Integrin involvement in keratocyte locomotion

Dual inhibition of TGFβR and ROCK reverses the epithelial to mesenchymal transition in collectively migrating zebrafish keratocytes

There is a growing controversy about the role of the epithelial to mesenchymal transition (EMT) in the fibrosis associated with chronic disease. Recent studies suggest that it is not the EMT transcriptional program but differentiation of progenitor cells, response to chronic inflammation, or some combination of both which cause the appearance of fibroblasts and the production of the extracellular matrix. To address this issue, we study the EMT process in the zebrafish keratocytes which migrate from primary explants of epithelial tissue as these cells are both terminally differentiated and able to divide. To firmly place this EMT process in the context of other systems, we first demonstrate that the zebrafish keratocyte EMT process involves nuclear accumulation of twist and snail/slug transcription factors as part of a TGFβR‐mediated EMT process. As assessed by the expression and localization of EMT transcription factors, the zebrafish keratocyte EMT process is reversed by the addition of Rho‐activated kinase (ROCK) in combination with TGFβR inhibitors. The complete cycle of EMT to MET observed in this system links these in vitro results more closely to the process of wound healing in vivo. However, the absence of observable activation of EMT transcription factors when keratocytes are cultured on compliant substrata in a TGFβ1‐containing medium suggests that ROCK signaling, initiated by tension within the sheet, is an essential contributor to the EMT process. Most importantly, the requirement for ROCK activation by culturing on noncompliant substrata suggests that EMT in these terminally differentiated cells would not occur in vivo.

Collective cell migration of primary zebrafish keratocytes

Fish keratocytes are an established model in single cell motility but little is known about their collective migration. Initially, sheets migrate from the scale at ~145 μm/h but over the course of 24h the rate of leading edge advance decreases to ~23 μm/h. During this period, leader cells retain their ability to migrate rapidly when released from the sheet and follower cell area increases. After the addition of RGD peptide, leader cell lamellae are lost, altering migratory forces within the sheet, resulting in rapid retraction. Leader and follower cell states interconvert within minutes with changes in cell-cell adhesions. Leader cells migrate as single cells when they detach from the leading edge and single cells appear to become leader cells if they rejoin the sheet. Follower cells rapidly establish leader cell morphology during closing of holes formed during sheet expansion and revert to follower cell morphology after hole-closure. Inhibition of Rho associated kinase releases leader cells and halts advancement of the leading edge suggesting an important role for the intercellular actomyosin cable at the leading edge. In addition, the presence of the stationary scale orients direction of sheet migration which is characterized by a more uniform advance of the leading edge than in some cell line systems. These data establish fish keratocyte explant cultures as a collective cell migration system and suggest that cell-cell interactions determine the role of keratocytes within the migrating sheet.

Cell migration

Cell migration is fundamental to establishing and maintaining the proper organization of multicellular organisms. Morphogenesis can be viewed as a consequence, in part, of cell locomotion, from large-scale migrations of epithelial sheets during gastrulation, to the movement of individual cells during development of the nervous system. In an adult organism, cell migration is essential for proper immune response, wound repair, and tissue homeostasis, while aberrant cell migration is found in various pathologies. Indeed, as our knowledge of migration increases, we can look forward to, for example, abating the spread of highly malignant cancer cells, retarding the invasion of white cells in the inflammatory process, or enhancing the healing of wounds. This article is organized in two main sections. The first section is devoted to the single-cell migrating in isolation such as occurs when leukocytes migrate during the immune response or when fibroblasts squeeze through connective tissue. The second section is devoted to cells collectively migrating as part of multicellular clusters or sheets. This second type of migration is prevalent in development, wound healing, and in some forms of cancer metastasis.

