Collagen modulates cell shape and cytoskeleton of embryonic corneal and fibroma fibroblasts: distribution of actin, alpha-actinin, and myosin

Trabecular meshwork cell differentiation in response to collagen and TGFβ-2 spatial interactions

Primary open angle glaucoma (POAG) is currently the most prevalent cause of irreversible blindness globally. To date, there are few in vitro models that can faithfully recapitulate the complex architecture of the trabecular meshwork (TM) and the specialized trabecular meshwork cell (TMC) characteristics that are local to structurally opposing regions. This study aimed to investigate the parameters that govern TMC phenotype by adapting the extracellular matrix structure to mimic the juxtacanalicular tissue (JCT) region of the TM. Initially, TMC phenotypic characteristics were investigated within type I collagen matrices of controlled fiber density and anisotropy, generated through confined plastic compression (PC). Notably, PC-collagen presented biophysical cues that induced JCT cellular characteristics (elastin, α-β-Crystallin protein expression, cytoskeletal remodeling and increased mesenchymal and JCT-specific genetic markers). In parallel, a pathological mesenchymal phenotype associated with POAG was induced through localized transforming growth factor -beta 2 (TGFβ-2) exposure. This resulted in a profile of alternative mesenchymal states (fibroblast/smooth muscle or myofibroblast) displayed by the TMC in vitro. Overall, the study provides an advanced insight into the biophysical cues that modulate TMC fate, demonstrating the induction of a JCT-specific TMC phenotype and transient mesenchymal characteristics that reflect both healthy or pathological scenarios. STATEMENT OF SIGNIFICANCE: Glaucoma is the most prevalent cause of blindness, with a lack of efficacy within current drug candidates. Reliable trabecular meshwork (TM) in vitro models will be critical for enhancing the fields understanding of healthy and disease states for pre-clinical testing. To date, trabecular meshwork cells (TMCs) display heterogeneity throughout the hierarchical TM, however our understanding into recapitulating these phenotypes in vitro, remains elusive. This study hypothesizes the importance of specific matrix/growth factor spatial stimuli in governing TMC phenotype. By emulating certain biophysical/biochemical in vivo parameters, we introduce an advanced profile of distinct TMC phenotypic states, reflecting healthy and disease scenarios. A notion that has not be stated prior and a fundamental consideration for future TM 3D in vitro modelling.

Differential expression of extracellular matrix proteins in the lesional skin of vitiligo patients

Skin pigmentation is regulated by intricate interaction of the dermis and epidermis. The extracellular components present in the dermis play a very important role in the maintenance of skin homeostasis. Therefore, our objective was to check the expression of various ECM components secreted by the dermal fibroblasts in the lesional skin and non-lesional skin of vitiligo patients. For this study, skin punch biopsies (4 mm) were collected from lesional skin (n = 12), non-lesional skin (n = 6) of non-segmental vitiligo patient's (NSV) and healthy control skin (n = 10). Masson's trichrome staining was performed to check the collagen fibre. The expression of collagen type 1, IV, elastin, fibronectin, E-cadherin and integrin β1 was checked by real-time PCR and immunohistochemistry. In this study, we demonstrated an increased expression of collagen type 1 in the lesional skin of vitiligo patients. The expression of collagen type IV, fibronectin, elastin and adhesion components such as E-cadherin and integrin β1 was observed to be significantly decreased in the lesional skin of NSV patients as compared to healthy control, whereas insignificant difference was observed between non-lesional and control skin. Increased expression of collagen type 1 in the lesional skin of vitiligo patients might be inhibiting the migration of melanocytes, whereas the decreased expression of elastin, collagen type IV, fibronectin, E-cadherins and integrins in the lesional skin may inhibit adhesion, migration, growth and differentiation of cells.

P300/CBP-Associated Factor Activates Cardiac Fibroblasts by SMAD2 Acetylation

Various heart diseases cause cardiac remodeling, which in turn leads to ineffective contraction. Although it is an adaptive response to injury, cardiac fibrosis contributes to this remodeling, for which the reactivation of quiescent myofibroblasts is a key feature. In the present study, we investigated the role of the p300/CBP-associated factor (PCAF), a histone acetyltransferase, in the activation of cardiac fibroblasts. An intraperitoneal (i.p.) injection of a high dose (160 mg/kg) of isoproterenol (ISP) induced cardiac fibrosis and reduced the amount of the PCAF in cardiac fibroblasts in the mouse heart. However, the PCAF activity was significantly increased in cardiac fibroblasts, but not in cardiomyocytes, obtained from ISP-administered mice. An in vitro study using human cardiac fibroblast cells recapitulated the in vivo results; an treatment with transforming growth factor-β1 (TGF-β1) reduced the PCAF, whereas it activated the PCAF in the fibroblasts. PCAF siRNA attenuated the TGF-β1-induced increase in and translocation of fibrosis marker proteins. PCAF siRNA blocked TGF-β1-mediated gel contraction and cell migration. The PCAF directly interacted with and acetylated mothers against decapentaplegic homolog 2 (SMAD2). PCAF siRNA prevented TGF-β1-induced phosphorylation and the nuclear localization of SMAD2. These results suggest that the increase in PCAF activity during cardiac fibrosis may participate in SMAD2 acetylation and thereby in its activation.

Epithelial Mesenchymal Transition in Embryonic Development, Tissue Repair and Cancer: A Comprehensive Overview

The epithelial mesenchymal transition (EMT) plays a central role in both normal physiological events (e.g., embryonic development) and abnormal pathological events (e.g., tumor formation and metastasis). The processes that occur in embryonic development are often reactivated under pathological conditions such as oncogenesis. Therefore, defining the regulatory networks (both gene and protein levels) involved in the EMT during embryonic development will be fundamental in understanding the regulatory networks involved in tumor development, as well as metastasis. There are many molecules, factors, mediators and signaling pathways that are involved in the EMT process. Although the EMT is a very old topic with numerous publications, recent new technologies and discoveries give this research area some new perspective and direction. It is now clear that these important processes are controlled by a network of transcriptional and translational regulators in addition to post-transcriptional and post-translational modifications that amplify the initial signals. In this review article, we will discuss some key concepts, historical findings, as well as some recent progresses in the EMT research field.

Rebooting the collagen gel: Artificial hydrogels for the study of epithelial mesenchymal transformation

The collagen gel has been used to study epithelial-mesenchymal transformation (EMT) for over 30 years. With advances in the field of materials sciences, new options are available to design optically clear, three-dimensional nature-inspired matrix mimetics to study EMT. Here, we review the history of the collagen gel assay, discuss its current use and how newer artificial matrices can be built to simulate in vivo extracellular environments and investigate important current questions in the EMT field. We suggest that further collaborations between materials scientists and biologists will be critical to move the field of EMT forward. Developmental Dynamics 247:332-339, 2018. © 2017 Wiley Periodicals, Inc.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

Comparative transcriptome analysis reveals molecular strategies of oriental river prawn Macrobrachium nipponense in response to acute and chronic nitrite stress

Macrobrachium nipponense is an economically and nutritionally important species threatened by ambient superfluous nitrite. De novo RNA-Seq was used to explore the molecular mechanism in M. nipponense exposed to the acute nitrite stress (26.05 mg/L nitrite-N) for 24 h and the chronic nitrite stress (1.38 mg/L nitrite-N) for 28 d A total of 175.13 million reads were obtained and assembled into 58,871 unigenes with an average length of 1028.7 bp and N50 of 1294 bp. Under the acute and chronic nitrite stress trials, 2824 and 2610 unigenes were significantly expressed. In GO analysis and KEGG pathway analysis, 30 pathways were significantly different between the two treatments while four pathways were in common and the markedly altered pathways were divided into four sections as immunity, metabolism, cell and others. The immunity section revealing the different depth of immunity provoked by nitrite stress contained the most pathways including the important pathways as phagosome, folate biosynthesis, glycerolipid metabolism, glycine, serine and threonine metabolism, amino sugar and nucleotide sugar metabolism under the acute nitrite stress, and lysosome, alanine, aspartate and glutamate metabolism, arginine and proline metabolism under the chronic nitrite stress. This is the first report of responses of M. nipponense under acute and chronic nitrite stress through de novo transcriptome sequencing on the transcriptome level. The results of transcriptome analysis improve our understanding on the underlying molecular mechanisms coping with nitrite stress in crustacean species.

