to study the impact of biomechanical cues on cell and tissue development under physiological conditions, and we point out directions, in which in vitro biomechanics can be of use in the future.

A microfluidic mammary gland coculture model using parallel 3D lumens for studying epithelial-endothelial migration in breast cancer

In breast cancer development, crosstalk between mammary epithelial cells and neighboring vascular endothelial cells is critical to understanding tumor progression and metastasis, but the mechanisms of this dynamic interplay are not fully understood. Current cell culture platforms do not accurately recapitulate the 3D luminal architecture of mammary gland elements. Here, we present the development of an accessible and scalable microfluidic coculture system that incorporates two parallel 3D luminal structures that mimic vascular endothelial and mammary epithelial cell layers, respectively. This parallel 3D lumen configuration allows investigation of endothelial-epithelial crosstalk and its effects of the comigration of endothelial and epithelial cells into microscale migration ports located between the parallel lumens. We describe the development and application of our platform, demonstrate generation of 3D luminal cell layers for endothelial cells and three different breast cancer cell lines, and quantify their migration profiles based on number of migrated cells, area coverage by migrated cells, and distance traveled by individual migrating cells into the migration ports. Our system enables analysis at the single-cell level, allows simultaneous monitoring of endothelial and epithelial cell migration within a 3D extracellular matrix, and has potential for applications in basic research on cellular crosstalk as well as drug development.

Dental Epithelial Stem Cells as a Source for Mammary Gland Regeneration and Milk Producing Cells In Vivo

The continuous growth of rodent incisors is ensured by clusters of mesenchymal and epithelial stem cells that are located at the posterior part of these teeth. Genetic lineage tracing studies have shown that dental epithelial stem cells (DESCs) are able to generate all epithelial cell populations within incisors during homeostasis. However, it remains unclear whether these cells have the ability to adopt alternative fates in response to extrinsic factors. Here, we have studied the plasticity of DESCs in the context of mammary gland regeneration. Transplantation of DESCs together with mammary epithelial cells into the mammary stroma resulted in the formation of chimeric ductal epithelial structures in which DESCs adopted all the possible mammary fates including milk-producing alveolar cells. In addition, when transplanted without mammary epithelial cells, DESCs developed branching rudiments and cysts. These in vivo findings demonstrate that when outside their niche, DESCs redirect their fates according to their new microenvironment and thus can contribute to the regeneration of non-dental tissues.

Light-inducible activation of cell cycle progression in Xenopus egg extracts under microfluidic confinement

Cell-free Xenopus egg extract is a widely used and biochemically tractable model system that allows recapitulation and elucidation of fundamental cellular processes. Recently, the introduction of microfluidic extract manipulation has enabled compartmentalization of bulk extract and a newfound ability to study organelles on length scales that recapitulate key features of cellular morphology. While the microfluidic confinement of extracts has produced a compelling platform for the in vitro study of cell processes at physiologically-relevant length scales, it also imposes experimental limitations by restricting dynamic control over extract properties. Here, we introduce photodegradable polyethylene glycol (PEG) hydrogels as a vehicle to passively and selectively manipulate extract composition through the release of proteins encapsulated within the hydrogel matrix. Photopatterned PEG hydrogels, passive to both extract and encapsulated proteins, serve as protein depots within microfluidic channels, which are subsequently flooded with extract. Illumination by ultraviolet light (UV) degrades the hydrogel structures and releases encapsulated protein. We show that an engineered fluorescent protein with a nuclear localization signal (GST-GFP-NLS) retains its ability to localize within nearby nuclei following UV-induced release from hydrogel structures. When diffusion is considered, the kinetics of nuclear accumulation are similar to those in experiments utilizing conventional, bulk fluid handling. Similarly, the release of recombinant cyclin B Δ90, a mutant form of the master cell cycle regulator cyclin B which lacks the canonical destruction box, was able to induce the expected cell cycle transition from interphase to mitosis. This transition was confirmed by the observation of nuclear envelope breakdown (NEBD), a phenomenological hallmark of mitosis, and the induction of mitosis-specific biochemical markers. This approach to extract manipulation presents a versatile and customizable route to regulating the spatial and temporal dynamics of cellular events in microfluidically confined cell-free extracts.

Adjustable viscoelasticity allows for efficient collective cell migration

Cell migration is essential for a wide range of biological processes such as embryo morphogenesis, wound healing, regeneration, and also in pathological conditions, such as cancer. In such contexts, cells are required to migrate as individual entities or as highly coordinated collectives, both of which requiring cells to respond to molecular and mechanical cues from their environment. However, whilst the function of chemical cues in cell migration is comparatively well understood, the role of tissue mechanics on cell migration is just starting to be studied. Recent studies suggest that the dynamic tuning of the viscoelasticity within a migratory cluster of cells, and the adequate elastic properties of its surrounding tissues, are essential to allow efficient collective cell migration in vivo. In this review we focus on the role of viscoelasticity in the control of collective cell migration in various cellular systems, mentioning briefly some aspects of single cell migration. We aim to provide details on how viscoelasticity of collectively migrating groups of cells and their surroundings is adjusted to ensure correct morphogenesis, wound healing, and metastasis. Finally, we attempt to show that environmental viscoelasticity triggers molecular changes within migrating clusters and that these new molecular setups modify clusters' viscoelasticity, ultimately allowing them to migrate across the challenging geometries of their microenvironment.