Zebrafish keratocyte explant cultures as a wound healing model system: differential gene expression & morphological changes support epithelial-mesenchymal transition

The control of collective cell migration of zebrafish keratocyte sheets in explant culture is of interest for cell migration and epithelial wound healing and depends on the gene expression profile. In a zebrafish genome array, ∼17.5% of the probe sets were differentially expressed greater than two-fold (p≤0.003) between 1 and 7 days of explant culture. Among the differentially expressed genes were a variety of wound healing-related genes and many of the biomarkers for epithelial-mesenchymal transition (EMT), including a switch from keratin and E-cadherin to vimentin and N-cadherin expression and several EMT-related transcription factors were found to be differentially expressed. Supporting evidence for EMT is seen in both morphological change and rearrangement of the actin cytoskeleton and in expression of cadherins during explant culture with a visible disassembly of the cell sheet. TGFβ1 and TNFα expression were analyzed by qPCR at various time points and peak differential expression of both cytokines occurred at 3 days, indicating that the EMT process is ongoing under conditions routinely used in the study of fish keratocyte motility. These data establish that an EMT process is occurring during zebrafish keratocyte explant culture and support the use of this system as a wound healing model.

Cell migration

Cell migration is fundamental to establishing and maintaining the proper organization of multicellular organisms. Morphogenesis can be viewed as a consequence, in part, of cell locomotion, from large-scale migrations of epithelial sheets during gastrulation, to the movement of individual cells during development of the nervous system. In an adult organism, cell migration is essential for proper immune response, wound repair, and tissue homeostasis, while aberrant cell migration is found in various pathologies. Indeed, as our knowledge of migration increases, we can look forward to, for example, abating the spread of highly malignant cancer cells, retarding the invasion of white cells in the inflammatory process, or enhancing the healing of wounds. This article is organized in two main sections. The first section is devoted to the single-cell migrating in isolation such as occurs when leukocytes migrate during the immune response or when fibroblasts squeeze through connective tissue. The second section is devoted to cells collectively migrating as part of multicellular clusters or sheets. This second type of migration is prevalent in development, wound healing, and in some forms of cancer metastasis.

TGFβ (transforming growth factor β) and keratocyte motility in 24 h zebrafish explant cultures

Fish keratocytes are used as a model system for the study of the mechanics of cell motility because of their characteristic rapid, smooth gliding motion, but little work has been done on the regulation of fish keratocyte movement. As TGFβ (transforming growth factor β) plays multiple roles in primary human keratinocyte cell migration, we investigated the possible involvement of TGFβ in fish keratocyte migration. Studying the involvement of TGFβ1 in 24 h keratocyte explant allows the examination of the cells before alterations in cellular physiology occur due to extended culture times. During this initial period, TGFβ levels increase 6.2-fold in SFM (serum-free medium) and 2.4-fold in SFM+2% FBS (fetal bovine serum), while TGFβ1 and TGFβRII (TGFβ receptor II) mRNA levels increase ∼3- and ∼5-fold respectively in each culture condition. Two measures of motility, cell sheet area and migration distance, vary with the amount of exogenous TGFβ1 and culture media. The addition of 100 ng/ml exogenous TGFβ1 in SFM increases both measures [3.3-fold (P = 4.5×10-5) and 26% (P = 2.1×10-2) respectively]. In contrast, 100 ng/ml of exogenous TGFβ1 in medium containing 2% FBS decreases migration distance by 2.1-fold (P = 1.7×10-7), but does not affect sheet area. TGFβ1 (10 ng/ml) has little effect on cell sheet area in SFM cultures, but leads to a 1.8-fold increase (P = 1.5×10-2) with 2% FBS. The variable response to TGFβ1 may be, at least in part, explained by the effect of 2% FBS on cell morphology, mode of motility and expression of endogenous TGFβ1 and TGFβRII. Together, these results suggest that expression of TGFβ and its receptor are up-regulated during zebrafish keratocyte explant culture and that TGFβ promotes fish keratocyte migration.

Mechanics model for actin-based motility

We present here a mechanics model for the force generation by actin polymerization. The possible adhesions between the actin filaments and the load surface, as well as the nucleation and capping of filament tips, are included in this model on top of the well-known elastic Brownian ratchet formulation. A closed form solution is provided from which the force-velocity relationship, summarizing the mechanics of polymerization, can be drawn. Model predictions on the velocity of moving beads driven by actin polymerization are consistent with experiment observations. This model also seems capable of explaining the enhanced actin-based motility of Listeria monocytogenes and beads by the presence of Vasodilator-stimulated phosphoprotein, as observed in recent experiments.