Bridging the Gap: From 2D Cell Culture to 3D Microengineered Extracellular Matrices

Historically the culture of mammalian cells in the laboratory has been performed on planar substrates with media cocktails that are optimized to maintain phenotype. However, it is becoming increasingly clear that much of biology discerned from 2D studies does not translate well to the 3D microenvironment. Over the last several decades, 2D and 3D microengineering approaches have been developed that better recapitulate the complex architecture and properties of in vivo tissue. Inspired by the infrastructure of the microelectronics industry, lithographic patterning approaches have taken center stage because of the ease in which cell-sized features can be engineered on surfaces and within a broad range of biocompatible materials. Patterning and templating techniques enable precise control over extracellular matrix properties including: composition, mechanics, geometry, cell-cell contact, and diffusion. In this review article we explore how the field of engineered extracellular matrices has evolved with the development of new hydrogel chemistry and the maturation of micro- and nano- fabrication. Guided by the spatiotemporal regulation of cell state in developing tissues, techniques for micropatterning in 2D, pseudo-3D systems, and patterning within 3D hydrogels will be discussed in the context of translating the information gained from 2D systems to synthetic engineered 3D tissues.

Mechanical interactions and crosstalk between corneal keratocytes and the extracellular matrix

The generation of cellular forces and the application of these physical forces to the ECM play a central role in mediating matrix patterning and remodeling during fundamental processes such as developmental morphogenesis and wound healing. In addition to growth factors and other biochemical factors that can modulate the keratocyte mechanical phenotype, another key player in the regulation of cell-induced ECM patterning is the mechanical state of the ECM itself. In this review we provide an overview of the biochemical and biophysical factors regulating the mechanical interactions between corneal keratocytes and the stromal ECM at the cellular level. We first provide an overview of how Rho GTPases regulate the sub-cellular pattern of force generation by corneal keratocytes, and the impact these forces have on the surrounding ECM. We next review how feedback from local matrix structural and mechanical properties can modulate keratocyte phenotype and mechanical activity. Throughout this review, we provide examples of how these biophysical interactions may contribute to clinical outcomes, with a focus on corneal wound healing.

The Role of Thrombin and Cell Contractility in Regulating Clustering and Collective Migration of Corneal Fibroblasts in Different ECM Environments

PURPOSE

We previously reported that extracellular matrix composition (fibrin versus collagen) modulates the pattern of corneal fibroblast spreading and migration in 3-D culture. In this study, we investigate the role of thrombin and cell contractility in mediating these differences in cell behavior.

METHODS

To assess cell spreading, corneal fibroblasts were plated on top of fibrillar collagen and fibrin matrices. To assess 3-dimensional cell migration, compacted collagen matrices seeded with corneal fibroblasts were embedded inside acellular collagen or fibrin matrices. Constructs were cultured in serum-free media containing platelet-derived growth factor (PDGF), with or without thrombin, the Rho kinase inhibitor Y-27632, and/or the myosin II inhibitor blebbistatin. We used 3-dimensional and 4-dimensional imaging to assess cell mechanical behavior, connectivity and cytoskeletal organization.

RESULTS

Thrombin stimulated increased contractility of corneal fibroblasts. Thrombin also induced Rho kinase-dependent clustering of cells plated on top of compliant collagen matrices, but not on rigid substrates. In contrast, cells on fibrin matrices coalesced into clusters even when Rho kinase was inhibited. In nested matrices, cells always migrated independently through collagen, even in the presence of thrombin. In contrast, cells migrating into fibrin formed an interconnected network. Both Y-27632 and blebbistatin reduced the migration rate in fibrin, but cells continued to migrate collectively.

CONCLUSIONS

The results suggest that while thrombin-induced actomyosin contraction can induce clustering of fibroblasts plated on top of compliant collagen matrices, it does not induce collective cell migration inside 3-D collagen constructs. Furthermore, increased contractility is not required for clustering or collective migration of corneal fibroblasts interacting with fibin.

3-D adipocyte differentiation and peri-adipocyte collagen turnover

Peri-adipocyte extracellular matrix (ECM) remodeling is a key biological process observed during adipose tissue development and expansion. The genetic loss of a pericellular collagenase, MMP14 (also known as MT1-MMP), renders mice lipodystrophic with the accumulation of undigested collagen fibers in adipose tissues. MMP14 is not necessary for adipocyte differentiation (adipogenesis) per se under a conventional two-dimensional (2-D) culture condition; however, MMP14 plays a critical role in adipogenesis in vivo. The role of MMP14 in adipogenesis and adipocyte gene expression was uncovered in vitro only when tested within a three-dimensional (3-D) collagen gel, which recapitulated the in vivo ECM-rich environment. Studying adipogenesis in 3-D may serve as an effective experimental approach to bridge gaps in our understanding of in vivo adipocyte biology. Moreover, by assessing the content of collagen family members and their rate of degradation in adipose tissues, we should be able to better define the role of dynamic ECM remodeling in the pathogenesis of obesity and diabetes.

MMP regulation of corneal keratocyte motility and mechanics in 3-D collagen matrices

Previous studies have shown that platelet derived growth factor (PDGF) can stimulate corneal keratocyte spreading and migration within 3-D collagen matrices, without inducing transformation to a contractile, fibroblastic phenotype. The goal of this study was to investigate the role of matrix metalloproteinases (MMPs) in regulating PDGF-induced changes in keratocyte motility and mechanical differentiation. Rabbit corneal keratocytes were isolated and cultured in serum-free media (S-) to maintain their quiescent phenotype. A nested collagen matrix construct was used to assess 3-D cell migration, and a standard collagen matrix model was used to assess cell morphology and cell-mediated matrix contraction. In both cases constructs were cultured in S- supplemented with PDGF, with or without the broad spectrum MMP inhibitors GM6001 or BB-94. After 4 days, f-actin, nuclei and collagen fibrils were imaged using confocal microscopy. To assess sub-cellular mechanical activity (extension and retraction of cell processes), time-lapse DIC imaging was also performed. MT1-MMP expression and MMP-mediated collagen degradation were also examined. Results demonstrated that neither GM6001 nor BB-94 affected corneal keratocyte viability or proliferation in 3-D culture. PDGF stimulated elongation and migration of corneal keratocytes within type I collagen matrices, without causing a loss of their dendritic morphology or inducing formation of intracellular stress fibers. Treatment with GM6001 and BB-94 inhibited PDGF-induced keratocyte spreading and migration. Relatively low levels of keratocyte-induced matrix contraction were also maintained in PDGF, and the amount of PDGF-induced collagen degradation was similar to that observed in S- controls. The collagen degradation pattern was consistent with membrane-associated MMP activity, and keratocytes showed positive staining for MT1-MMP, albeit weak. Both matrix contraction and collagen degradation were reduced by MMP inhibition. For most outcome measures, the inhibitory effect of BB-94 was significantly greater than that of GM6001. Overall, the data demonstrate for the first time that even under conditions in which low levels of contractility and extracellular matrix proteolysis are maintained, MMPs still play an important role in mediating cell spreading and migration within 3-D collagen matrices. This appears to be mediated at least in part by membrane-tethered MMPs, such as MT1-MMP.