Engineering Functional Cardiac Tissues for Regenerative Medicine Applications

PURPOSE OF REVIEW

Tissue engineering has expanded into a highly versatile manufacturing landscape that holds great promise for advancing cardiovascular regenerative medicine. In this review, we provide a summary of the current state-of-the-art bioengineering technologies used to create functional cardiac tissues for a variety of applications in vitro and in vivo.

RECENT FINDINGS

Studies over the past few years have made a strong case that tissue engineering is one of the major driving forces behind the accelerating fields of patient-specific regenerative medicine, precision medicine, compound screening, and disease modeling. To date, a variety of approaches have been used to bioengineer functional cardiac constructs, including biomaterial-based, cell-based, and hybrid (using cells and biomaterials) approaches. While some major progress has been made using cellular approaches, with multiple ongoing clinical trials, cell-free cardiac tissue engineering approaches have also accomplished multiple breakthroughs, although drawbacks remain. This review summarizes the most promising methods that have been employed to generate cardiovascular tissue constructs for basic science or clinical applications. Further, we outline the strengths and challenges that are inherent to this field as a whole and for each highlighted technology.

Fibronectin-induced ductal formation in salivary gland self-organization model

BACKGROUND

Recent advances in tissue regeneration approaches including 3D organoids, were based on various 3D organogenesis models. However, 3D models are generally technique-sensitive and time-consuming. Thus, we utilized an existing model of submandibular salivary gland (SMG) to modify a simple and highly reproducible in vitro 3D culture model of primary SMG cells self-organization into a well-developed cell spheroid inside Matrigel substrate. We used this model to observe the collective multicellular behavior during spheroid formation. Further, we applied various quantitative approaches including real-time live imaging and immune histochemical image analysis to dissect the cellular dynamics during tissue patterning.

RESULTS

On a time-scale of hours, we observed marked size and shape transformations in the developed 3D spheroid which resulted in a spatially-controlled growth differential from the canter to the periphery of the formed aggregates. Moreover, we investigated the effect of fibronectin (FN) on SMG cells self-organization using our simplified culture model. Interestingly, we discovered a novel role of FN in inducing duct-like elongation during initial stages of SMG bud formation.

CONCLUSION

This in vitro model provides an excellent tool for analyzing the intercellular dynamics during early SMG tissue development as well as revealing a novel role of FN in SMG ductal expansion.

Organoid technology in cancer precision medicine

Organoid technology has been remarkably improved over the last decade. Various organoids have been derived from different types of tissues and recapitulate their organ-specific gene expression signatures, particular tissue spatial structures and functions of their original tissue. The patient-derived organoids (PDOs) have been used to elucidate crucial scientific questions, including the relationships between genetic/epigenetic alterations and drug responses, cell plasticity during disease progressions, and mechanisms of drug resistances. With the great expectations, PDOs will be widely used to facilitate the personalized medical decisions, which have the potential to profoundly improve patient outcomes. In this review, we will discuss the developmental details, current achievements, applications and challenges of organoid technology in precision cancer medicine.

Neural crest streaming as an emergent property of tissue interactions during morphogenesis

A fundamental question in embryo morphogenesis is how a complex pattern is established in seemingly uniform tissues. During vertebrate development, neural crest cells differentiate as a continuous mass of tissue along the neural tube and subsequently split into spatially distinct migratory streams to invade the rest of the embryo. How these streams are established is not well understood. Inhibitory signals surrounding the migratory streams led to the idea that position and size of streams are determined by a pre-pattern of such signals. While clear evidence for a pre-pattern in the cranial region is still lacking, all computational models of neural crest migration published so far have assumed a pre-pattern of negative signals that channel the neural crest into streams. Here we test the hypothesis that instead of following a pre-existing pattern, the cranial neural crest creates their own migratory pathway by interacting with the surrounding tissue. By combining theoretical modeling with experimentation, we show that streams emerge from the interaction of the hindbrain neural crest and the neighboring epibranchial placodal tissues, without the need for a pre-existing guidance cue. Our model suggests that the initial collective neural crest invasion is based on short-range repulsion and asymmetric attraction between neighboring tissues. The model provides a coherent explanation for the formation of cranial neural crest streams in concert with previously reported findings and our new in vivo observations. Our results point to a general mechanism of inducing collective invasion patterns.

High repeatability from 3D experimental platform for quantitative analysis of cellular branch pattern formations

Three-dimensional (3D) cell and tissue cultures more closely mimic biological environments than two-dimensional (2D) cultures and are therefore highly desirable in culture experiments. However, 3D cultures often fail to yield repeatable experimental results because of variation in the initial culture conditions, such as cell density and distribution in the extracellular matrix, and therefore reducing such variation is a paramount concern. Here, we present a 3D culture platform that demonstrates highly repeatable experimental results, obtained by controlling the initial cell cluster shape in the gel cube culture device. A micro-mould with the desired shape was fabricated by photolithography or machining, creating a 3D pocket in the extracellular matrix contained in the device. Highly concentrated human bronchial epithelial cells were then injected in the pocket so that the cell cluster shape matched the fabricated mould shape. Subsequently, the cubic device supplied multi-directional scanning, enabling high-resolution capture of the whole tissue structure with only a low-magnification lens. The proposed device significantly improved the repeatability of the developed branch pattern, and multi-directional scanning enabled quantitative analysis of the developed branch pattern formations. A mathematical simulation was also conducted to reveal the mechanisms of branch pattern formation. The proposed platform offers the potential to accelerate any research field that conducts 3D culture experiments, including tissue regeneration and drug development.

