Cell crawling: first the motor, now the transmission

Complex systems biology

Complex systems theory is concerned with identifying and characterizing common design elements that are observed across diverse natural, technological and social complex systems. Systems biology, a more holistic approach to study molecules and cells in biology, has advanced rapidly in the past two decades. However, not much appreciation has been granted to the realization that the human cell is an exemplary complex system. Here, I outline general design principles identified in many complex systems, and then describe the human cell as a prototypical complex system. Considering concepts of complex systems theory in systems biology can illuminate our overall understanding of normal cell physiology and the alterations that lead to human disease.

Persistence of fan-shaped keratocytes is a matrix-rigidity-dependent mechanism that requires α5β1 integrin engagement

Despite the importance of matrix rigidity on cell functions, many aspects of the mechanosensing process in highly migratory cells remain elusive. Here, we studied the migration of highly motile keratocytes on culture substrates with similar biochemical properties and rigidities spanning the range between soft tissues (~kPa) and stiff culture substrates (~GPa). We show that morphology, polarization and persistence of motile keratocytes are regulated by the matrix stiffness over seven orders of magnitude, without changing the cell spreading area. Increasing the matrix rigidity leads to more F-actin in the lamellipodia and to the formation of mature contractile actomyosin fibers that control the cell rear retraction. Keratocytes remain rounded and form nascent adhesions on compliant substrates, whereas large and uniformly distributed focal adhesions are formed on fan-shaped keratocytes migrating on rigid surfaces. By combining poly-L-lysine, fibronectin and vitronectin coatings with selective blocking of α_vβ₃ or α₅β₁ integrins, we show that α_Vβ₃ integrins permit the spreading of keratocytes but are not sufficient for polarization and rigidity sensing that require the engagement of α₅β₁ integrins. Our study demonstrates a matrix rigidity-dependent regulation of the directional persistence in motile keratocytes and refines the role of α_vβ₃ and α₅β₁ integrins in the molecular clutch model.

Coupling actin flow, adhesion, and morphology in a computational cell motility model

Cell migration is a pervasive process in many biology systems and involves protrusive forces generated by actin polymerization, myosin dependent contractile forces, and force transmission between the cell and the substrate through adhesion sites. Here we develop a computational model for cell motion that uses the phase-field method to solve for the moving boundary with physical membrane properties. It includes a reaction-diffusion model for the actin-myosin machinery and discrete adhesion sites which can be in a "gripping" or "slipping" mode and integrates the adhesion dynamics with the dynamics of the actin filaments, modeled as a viscous network. To test this model, we apply it to fish keratocytes, fast moving cells that maintain their morphology, and show that we are able to reproduce recent experimental results on actin flow and stress patterns. Furthermore, we explore the phase diagram of cell motility by varying myosin II activity and adhesion strength. Our model suggests that the pattern of the actin flow inside the cell, the cell velocity, and the cell morphology are determined by the integration of actin polymerization, myosin contraction, adhesion forces, and membrane forces.

Alphaherpesviruses and the cytoskeleton in neuronal infections

Following infection of exposed peripheral tissues, neurotropic alphaherpesviruses invade nerve endings and deposit their DNA genomes into the nuclei of neurons resident in ganglia of the peripheral nervous system. The end result of these events is the establishment of a life-long latent infection. Neuroinvasion typically requires efficient viral transmission through a polarized epithelium followed by long-distance transport through the viscous axoplasm. These events are mediated by the recruitment of the cellular microtubule motor proteins to the intracellular viral particle and by alterations to the cytoskeletal architecture. The focus of this review is the interplay between neurotropic herpesviruses and the cytoskeleton.

Adhesion dynamics and durotaxis in migrating cells

When tissue cells are plated on a flexible substrate, durotaxis, the directed migration of cells toward mechanically stiff regions, has been observed. Environmental mechanical signals are not only important in cell migration but also seem to influence all aspects of cell differentiation and development, including the metastatic process in cancer cells. Based on a theoretical model suggesting that this mechanosensation has a mechanical basis, we introduce a simple model of a cell by considering the contraction of F-actin bundles containing myosin motors (stress fibers) mediated by the movement of adhesions. We show that, when presented with a linear stiffness gradient, this simple model exhibits durotaxis. Interestingly, since stress fibers do not form on soft surfaces and since adhesion sliding occurs very slowly on hard surfaces, the model predicts that the expected cell velocity reaches a maximum at an intermediate stiffness. This prediction can be experimentally tested. We therefore argue that stiffness-dependent cellular adaptations (mechanosensation) and durotaxis are intimately related and may share a mechanical basis. We therefore identify the essential physical ingredients, which combined with additional biochemical mechanisms can explain durotaxis and mechanosensation in cells.

Single-cell bioelectrical impedance platform for monitoring cellular response to drug treatment

The response of cells to a chemical or biological agent in terms of their impedance changes in real-time is a useful mechanism that can be utilized for a wide variety of biomedical and environmental applications. The use of a single-cell-based analytical platform could be an effective approach to acquiring more sensitive cell impedance measurements, particularly in applications where only diminutive changes in impedance are expected. Here, we report the development of an on-chip cell impedance biosensor with two types of electrodes that host individual cells and cell populations, respectively, to study its efficacy in detecting cellular response. Human glioblastoma (U87MG) cells were patterned on single- and multi-cell electrodes through ligand-mediated natural cell adhesion. We comparatively investigated how these cancer cells on both types of electrodes respond to an ion channel inhibitor, chlorotoxin (CTX), in terms of their shape alternations and impedance changes to exploit the fine detectability of the single-cell-based system. The detecting electrodes hosting single cells exhibited a significant reduction in the real impedance signal, while electrodes hosting confluent monolayer of cells showed little to no impedance change. When single-cell electrodes were treated with CTX of different doses, a dose-dependent impedance change was observed. This enables us to identify the effective dose needed for this particular treatment. Our study demonstrated that this single-cell impedance system may potentially serve as a useful analytical tool for biomedical applications such as environmental toxin detection and drug evaluation.

Force transmission in migrating cells

During cell migration, forces generated by the actin cytoskeleton are transmitted through adhesion complexes to the substrate. To investigate the mechanism of force generation and transmission, we analyzed the relationship between actin network velocity and traction forces at the substrate in a model system of persistently migrating fish epidermal keratocytes. Front and lateral sides of the cell exhibited much stronger coupling between actin motion and traction forces than the trailing cell body. Further analysis of the traction-velocity relationship suggested that the force transmission mechanisms were different in different cell regions: at the front, traction was generated by a gripping of the actin network to the substrate, whereas at the sides and back, it was produced by the network's slipping over the substrate. Treatment with inhibitors of the actin-myosin system demonstrated that the cell body translocation could be powered by either of the two different processes, actomyosin contraction or actin assembly, with the former associated with significantly larger traction forces than the latter.