The Ets transcription factor EHF as a regulator of cornea epithelial cell identity

The cornea is the clear, outermost portion of the eye composed of three layers: an epithelium that provides a protective barrier while allowing transmission of light into the eye, a collagen-rich stroma, and an endothelium monolayer. How cornea development and aging is controlled is poorly understood. Here we characterize the mouse cornea transcriptome from early embryogenesis through aging and compare it with transcriptomes of other epithelial tissues, identifying cornea-enriched genes, pathways, and transcriptional regulators. Additionally, we profiled cornea epithelium and stroma, defining genes enriched in these layers. Over 10,000 genes are differentially regulated in the mouse cornea across the time course, showing dynamic expression during development and modest expression changes in fewer genes during aging. A striking transition time point for gene expression between postnatal days 14 and 28 corresponds with completion of cornea development at the transcriptional level. Clustering classifies co-expressed, and potentially co-regulated, genes into biologically informative categories, including groups that exhibit epithelial or stromal enriched expression. Based on these findings, and through loss of function studies and ChIP-seq, we show that the Ets transcription factor EHF promotes cornea epithelial fate through complementary gene activating and repressing activities. Furthermore, we identify potential interactions between EHF, KLF4, and KLF5 in promoting cornea epithelial differentiation. These data provide insights into the mechanisms underlying epithelial development and aging, identifying EHF as a regulator of cornea epithelial identity and pointing to interactions between Ets and KLF factors in promoting epithelial fate. Furthermore, this comprehensive gene expression data set for the cornea is a powerful tool for discovery of novel cornea regulators and pathways.

Techniques for assessing 3-D cell-matrix mechanical interactions in vitro and in vivo

Cellular interactions with extracellular matrices (ECM) through the application of mechanical forces mediate numerous biological processes including developmental morphogenesis, wound healing and cancer metastasis. They also play a key role in the cellular repopulation and/or remodeling of engineered tissues and organs. While 2-D studies can provide important insights into many aspects of cellular mechanobiology, cells reside within 3-D ECMs in vivo, and matrix structure and dimensionality have been shown to impact cell morphology, protein organization and mechanical behavior. Global measurements of cell-induced compaction of 3-D collagen matrices can provide important insights into the regulation of overall cell contractility by various cytokines and signaling pathways. However, to understand how the mechanics of cell spreading, migration, contraction and matrix remodeling are regulated at the molecular level, these processes must also be studied in individual cells. Here we review the evolution and application of techniques for imaging and assessing local cell-matrix mechanical interactions in 3-D culture models, tissue explants and living animals.

Injectable biodegradable hydrogels: progress and challenges

Over the past decades, injectable hydrogels have emerged as promising biomaterials because of their biocompatibility, excellent permeability, minimal invasion, and easy integration into surgical procedures. These systems provide an effective and convenient way to administer a wide variety of bioactive agents such as proteins, genes, and even living cells. Additionally, they can be designed to be degradable and eventually cleared from the body after completing their missions. Given their unique characteristics, injectable biodegradable hydrogels have been actively explored as drug reservoir systems for sustained release of bioactive agents and temporary extracellular matrices for tissue engineering. This review provides an overview of state-of-the-art strategies towards constructing a rational design of injectable biodegradable hydrogels for protein drug delivery and tissue engineering. We also discuss the use of injectable hydrogels for gene delivery systems and biomedical adhesives.

At the leading edge of three-dimensional cell migration

Cells migrating on flat two-dimensional (2D) surfaces use actin polymerization to extend the leading edge of the plasma membrane during lamellipodia-based migration. This mode of migration is not universal; it represents only one of several mechanisms of cell motility in three-dimensional (3D) environments. The distinct modes of 3D migration are strongly dependent on the physical properties of the extracellular matrix, and they can be distinguished by the structure of the leading edge and the degree of matrix adhesion. How are these distinct modes of cell motility in 3D environments related to each other and regulated? Recent studies show that the same type of cell migrating in 3D extracellular matrix can switch between different leading edge structures. This mode-switching behavior, or plasticity, by a single cell suggests that the apparent diversity of motility mechanisms is integrated by a common intracellular signaling pathway that governs the mode of cell migration. In this Commentary, we propose that the mode of 3D cell migration is governed by a signaling axis involving cell-matrix adhesions, RhoA signaling and actomyosin contractility, and that this might represent a universal mechanism that controls 3D cell migration.

Cell-matrix and cell-cell interactions of human gingival fibroblasts on three-dimensional nanofibrous gelatin scaffolds

An in-depth understanding of the interactions between cells and three-dimensional (3D) matrices (scaffolds) is pivotal to the development of novel biomaterials for tissue regeneration. However, it remains a challenge to find suitable biomimetic substrates and tools to observe cell-material and cell-cell interactions on 3D matrices. In the present study, we developed biomimetic nanofibrous 3D gelatin scaffolds (3D-NF-GS) and utilized confocal microscopy combined with a quantitative analysis approach to explore cell-matrix and cell-cell interactions on the 3D-NF-GS. Human gingival fibroblasts (HGFs) migrated throughout the 3D-NF-GS by 5 days and formed stable focal adhesions by 14 days. The focal adhesions were detected using integrin-β1, phospho-paxillin and vinculin expression, which were quantified from specific wavelength photon data generated using a spectral separation confocal microscope. As the cells became more confluent after 14 days of culture, cell-cell communication via gap junctions increased significantly. Collagen I matrix production by HGFs on 3D-NF-GS was visualized and quantified using a novel approach incorporating TRITC label in the scaffolds. Based on confocal microscopy, this study has developed qualitative and quantitative methods to study cell-matrix and cell-cell interactions on biomimetic 3D matrices, which provides valuable insights for the development of appropriate scaffolds for tissue regeneration.

Individual versus collective fibroblast spreading and migration: regulation by matrix composition in 3D culture

Extracellular matrix (ECM) supplies both physical and chemical signals to cells and provides a substrate through which fibroblasts migrate during wound repair. To directly assess how ECM composition regulates this process, we used a nested 3D matrix model in which cell-populated collagen buttons were embedded in cell-free collagen or fibrin matrices. Time-lapse microscopy was used to record the dynamic pattern of cell migration into the outer matrices, and 3D confocal imaging was used to assess cell connectivity and cytoskeletal organization. Corneal fibroblasts stimulated with PDGF migrated more rapidly into collagen as compared to fibrin. In addition, the pattern of fibroblast migration into fibrin and collagen ECMs was strikingly different. Corneal fibroblasts migrating into collagen matrices developed dendritic processes and moved independently, whereas cells migrating into fibrin matrices had a more fusiform morphology and formed an interconnected meshwork. A similar pattern was observed when using dermal fibroblasts, suggesting that this response is not unique to corneal cells. We next cultured corneal fibroblasts within and on top of standard collagen and fibrin matrices to assess the impact of ECM composition on the cell spreading response. Similar differences in cell morphology and connectivity were observed – cells remained separated on collagen but coalesced into clusters on fibrin. Cadherin was localized to junctions between interconnected cells, whereas fibronectin was present both between cells and at the tips of extending cell processes. Cells on fibrin matrices also developed more prominent stress fibers than those on collagen matrices. Importantly, these spreading and migration patterns were consistently observed on both rigid and compliant substrates, thus differences in ECM mechanical stiffness were not the underlying cause. Overall, these results demonstrate for the first time that ECM protein composition alone (collagen vs. fibrin) can induce a switch from individual to collective fibroblast spreading and migration in 3D culture. Similar processes may also influence cell behavior during wound healing, development, tumor invasion and repopulation of engineered tissues.