Cysteine Dioxygenase Regulates the Epithelial Morphogenesis of Mammary Gland via Cysteine Sulfinic Acid

Epithelial morphogenesis is a common feature in various organs and contributes to functional formation. However, the molecular mechanisms behind epithelial morphogenesis remain largely unknown. Mammary gland is an excellent model system to investigate the molecular mechanisms of epithelial morphogenesis. In this study, we found that cysteine dioxygenase (CDO), a key enzyme in cysteine oxidative metabolism, was involved in mammary epithelial morphogenesis. CDO knockout (KO) females exhibited severe defects in mammary branching morphogenesis and ductal elongation, resulting in poor lactation. CDO contributes to the luminal epithelial cell differentiation, proliferation, and apoptosis mainly through its downstream product cysteine sulfinic acid (CSA). Exogenous supplementation of CSA not only rescued the defects in CDO KO mouse but also enhanced ductal growth in wild-type mouse. It suggests that CDO regulates luminal epithelial differentiation and regeneration via CSA and consequently contributes to mammary development, which raises important implications for epithelial morphogenesis and pathogenesis of breast cancer.

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Cysteine dioxygenase (CDO) is necessary for mammary epithelial morphogenesis

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Cysteine sulfinic acid (CSA) supplementation rescues the mammary defects in CDO KO mouse

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CDO retains lumen character and maintains luminal cell differentiation via CSA

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CDO maintains epithelial cell renewal via CSA

Image-based modeling of kidney branching morphogenesis reveals GDNF-RET based Turing-type mechanism and pattern-modulating WNT11 feedback

Branching patterns and regulatory networks differ between branched organs. It has remained unclear whether a common regulatory mechanism exists and how organ-specific patterns can emerge. Of all previously proposed signalling-based mechanisms, only a ligand-receptor-based Turing mechanism based on FGF10 and SHH quantitatively recapitulates the lung branching patterns. We now show that a GDNF-dependent ligand-receptor-based Turing mechanism quantitatively recapitulates branching of cultured wildtype and mutant ureteric buds, and achieves similar branching patterns when directing domain outgrowth in silico. We further predict and confirm experimentally that the kidney-specific positive feedback between WNT11 and GDNF permits the dense packing of ureteric tips. We conclude that the ligand-receptor based Turing mechanism presents a common regulatory mechanism for lungs and kidneys, despite the differences in the molecular implementation. Given its flexibility and robustness, we expect that the ligand-receptor-based Turing mechanism constitutes a likely general mechanism to guide branching morphogenesis and other symmetry breaks during organogenesis.

Many organs develop through branching morphogenesis, but whether the underlying mechanisms are shared is unknown. Here, the authors show that a ligand-receptor based Turing mechanisms, similar to that observed in lung development, likely underlies branching morphogenesis of the kidney.

Dispersible hydrogel force sensors reveal patterns of solid mechanical stress in multicellular spheroid cultures

Understanding how forces orchestrate tissue formation requires technologies to map internal tissue stress at cellular length scales. Here, we develop ultrasoft mechanosensors that visibly deform under less than 10 Pascals of cell-generated stress. By incorporating these mechanosensors into multicellular spheroids, we capture the patterns of internal stress that arise during spheroid formation. We experimentally demonstrate the spontaneous generation of a tensional 'skin', only a few cell layers thick, at the spheroid surface, which correlates with activation of mechanobiological signalling pathways, and balances a compressive stress profile within the tissue. These stresses develop through cell-driven mechanical compaction at the tissue periphery, and suggest that the tissue formation process plays a critically important role in specifying mechanobiological function. The broad applicability of this technique should ultimately provide a quantitative basis to design tissues that leverage the mechanical activity of constituent cells to evolve towards a desired form and function.

Regulation of YAP/TAZ Activity by Mechanical Cues: An Experimental Overview

YAP/TAZ activity is regulated by a complex network of signals that include the Hippo pathway, cell polarity complexes, and signaling receptors of the RTK, GPCR, and WNT pathways and by a seamlessly expanding number of intracellular cues including energy and mevalonate metabolism. Among these inputs, we here concentrate on mechanical cues embedded in the extracellular matrix (ECM) microenvironment, which are key regulators of YAP/TAZ activity. We review the techniques that have been used to study mechano-regulation of YAP/TAZ, including conceptual and practical considerations on how these experiments should be designed and controlled. Finally, we briefly review the most appropriate techniques to monitor YAP/TAZ activity in these experiments and their significance to study the mechanisms linking YAP/TAZ to mechanical cues.

Mechanically robust photodegradable gelatin hydrogels for 3D cell culture and in situ mechanical modification

Highly conjugated, hydrophobically modified gelatin hydrogels were synthesized, polymerized and degraded with orthogonal wavelengths of light.

Spatial patterning of liver progenitor cell differentiation mediated by cellular contractility and Notch signaling

The progenitor cells of the developing liver can differentiate toward both hepatocyte and biliary cell fates. In addition to the established roles of TGFβ and Notch signaling in this fate specification process, there is increasing evidence that liver progenitors are sensitive to mechanical cues. Here, we utilized microarrayed patterns to provide a controlled biochemical and biomechanical microenvironment for mouse liver progenitor cell differentiation. In these defined circular geometries, we observed biliary differentiation at the periphery and hepatocytic differentiation in the center. Parallel measurements obtained by traction force microscopy showed substantial stresses at the periphery, coincident with maximal biliary differentiation. We investigated the impact of downstream signaling, showing that peripheral biliary differentiation is dependent not only on Notch and TGFβ but also E-cadherin, myosin-mediated cell contractility, and ERK. We have therefore identified distinct combinations of microenvironmental cues which guide fate specification of mouse liver progenitors toward both hepatocyte and biliary fates.

