The urokinase plasminogen activator receptor in the regulation of the actin cytoskeleton and cell motility

Cell migration is a complex process requiring tight control of several mechanisms including dynamic reorganization of the actin cytoskeleton and adhesion to the extracellular matrix. The GPI-anchored urokinase plasminogen activator receptor (uPAR) has an important role in the regulation of cell motility in many cell types. This is partly due to the localization of proteolytic activity on the cell surface by binding of the serine protease uPA. Results accumulated over the last decade suggest that uPAR is also involved in motility control through other mechanisms. These include induction of signal transduction events after ligation with uPA, binding to the extracellular matrix molecule vitronectin (VN), and association with integrins and other transmembrane partners. In this review these mechanisms will be discussed with a special emphasis on how the GPI-linked receptor transmits signals to the intracellular milieu and how uPAR participates in the regulation of actin cytoskeleton reorganization and cell adhesion during cell migration.     

Golgi body motility in the plant cell cortex correlates with actin cytoskeleton organization

The actin cytoskeleton is involved in the transport and positioning of Golgi bodies, but the actin-based processes that determine the positioning and motility behavior of Golgi bodies are not well understood. In this work, we have studied the relationship between Golgi body motility behavior and actin organization in intercalary growing root epidermal cells during different developmental stages. We show that in these cells two distinct actin configurations are present, depending on the developmental stage. In small cells of the early root elongation zone, fine filamentous actin (F-actin) occupies the whole cell, including the cortex. In larger cells in the late elongation zone that have almost completed cell elongation, actin filament bundles are interspersed with areas containing this fine F-actin and areas without F-actin. Golgi bodies in areas with the fine F-actin exhibit a non-directional, wiggling type of motility. Golgi bodies in areas containing actin filament bundles move up to 7 μm s?1. Since the motility of Golgi bodies changes when they enter an area with a different actin configuration, we conclude that the type of movement depends on the actin organization and not on the individual organelle. Our results show that the positioning of Golgi bodies depends on the local actin organization.     

The role of palladin in actin organization and cell motility

Palladin is a widely expressed protein found in stress fibers, focal adhesions, growth cones, Z-discs, and other actin-based subcellular structures. It belongs to a small gene family that includes the Z-disc proteins myopalladin and myotilin, all of which share similar Ig-like domains. Recent advances have shown that palladin shares with myotilin the ability to bind directly to F-actin, and to crosslink actin filaments into bundles, in vitro. Studies in a variety of cultured cells suggest that the actin-organizing activity of palladin plays a central role in promoting cell motility. Correlative evidence also supports this hypothesis, as palladin levels are typically up-regulated in cells that are actively migrating: in developing vertebrate embryos, in cells along a wound edge, and in metastatic cancer cells. Recently, a mutation in the human palladin gene was implicated in an unusually penetrant form of inherited pancreatic cancer, which has stimulated new ideas about the role of palladin in invasive cancer.     

A critical role of tropomyosins in TGF-beta regulation of the actin cytoskeleton and cell motility in epithelial cells

We have investigated transforming growth factor beta (TGF-beta)-mediated induction of actin stress fibers in normal and metastatic epithelial cells. We found that stress fiber formation requires de novo protein synthesis, p38Mapk and Smad signaling. We show that TGF-beta via Smad and p38Mapk up-regulates expression of actin-binding proteins including high-molecular-weight tropomyosins, alpha-actinin and calponin h2. We demonstrate that, among these proteins, tropomyosins are both necessary and sufficient for TGF-beta induction of stress fibers. Silencing of tropomyosins with short interfering RNAs (siRNAs) blocks stress fiber assembly, whereas ectopic expression of tropomyosins results in stress fibers. Ectopic-expression and siRNA experiments show that Smads mediate induction of tropomyosins and stress fibers. Interestingly, TGF-beta induction of stress fibers was not accompanied by changes in the levels of cofilin phosphorylation. TGF-beta induction of tropomyosins and stress fibers are significantly inhibited by Ras-ERK signaling in metastatic breast cancer cells. Inhibition of the Ras-ERK pathway restores TGF-beta induction of tropomyosins and stress fibers and thereby reduces cell motility. These results suggest that induction of tropomyosins and stress fibers play an essential role in TGF-beta control of cell motility, and the loss of this TGF-beta response is a critical step in the acquisition of metastatic phenotype by tumor cells.     