Corneal stromal cells use both high- and low-contractility migration mechanisms in 3-D collagen matrices

Corneal keratocyte migration can impact both corneal clarity and refractive outcome following injury or refractive surgery. In this study, we investigated how culture conditions, ECM properties, and Rho kinase activity regulate the mechanics of keratocyte migration, using a nested collagen matrix model. Time-lapse imaging demonstrated that both serum and PDGF stimulate keratocyte migration into the outer matrix. Although the velocity of cell migration was similar, cells in serum were bipolar and induced significant matrix deformation during migration, whereas PDGF induced extension of branching dendritic processes with smaller, more localized force generation. These differences in cell-induced matrix reorganization were verified with a global matrix contraction assay and confocal reflection imaging, using both bovine and rat tail collagen. When constructs were detached from the substrate to lower the effective stiffness, migration was significantly reduced in serum; but was unchanged in PDGF. These differences in migration mechanics were mediated, in part, by Rho kinase. Overall, corneal keratocytes can effectively migrate through collagen matrices using varying degrees of cellular force generation. Low-contractility migration may facilitate keratocyte repopulation of the stroma following surgery or injury, without altering the structural and mechanical properties that are critical to maintaining corneal transparency.

Experimental models for investigating intra-stromal migration of corneal keratocytes, fibroblasts and myofibroblasts

Following laser vision correction, corneal keratocytes must repopulate areas of cell loss by migrating through the intact corneal stroma, and this can impact corneal shape and transparency. In this study, we evaluate 3D culture models for simulating this process in vitro. Buttons (8 mm diameter) were first punched out of keratocyte populated compressed collagen matrices, exposed to a 3mm diameter freeze injury, and cultured in serum-free media (basal media) or media supplemented with 10% FBS, TGFβ1 or PDGF BB. Following freeze injury, a region of cell death was observed in the center of the constructs. Although cells readily migrated on top of the matrices to cover the wound area, a limited amount of cell migration was observed within the constructs. We next developed a novel "sandwich" model, which better mimics the native lamellar architecture of the cornea. Using this model, significant migration was observed under all conditions studied. In both models, cells in TGFβ and 10% FBS developed stress fibers; whereas cells in PDGF were more dendritic. PDGF stimulated the most inter-lamellar migration in the sandwich construct. Overall, these models provide insights into the complex interplay between growth factors, cell mechanical phenotypes and the structural properties of the ECM.

Growth factor regulation of corneal keratocyte mechanical phenotypes in 3-D collagen matrices

PURPOSE

To assess how wound healing cytokines and the extracellular matrix (ECM) environment regulate the keratocyte mechanical phenotype.

METHODS

Rabbit corneal keratocytes were plated within standard bovine or rat tail type I collagen matrices (2.5 mg/mL), compressed collagen matrices (approximately 100 mg/mL), or on collagen-coated dishes and cultured for up to 7 days in serum-free media, platelet derived growth factor BB (PDGF BB), insulin-like growth factor (IGF), TGFβ1, TGFβ2, or FGF2. F-actin, α-smooth muscle actin (α-SMA) and collagen fibrils were imaged using confocal microscopy. Cell morphology, local matrix reorganization, and global matrix contraction were quantified digitally.

RESULTS

IGF and PDGF BB stimulated elongation of keratocytes and extension of dendritic processes within 3-D matrices, without inducing stress fiber formation or collagen reorganization. In contrast, treatment with TGFβ1 and TGFβ2 increased keratocyte contractility, as indicated by stress fiber formation and matrix compaction and alignment. This transformation was enhanced at higher cell densities within standard 3-D matrices, in which α-SMA was incorporated into stress fibers. In contrast, α-SMA was expressed within compressed 3-D matrices even at low cell density. FGF2 did not produce significant cytoskeletal or matrix reorganization in standard 3-D matrices; however, stress fibers were consistently expressed within compressed collagen matrices and on rigid two-dimensional substrates. Inhibiting Rho kinase blocked both TGFβ- and FGF2-induced stress fiber formation.

CONCLUSIONS

Keratocytes cultured in IGF or PDGF BB maintain a quiescent mechanical phenotype over a range of matrix environments. In contrast, the mechanical phenotypes induced by FGF and TGFβ vary in response to the structural and/or mechanical properties of the ECM.

Collagenous Materials Enhance Healing of Chronic Skin Ulcers

Type I collagen in a porous sponge form attracts fibroblasts in culture and accelerates repair of animal wounds. This study examines the effect of type I collagen sponge and flakes on healing of chronic skin ulcers. Patients included in this study had skin ulcers.

Patients included in this study had skin ulcers characterized by loss of dermis and epidermis without exposure of muscle, tendon or bone. Patients showing evidence of systemic infection or patients with ulcers that decreased in area during an initial three week observation period were excluded.

Three out of seven patients treated with a collagen sponge and twelve out of fourteen patients treated with collagen flakes showed a 40% decrease in wound area after six weeks of treatment. In comparison, eighteen control patients showed no change in wound area over the same time interval. These results suggest that collagen flakes are effective in initiating healing of chronic skin ulcers.

Mammary gland ECM remodeling, stiffness, and mechanosignaling in normal development and tumor progression

Cells of the mammary gland are in intimate contact with other cells and with the extracellular matrix (ECM), both of which provide not only a biochemical context, but a mechanical context as well. Cell-mediated contraction allows cells to sense the stiffness of their microenvironment, and respond with appropriate mechanosignaling events that regulate gene expression and differentiation. ECM composition and organization are tightly regulated throughout development of the mammary gland, resulting in corresponding regulation of the mechanical environment and proper tissue architecture. Mechanical regulation is also at play during breast carcinoma progression, as changes in ECM deposition, composition, and organization accompany breast carcinoma. These changes result in stiffer matrices that activate mechanosignaling pathways and thereby induce cell proliferation, facilitate local tumor cell invasion, and promote progression. Thus, understanding the role of forces in the mammary gland is crucial to understanding both normal developmental and pathological processes.

Characterization of dense artificial connective tissues generated in a newly designed bioreactor

Dense connective tissues were generated simultaneously by accumulating collagen fibrils and fibroblasts on stainless steel mesh using a bioreactor system that we designed. The advantage of our system is that the artificial connective tissues can be generated within 24 hr in the absence of inhibitors against matrix metalloproteinases. The fibroblasts were suspended in 200 mL of Dulbecco's modified Eagle's medium containing 10% fetal bovine serum and 0.5 mg/mL type I collagen. The mixed solution was circulated in two types of bioreactors with cylindrical or vertical configurations to generate luminal or parenchymal tissues, respectively. The gelatin zymography showed that MMPs were first detected in the media after 8 hr from the start of circulation and reached the highest levels on day 3. Glossy white aggregates, 1-3 mm in thickness, depending on the circulation period, accumulated on mesh grids. Fibroblasts were embedded in the network of collagen fibrils and possessed oval nuclei with or without prominent cell processes to form a bipolar shape. We could not observe distended cisternae of the endoplasmic reticula, the Golgi apparatus, or exploded mitochondria, showing hypoxic degenerative alterations of fibroblasts in dense connective tissues. The artificial tissues generated by our system will be useful for biological studies and transplantation therapy.