Microglia processes block the spread of damage in the brain and require functional chloride channels

Microglia cells exhibit two forms of motility, constant movement of filopodia probing surrounding brain tissue, and outgrowth of larger processes in response to nearby damage. The mechanisms and functions of filopodia sensing and process outgrowth are not well characterized but are likely critical for normal immune function in the brain. Using two photon laser scanning microscopy we investigated microglia process outgrowth in response to damage, and explored the relationship between process outgrowth and filopodia movement. Further, we examined the roles of Cl(-) or K(+) channel activation, as well as actin polymerization in these two distinct processes, because mechanistic understanding could provide a strategy to modulate microglia function. We found that volume sensitive Cl(-) channel blockers (NPPB, tamoxifen, DIDS) prevented the rapid process outgrowth of microglia observed in response to damage. In contrast, filopodia extension during sensing was resistant to Cl(-) channel inhibitors, indicating that these motile processes have different cellular mechanisms. However, both filopodia sensing and rapid process outgrowth were blocked by inhibition of actin polymerization. Following lesion formation under control conditions, rapidly outgrowing processes contacted the damaged area and this was associated with a 37% decrease in lesion volume. Inhibition of process outgrowth by Cl(-) channel block, prevention of actin polymerization, or by selectively ablating microglia all allowed lesion volume to increase and spread into the surrounding tissue. Therefore, process outgrowth in response to focal brain damage is beneficial by preventing lesion expansion and suggests microglia represent a front line defence against damage in the brain.

Dephosphorylation of major sperm protein (MSP) fiber protein 3 by protein phosphatase 2A during cell body retraction in the MSP-based amoeboid motility of Ascaris sperm

The crawling movement of nematode sperm requires coordination of leading edge protrusion with cell body retraction, both of which are powered by modulation of a cytoskeleton based on major sperm protein (MSP) filaments. We used a cell-free in vitro motility system in which both protrusion and retraction can be reconstituted, to identify two proteins involved in cell body retraction. Pharmacological and depletion-add back assays showed that retraction was triggered by a putative protein phosphatase 2A (PP2A, a Ser/Thr phosphatase activated by tyrosine dephosphorylation). Immunofluorescence showed that PP2A was present in the cell body and was concentrated at the base of the lamellipod where the force for retraction is generated. PP2A targeted MSP fiber protein 3 (MFP3), a protein unique to nematode sperm that binds to the MSP filaments in the motility apparatus. Dephosphorylation of MFP3 caused its release from the cytoskeleton and generated filament disassembly. Our results suggest that interaction between PP2A and MFP3 leads to local disassembly of the MSP cytoskeleton at the base of the lamellipod in sperm that in turn pulls the trailing cell body forward.

Cellular impedance biosensors for drug screening and toxin detection

Cell-based impedance biosensing is an emerging technology that can be used to non-invasively and instantaneously detect and analyze cell responses to chemical and biological agents. This article highlights the fabrication and measurement technologies of cell impedance sensors, and their application in toxin detection and anti-cancer drug screening. We start with an introduction that describes the capability and advantages of cell-based sensors over conventional sensing technology, followed by a discussion of the influence of cell adhesion, spreading and viability during cell patterning on the subsequent impedance measurements and sensing applications. We then present an electronic circuit that models the cell-electrode system, by which the cellular changes can be detected in terms of impedance changes of the circuit. Finally, we discuss the current status on using cell impedance sensors for toxin detection and anti-cancer drug screening.

Attenuation of actinomyosinII contractile activity in growth cones accelerates filopodia-guided and microtubule-based neurite elongation

The myosinII-specific inhibitor blebbistatin was used to attenuate actinomyosinII contractility in E7-chicken retina explant, medulla and spinal cord neuronal cell cultures. Addition of 20-100 microM blebbistatin, a concentration range that reversibly disrupts actin stress fibers, led to a reduction of growth cone lamellipodial areas and to an elongation of filopodia within 5 to 10 min. These morphological changes were completely reversed after removing the inhibitor. In the continued presence of blebbistatin for several hours, a dose-dependent acceleration (up to 6-fold) of neurite outgrowth was observed. The rapidly elongating neuritic processes displayed narrowed growth cones with one to three long filopodia at the leading edge. At the same time, thin neuritic branches emerged in a "push"-like fashion guided by filopodial extensions. Immunocytochemical characterization of these thin sprouts revealed that they contained actin filaments, myosinIIA, phosphorylated neurofilament/tau epitopes, MAP2, NCAM-PSA, and microtubules, demonstrating that these processes presented neurites and not filopodia. The crucial involvement of microtubules in blebbistatin-induced accelerated neurite extension was confirmed by its inhibition in the presence of nocodazole or taxol. The promotion by blebbistatin of neurite outgrowth occurred on polylysine, laminin, as well as on fibronectin as substrate. The presence of the Rho/ROCK-inhibitor Y-27632 also caused a dose-dependent promotion of neurite growth which was, however, 3-fold less pronounced as compared to blebbistatin. In contrast to blebbistatin, Y-27632 led to the enlargement of growth cone lamellipodial extensions. Our data demonstrate that neurite outgrowth and branching are inversely correlated with the degree of actinomyosinII contractility which determines the speed of retrograde flow and turnover of actin filaments and, by this, microtubule extension.

Neurite outgrowth and branching of PC12 cells on very soft substrates sharply decreases below a threshold of substrate rigidity

Rationally designed matrices for nerve tissue engineering and encapsulated cell therapies critically rely on a comprehensive understanding of neural response to biochemical as well as biophysical cues. Whereas biochemical cues are established mediators of neuronal behavior (e.g., outgrowth), physical cues such as substrate stiffness have only recently been recognized to influence cell behavior. In this work, we examine the response of PC12 neurites to substrate stiffness. We quantified and controlled fibronectin density on the substrates and measured multiple neurite behaviors (e.g., growth, branching, neurites per cell, per cent cells expressing neurites) in a large sample population. We found that PC12 neurons display a threshold response to substrate stiffness. On the softest substrates tested (shear modulus approximately 10 Pa), neurites were relatively few, short in length and unbranched. On stiffer substrates (shear modulus approximately 10(2)-10(4) Pa), neurites were longer and more branched and a greater percentage of cells expressed neurites; significant differences in these measures were not found on substrates with a shear modulus >10(2) Pa. Based on these data and comparisons with published neurobiology and neuroengineering reports of neurite mechanotransduction, we hypothesize that results from studies of neuronal response to compliant substrates are cell-type dependent and sensitive to ligand density, sample size and the range of stiffness investigated.