Mathematical Approaches of Branching Morphogenesis

Many organs require a high surface to volume ratio to properly function. Lungs and kidneys, for example, achieve this by creating highly branched tubular structures during a developmental process called branching morphogenesis. The genes that control lung and kidney branching share a similar network structure that is based on ligand-receptor reciprocal signalling interactions between the epithelium and the surrounding mesenchyme. Nevertheless, the temporal and spatial development of the branched epithelial trees differs, resulting in organs of distinct shape and size. In the embryonic lung, branching morphogenesis highly depends on FGF10 signalling, whereas GDNF is the driving morphogen in the kidney. Knockout of Fgf10 and Gdnf leads to lung and kidney agenesis, respectively. However, FGF10 plays a significant role during kidney branching and both the FGF10 and GDNF pathway converge on the transcription factors ETV4/5. Although the involved signalling proteins have been defined, the underlying mechanism that controls lung and kidney branching morphogenesis is still elusive. A wide range of modelling approaches exists that differ not only in the mathematical framework (e.g., stochastic or deterministic) but also in the spatial scale (e.g., cell or tissue level). Due to advancing imaging techniques, image-based modelling approaches have proven to be a valuable method for investigating the control of branching events with respect to organ-specific properties. Here, we review several mathematical models on lung and kidney branching morphogenesis and suggest that a ligand-receptor-based Turing model represents a potential candidate for a general but also adaptive mechanism to control branching morphogenesis during development.

Mimicking Epithelial Tissues in Three-Dimensional Cell Culture Models

Epithelial tissues are composed of layers of tightly connected cells shaped into complex three-dimensional (3D) structures such as cysts, tubules, or invaginations. These complex 3D structures are important for organ-specific functions and often create biochemical gradients that guide cell positioning and compartmentalization within the organ. One of the main functions of epithelia is to act as physical barriers that protect the underlying tissues from external insults. In vitro, epithelial barriers are usually mimicked by oversimplified models based on cell lines grown as monolayers on flat surfaces. While useful to answer certain questions, these models cannot fully capture the in vivo organ physiology and often yield poor predictions. In order to progress further in basic and translational research, disease modeling, drug discovery, and regenerative medicine, it is essential to advance the development of new in vitro predictive models of epithelial tissues that are capable of representing the in vivo-like structures and organ functionality more accurately. Here, we review current strategies for obtaining biomimetic systems in the form of advanced in vitro models that allow for more reliable and safer preclinical tests. The current state of the art and potential applications of self-organized cell-based systems, organ-on-a-chip devices that incorporate sensors and monitoring capabilities, as well as microfabrication techniques including bioprinting and photolithography, are discussed. These techniques could be combined to help provide highly predictive drug tests for patient-specific conditions in the near future.

Dachshund Depletion Disrupts Mammary Gland Development and Diverts the Composition of the Mammary Gland Progenitor Pool

DACH1 abundance is reduced in human malignancies, including breast cancer. Herein DACH1 was detected among multipotent fetal mammary stem cells in the embryo, among mixed lineage precursors, and in adult basal cells and (ERα⁺) luminal progenitors. Dach1 gene deletion at 6 weeks in transgenic mice reduced ductal branching, reduced the proportion of mammary basal cells (Lin⁻ CD24^med CD29^high) and reduced abundance of basal cytokeratin 5, whereas DACH1 overexpression induced ductal branching, increased Gata3 and Notch1, and expanded mammosphere formation in LA-7 breast cells. Mammary gland-transforming growth factor β (TGF-β) activity, known to reduce ductal branching and to reduce the basal cell population, increased upon Dach1 deletion, associated with increased SMAD phosphorylation. Association of the scaffold protein Smad anchor for receptor activation with Smad2/3, which facilitates TGF-β activation, was reduced by endogenous DACH1. DACH1 increases basal cells, enhances ductal formation and restrains TGF-β activity in vivo.

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Dach1 is expressed in mammary gland fetal stem cells and adult luminal cells

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Dach1 expands mammary gland basal/myoepithelial cells

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Dach1 induces post-natal mammary gland ductal formation

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Dach1 retrains TGF-β activity in the mammary gland in vivo

Biomaterials and engineered microenvironments to control YAP/TAZ-dependent cell behaviour

Mechanical signals are increasingly recognized as overarching regulators of cell behaviour, controlling stemness, organoid biology, tissue development and regeneration. Moreover, aberrant mechanotransduction is a driver of disease, including cancer, fibrosis and cardiovascular defects. A central question remains how cells compute a host of biomechanical signals into meaningful biological behaviours. Biomaterials and microfabrication technologies are essential to address this issue. Here we review a large body of evidence that connects diverse biomaterial-based systems to the functions of YAP/TAZ, two highly related mechanosensitive transcriptional regulators. YAP/TAZ orchestrate the response to a suite of engineered microenviroments, emerging as a universal control system for cells in two and three dimensions, in static or dynamic fashions, over a range of elastic and viscoelastic stimuli, from solid to fluid states. This approach may guide the rational design of technological and material-based platforms with dramatically improved functionalities and inform the generation of new biomaterials for regenerative medicine applications.