Growth factor regulation of corneal keratocyte differentiation and migration in compressed collagen matrices

PURPOSE

To evaluate a novel 3D culture model of the corneal stroma and apply it to investigate how key wound-healing growth factors regulate the mechanics of corneal keratocyte migration.

METHODS

Rabbit corneal keratocytes were seeded within collagen matrices that were compacted using external compression. Six-millimeter-diameter buttons were then incubated in media supplemented with 10% FBS, TGFbeta1, TGFbeta2, platelet-derived growth factor (PDGF), or no growth factor (control). After 1, 3, or 7 days, matrices were labeled with phalloidin and a nucleic acid dye, and were imaged using laser confocal microscopy. To study cell migration, buttons were nested within acellular uncompressed outer collagen matrices before growth factor stimulation.

RESULTS

Corneal keratocytes in basal media within compressed matrices had a broad, convoluted cell body and thin dendritic processes. In contrast, cells in 10% FBS developed a bipolar fibroblastic morphology. Treatment with TGFbeta induced the formation of stress fibers expressing alpha-smooth muscle actin, suggesting myofibroblast transformation. PDGF induced keratocyte elongation without inducing stress fiber formation. Both 10% FBS and PDGF stimulated significant keratocyte migration through the uncompressed outer matrix, but 10% FBS produced more cell-induced collagen matrix reorganization. TGFbeta induced the smallest increase in migration and the greatest matrix reorganization.

CONCLUSIONS

Corneal keratocytes are able to differentiate normally and respond to growth factors within compressed collagen matrices, which provide a high-stiffness, 3D environment, similar to native stromal tissue. In addition, nesting these matrices provides a unique platform for investigating the mechanics of keratocyte migration after exposure to specific wound-healing cytokines.

Characterization of corneal keratocyte morphology and mechanical activity within 3-D collagen matrices

The purpose of this study was to assess quantitatively the differences in morphology, cytoskeletal organization and mechanical behavior between quiescent corneal keratocytes and activated fibroblasts in a 3-D culture model. Primary cultures of rabbit corneal keratocytes and fibroblasts were plated inside type I collagen matrices in serum-free media or 10% FBS, and allowed to spread for 1-5 days. Following F-actin labeling using phalloidin, and immunolabeling of tubulin, alpha-smooth muscle actin or connexin 43, fluorescent and reflected light (for collagen fibrils) 3-D optical section images were acquired using laser confocal microscopy. In other experiments, dynamic imaging was performed using differential interference contrast microscopy, and finite element modeling was used to map ECM deformations. Corneal keratocytes developed a stellate morphology with numerous cell processes that ran a tortuous path between and along collagen fibrils without any apparent impact on their alignment. Fibroblasts on the other hand, had a more bipolar morphology with pseudopodial processes (P </= 0.001). Time-lapse imaging of keratocytes revealed occasional extension and retraction of dendritic processes with only transient displacements of collagen fibrils, whereas fibroblasts exerted stronger myosin II-dependent contractile forces (P < 0.01), causing increased compaction and alignment of collagen at the ends of the pseudopodia (P < 0.001). At high cell density, both keratocytes and fibroblasts appeared to form a 3-D network connected via gap junctions. Overall, this experimental model provides a unique platform for quantitative investigation of the morphological, cytoskeletal and contractile behavior of corneal keratocytes (i.e. their mechanical phenotype) in a 3-D microenvironment.

Nanoscale topography-induced modulation of fundamental cell behaviors of rabbit corneal keratocytes, fibroblasts, and myofibroblasts

PURPOSE

Keratocyte-to-myofibroblast differentiation is a key factor in corneal wound healing. The purpose of this study was to determine the influence of environmental nanoscale topography on keratocyte, fibroblast, and myofibroblast cell behavior.

METHODS

Primary rabbit corneal keratocytes, fibroblasts, and myofibroblasts were seeded onto planar polyurethane surfaces with six patterned areas, composed of anisotropically ordered grooves and ridges with a 400-, 800-, 1200-, 1600-, 2000-, and 4000-nm pitch (pitch = groove + ridge width). After 24 hours cells were fixed, stained, imaged, and analyzed for cell shape and orientation. For migration studies, cells on each patterned surface were imaged every 10 minutes for 12 hours, and individual cell trajectories and migration rates were calculated.

RESULTS

Keratocytes, fibroblasts, and myofibroblasts aligned and elongated to pitch sizes larger than 1000 nm. A lower limit to the topographic feature sizes that the cells responded to was identified for all three phenotypes, with a transition zone around the 800- to 1200-nm pitch size. Fibroblasts and myofibroblasts migrated parallel to surface ridges larger than 1000 nm but lacked directional guidance on submicron and nanoscale topographic features and on planar surfaces. Keratocytes remained essentially immobile.

CONCLUSIONS

Corneal stromal cells elongated, aligned, and migrated, differentially guided by substratum topographic features. All cell types failed to respond to topographic features approximating the dimensions of individual stromal fibers. These findings contribute to our understanding of corneal stromal cell biology in health and disease and their interaction with biomaterials and their native extracellular matrix.

The hard life of soft cells

Cells are mechanical as well as chemical machines, and much of the energy they consume is used to apply forces to each other and to the extracellular matrix around them. The cytoskeleton, the cell membrane, and the macromolecules composing the extracellular matrix form networks that in concert with the forces generated by the cell create dynamic materials with viscoelastic properties unique to each tissue. Numerous recent studies suggest that the forces that cells create and are subjected to, as well as the mechanical properties of the materials to which they adhere, can have large effects on cell structure and function that can act in concert with or override signals from soluble stimuli. This brief review summarizes recent studies of the effects of substrate mechanics on cell motility, differentiation, and proliferation, and discusses possible mechanisms by which a cell can probe the stiffness of its surroundings. Cell Motil. Cytoskeleton, 2009. (c) 2009 Wiley-Liss, Inc.

Production of heparin-containing hydrogels for modulating cell responses

Successful tissue regeneration requires that biomaterials have optimal bioactivity and mechanical properties. Heparin-containing hydrogels that can be crosslinked in situ were designed to contain tunable amounts of biological components (e.g. heparin, arginine-glycine-aspartate (RGD)) as well as to exhibit controlled mechanical properties (e.g. shear modulus). These gel parameters can also be tuned to provide controlled delivery of proteins, such as growth factors, for regulating cellular behavior. Maleimide-functionalized low-molecular-weight heparin (LWMH) was conjugated to a poly(ethylene glycol) (PEG) hydrogel. The elastic shear modulus, as assessed via oscillatory rheology experiments, could be tuned by the concentration of polymer in the hydrogel, and by the end group functionality of PEG. Hydrogels of two different moduli (2.8 and 0.4kPa) were used to study differences in the response of human aortic adventitial fibroblasts (AoAF) in two-dimensional cell culture experiments. These experiments indicated that the AoAFs show improved adhesion to materials with the higher modulus. Evaluation of cell responses to hydrogels with RGD linked to the hydrogels via conjugation to PEG or to LMWH indicated improved cellular responses to these materials when the bioactive ligands were chemically attached through linkage to the PEG rather than to the LMWH. These results highlight important design considerations in the tailoring of these materials for cardiovascular tissue engineering applications.