A role for the neuronal protein collapsin response mediator protein 2 in T lymphocyte polarization and migration

The semaphorin-signaling transducer collapsin response mediator protein 2 (CRMP2) has been identified in the nervous system where it mediates Sema3A-induced growth cone navigation. In the present study, we provide first evidence that CRMP2 is present in the immune system and plays a critical role in T lymphocyte function. CRMP2 redistribution at the uropod in polarized T cells, a structural support of lymphocyte motility, suggests that it may regulate T cell migration. This was evidenced in primary T cells by small-interfering RNA-mediated CRMP2 gene silencing and blocking Ab, as well as CRMP2 overexpression in Jurkat T cells tested in a chemokine- and semaphorin-mediated transmigration assay. Expression analysis in PBMC from healthy donors showed that CRMP2 is enhanced in cell subsets bearing the activation markers CD69+ and HLA-DR+. Heightened expression in T lymphocytes of patients suffering from neuroinflammatory disease with enhanced T cell-transmigrating activity points to a role for CRMP2 in pathogenesis. The elucidation of the signals and mechanisms that control this pathway will lead to a better understanding of T cell trafficking in physiological and pathological situations.

Actin cytoskeleton disassembly affects conductive properties of stretch-activated cation channels in leukaemia cells

Mechanosensitive channels in various eucaryotic cells are thought to be functionally and structurally coupled to the cortical cytoskeleton. However, the results of electrophysiological studies are rather controversial and the functional impact of cytoskeleton assembly-disassembly on stretch-activated channel properties remains unclear. Here, the possible involvement of cytoskeletal elements in the regulation of stretch-activated Ca2+-permeable channels was studied in human leukaemia K562 cells with the use of agents that selectively modify the actin or tubulin system. F-actin disassembly resulted in a considerable reduction of the amplitude of stretch-activated currents without significant change in channel open probability. The effects of treatments with cytochalasins or latrunculin were principally similar, developed gradually and consisted a strong decrease of single channel conductance. Microtubule disruption did not affect stretch-activated channels. The data presented here are in principal agreement with the general conclusion that mechanosensitive channel functions are largely dependent on the integrity of the cortical actin cytoskeleton. Specifically, changes in conductive properties of the pore may provide an essential mechanism of channel regulation underlying functional modulation of membrane currents. Our results allow one to speculate that microfilament organization may be an important determinant in modulating biophysical characteristics of stretch-activated cation channels in cells of blood origin.

Slipping or gripping? Fluorescent speckle microscopy in fish keratocytes reveals two different mechanisms for generating a retrograde flow of actin

Fish keratocytes can generate rearward directed traction forces within front portions of the lamellipodium, suggesting that a retrograde flow of actin may also occur here but this was not detected by previous photoactivation experiments. To investigate the relationship between retrograde flow and traction force generation, we have transfected keratocytes with GFP-actin and used fluorescent speckle microscopy, to observe speckle flow. We detected a retrograde flow of actin within the leading lamellipodium that is inversely proportional to both protrusion rate and cell speed. To observe the effect of reducing contractility, we treated transfected cells with ML7, a potent inhibitor of myosin II. Surprisingly, ML7 treatment led to an increase in retrograde flow rate, together with a decrease in protrusion and cell speed, but only in rapidly moving cells. In slower moving cells, retrograde flow decreased, whereas protrusion rate and cell speed increased. These results suggest that there are two mechanisms for producing retrograde flow. One involves slippage between the cytoskeleton and adhesions, that decreases traction force production. The other involves slippage between adhesions and the substratum, which increases traction force production. We conclude that a biphasic relationship exists between retrograde actin flow and adhesiveness in moving keratocytes.

Actin-based centripetal flow: phosphatase inhibition by calyculin-A alters flow pattern, actin organization, and actomyosin distribution

Previous studies have suggested that the actin-based centripetal flow process in sea urchin coelomocytes is the result of a two-part mechanism, actin polymerization at the cell edge coupled with actomyosin contraction at the cell center. In the present study, we have extended the testing of this two-part model by attempting to stimulate actomyosin contraction via treatment of coelomocytes with the phosphatase inhibitor Calyculin A (CalyA). The effects of this drug were studied using digitally-enhanced video microscopy of living cells combined with immunofluorescent localization and scanning electron microscopy. Under the influence of CalyA, the coelomocyte actin cytoskeleton undergoes a radical reorganization from a dense network to one displaying an array of tangential arcs and radial rivulets in which actin and the Arp2/3 complex concentrate. In addition, the structure and dynamics of the cell center are transformed due to the accumulation of actin and membrane in this region and the constriction of the central actomyosin ring. Physiological evidence of an increase in actomyosin-based contractility following CalyA treatment was demonstrated in experiments in which cells generated tears in their cell centers in response to the drug. Western blotting and immunofluorescent localization with antibodies against the phosphorylated form of the myosin regulatory light chain (MRLC) suggested that the demonstrated constriction of actomyosin distribution was the result of CalyA-induced phosphorylation of MRLC. Overall, the results suggest that there is significant cross talk between the two underlying mechanisms of actin polymerization and actomyosin contraction, and indicate that changes in actomyosin tension may be translated into alterations in the structural organization of the actin cytoskeleton.

A single class II myosin modulates T cell motility and stopping, but not synapse formation

Upon encountering an antigen, motile T cells stop crawling, change morphology and ultimately form an 'immunological synapse'. Although myosin motors are thought to mediate various aspects of this process, the molecules involved and their exact roles are not defined. Here we show that nonmuscle myosin heavy chain IIA, or MyH9, is the only class II myosin expressed in T cells and is associated with the uropod during crawling. MyH9 function is required for maintenance of the uropod and for T cell motility but is dispensable for synapse formation. Phosphorylation of MyH9 in its multimerization domain by T cell receptor-generated signals indicates that inactivation of this motor may be a key step in the 'stop' response during antigen recognition.