The actin cytoskeleton in ageing and apoptosis

Regulated cell death, or apoptosis, has evolved to fulfil a myriad of functions amongst multicellular organisms. It is now apparent that programmed cell death occurs in unicellular organisms such as yeast. In yeast, as in higher eukaryotes, the actin cytoskeleton is an essential component of a number of cellular activities, and many of the regulatory proteins involved are highly conserved. Recent evidence from diverse eukaryotic systems suggests that the actin cytoskeleton has a role in regulating apoptosis via interactions with the mitochondria. This interaction also appears to have a significant impact on the management of oxidative stress and so cellular ageing. In this mini-review we summarise some of the work, which suggests that actin is a key regulator of apoptosis and ageing in eukaryotic cells.     

The actin cytoskeleton in presynaptic assembly

Dramatic morphogenetic processes underpin nearly every step of nervous system development, from initial neuronal migration and axon guidance to synaptogenesis. Underlying this morphogenesis are dynamic rearrangements of cytoskeletal architecture. Here we discuss the roles of the actin cytoskeleton in the development of presynaptic terminals, from the elaboration of terminal arbors to the recruitment of presynaptic vesicles and active zone components. The studies discussed here underscore the importance of actin regulation at every step in neuronal circuit assembly.     

The nucleation-release model of actin filament dynamics in cell motility

The actin cytoskeleton is intimately involved in the motile behaviour of animal cells. The structure and dynamic behaviour of actin and its binding proteins have been intensively studied in vitro over the past several decades, culminating in achievements such as an atomic model of the actin filament. Despite this progress, it is not yet clear how the behaviour of these purified proteins in vitro relates to the dynamic behaviour of actin inside living, moving cells. Here we discuss a new model that relates the known dynamic parameters for pure actin to the observed behaviour of actin filaments inside motile cells.     

Cyclooxygenase and cAMP-dependent protein kinase reorganize the actin cytoskeleton for motility in HeLa cells

The adhesion of a cell to its surrounding matrix is a key determinant in many aspects of cell behavior. Adhesion consists of distinct stages : attachment, cell spreading, motility, and/or immobilization. Interrelated signaling pathways regulate these stages, and many adhesion-related signals control the architecture of the cytoskeleton. The various cytoskeletal organizations then give rise to the specific stages of adhesion. It has been shown that arachidonic acid acts at a signaling branch point during cell attachment. Arachidonic acid is metabolized via lipoxygenase to activate actin polymerization and cell spreading. It is also metabolized by cyclooxygenase to generate small actin bundles. We have used confocal microscopy and indirect immunofluorescence to investigate the structure of these cyclooxygenase dependent actin bundles in HeLa cells. We have also employed cell migration assays and pharmacological modulation of cyclooxygenase and downstream signals. The results indicate that cyclooxygenase and PKA stimulate the formation of actin bundles that contain myosin II and associate with small focal adhesions. In addition, we demonstrate that this cytoskeletal organization correlates with increased cell motility.     

Trophoblast giant-cell differentiation involves changes in cytoskeleton and cell motility

Trophoblast giant-cell differentiation is well-characterized at the molecular level, yet very little is known about how molecular changes affect the cellular functions of trophoblast in embryo implantation. We have found, using both explanted E7.5 mouse embryo ectoplacental cone and the rat choriocarcinoma (Rcho-1) cell line, that trophoblast differentiation is distinguished by dramatic changes in cytoarchitecture and cell behavior. Undifferentiated trophoblast cells contain little organized actin and few small, peripheral focal complexes and exhibit high membrane protrusive activity, while differentiated trophoblast giant cells contain prominent stress fibers, large internal as well as peripheral focal adhesions, and become immotile. The dramatic changes in cell behavior and cytoskeletal organization of giant cells correlate with changes in the activities of the Rho family of small GTPases and a decrease in tyrosine phosphorylation of focal adhesion kinase. Together, these data provide detailed insight into the cellular properties of trophoblast giant cells and suggest that giant-cell differentiation is characterized by a transition from a motile to a specialized epithelial phenotype. Furthermore, our data support a phagocytic erosion, rather than a migratory infiltration, mechanism for trophoblast giant-cell invasion of the uterine stroma.