Vessel-on-a-chip with Hydrogel-based Microfluidics

Hydrogel structures equipped with internal microchannels offer more in vivo-relevant models for construction of tissues and organs in vitro. However, currently used microfabrication methods of constructing microfluidic devices are not suitable for the handling of hydrogel. This study presents a novel method of fabricating hydrogel-based microfluidic chips by combining the casting and bonding processes. A twice cross-linking strategy is designed to obtain a bonding interface that has the same strength with the hydrogel bulk, which can be applied to arbitrary combinations of hydrogels. It is convenient to achieve the construction of hydrogel structures with channels in branched, spiral, serpentine, and multilayer forms. The experimental results show that the combination of gelatin and gelatin methacrylate (GelMA) owns the best biocompatibility and can promote cell functionalization. Based on these, a vessel-on-a-chip system with vascular function in both physiological and pathological situations is established, providing a promising model for further investigations such as vascularization, vascular inflammation, tissue engineering, and drug development. Taken together, a facile and cytocompatible approach is developed for engineering a user-defined hydrogel-based chip that can be potentially useful in developing vascularized tissue or organ models.

Divergent Matrix-Remodeling Strategies Distinguish Developmental from Neoplastic Mammary Epithelial Cell Invasion Programs

Metastasizing breast carcinoma cells have been hypothesized to mobilize tissue-invasive activity by co-opting the proteolytic systems employed by normal mammary epithelial cells undergoing branching morphogenesis. However, the critical effectors underlying morphogenesis remain unidentified, and their relationship to breast cancer invasion programs is yet to be established. Here, we identify the membrane-anchored matrix metalloproteinase, Mmp14/MT1-MMP, but not the closely related proteinase Mmp15/MT2-MMP, as the dominant proteolytic effector of both branching morphogenesis and carcinoma cell invasion in vivo. Unexpectedly, however, epithelial cell-specific targeting of Mmp14/MT1-MMP in the normal mammary gland fails to impair branching, whereas deleting the proteinase in carcinoma cells abrogates invasion, preserves matrix architecture, and completely blocks metastasis. By contrast, in the normal mammary gland, extracellular matrix remodeling and morphogenesis are ablated only when Mmp14/MT1-MMP expression is specifically deleted from the periductal stroma. Together, these findings uncover the overlapping but divergent strategies that underlie developmental versus neoplastic matrix remodeling programs.

Role of Extracellular Matrix in Development and Cancer Progression

The immense diversity of extracellular matrix (ECM) proteins confers distinct biochemical and biophysical properties that influence cell phenotype. The ECM is highly dynamic as it is constantly deposited, remodelled, and degraded during development until maturity to maintain tissue homeostasis. The ECM’s composition and organization are spatiotemporally regulated to control cell behaviour and differentiation, but dysregulation of ECM dynamics leads to the development of diseases such as cancer. The chemical cues presented by the ECM have been appreciated as key drivers for both development and cancer progression. However, the mechanical forces present due to the ECM have been largely ignored but recently recognized to play critical roles in disease progression and malignant cell behaviour. Here, we review the ways in which biophysical forces of the microenvironment influence biochemical regulation and cell phenotype during key stages of human development and cancer progression.

Geometrical confinement controls the asymmetric patterning of brachyury in cultures of pluripotent cells

Diffusible signals are known to orchestrate patterning during embryogenesis, yet diffusion is sensitive to noise. The fact that embryogenesis is remarkably robust suggests that additional layers of regulation reinforce patterning. Here, we demonstrate that geometrical confinement orchestrates the spatial organisation of initially randomly positioned subpopulations of spontaneously differentiating mouse embryonic stem cells. We use micropatterning in combination with pharmacological manipulations and quantitative imaging to dissociate the multiple effects of geometry. We show that the positioning of a pre-streak-like population marked by brachyury (T) is decoupled from the size of its population, and that breaking radial symmetry of patterns imposes polarised patterning. We provide evidence for a model in which the overall level of diffusible signals together with the history of the cell culture define the number of T+ cells, whereas geometrical constraints guide patterning in a multi-step process involving a differential response of the cells to multicellular spatial organisation. Our work provides a framework for investigating robustness of patterning and provides insights into how to guide symmetry-breaking events in aggregates of pluripotent cells.

3D culture models for studying branching morphogenesis in the mammary gland and mammalian lung

The intricate architecture of branched tissues and organs has fascinated scientists and engineers for centuries. Yet-despite their ubiquity-the biophysical and biochemical mechanisms by which tissues and organs undergo branching morphogenesis remain unclear. With the advent of three-dimensional (3D) culture models, an increasingly powerful and diverse set of tools are available for investigating the development and remodeling of branched tissues and organs. In this review, we discuss the application of 3D culture models for studying branching morphogenesis of the mammary gland and the mammalian lung in the context of normal development and disease. While current 3D culture models lack the cellular and molecular complexity observed in vivo, we emphasize how these models can be used to answer targeted questions about branching morphogenesis. We highlight the specific advantages and limitations of using 3D culture models to study the dynamics and mechanisms of branching in the mammary gland and mammalian lung. Finally, we discuss potential directions for future research and propose strategies for engineering the next generation of 3D culture models for studying tissue morphogenesis.