Cdc42 and RhoA are differentially regulated during arachidonate-mediated HeLa cell adhesion

Cell adhesion to extracellular matrix requires stimulation of an eicosanoid signaling pathway through the metabolism of arachidonate by 5-lipoxygenase to leukotrienes and cyclooxygenase-1/2 to prostaglandins, as well as activation of the small GTPase signaling pathway involving Cdc42 and Rho. These signaling pathways direct remodeling of the actin cytoskeleton during the adhesion process, specifically the polymerization of actin during cell spreading and the bundling of actin filaments when cells migrate. However, few studies linking these signaling pathways have been described in the literature. We have previously shown that HeLa cell adhesion to collagen requires oxidation of arachidonic acid (AA) by lipoxygenase for actin polymerization and cell spreading, and cyclooxygenase for bundling actin filaments during cell migration. We demonstrate that small GTPase activity is required for HeLa cell spreading upon gelatin, and that Cdc42 is activated while Rho is downregulated during the spreading process. Using constitutively active and dominant negative expression studies, we show that Cdc42 is required for HeLa cell spreading and migration, while activated RhoA is antagonistic towards spreading. Constitutively active RhoA promotes cell migration and increases the degree of actin bundling in HeLa cells. Further, we demonstrate that activation of either the AA oxidation pathway or the small GTPase pathway cannot rescue inhibition of spreading when the alternate pathway is blocked. Our results suggest (1) both the eicosanoid signaling pathway and small GTPase activation are required during HeLa cell adhesion, and (2) these signaling pathways converge to properly direct remodeling of the actin cytoskeleton during HeLa cell spreading and migration upon collagen.

Actin dynamics is controlled by a casein kinase II and phosphatase 2C interplay on Toxoplasma gondii Toxofilin

Actin polymerization in Apicomplexa protozoa is central to parasite motility and host cell invasion. Toxofilin has been characterized as a protein that sequesters actin monomers and caps actin filaments in Toxoplasma gondii. Herein, we show that Toxofilin properties in vivo as in vitro depend on its phosphorylation. We identify a novel parasitic type 2C phosphatase that binds the Toxofilin/G-actin complex and a casein kinase II-like activity in the cytosol, both of which modulate the phosphorylation status of Toxofilin serine53. The interplay of these two molecules controls Toxofilin binding of G-actin as well as actin dynamics in vivo. Such functional interactions should play a major role in actin sequestration, a central feature of actin dynamics in Apicomplexa that underlies the spectacular speed and nature of parasite gliding motility.

Role of myosin II in axon outgrowth

The initial stages of nerve outgrowth carried out by growth cones occur in three fundamental cyclic steps. Each of these steps appears to require myosin II activity to variable degrees. The steps include the following: (a) exploration, involving extensions and retractions that are driven and controlled by the interaction of actin retrograde flow and polymerization; (b) adhesion of new extensions to the substrate, which has been shown to be mediated by complex interactions between extracellular matrix proteins, cell adhesion proteins, and the actin cytoskeleton; and (c) traction force generated during forward advance of the growth cone, resulting in the production of tension on the neurite.

A talin fragment as an actin trap visualizing actin flow in chemotaxis, endocytosis, and cytokinesis

A C-terminal 63-kDa fragment of talin A from Dictyostelium discoideum forms a slowly dissociating complex with F-actin in vitro. This talin fragment (TalC63) has been tagged with GFP and used as a trap for actin filaments in chemotactic cell movement, endocytosis, and mitotic cell division. TalC63 efficiently sequesters actin filaments in vivo. Its translocation reflects the direction and efficiency of an actin flow. Along the body of a migrating Dictyostelium cell, this flow is directed from the front to the tail. If during chemotaxis one or two new fronts are induced, the flow is always directed away from these fronts. The flow thus reflects the re-programming of cell polarity in response to changing gradients of chemoattractant. In endocytosis, the fluorescent complexes are translocated to the base of a phagocytic or macropinocytic cup. During mitosis, the complexes of F-actin with TalC63 accumulate within the midzone of anaphase cells. If TalC63 is strongly expressed, the entire cleavage furrow is filled out by sequestered actin filaments, and cytokinesis is severely impaired. These cells are considered to mimic the phenotype of mutants deficient in the shredding of actin filaments that normally occurs in the mid-zone of a dividing cell.

Growth cones contain myosin II bipolar filament arrays

Nonmuscle myosin II is among the most abundant forms of myosin in nerve growth cones. At least two isoforms of myosin II (A and B) that have overlapping but distinct distributions are found in growth cones. It appears that both myosin IIA and IIB may be necessary for normal nerve outgrowth and motility, but the molecular interactions responsible for their activity remain unclear. For instance, it is unknown if these myosin II isoforms produce bipolar "minifilaments" in growth cones similar to those observed in other nonmuscle cells. To determine if minifilaments are present in growth cones, we modified the electron microscopy preparative procedures used to detect minifilaments in other cell types. We found structures that appeared very similar to bipolar minifilaments found in noneuronal cells. They also labeled with antibodies to either myosin IIA or IIB. Thus, the activity of myosin II in growth cones is likely to be similar to that in other nonmuscle cells. Bipolar filaments interacting with oppositely oriented actin filaments will produce localized contractions or exert tension on actin networks. This activity will be responsible for the myosin II dependent motility in growth cones.

On the mechanism of cleavage furrow ingression in Dictyostelium

The ability of Dictyostelium cells to divide without myosin II in a cell cycle-coupled manner has opened two questions about the mechanism of cleavage furrow ingression. First, are there other possible functions for myosin II in this process except for generating contraction of the furrow by a sliding filament mechanism? Second, what could be an alternative mechanical basis for the furrowing? Using aberrant changes of the cell shape and anomalous localization of the actin-binding protein cortexillin I during asymmetric cytokinesis in myosin II-deficient cells as clues, it is proposed that myosin II filaments act as a mechanical lens in cytokinesis. The mechanical lens serves to focus the forces that induce the furrowing to the center of the midzone, a cortical region where cortexillins are enriched in dividing cells. Additionally, continual disassembly of a filamentous actin meshwork at the midzone is a prerequisite for normal ingression of the cleavage furrow and a successful cytokinesis. If this process is interrupted, as it occurs in cells that lack cortexillins, an overassembly of filamentous actin at the midzone obstructs the normal cleavage. Disassembly of the crosslinked actin network can generate entropic contractile forces in the cortex, and may be considered as an alternative mechanism for driving ingression of the cleavage furrow. Instead of invoking different types of cytokinesis that operate under attached and unattached conditions in Dictyostelium, it is anticipated that these cells use a universal multifaceted mechanism to divide, which is only moderately sensitive to elimination of its constituent mechanical processes.