Emergence of large-scale cell morphology and movement from local actin filament growth dynamics

Variations in cell migration and morphology are consequences of changes in underlying cytoskeletal organization and dynamics. We investigated how these large-scale cellular events emerge as direct consequences of small-scale cytoskeletal molecular activities. Because the properties of the actin cytoskeleton can be modulated by actin-remodeling proteins, we quantitatively examined how one such family of proteins, enabled/vasodilator-stimulated phosphoprotein (Ena/VASP), affects the migration and morphology of epithelial fish keratocytes. Keratocytes generally migrate persistently while exhibiting a characteristic smooth-edged “canoe” shape, but may also exhibit less regular morphologies and less persistent movement. When we observed that the smooth-edged canoe keratocyte morphology correlated with enrichment of Ena/VASP at the leading edge, we mislocalized and overexpressed Ena/VASP proteins and found that this led to changes in the morphology and movement persistence of cells within a population. Thus, local changes in actin filament dynamics due to Ena/VASP activity directly caused changes in cell morphology, which is coupled to the motile behavior of keratocytes. We also characterized the range of natural cell-to-cell variation within a population by using measurable morphological and behavioral features—cell shape, leading-edge shape, filamentous actin (F-actin) distribution, cell speed, and directional persistence—that we have found to correlate with each other to describe a spectrum of coordinated phenotypes based on Ena/VASP enrichment at the leading edge. This spectrum stretched from smooth-edged, canoe-shaped keratocytes—which had VASP highly enriched at their leading edges and migrated fast with straight trajectories—to more irregular, rounder cells migrating slower with less directional persistence and low levels of VASP at their leading edges. We developed a mathematical model that accounts for these coordinated cell-shape and behavior phenotypes as large-scale consequences of kinetic contributions of VASP to actin filament growth and protection from capping at the leading edge. This work shows that the local effects of actin-remodeling proteins on cytoskeletal dynamics and organization can manifest as global modifications of the shape and behavior of migrating cells and that mathematical modeling can elucidate these large-scale cell behaviors from knowledge of detailed multiscale protein interactions.

The shape of animal cells is largely determined by the organization of their internal structural elements, including the filamentous structures of their cytoskeleton. Motile cells that crawl across solid substrates must assemble their cytoskeletal actin filaments in a spatially organized way, such that net filament growth and cell protrusion occur at the front of the cell. Actin filament dynamics, in turn, influence the overall shape of the cell by pushing on the plasma membrane. In this work, we have explored the ways that variations in small-scale actin filament growth dynamics are coupled to large-scale changes in cell shape and behavior. By manipulating the availability of a family of actin-binding proteins (Ena/VASP) that regulate actin filament growth, we can alter the overall cell shape and motile behavior of epithelial fish keratocytes—unusually fast-moving and regularly shaped cells. We have also found that unperturbed keratocytes in a population exhibit a continuum of shape and behavioral variations that can be correlated with differences in Ena/VASP levels. We have developed a mathematical model that allows us to explain our observations of intrinsic cell-to-cell shape variation, motile behavior, and cell responses to molecular perturbations as a function of actin filament growth dynamics in motile cells.

Mathematical modeling predicts global modifications of the shape and behavior of migrating cells from knowledge of detailed multiscale protein interactions.

Encapsulation of attached ectoparasitic glochidia larvae of freshwater mussels by epithelial tissue on fins of naive and resistant host fish

To metamorphose into juveniles and subsequently mature into adults, the glochidia larvae of freshwater mussels in the order Unionoida must temporarily parasitize the gills, fins, or other external structures of fish. Once attached to the fish, the glochidium is encapsulated by host fish epithelial tissue. The migration of epithelial cells of the bluegill sunfish Lepomis macrochirus over glochidia of Utterbackia imbecillis was examined by time-lapse video microscopy, and the morphology was examined by scanning electron microscopy. Initially, the leading edge epithelial cells migrating over the larvae became rounded and the cells moved as a sheet until the attached glochidium was completely covered. Cyst formation on host fish that had been repeatedly exposed to mussel larvae was significantly delayed and morphologically irregular compared to that on naïve fish. Cyst formation on other species of fish that are less successful as hosts was examined. In general, it took longer for glochidia to become encapsulated on these less suitable potential hosts. The delay and irregularities in cyst formation on resistant fish and nonhost fish species may result in increased mortality and reduced success of metamorphosis of glochidia.