Sleeping Beauty and the Microenvironment Enchantment: Microenvironmental Regulation of the Proliferation-Quiescence Decision in Normal Tissues and in Cancer Development

Cells from prokaryota to the more complex metazoans cease proliferating at some point in their lives and enter a reversible, proliferative-dormant state termed quiescence. The appearance of quiescence in the course of evolution was essential to the acquisition of multicellular specialization and compartmentalization and is also a central aspect of tissue function and homeostasis. But what makes a cell cease proliferating even in the presence of nutrients, growth factors, and mitogens? And what makes some cells "wake up" when they should not, as is the case in cancer? Here, we summarize and discuss evidence showing how microenvironmental cues such as those originating from metabolism, extracellular matrix (ECM) composition and arrangement, neighboring cells and tissue architecture control the cellular proliferation-quiescence decision, and how this complex regulation is corrupted in cancer.

Engineering Hydrogel Microenvironments to Recapitulate the Stem Cell Niche

Stem cells are a powerful resource for many applications including regenerative medicine, patient-specific disease modeling, and toxicology screening. However, eliciting the desired behavior from stem cells, such as expansion in a naïve state or differentiation into a particular mature lineage, remains challenging. Drawing inspiration from the native stem cell niche, hydrogel platforms have been developed to regulate stem cell fate by controlling microenvironmental parameters including matrix mechanics, degradability, cell-adhesive ligand presentation, local microstructure, and cell-cell interactions. We survey techniques for modulating hydrogel properties and review the effects of microenvironmental parameters on maintaining stemness and controlling differentiation for a variety of stem cell types. Looking forward, we envision future hydrogel designs spanning a spectrum of complexity, ranging from simple, fully defined materials for industrial expansion of stem cells to complex, biomimetic systems for organotypic cell culture models.

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.

Regulation of cell cycle progression by cell-cell and cell-matrix forces

It has long been proposed that the cell cycle is regulated by physical forces at the cell-cell and cell-extracellular matrix (ECM) interfaces^1-12. However, the evolution of these forces during the cycle has never been measured in a tissue, and whether this evolution affects cell cycle progression is unknown. Here, we quantified cell-cell tension and cell-ECM traction throughout the complete cycle of a large cell population in a growing epithelium. These measurements unveil temporal mechanical patterns that span the entire cell cycle and regulate its duration, the G1-S transition and mitotic rounding. Cells subjected to higher intercellular tension exhibit a higher probability to transition from G1 to S, as well as shorter G1 and S-G2-M phases. Moreover, we show that tension and mechanical energy are better predictors of the duration of G1 than measured geometric properties. Tension increases during the cell cycle but decreases 3 hours before mitosis. Using optogenetic control of contractility, we show that this tension drop favours mitotic rounding. Our results establish that cell cycle progression is regulated cooperatively by forces between the dividing cell and its neighbours.

R-spondin3 Is Associated with Basal-Progenitor Behavior in Normal and Tumor Mammary Cells

R-spondin3 (RSPO3) is a member of a family of secreted proteins that enhance Wnt signaling pathways in diverse processes, including cancer. However, the role of RSPO3 in mammary gland and breast cancer development remains unclear. In this study, we show that RSPO3 is expressed in the basal stem cell-enriched compartment of normal mouse mammary glands but is absent from committed mature luminal cells in which exogenous RSPO3 impairs lactogenic differentiation. RSPO3 knockdown in basal-like mouse mammary tumor cells reduced canonical Wnt signaling, epithelial-to-mesenchymal transition-like features, migration capacity, and tumor formation in vivo Conversely, RSPO3 overexpression, which was associated with some LGR and RUNX factors, highly correlated with the basal-like subtype among patients with breast cancer. Thus, we identified RSPO3 as a novel key modulator of breast cancer development and a potential target for treatment of basal-like breast cancers.Significance: These findings identify RSPO3 as a potential therapetuic target in basal-like breast cancers.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/16/4497/F1.large.jpg Cancer Res; 78(16); 4497-511. ©2018 AACR.

Disparate levels of beta-catenin activity determine nephron progenitor cell fate

Formation of a functional kidney depends on the balance between renewal and differentiation of nephron progenitors. Failure to sustain this balance can lead to kidney failure or stem cell tumors. For nearly 60 years, we have known that signals from an epithelial structure known as the ureteric bud were essential for maintaining this balance. More recently it was discovered that one molecule, Wnt9b, was necessary for both renewal and differentiation of the nephron progenitor cells. How one ligand signaling through one transcription factor promoted two seemingly contradictory cellular processes was unclear. In this study, we show that Wnt9b/beta-catenin signaling alone is sufficient to promote both renewal and differentiation. Moreover, we show that discrete levels of beta-catenin can promote these two disparate fates, with low levels fostering progenitor renewal and high levels driving differentiation. These results provide insight into how Wnt9b regulates distinct target genes that balance nephron progenitor renewal and differentiation.

Microfabrication-Based Three-Dimensional (3-D) Extracellular Matrix Microenvironments for Cancer and Other Diseases

Exploring the complicated development of tumors and metastases needs a deep understanding of the physical and biological interactions between cancer cells and their surrounding microenvironments. One of the major challenges is the ability to mimic the complex 3-D tissue microenvironment that particularly influences cell proliferation, migration, invasion, and apoptosis in relation to the extracellular matrix (ECM). Traditional cell culture is unable to create 3-D cell scaffolds resembling tissue complexity and functions, and, in the past, many efforts were made to realize the goal of obtaining cell clusters in hydrogels. However, the available methods still lack a precise control of cell external microenvironments. Recently, the rapid development of microfabrication techniques, such as 3-D printing, microfluidics, and photochemistry, has offered great advantages in reconstructing 3-D controllable cancer cell microenvironments in vitro. Consequently, various biofunctionalized hydrogels have become the ideal candidates to help the researchers acquire some new insights into various diseases. Our review will discuss some important studies and the latest progress regarding the above approaches for the production of 3-D ECM structures for cancer and other diseases. Especially, we will focus on new discoveries regarding the impact of the ECM on different aspects of cancer metastasis, e.g., collective invasion, enhanced intravasation by stress and aligned collagen fibers, angiogenesis regulation, as well as on drug screening.