Myr 8, a novel unconventional myosin expressed during brain development associates with the protein phosphatase catalytic subunits 1alpha and 1gamma1

Directed neuronal, astroglial, and oligodendroglial cell migrations comprise a prominent feature of mammalian brain development. Because molecular motor proteins have been implicated in a wide spectrum of processes associated with cell motility, we initiated studies to define the pool of myosins in migrating cerebellar granule neurons and type-1 neocortical astrocytes. Our analyses identified two isoforms of a novel unconventional myosin, which we have cloned, sequenced, and designated myr 8a and 8b (eighth unconventional myosin from rat). Phylogenetic analysis indicates that myr 8 myosins comprise a new class of myosins, which we have designated class XVI. The head domain contains a large N-terminal extension composed of multiple ankyrin repeats, which are implicated in mediating an association with the protein phosphatase 1 (PP1) catalytic subunits 1alpha and 1gamma. The motor domain is followed by a single putative light-chain binding domain. The tail domain of myr 8a is comparatively short with a net positive charge, whereas the tail domain of myr 8b is extended, bears an overall neutral charge, and reveals several stretches of poly-proline residues. Neither the myr 8a nor the myr 8b sequence reveals alpha-helical coiled-coil motifs, suggesting that these myosins exist as monomers. Both immunoblot and Northern blot analyses indicate that myr 8b is the predominant isoform expressed in brain, principally at developmental time periods. The structural features and restricted expression patterns suggest that members of this novel class of unconventional myosins comprise a mechanism to target selectively the protein phosphatase 1 catalytic subunits 1alpha and/or 1gamma in developing brain.

Myr 8, a novel unconventional myosin expressed during brain development associates with the protein phosphatase catalytic subunits 1alpha and 1gamma1

Directed neuronal, astroglial, and oligodendroglial cell migrations comprise a prominent feature of mammalian brain development. Because molecular motor proteins have been implicated in a wide spectrum of processes associated with cell motility, we initiated studies to define the pool of myosins in migrating cerebellar granule neurons and type-1 neocortical astrocytes. Our analyses identified two isoforms of a novel unconventional myosin, which we have cloned, sequenced, and designated myr 8a and 8b (eighth unconventional myosin from rat). Phylogenetic analysis indicates that myr 8 myosins comprise a new class of myosins, which we have designated class XVI. The head domain contains a large N-terminal extension composed of multiple ankyrin repeats, which are implicated in mediating an association with the protein phosphatase 1 (PP1) catalytic subunits 1alpha and 1gamma. The motor domain is followed by a single putative light-chain binding domain. The tail domain of myr 8a is comparatively short with a net positive charge, whereas the tail domain of myr 8b is extended, bears an overall neutral charge, and reveals several stretches of poly-proline residues. Neither the myr 8a nor the myr 8b sequence reveals alpha-helical coiled-coil motifs, suggesting that these myosins exist as monomers. Both immunoblot and Northern blot analyses indicate that myr 8b is the predominant isoform expressed in brain, principally at developmental time periods. The structural features and restricted expression patterns suggest that members of this novel class of unconventional myosins comprise a mechanism to target selectively the protein phosphatase 1 catalytic subunits 1alpha and/or 1gamma in developing brain.

Myosin IIB is required for growth cone motility

Growth cones are required for the forward advancement and navigation of growing axons. Modulation of growth cone shape and reorientation of the neurite are responsible for the change of outgrowth direction that underlies navigation. Change of shape involves the reordering of the cytoskeleton. Reorientation of the neurite requires the generation of tension, which is supplied by the ability of the growth cone to crawl on a substrate. The specific molecular mechanisms responsible for these activities are unknown but are thought to involve actomyosin-generated force combined with linkage to the cell surface receptors that are responsible for adhesion (Heidemann and Buxbaum, 1998). To test whether myosin IIB is responsible for the force generation, we quantified shape dynamics and filopodial-mediated traction force in growth cones from myosin IIB knock-out (KO) mice and compared them with neurons from normal littermates. Growth cones from the KO mice spread less, showed alterations in shape dynamics and actin organization, and had reduced filopodial-mediated traction force. Although peak traction forces produced by filopodia of KO cones were decreased significantly, KO filopodia occasionally developed forces equivalent to those in the wild type. This indicates that other myosins participate in filopodial-dependent traction force. Therefore, myosin IIB is necessary for normal growth cone spreading and the modulation of shape and traction force but acts in combination with other myosins for some or all of these activities. These activities are essential for growth cone forward advancement, which is necessary for outgrowth. Thus outgrowth is slowed, but not eliminated, in neurons from the myosin IIB KO mice.

Myosin IIB is required for growth cone motility

Growth cones are required for the forward advancement and navigation of growing axons. Modulation of growth cone shape and reorientation of the neurite are responsible for the change of outgrowth direction that underlies navigation. Change of shape involves the reordering of the cytoskeleton. Reorientation of the neurite requires the generation of tension, which is supplied by the ability of the growth cone to crawl on a substrate. The specific molecular mechanisms responsible for these activities are unknown but are thought to involve actomyosin-generated force combined with linkage to the cell surface receptors that are responsible for adhesion (Heidemann and Buxbaum, 1998). To test whether myosin IIB is responsible for the force generation, we quantified shape dynamics and filopodial-mediated traction force in growth cones from myosin IIB knock-out (KO) mice and compared them with neurons from normal littermates. Growth cones from the KO mice spread less, showed alterations in shape dynamics and actin organization, and had reduced filopodial-mediated traction force. Although peak traction forces produced by filopodia of KO cones were decreased significantly, KO filopodia occasionally developed forces equivalent to those in the wild type. This indicates that other myosins participate in filopodial-dependent traction force. Therefore, myosin IIB is necessary for normal growth cone spreading and the modulation of shape and traction force but acts in combination with other myosins for some or all of these activities. These activities are essential for growth cone forward advancement, which is necessary for outgrowth. Thus outgrowth is slowed, but not eliminated, in neurons from the myosin IIB KO mice.