Human primary corneal fibroblasts synthesize and deposit proteoglycans in long-term 3-D cultures

Our goal was to develop a 3-D multi-cellular construct using primary human corneal fibroblasts cultured on a disorganized collagen substrate in a scaffold-free environment and to use it to determine the regulation of proteoglycans over an extended period of time (11 weeks). Electron micrographs revealed multi-layered constructs with cells present in between alternating parallel and perpendicular arrays of fibrils. Type I collagen increased 2-4-fold. Stromal proteoglycans including lumican, syndecan4, decorin, biglycan, mimecan, and perlecan were expressed. The presence of glycosaminoglycan chains was demonstrated for a subset of the core proteins (lumican, biglycan, and decorin) using lyase digestion. Cuprolinic blue-stained cultures showed that sulfated proteoglycans were present throughout the construct and most prominent in its mid-region. The size of the Cuprolinic-positive filaments resembled those previously reported in a human corneal stroma. Under the current culture conditions, the cells mimic a development or nonfibrotic repair phenotype.

Localized application of mechanical and biochemical stimuli in 3-D culture

The goal of this study was to investigate the responses of isolated cells in 3-D culture to localized application of mechanical and biochemical signals. Corneal fibroblasts were plated inside collagen matrices for 24 hours, then imaged using time-lapse DIC. For mechanical perturbation, a microinjection needle (Femtotip) was inserted axially into the ECM, then displaced laterally to alter local ECM stress. For biochemical stimulation, PDGF or vehicle control solution was microinjected into the matrix. Compressing the ECM perpendicular to the cell axis had no appreciable effect on cell behavior. However, pushing the ECM parallel to the cell axis induced rapid cellular contraction, followed by secondary cell spreading and tractional force generation. Injection of PDGF induced a similar cell spreading response. Cells in 3-D matrices showed remarkable plasticity, and extension of pseudopodia could be induced at both the leading and trailing edges of migrating cells.

Dynamic assessment of fibroblast mechanical activity during Rac-induced cell spreading in 3-D culture

The goal of this study was to determine the morphological and sub-cellular mechanical effects of Rac activation on fibroblasts within 3-D collagen matrices. Corneal fibroblasts were plated at low density inside 100 microm thick fibrillar collagen matrices and cultured for 1-2 days in serum-free media. Time-lapse imaging was then performed using Nomarski DIC. After an acclimation period, perfusion was switched to media containing PDGF. In some experiments, Y-27632 or blebbistatin were used to inhibit Rho-kinase (ROCK) or myosin II, respectively. PDGF activated Rac and induced cell spreading, which resulted in an increase in cell length, cell area, and the number of pseudopodial processes. Tractional forces were generated by extending pseudopodia, as indicated by centripetal displacement and realignment of collagen fibrils. Interestingly, the pattern of pseudopodial extension and local collagen fibril realignment was highly dependent upon the initial orientation of fibrils at the leading edge. Following ROCK or myosin II inhibition, significant ECM relaxation was observed, but small displacements of collagen fibrils continued to be detected at the tips of pseudopodia. Taken together, the data suggests that during Rac-induced cell spreading within 3-D matrices, there is a shift in the distribution of forces from the center to the periphery of corneal fibroblasts. ROCK mediates the generation of large myosin II-based tractional forces during cell spreading within 3-D collagen matrices, however residual forces can be generated at the tips of extending pseudopodia that are both ROCK and myosin II-independent.

Retrograde fluxes of focal adhesion proteins in response to cell migration and mechanical signals

Recent studies suggest that mechanical signals mediated by the extracellular matrix play an essential role in various physiological and pathological processes; yet, how cells respond to mechanical stimuli remains elusive. Using live cell fluorescence imaging, we found that actin filaments, in association with a number of focal adhesion proteins, including zyxin and vasodilator-stimulated phosphoprotein, undergo retrograde fluxes at focal adhesions in the lamella region. This flux is inversely related to cell migration, such that it is amplified in fibroblasts immobilized on micropatterned islands. In addition, the flux is regulated by mechanical signals, including stretching forces applied to flexible substrates and substrate stiffness. Conditions favoring the flux share the common feature of causing large retrograde displacements of the interior actin cytoskeleton relative to the substrate anchorage site, which may function as a switch translating mechanical input into chemical signals, such as tyrosine phosphorylation. In turn, the stimulation of actin flux at focal adhesions may function as part of a feedback mechanism, regulating structural assembly and force production in relation to cell migration and mechanical load. The retrograde transport of associated focal adhesion proteins may play additional roles in delivering signals from focal adhesions to the interior of the cell.

Hidden in plain sight: Borrelia burgdorferi and the extracellular matrix

Borrelia burgdorferi, the tick-transmitted etiologic agent of Lyme borreliosis, can colonize and persist in multiple tissue sites despite vigorous host immune responses. The extracellular matrix appears to provide a protective niche for the spirochete. Recent studies in mice suggest that B. burgdorferi interacts in various ways with collagen and its associated molecules, exploiting molecular and structural features to establish microcolonial refugia. Better knowledge of the genetic and structural bases for these interactions of B. burgdorferi with the extracellular matrix will be required before an understanding of the persistence of B. burgdorferi in the tissues and development of chronic infections can be achieved.

Microtubule regulation of corneal fibroblast morphology and mechanical activity in 3-D culture

The purpose of this study was to investigate the role of microtubules in regulating corneal fibroblast structure and mechanical behavior using static (3-D) and dynamic (4-D) imaging of both cells and their surrounding matrix. Human corneal fibroblasts transfected to express GFP-zyxin (to label focal adhesions) or GFP-tubulin (to label microtubules) were plated at low density inside 100 microm thick type I collagen matrices. After 24h, the effects of nocodazole (to depolymerize microtubules), cytochalasin D (to disrupt f-actin), and/or Y-27632 (to block Rho-kinase) were evaluated using 3-D and 4-D imaging of both cells and ECM. After 24h of incubation, cells had well organized microtubules and prominent focal adhesions, and significant cell-induced matrix compaction was observed. Addition of nocodazole induced rapid microtubule disruption which resulted in Rho activation and additional cellular contraction. The matrix was pulled inward by retracting pseudopodial processes, and focal adhesions appeared to mediate this process. Following 24h exposure to nocodazole, there was an even greater increase in both the number of stress fibers and the amount of matrix compaction and alignment at the ends of cells. When Rho-kinase was inhibited, disruption of microtubules resulted in retraction of dendritic cell processes, and rapid formation and extension of lamellipodial processes at random locations along the cell body, eventually leading to a convoluted, disorganized cell shape. These data suggest that microtubules modulate both cellular contractility and local collagen matrix reorganization via regulation of Rho/Rho-kinase activity. In addition, microtubules appear to play a central role in dynamic regulation of cell spreading mechanics, morphology and polarity in 3-D culture.

Lysophosphatidic acid down-regulates stress fibers and up-regulates pro-matrix metalloproteinase-2 activation in ovarian cancer cells

Epithelial ovarian cancer (EOC) is asymptomatic at early stages and is often diagnosed late when tumor cells are highly metastatic. Lysophosphatidic acid (LPA) has been implicated in ovarian oncogenesis as levels of this lipid are elevated in patient ascites and plasma. Because the underlying mechanism governing LPA regulation of matrix metalloproteinase-2 (MMP-2) activation remains undefined, we investigated the relationship between LPA-induced changes in actin microfilament organization and MMP-2 enzymatic activity. We report that when cells were cultured at a high density, LPA mediated stress fiber and focal adhesion disassembly and significantly repressed RhoA activity in EOC cells. Inhibition of Rho-kinase/ROCK enhanced both LPA-stimulated loss of stress fibers and pro-MMP-2 activation. In contrast, expression of the constitutively active RhoA(G14V) mutant diminished LPA-induced pro-MMP-2 activation. LPA had no effects on membrane type 1-MMP or tissue inhibitor of metalloproteinase-2 expression, but up-regulated MMP-2 levels, contributing to the induction of MMP-2 activation. Interestingly, when cells were cultured at a low density, stress fibers were present after LPA stimulation, and ROCK activity was required for EOC cell migration. Collectively, these results were consistent with a model in which LPA stimulates the metastatic dissemination of EOC cells by initiating loss of adhesion and metalloproteinase activation.