c-Abl inhibits breast cancer tumorigenesis through reactivation of p53-mediated p21 expression

We previously reported that constitutive c-Abl activity (CST-Abl) abrogates the tumorigenicity of triple-negative breast cancer cells through the combined actions of two cellular events: downregulated matrix metalloproteinase (MMP) and upregulated p21^Waf1/Cip1 expression. We now find decreased c-Abl expression to be significantly associated with diminished relapse-fee survival in breast cancer patients, particularly those exhibiting invasive and basal phenotypes. Moreover, CST-Abl expression enabled 4T1 cells to persist innocuously in the mammary glands of mice, doing so by exhausting their supply of cancer stem cells. Restoring MMP-9 expression and activity in CST-Abl-expressing 4T1 cells failed to rescue their malignant phenotypes; however, rendering these same cells deficient in p21 expression not only delayed their acquisition of senescent phenotypes, but also partially restored their tumorigenicity in mice. Although 4T1 cells lacked detectable expression of p53, those engineered to express CST-Abl exhibited robust production and secretion of TGF-β1 that engendered the reactivated expression of p53. Mechanistically, TGF-β-mediated p53 expression transpired through the combined actions of Smad1/5/8 and Smad2, leading to the dramatic upregulation of p21 and its stimulation of TNBC senescence. Collectively, we identified a novel c-Abl:p53:p21 signaling axis that functions as a powerful suppressor of mammary tumorigenesis and metastatic progression.

Fabrication of highly modulable fibrous 3D extracellular microenvironments

Three-dimensional (3D) in vitro scaffolds that mimic the irregular fibrous structures of in vivo extracellular matrix (ECM) are critical for many important biological applications. However, structural properties modulation of fibrous 3D scaffolds remains a challenge. Here, we report the first highly modulable 3D fibrous scaffolds self-assembled by high-aspect-ratio (HAR) microfibers. The scaffolds structural properties can be easily tailored to incorporate various physical cues, including geometry, stiffness, heterogeneity and nanotopography. Moreover, the fibrous scaffolds are readily and accurately patterned on desired locations of the substrate. Cell culture exhibits that our scaffolds can elicit strong bidirectional cell-material interactions. Furthermore, a functional disparity between the two-dimensional substrate and our 3D scaffolds is identified by cell spreading and proliferation data. These results prove the potential of the proposed scaffold as a biomimetic extracellular microenvironment for cell study.

3D microniches reveal the importance of cell size and shape

Geometrical cues have been shown to alter gene expression and differentiation on 2D substrates. However, little is known about how geometrical cues affect cell function in 3D. One major reason for this lack of understanding is rooted in the difficulties of controlling cell geometry in a complex 3D setting and for long periods of culture. Here, we present a robust method to control cell volume and shape of individual human mesenchymal stem cells (hMSCs) inside 3D microniches with a range of different geometries (e.g., cylinder, triangular prism, cubic, and cuboid). We find that the actin filaments, focal adhesions, nuclear shape, YAP/TAZ localization, cell contractility, nuclear accumulation of histone deacetylase 3, and lineage selection are all sensitive to cell volume. Our 3D microniches enable fundamental studies on the impact of biophysical cues on cell fate, and have potential applications in investigating how multicellular architectures organize within geometrically well-defined 3D spaces.

Emergent patterns of collective cell migration under tubular confinement

Collective epithelial behaviors are essential for the development of lumens in organs. However, conventional assays of planar systems fail to replicate cell cohorts of tubular structures that advance in concerted ways on out-of-plane curved and confined surfaces, such as ductal elongation in vivo. Here, we mimic such coordinated tissue migration by forming lumens of epithelial cell sheets inside microtubes of 1-10 cell lengths in diameter. We show that these cell tubes reproduce the physiological apical-basal polarity, and have actin alignment, cell orientation, tissue organization, and migration modes that depend on the extent of tubular confinement and/or curvature. In contrast to flat constraint, the cell sheets in a highly constricted smaller microtube demonstrate slow motion with periodic relaxation, but fast overall movement in large microtubes. Altogether, our findings provide insights into the emerging migratory modes for epithelial migration and growth under tubular confinement, which are reminiscent of the in vivo scenario.

Probing three-dimensional collective cancer invasion with DIGME

BACKGROUND

Multicellular pattern formation plays an important role in developmental biology, cancer metastasis and wound healing. While many physical factors have been shown to regulate these multicellular processes, the role of ECM micro-to-meso scale geometry has been poorly understood in 3D collective cancer invasion.

RESULTS

We have developed a mechanical-based strategy, Diskoid In Geometrically Micropatterned ECM (DIGME). DIGME allows easy engineering of the shape of 3D tissue organoid, the mesoscale ECM heterogeneity, and the fiber alignment of collagen-based ECM all at the same time. We have employed DIGME to study the 3D invasion of MDA-MB-231 diskoids in engineered collagen matrix. We find that the collective cancer invasion is closely regulated by the micro-to-meso scale geometry of the ECM.

CONCLUSIONS

We conclude that DIGME provides a simple yet powerful tool to probe 3D dynamics of tissue organoids in physically patterned microenvironments.