Modulation of cell-substrate adhesion by arachidonic acid: lipoxygenase regulates cell spreading and ERK1/2-inducible cyclooxygenase regulates cell migration in NIH-3T3 fibroblasts

Adhesion of cells to an extracellular matrix is characterized by several discrete morphological and functional stages beginning with cell-substrate attachment, followed by cell spreading, migration, and immobilization. We find that although arachidonic acid release is rate-limiting in the overall process of adhesion, its oxidation by lipoxygenase and cyclooxygenases regulates, respectively, the cell spreading and cell migration stages. During the adhesion of NIH-3T3 cells to fibronectin, two functionally and kinetically distinct phases of arachidonic acid release take place. An initial transient arachidonate release occurs during cell attachment to fibronectin, and is sufficient to signal the cell spreading stage after its oxidation by 5-lipoxygenase to leukotrienes. A later sustained arachidonate release occurs during and after spreading, and signals the subsequent migration stage through its oxidation to prostaglandins by newly synthesized cyclooxygenase-2. In signaling migration, constitutively expressed cyclooxygenase-1 appears to contribute approximately 25% of prostaglandins synthesized compared with the inducible cyclooxygenase-2. Both the second sustained arachidonate release, and cyclooxygenase-2 protein induction and synthesis, appear to be regulated by the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK)1/2. The initial cell attachment-induced transient arachidonic acid release that signals spreading through lipoxygenase oxidation is not sensitive to ERK1/2 inhibition by PD98059, whereas PD98059 produces both a reduction in the larger second arachidonate release and a blockade of induced cyclooxygenase-2 protein expression with concomitant reduction of prostaglandin synthesis. The second arachidonate release, and cyclooxygenase-2 expression and activity, both appear to be required for cell migration but not for the preceding stages of attachment and spreading. These data suggest a bifurcation in the arachidonic acid adhesion-signaling pathway, wherein lipoxygenase oxidation generates leukotriene metabolites regulating the spreading stage of cell adhesion, whereas ERK 1/2-induced cyclooxygenase synthesis results in oxidation of a later release, generating prostaglandin metabolites regulating the later migration stage.

Cytochalasin D induces edema formation and lowering of interstitial fluid pressure in rat dermis

The increased capillary fluid filtration required to create a rapid edema formation in acute inflammation can be generated by lowering the interstitial fluid pressure (P(IF)). The lowering of P(IF) appears to involve dynamic beta(1)-integrin-mediated interactions between dermal cells and extracellular matrix fibers. The present study specifically investigates the role of the cell cytoskeleton, i.e., the contractile apparatus of cells, in controlling P(IF) in rat skin as the integrins are linked to both the cytoskeleton and the extracellular matrix. P(IF) was measured using a micropuncture technique in the dorsal skin of the hind paw at a depth of 0.2--0.5 mm and following the induction of circulatory arrest with the intravenous injection of KCl in pentobarbital anesthesia. This procedure prevented the transcapillary flux of fluid and protein leading to edema formation in acute inflammation, which in turn can increase the P(IF) and therefore potentially mask a decrease of P(IF). Control P(IF) (n = 42) averaged -0.8 +/- 0.5 (means +/- SD) mmHg. In the first group of experiments, subdermal injection of 2 microl cytochalasin D, a microfilament-disrupting drug, lowered P(IF) to an average of -2.8 +/- 0.7 mmHg within 40 min postinjection (P < 0.05 compared with control). Subdermal injection of vehicle (10% DMSO in PBS or PBS alone) did not change the P(IF) (P > 0.05). Lowering of the P(IF) was not observed after the injection of colchicine or nocodazole, which specifically disrupts microtubuli in cultured cells. In the second group of experiments, 2 microl of cytochalasin D injected subdermally into rats with intact circulation increased the total tissue water (TTW) and albumin extravasation rate (E(ALB)) by 0.7 +/- 0.2 and 0.4 +/- 0.3 ml/g dry wt, respectively (P < 0.05 compared with vehicle). Nocodazole and colchicine did not significantly alter the TTW or E(ALB) compared with the vehicle (P > 0.05). Taken together, these findings strongly suggest that the connective tissue cells can participate in control of P(IF) via the actin filament system. In addition, the observation that subdermal injection of cytochalasin D lowered P(IF) indicates that a dynamic assembly and disassembly of actin filaments also occurs in the cells of dermal tissues in vivo.

An elastic analysis of Listeria monocytogenes propulsion

The bacterium Listeria monocytogenes uses the energy of the actin polymerization to propel itself through infected tissues. In steady state, it continuously adds new polymerized filaments to its surface, pushing on its tail, which is made from previously cross-linked actin filaments. In this paper we introduce an elastic model to describe how the addition of actin filaments to the tail results in the propulsive force on the bacterium. Filament growth on the bacterial surface produces stresses that are relieved at the back of the bacterium as it moves forward. The model leads to a natural competition between growth from the sides and growth from the back of the bacterium, with different velocities and strengths for each. This competition can lead to the periodic motion observed in a Listeria mutant.

Actin filament disruption blocks cerebellar granule neurons at the unipolar stage of differentiation in vitro

Cerebellar granule neurons developing in vitro initially extend a single axon, with the Golgi apparatus and centrosome positioned at the base of this axon and then begin the transition to a bipolar morphology by rotating the Golgi-centrosome to the opposite pole of the cell and extending a secondary axon; granule cells reach a mature, complex morphology by extending multiple, short dendrites by 5-6 days in vitro. (Zmuda and Rivas, 1998. Cell Motil Cytoskel 41:18-38). To test the effects of actin depolymerization on this characteristic pattern of granule cell axonogenesis, cultured granule cells were treated with either cytochalasin D (CD) or latrunculin A (Lat A) to depolymerize filamentous actin. Although actin depolymerization did not inhibit initial axon extension, it prevented the cells from proceeding on to the transitional, bipolar, or complex stages of differentiation, effectively blocking the cells at the unipolar stage of differentiation. Although the Golgi apparatus resided at the base of the axon in nontreated unipolar cells, or at the opposite pole of the cell body in nontreated transitional cells, the Golgi was randomly localized within the cytoplasm of cells that had been treated with either CD or Lat A. These results show that the transition from the unipolar to the bipolar stage and on to more mature stages of granule cell differentiation is dependent on an intact actin cytoskeleton and suggest that the characteristic pattern of granule cell differentiation may be dependent on the repositioning of the Golgi-centrosome during morphological development.