Dynamic assessment of cell-matrix mechanical interactions in three-dimensional culture

Cell-matrix mechanical interactions play a defining role in a range of biological processes such as developmental morphogenesis and wound healing. Despite current agreement that fibroblasts exert mechanical forces on the extracellular matrix (ECM) to promote structural organization of the collagen architecture, the underlying mechanisms of force generation and transduction to the ECM are not completely understood. Investigation of these processes has been limited, in part, by the technical challenges associated with simultaneous imaging of cell activity and fibrillar collagen organization. To overcome these limitations, we have developed an experimental model in which cells expressing proteins tagged with enhanced green fluorescent protein are plated inside fibrillar collagen matrices, and high magnification time-lapse differential interference contrast and fluorescent imaging is then performed. Using this system, focal adhesion movement and reorganization in isolated cells can be directly correlated with collagen matrix deformation and changes in the mechanical behavior of fibroblasts can be assessed over time.

Quantitative assessment of local collagen matrix remodeling in 3-D culture: the role of Rho kinase

The purpose of this study was to quantitatively assess the role of Rho kinase in modulating the pattern and amount of local cell-induced collagen matrix remodeling. Human corneal fibroblasts were plated inside 100-microm thick fibrillar collagen matrices and cultured for 24 h in media with or without the Rho kinase inhibitor Y-27632. Cells were then fixed and stained with phalloidin. Fluorescent (for f-actin) and reflected light (for collagen fibrils) 3-D optical section images were acquired using laser confocal microscopy. Fourier transform analysis was used to assess collagen fibril alignment, and 3-D cell morphology and local collagen density were measured using MetaMorph. Culture in serum-containing media induced significant global matrix contraction, which was inhibited by blocking Rho kinase (p<0.001). Fibroblasts generally had a bipolar morphology and intracellular stress fibers. Collagen fibrils were compacted and aligned parallel to stress fibers and pseudopodia. When Rho kinase was inhibited, cells had a more cortical f-actin distribution and dendritic morphology. Both local collagen fibril density and alignment were significantly reduced (p<0.01). Overall, the data suggests that Rho kinase-dependent contractile force generation leads to co-alignment of cells and collagen fibrils along the plane of greatest resistance, and that this process contributes to global matrix contraction.

Extracellular matrix macroassembly dynamics in early vertebrate embryos

This chapter focuses on the in vivo macroassembly dynamics of fibronectin and fibrillin-2--two prominent extracellular matrix (ECM) components, present in vertebrate embryos at the earliest stages of development. The ECM is an inherently dynamic structure with a well-defined position fate: ECM filaments are not only anchored to and move with established tissue boundaries, but are repositioned prior to the formation of new anatomical features. We distinguish two ECM filament relocation processes-each operating on different length scales. First, ECM filaments are moved by large-scale tissue motion, which rearranges major organ primordia within the embryo. The second type of motion, on the scale of the individual ECM filaments, is driven by local motility and protrusive activity of nearby cells. The motion decomposition is made practically possible by recent advances in microscopy and high-resolution particle image velocimetry algorithms. We demonstrate that both kinds of motion contribute substantially to the establishment of normal ECM structure, and both must be taken into account when attempting to understand ECM macroassembly during embryonic morphogenesis. The tissue-scale motion changes the local amount (density) and the tissue-level structure (e.g., orientation) of ECM fibers. Local reorganization includes filament assembly and the segregation of ECM into specific patterns. Local reorganization takes place most actively at Hensen's node and around the primitive streak. These regions are also sites of active cell migration, where fibrillin-2 and fibronectin are often colocalized in ECM globules, and new fibrillin-2 foci are deposited. During filament assembly, the globular patches of ECM are joined into larger linear structures in a hierarchical process: increasingly larger structures are created by the aggregation of smaller units. A future understanding of ECM assembly thus requires the study of the complex interactions between biochemical assembly steps, local cell action, and tissue motion.

The mesenchymal cell, its role in the embryo, and the remarkable signaling mechanisms that create it

This review centers on the role of the mesenchymal cell in development. The creation of this cell is a remarkable process, one where a tightly knit, impervious epithelium suddenly extends filopodia from its basal surface and gives rise to migrating cells. The ensuing process of epithelial-mesenchymal transformation (EMT) creates the mechanism that makes it possible for the mesenchymal cell to become mobile, so as to leave the epithelium and move through the extracellular matrix. EMT is now recognized as a very important mechanism for the remodeling of embryonic tissues, with the power to turn an epithelial somite into sclerotome mesenchyme, and the neural crest into mesenchyme that migrates to many targets. Thus, the time has come for serious study of the underlying mechanisms and the signaling pathways that are used to form the mesenchymal cell in the embryo. In this review, I discuss EMT centers in the embryo that are ready for such serious study and review our current understanding of the mechanisms used for EMT in vitro, as well as those that have been implicated in EMT in vivo. The purpose of this review is not to describe every study published in this rapidly expanding field but rather to stimulate the interest of the reader in the study of the role of the mesenchymal cell in the embryo, where it plays profound roles in development. In the adult, mesenchymal cells may give rise to metastatic tumor cells and other pathological conditions that we will touch on at the end of the review.

Dynamic three-dimensional visualization of collagen matrix remodeling and cytoskeletal organization in living corneal fibroblasts

The remodeling of extracellular matrices by cells plays a defining role in developmental morphogenesis and wound healing, as well as in tissue engineering. Three-dimensional (3-D) type I collagen matrices have been used extensively as an in vitro model for studying cell-induced matrix reorganization at the macroscopic level. However, few studies have directly assessed the dynamic process of 3-D matrix remodeling at the cellular and subcellular level. We recently developed an experimental model for investigating cell-matrix mechanical interactions by plating green fluorescen protein (GFP)-zyxin transfected cells inside fibrillar collagen matrices and performing high-magnification time-lapse differential interference microscopy (DIC) and wide-field fluorescent imaging. In this study, we extend this experimental model by performing four-dimensional (4-D) reflected light and fluorescent confocal imaging (using either visible light or multiphoton excitation) of living corneal fibroblasts transfected to express GFP-zyxin or GFP-alpha-actinin, 18 h after plating inside 3-D collagen matrices. Reflected light confocal imaging allowed detailed visualization of the cells and the fibrillar collagen surrounding them. By overlaying maximum intensity projections of reflected light and GFP-zyxin or GFP-alpha-actinin images and generating stereo pair reconstructions, 3-D interactions between focal adhesions and collagen fibrils in living cells could be visualized directly. Focal adhesions were generally oriented parallel to the direction of collagen fibril alignment in front of the cell. Killing the cells induced relaxation of transient cell-induced tension on the matrix; however, significant permanent remodeling always remained. Time-lapse 3-D imaging demonstrated an active response to the Rho-kinase inhibitor Y-27632, as indicated by cell elongation, extracellular matrix relaxation, and extension of pseudopodial processes. It is interesting that, at higher cell densities, groups of collagen fibrils were compacted and aligned into straps between neighboring cells. Overall, the continued development and application of this new approach should provide important insights into the basic underlying biochemical and biomechanical regulatory mechanisms controlling matrix remodeling by corneal fibroblasts.