Adaptive self-organization in the embryo: its importance to adult anatomy and to tissue engineering

The anatomy of healthy humans shows much minor variation, and twin‐studies reveal at least some of this variation cannot be explained genetically. A plausible explanation is that fine‐scale anatomy is not specified directly in a genetic programme, but emerges from self‐organizing behaviours of cells that, for example, place a new capillary where it happens to be needed to prevent local hypoxia. Self‐organizing behaviour can be identified by manipulating growing tissues (e.g. putting them under a spatial constraint) and observing an adaptive change that conserves the character of the normal tissue while altering its precise anatomy. Self‐organization can be practically useful in tissue engineering but it is limited; generally, it is good for producing realistic small‐scale anatomy but large‐scale features will be missing. This is because self‐organizing organoids miss critical symmetry‐breaking influences present in the embryo: simulating these artificially, for example, with local signal sources, makes anatomy realistic even at large scales. A growing understanding of the mechanisms of self‐organization is now allowing synthetic biologists to take their first tentative steps towards constructing artificial multicellular systems that spontaneously organize themselves into patterns, which may soon be extended into three‐dimensional shapes.

A Unifying Theory of Branching Morphogenesis

The morphogenesis of branched organs remains a subject of abiding interest. Although much is known about the underlying signaling pathways, it remains unclear how macroscopic features of branched organs, including their size, network topology, and spatial patterning, are encoded. Here, we show that, in mouse mammary gland, kidney, and human prostate, these features can be explained quantitatively within a single unifying framework of branching and annihilating random walks. Based on quantitative analyses of large-scale organ reconstructions and proliferation kinetics measurements, we propose that morphogenesis follows from the proliferative activity of equipotent tips that stochastically branch and randomly explore their environment but compete neutrally for space, becoming proliferatively inactive when in proximity with neighboring ducts. These results show that complex branched epithelial structures develop as a self-organized process, reliant upon a strikingly simple but generic rule, without recourse to a rigid and deterministic sequence of genetically programmed events.

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Branching morphogenesis follows conserved statistical rules in multiple organs

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Ductal tips grow and branch as default state and stop dividing in high-density regions

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Model reproduces quantitatively organ properties in a parameter-free manner

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Shows that complex organ formation proceeds in a stochastic, self-organized manner

The development of cell-initiated degradable hydrogel based on methacrylated alginate applicable to multiple microfabrication technologies

Biomimetic multicellular complex tissues can be obtained via 3D microfabrication approaches, although the lack of active materials that meet the needs of both molding and the maintenance of cell function is the main bottleneck in their application. The development of 3D cell culture platforms, which are suitable for multiple cell types and various microfabrication technologies, would open the way to the biomimetic construction of multicellular complex tissues. In this study, a MMP-sensitive photocrosslinkable hydrogel system based on methacrylated alginate, which is suitable for the survival of multiple cell types and can be employed in a variety of micro-manufacturing technologies, was designed for the construction of 3D microfeatures to meet both the demands of molding and the requirement for biomimetic function. By employing the characteristics of photocrosslinking and ion crosslinking, the gel system was combined with a variety of microfabrication techniques to construct biomimetic microstructures with a spatial distribution of multiple types of cell. The diversity of shapes and cell types in these assemblies, as well as the maintenance of cell activity, collectively show that the functional MMP-sensitive gel system has the potential to be used as a cell culture platform for the construction of tissue microstructures.

Scaffold curvature-mediated novel biomineralization process originates a continuous soft tissue-to-bone interface

A myriad of shapes are found in biological tissues, often naturally evolved to fulfill a particular function. In the field of tissue engineering, substrate geometry influences cell behavior and tissue formation in vitro, yet little is known how this translates to an in vivo scenario. Here we investigate scaffold curvature-induced tissue growth, without additional growth factors or cells, in an ovine animal model. We show that soft tissue formation follows a curvature-driven tissue growth model. The highly organized endogenous soft matrix, potentially under mechanical strain, leads to a non-standard form of biomineralization, whereby the pre-existing organic matrix is mineralized without collagen remodeling and without an intermediate cartilage ossification phase. Micro- and nanoscale characterization of the tissue microstructure using histology, backscattered electron (BSE) and second-harmonic generation (SHG) imaging and synchrotron small angle X-ray scattering (SAXS) revealed (i) continuous collagen fibers across the soft-hard tissue interface on the tip of mineralized cones, and (ii) bone remodeling by basic multicellular units (BMUs) in regions adjacent to the native cortical bone. Thus, features of soft tissue-to-bone interface resembling the insertion sites of ligaments and tendons into bone were created, using a scaffold that did not mimic the structural or biological gradients across such a complex interface at its mature state. This study provides fundamental knowledge for biomimetic scaffold design in the fields of bone regeneration and soft tissue-to-bone interface tissue engineering.

STATEMENT OF SIGNIFICANCE

Geometry influences cell behavior and tissue formation in vitro. However, little is known how this translates to an in vivo scenario. Here we investigate the influence of scaffold mean surface curvature on in vivo tissue growth using an ovine animal model. Based on a multiscale tissue microstructure characterization, we show a seamless integration of soft tissue into newly formed bone, resembling the insertion sites of ligaments and tendons into bone. This interface was created using a scaffold without additional growth factors or cells that did not recapitulate the structural or biological gradients across such a complex tissue interface at its mature state. These findings have important implications for biomimetic scaffold design for bone regeneration and soft tissue-to-bone interface tissue engineering.

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.