In Situ Photoactivation of a Caged Phosphotyrosine Peptide Derived from Focal Adhesion Kinase Temporarily Halts Lamellar Extension of Single Migrating Tumor Cells

Focal adhesion kinase (FAK), a non-receptor tyrosine kinase, mediates integrin-based cell signaling by transferring signals regulating cell migration, adhesion, and survival from the extracellular matrix to the cytoplasm. Following autophosphorylation at tyrosine 397, FAK binds the Src homology 2 domains of Src and phosphoinositide 3-kinase, among several other possible binding partners. To further investigate the role of phosphorylated FAK in cell migration in situ, peptides comprising residues 391-406 of human FAK with caged phosphotyrosine 397 were synthesized. Although the caged phosphopeptides were stable to phosphatase activity, the free phosphopeptides showed a half-life of approximately 10-15 min in cell lysates. Migrating NBT-II cells (a rat bladder tumor cell line) were microinjected with the caged FAK peptide and locally photoactivated using a focused laser beam. The photoactivation of caged FAK peptide in 8-microm diameter spots over the cell body led to the temporary arrest of the leading edge migration within approximately 1 min of irradiation. In contrast, cell body migration was not inhibited. Microinjection of a non-caged phosphorylated tyrosine 397 FAK peptide into migrating NBT-II cells also led to lamellar arrest; however, this approach lacks the temporal control afforded by the caged phosphopeptides. Photoactivation of related phosphotyrosine peptides with altered sequences did not result in transient lamellar arrest. We hypothesize that the phosphorylated FAK peptide competes with the endogenous FAK for binding to FAK effectors including, but not limited to, Src and phosphoinositide 3-kinase, causing spatiotemporal misregulation and subsequent lamellar arrest.

Crawling toward a unified model of cell mobility: spatial and temporal regulation of actin dynamics

Crawling cells of various morphologies displace themselves in their biological environments by a similar overall mechanism of protrusion through actin assembly at the front coordinated with retraction at the rear. Different cell types organize very distinct protruding structures, yet they do so through conserved biochemical mechanisms to regulate actin polymerization dynamics and vary the mechanical properties of these structures. The moving cell must spatially and temporally regulate the biochemical interactions of its protein components to exert control over higher-order dynamic structures created by these proteins and global cellular responses four or more orders of magnitude larger in scale and longer in time than the individual protein-protein interactions that comprise them. To fulfill its biological role, a cell globally responds with high sensitivity to a local perturbation or signal and coordinates its many intracellular actin-based functional structures with the physical environment it experiences to produce directed movement. This review attempts to codify some unifying principles for cell motility that span organizational scales from single protein polymer filaments to whole crawling cells.

Distinct roles of frontal and rear cell-substrate adhesions in fibroblast migration

Cell migration involves complex physical and chemical interactions with the substrate. To probe the mechanical interactions under different regions of migrating 3T3 fibroblasts, we have disrupted cell-substrate adhesions by local application of the GRGDTP peptide, while imaging stress distribution on the substrate with traction force microscopy. Both spontaneous and GRGDTP-induced detachment of the trailing edge caused extensive cell shortening, without changing the overall level of traction forces or the direction of migration. In contrast, disruption of frontal adhesions caused dramatic, global loss of traction forces before any significant shortening of the cell. Although traction forces and cell migration recovered within 10-20 min of transient frontal treatment, persistent treatment with GRGDTP caused the cell to develop traction forces elsewhere and reorient toward a new direction. We conclude that contractile forces of a fibroblast are transmitted to the substrate through two distinct types of adhesions. Leading edge adhesions are unique in their ability to transmit active propulsive forces. Their functions cannot be transferred directly to existing adhesions upon detachment. Trailing end adhesions create passive resistance during cell migration and readily redistribute their loads upon detachment. Our results indicate the distinct nature of mechanical interactions at the leading versus trailing edges, which together generate the mechanical interactions for fibroblast migration.