Role of p120 Ras-GAP in directed cell movement

We have used cell lines deficient in p120 Ras GTPase activating protein (Ras-GAP) to investigate the roles of Ras-GAP and the associated p190 Rho-GAP (p190) in cell polarity and cell migration. Cell wounding assays showed that Ras-GAP-deficient cells were incapable of establishing complete cell polarity and migration into the wound. Stimulation of mutant cells with growth factor rescued defects in cell spreading, Golgi apparatus fragmentation, and polarized vesicular transport and partially rescued migration in a Ras-dependent manner. However, for directional movement, the turnover of stress fibers and focal adhesions to produce an elongate morphology was dependent on the constitutive association between Ras-GAP and p190, independent of Ras regulation. Disruption of the phosphotyrosine-mediated Ras-GAP/p190 complex by microinjecting synthetic peptides derived from p190 sequences in wild-type cells caused a suppression of actin filament reorientation and migration. From these observations we suggest that although Ras-GAP is not directly required for motility per se, it is important for cell polarization by regulating actin stress fiber and focal adhesion reorientation when complexed with 190. This observation suggests a specific function for Ras-GAP separate from Ras regulation in cell motility.

Actin cytoskeleton and cell signaling

The role of the actin cytoskeleton in the function of eukaryotic cells is ubiquitous. Regulation of actin polymerization allows cells to control their shape, to move, divide, secrete, and phagocytose. Actin filaments provide strength, connections to other cells and the extracellular matrix, paths for intracellular transport and a scaffold for generating force. Recently, a number of signal transduction pathways have been identified that regulate actin polymerization and contractility. GTP-binding proteins, protein kinases, phosphoinositide kinases, and protein phosphatases all play important roles in determining the location and extent of actin polymerization and contractility of actin/myosin filaments. These pathways allow cells to respond to extracellular signals to regulate movement, the tone of vascular smooth muscle cells, secretion, and phagocytosis. Some pathogens use signal transduction pathways that regulate actin polymerization to invade cells. The signal transduction pathways that regulate actin-dependent events are the focus of this review.

Cell spreading and lamellipodial extension rate is regulated by membrane tension

Cell spreading and motility require the extension of the plasma membrane in association with the assembly of actin. In vitro, extension must overcome resistance from tension within the plasma membrane. We report here that the addition of either amphiphilic compounds or fluorescent lipids that expanded the plasma membrane increased the rate of cell spreading and lamellipodial extension, stimulated new lamellipodial extensions, and caused a decrease in the apparent membrane tension. Further, in PDGF-stimulated motility, the increase in the lamellipodial extension rate was associated with a decrease in the apparent membrane tension and decreased membrane-cytoskeleton adhesion through phosphatidylinositol diphosphate hydrolysis. Conversely, when membrane tension was increased by osmotically swelling cells, the extension rate decreased. Therefore, we suggest that the lamellipodial extension process can be activated by a physical signal (perhaps secondarily), and the rate of extension is directly dependent upon the tension in the plasma membrane. Quantitative analysis shows that the lamellipodial extension rate is inversely correlated with the apparent membrane tension. These studies describe a physical chemical mechanism involving changes in membrane-cytoskeleton adhesion through phosphatidylinositol 4,5-biphosphate-protein interactions for modulating and stimulating the biochemical processes that power lamellipodial extension.

Two components of actin-based retrograde flow in sea urchin coelomocytes

Sea urchin coelomocytes represent an excellent experimental model system for studying retrograde flow. Their extreme flatness allows for excellent microscopic visualization. Their discoid shape provides a radially symmetric geometry, which simplifies analysis of the flow pattern. Finally, the nonmotile nature of the cells allows for the retrograde flow to be analyzed in the absence of cell translocation. In this study we have begun an analysis of the retrograde flow mechanism by characterizing its kinetic and structural properties. The supramolecular organization of actin and myosin II was investigated using light and electron microscopic methods. Light microscopic immunolocalization was performed with anti-actin and anti-sea urchin egg myosin II antibodies, whereas transmission electron microscopy was performed on platinum replicas of critical point-dried and rotary-shadowed cytoskeletons. Coelomocytes contain a dense cortical actin network, which feeds into an extensive array of radial bundles in the interior. These actin bundles terminate in a perinuclear region, which contains a ring of myosin II bipolar minifilaments. Retrograde flow was arrested either by interfering with actin polymerization or by inhibiting myosin II function, but the pathway by which the flow was blocked was different for the two kinds of inhibitory treatments. Inhibition of actin polymerization with cytochalasin D caused the actin cytoskeleton to separate from the cell margin and undergo a finite retrograde retraction. In contrast, inhibition of myosin II function either with the wide-spectrum protein kinase inhibitor staurosporine or the myosin light chain kinase-specific inhibitor KT5926 stopped flow in the cell center, whereas normal retrograde flow continued at the cell periphery. These differential results suggest that the mechanism of retrograde flow has two, spatially segregated components. We propose a "push-pull" mechanism in which actin polymerization drives flow at the cell periphery, whereas myosin II provides the tension on the actin cytoskeleton necessary for flow in the cell interior.

Rho GTPases control migration and polarization of adhesion molecules and cytoskeletal ERM components in T lymphocytes

Motile lymphocytes adopt a polarized morphology with different adhesion molecules (ICAM, CD43 and CD44) and ERM actin-binding proteins concentrated on the uropod, a slender posterior appendage with important functions in cell-cell interactions and lymphocyte recruitment. We have studied the role of Rho family of GTPases (Rho, Rac and Cdc42) in the control of lymphocyte polarity and migration by analyzing the effects of exogenously introduced Rho GTPase mutants. Transfection of T cell lines that constitutively display a polarized motile morphology with activated mutants of RhoA, Rac1 and Cdc42 impaired cell polarization. A guanosine nucleotide exchange factor for Rac, Tiam-1, induced the same effect as activated Rac1. Conversely, dominant negative forms of the three GTP-binding proteins induced a polarized phenotype in constitutively round-shaped T cells with redistribution of ICAM-3 and moesin to the uropod in an integrin-dependent manner. On the other hand, overexpression of dominant negative Cdc42 and activated mutants of all three Rho GTPases significantly inhibited SDF-1alpha-induced T cell chemotaxis. Together, these data demonstrate that Rho GTPases regulate lymphocyte polarization and chemokine-induced migration, and underscore the key role of Cdc42 in lymphocyte directional migration.

Mechanisms of hyphal tip growth: tube dwelling amebae revisited

Over 100 years ago, Reinhardt suggested that hyphal tip growth is comparable to ameboid movement inside a tube; the apical cytoplasm being protruded like a pseudopodium with the wall assembled on its surface. There are increasing data from hyphae which are explicable by this model. Fungi produce pseudopodia-like structures and their cytoplasm contains all of the major components implicated in pseudopodium production in animal cells. Most of these components are concentrated in hyphal tips and tip growth involves actin, a major component of pseudopodia. Together these data indicate that the essence of the ameboid model is still tenable. However, detailed mechanisms of tip growth remain too poorly known to provide definitive proof of the model and the behavior of the trailing cytoplasm indicates differences which are probably a response to the walled lifestyle.