Direct, dynamic assessment of cell-matrix interactions inside fibrillar collagen lattices

Cell mechanical behavior has traditionally been studied using 2-D planar elastic substrates. The goal of this study was to directly assess cell-matrix mechanical interactions inside more physiologic 3-D collagen matrices. Rabbit corneal fibroblasts transfected to express GFP-zyxin were plated at low density inside 100 micro m-thick type I collagen matrices. 3-D datasets of isolated cells were acquired at 1-3-min intervals for up to 5 h using fluorescent and Nomarski DIC imaging. Unlike cells on 2-D substrates, cells inside the collagen matrices had a bipolar morphology with thin pseudopodial processes, and without lamellipodia. The organization of the collagen fibrils surrounding each cell was clearly visualized using DIC. Using time-lapse color overlays of GFP and DIC images, displacement and/or realignment of collagen fibrils by focal adhesions could be directly visualized. During pseudopodial extension, new focal adhesions often formed in a line along collagen fibrils in front of the cell, while existing adhesions moved backward. This process generated tractional forces as indicated by the pulling in of collagen fibrils in front of the cell. Meanwhile, adhesions on both the dorsal and ventral surface of the cell body generally moved forward, resulting in contractile shortening along the pseudopodia and localized extracellular matrix (ECM) compression. Cytochalasin D induced rapid disassembly of focal adhesions, cell elongation, and ECM relaxation. This experimental model allows direct, dynamic assessment of cell-matrix interactions inside a 3-D fibrillar ECM. The data suggest that adhesions organize along actin-based contractile elements that are much less complex than the network of actin filaments that mechanically links lamellar adhesions on 2-D substrates.

Contractile responses of smooth muscle cells differentiated from rat neural stem cells

To characterize the functional differentiation of neural stem cells into smooth muscle cells, multipotent stem cells in the central nervous system (CNS) were isolated from rat embryonic day 14 (E14) cortex and cultured by neurosphere formation in serum-free medium in the presence of 10 ng ml(-1) of basic fibroblast growth factor. Differentiation was induced by the addition of 10 % fetal bovine serum to low-density cultures (2.5 x 10(3) cells cm(-2)). Immunological analyses and reverse transcriptase-polymerase chain reaction indicated that the differentiated cells expressed smooth-muscle-specific marker proteins such as SM-1, SM-2, and SMemb myosin heavy chains, SM-22, basic calponin and alpha-smooth-muscle actin, but not the astrocyte marker glial fibrillary acidic protein. To examine whether smooth-muscle-like cells that are differentiated from CNS stem cells possess the characteristics of contractile smooth muscle, we prepared reconstituted collagen gel fibres and measured their contractile tension. The reconstituted fibres were prepared by thermal gelation of collagen and the differentiated cells. The fibres contracted in response to treatment with KCl (80 mM), ACh (100 microM), endothelin-1 (10 nM), endothelin-2 (10 nM), and prostaglandin F2alpha (100 microM). ACh-induced contraction was partially inhibited by the L-type voltage-dependent Ca(2+) channel inhibitor nifedipine and by the intracellular Ca(2+) chelator 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, the myosin light chain kinase inhibitor ML-9, the Rho kinase inhibitor Y-27632, dibutyryl cAMP and 8-bromo-cGMP. These results suggest that CNS stem cells give rise to smooth muscle cells in vitro that have an identical contractile function to smooth muscle in vivo.

Studying actin-dependent processes in tissue culture

The cytoskeletal organization of cells that are grown in tissue culture is often very different from that of cells in living organisms. This casts some doubt as to whether information that comes from studying actin-dependent cellular processes--such as cell motility or differentiation--in cells that are cultured under these conditions is physiologically relevant. Studies on cells grown in improved two-dimensional- and three-dimensional-culture systems that closely mimic the in vivo extracellular-matrix environment should provide a more accurate picture of actin-cytoskeletal function in the living organism.

Effects of pdgf-bb on rat dermal fibroblast behavior in mechanically stressed and unstressed collagen and fibrin gels

The dose-response effects of platelet-derived growth factor BB (PDGF-BB) on rat dermal fibroblast (RDF) behavior in mechanically stressed and unstressed type I collagen and fibrin were investigated using quantitative assays developed in our laboratory. In chemotaxis experiments, RDFs responded optimally (P < 0.05) to a gradient of 10 ng/ml PDGF-BB in both collagen and fibrin. In separate experiments, the migration of RDFs and the traction exerted by RDFs in the presence of PDGF-BB (0, 0.1, 1, 10, or 100 ng/ml) were assessed simultaneously in the presence or absence of stress. RDF migration increased significantly (P < 0.05) at doses of 10 and 100 ng/ml PDGF-BB in collagen and fibrin in the presence and absence of stress. In contrast, the effects of PDGF-BB on RDF traction depended on the gel type and stress state. PDGF-BB decreased fibroblast traction in stressed collagen, but increased traction in unstressed collagen (P < 0.05). No statistical conclusion could be inferred for stressed fibrin, but increasing PDGF-BB decreased traction in unstressed fibrin (P < 0.05). These results demonstrate the complex response of fibroblasts to environmental cues and suggest that mechanical resistance to compaction may be a crucial element in dictating fibroblast behavior.

Adhesion between cells and extracellular matrix with special reference to hepatic stellate cell adhesion to three-dimensional collagen fibers

Hepatic stellate cells are located in the perisinusoidal space (space of Disse), and extend their dendritic, thin membranous processes and fine fibrillar processes into this space. The stellate cells coexist with a three-dimensional extracellular matrix (ECM) in the perisinusoidal space. In turn the three-dimensional structure of the ECM regulates the proliferation, morphology, and functions of the stellate cell. In this review, the morphology of sites of adhesion between hepatic stellate cells and extracellular matrix is described. Hepatic stellate cells cultured in polystyrene dishes spread well, whereas the cells cultured on or in type I collagen gel become slender and elongate their long cellular processes which adhere directly to the collagen fibers. Cells in type I collagen gel form a large number of adhesive structures, each adhesive area forming a face but not a point. Adhesion molecules, integrins, for the ECM are localized on the cell surface. Elongation of the cellular processes occurs via integrin-binding to type I collagen fibers. The signal transduction mechanism, including protein and phosphatidylinositol phosphorylation, is critical to induce and sustain the cellular processes. Information on the three-dimensional structures of ECM is transmitted via three-dimensional adhesive structures containing the integrins.

Agonist-induced isometric contraction of smooth muscle cell-populated collagen gel fiber

String-shaped reconstituted smooth muscle (SM) fibers were prepared in rectangular wells by thermal gelation of a mixed solution of collagen and cultured SM cells derived from guinea pig stomach. The cells in the fiber exhibited an elongated spindle shape and were aligned along the long axis. The fiber contracted in response to KCl (140 mM), norepinephrine (NE; 10(-7) M), epinephrine (10(-7) M), phenylephrine (10(-6) M), serotonin (10(-6) M), and histamine (10(-5) M), but not acetylcholine (10(-5) M). Phentolamine (10(-7) M) produced a parallel rightward shift of the NE dose-response curve. Moreover, NE-induced contraction was partially inhibited by nifedipine and completely abolished by the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, the myosin light chain kinase inhibitor ML-9, the Rho kinase inhibitor Y-27632, and papaverine. A [(3)H]quinuclidinyl benzilate binding study revealed that the loss of response to acetylcholine was due to the loss of muscarinic receptor expression during culture. The expression of contractile proteins in the fibers was similar to that in cultured SM cells. These results suggest that, although the fiber is not a model for fully differentiated SM, contractile mechanisms are maintained.