Keratocytes pull with similar forces on their dorsal and ventral surfaces

As cells move forward, they pull rearward against extracellular matrices (ECMs), exerting traction forces. However, no rearward forces have been seen in the fish keratocyte. To address this discrepancy, we have measured the propulsive forces generated by the keratocyte lamella on both the ventral and the dorsal surfaces. On the ventral surface, a micromachined device revealed that traction forces were small and rearward directed under the lamella, changed direction in front of the nucleus, and became larger under the cell body. On the dorsal surface of the lamella, an optical gradient trap measured rearward forces generated against fibronectin-coated beads. The retrograde force exerted by the cell on the bead increased in the thickened region of the lamella where myosin condensation has been observed (Svitkina, T.M., A.B. Verkhovsky, K.M. McQuade, and G. G. Borisy. 1997. J. Cell Biol. 139:397-415). Similar forces were generated on both the ventral (0.2 nN/microm(2)) and the dorsal (0.4 nN/microm(2)) surfaces of the lamella, suggesting that dorsal matrix contacts are as effectively linked to the force-generating cytoskeleton as ventral contacts. The correlation between the level of traction force and the density of myosin suggests a model for keratocyte movement in which myosin condensation in the perinuclear region generates rearward forces in the lamella and forward forces in the cell rear.

Keratocytes generate traction forces in two phases

Forces generated by goldfish keratocytes and Swiss 3T3 fibroblasts have been measured with nanonewton precision and submicrometer spatial resolution. Differential interference contrast microscopy was used to visualize deformations produced by traction forces in elastic substrata, and interference reflection microscopy revealed sites of cell-substratum adhesions. Force ranged from a few nanonewtons at submicrometer spots under the lamellipodium to several hundred nanonewtons under the cell body. As cells moved forward, centripetal forces were applied by lamellipodia at sites that remained stationary on the substratum. Force increased and abruptly became lateral at the boundary of the lamellipodium and the cell body. When the cell retracted at its posterior margin, cell-substratum contact area decreased more rapidly than force, so that stress (force divided by area) increased as the cell pulled away. An increase in lateral force was associated with widening of the cell body. These mechanical data suggest an integrated, two-phase mechanism of cell motility: (1) low forces in the lamellipodium are applied in the direction of cortical flow and cause the cell body to be pulled forward; and (2) a component of force at the flanks pulls the rear margins forward toward the advancing cell body, whereas a large lateral component contributes to detachment of adhesions without greatly perturbing forward movement.

Separation of propulsive and adhesive traction stresses in locomoting keratocytes

Strong, actomyosin-dependent, pinching tractions in steadily locomoting (gliding) fish keratocytes revealed by traction imaging present a paradox, since only forces perpendicular to the direction of locomotion are apparent, leaving the actual propulsive forces unresolved. When keratocytes become transiently "stuck" by their trailing edge and adopt a fibroblast-like morphology, the tractions opposing locomotion are concentrated into the tail, leaving the active pinching and propulsive tractions clearly visible under the cell body. Stuck keratocytes can develop approximately 1 mdyn (10,000 pN) total propulsive thrust, originating in the wings of the cell. The leading lamella develops no detectable propulsive traction, even when the cell pulls on its transient tail anchorage. The separation of propulsive and adhesive tractions in the stuck phenotype leads to a mechanically consistent hypothesis that resolves the traction paradox for gliding keratocytes: the propulsive tractions driving locomotion are normally canceled by adhesive tractions resisting locomotion, leaving only the pinching tractions as a resultant. The resolution of the traction pattern into its components specifies conditions to be met for models of cytoskeletal force production, such as the dynamic network contraction model (Svitkina, T.M., A.B. Verkhovsky, K.M. McQuade, and G.G. Borisy. 1997. J. Cell Biol. 139:397-415). The traction pattern associated with cells undergoing sharp turns differs markedly from the normal pinching traction pattern, and can be accounted for by postulating an asymmetry in contractile activity of the opposed lateral wings of the cell.