Relationships between the actin cytoskeleton and cell volume regulation

The actin cytoskeleton mediates a variety of essential biological functions in cells, including division, shape changes, and movement. A number of studies have suggested that the abundant submembranous actin cytoskeleton present in the cortex of many cell types is involved in the regulation of cell volume. This relationship is supported by numerous works which document the changes in the structural organization of the actin cytoskeleton which accompany cell volume changes and the F-actin-dependence of the regulatory volume responses. In addition, other studies demonstrate structural and functional relationships between the actin cytoskeleton and the membrane transporters known to be involved in cell volume homeostasis. This review provides a summary of the current level of knowledge in this area and discusses the mechanisms which may underlie the linkage between the actin cytoskeleton and cell volume regulation.

Quantifying lamella dynamics of cultured cells by SACED, a new computer-assisted motion analysis

Formation of lamellipodia and the retraction of ruffles are essential activities during motility and migration of eukaryotic cells. We have developed a computer-assisted stroboscopic method for the continuous observation of cell dynamics (stroboscopic analysis of cell dynamics, SACED) that allows one to analyze changes in lamellipodia protrusion and ruffle retraction with high resolution in space and time. To demonstrate the potential of this method we analyzed keratinocytes in culture, unstimulated or stimulated with epidermal growth factor (EGF), which is known to induce cell motility and migration. Keratinocytes stimulated with EGF exhibited a 2.6-fold increase in their migration velocity, which coincided with enhanced ruffle retraction velocity (144%) and increased ruffle frequency (135%) compared to control cells. We also recorded an enhanced frequency of lamellipodia (135%), whereas the velocity of lamellipodia protrusion remained unchanged. These results on ruffle and lamellipodia dynamics in epidermal cells show that SACED is at least equal to established methods in terms of accuracy. SACED is, however, advantageous concerning resolution in time and therefore allows one to analyze the activity of lamellipodia and ruffles in as yet unknown detail. Moreover, SACED offers two opportunities that render this technique superior to established methods: First, several parameters relevant to cell motility can be analyzed simultaneously. Second, a large number of cells can conveniently be examined, which facilitates the compilation of statistically significant data.

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.

Leukocyte polarization in cell migration and immune interactions

Cell migration plays a key role in a wide variety of biological phenomena. This process is particularly important for leukocyte function and the inflammatory response. Prior to migration leukocytes undergo polarization, with the formation of a lamellipodium at the leading edge and a uropod at the trailing edge. This cell shape allows them to convert cytoskeletal forces into net cell-body displacement. Leukocyte chemoattractants, including chemokines, provide directional cues for leukocyte motility, and concomitantly induce polarization. Chemoattractant receptors, integrins and other adhesion molecules, cytoskeletal proteins and intracellular regulatory molecules change their cellular localization during cell polarization. A complex system of signal transduction molecules, including tyrosine kinases, lipid kinases, second messengers and members of the Rho family of small GTPases is thought to regulate the cytoskeletal rearrangements underlying leukocyte polarization and migration. The elucidation of the mechanisms and signals that control this complex reorganization will lead to a better understanding of critical questions in cell biology of leukocyte migration and polarity.

Polyamine depletion alters the relationship of F-actin, G-actin, and thymosin beta4 in migrating IEC-6 cells

The cause of reduced migration ability in polyamine-deficient cells is not known, but their actin cytoskeleton is clearly abnormal. We depleted polyamines with alpha-difluoromethylornithine (DFMO) in migrating cells with or without stimulation by epidermal growth factor (EGF) and investigated filamentous (F-) actin, monomeric (G-) actin, and thymosin beta4 (Tbeta4), using immunofluorescent confocal microscopy, DNase assay, and immunoblot analysis. DFMO reduced F-actin in the cell interior, increased it in the cell cortex, redistributed G-actin, and increased nuclear staining of Tbeta4. However, DFMO did not affect the amount of Tbeta4 mRNA. EGF caused a rapid increase in the staining of F-actin in control cells, but DFMO prevented this response to EGF. Despite the visible changes shown by immunocytochemistry, statistically significant changes in the amount of either actin isoform or of total actin did not occur. We propose that DFMO reduces migration by interfering with the sequestration of G-actin by Tbeta4 and the association of F-actin with activated EGF receptors.

Cell migration strategies in 3-D extracellular matrix: differences in morphology, cell matrix interactions, and integrin function

Cell migration in extracellular matrix is a complex process of adhesion and deadhesion events combined with cellular strategies to overcome the biophysical resistance imposed by three-dimensionally interconnected matrix ligands. Using a 3-D collagen matrix migration model in combination with computer-assisted cell tracking for reconstruction of migration paths and confocal microscopy, we investigated molecular principles governing cell-matrix interactions and migration of different cell types. Highly invasive MV3 melanoma cells and fibroblasts are large and highly polarized cells migrating at low speed (0.1-0.5 microm/min) and at high directional persistence. MV3 melanoma cells utilize adhesive migration strategies as characterized by high beta1 integrin surface expression, beta1 integrin clustering at interactions with matrix fibers, and beta1 integrin-mediated adhesion for force generation and migration. In contrast, T lymphocytes and dendritic cells are highly mobile cells of lower beta1 integrin expression migrating at 10- to 40-fold higher velocities, and directionally unpredictable path profiles. This migration occurs in the absence of focal adhesions and largely independent of beta1 integrin-mediated adhesion. Whereas cell-matrix interactions of migrating tumor cells result in traction and reorientation of collagen fibers, partial matrix degradation, and pore formation, leukocytes form transient and short-lived interactions with the collagen lacking structural proteolysis and matrix remodeling. In conclusion, the 3-D extracellular matrix provides a spatially complex and biomechanically demanding substrate for cell migration, thereby differing from cell migration across planar ligands. Highly adhesive and integrin-dependent migration strategies detected in morphologically large and slowly migrating cells may result in reorganization of the extracellular matrix, whereas leukocytes favor largely integrin-independent, rapid, and flexible migration strategies lacking typical focal adhesions and structural matrix remodeling.

Cytosolic free calcium and the cytoskeleton in the control of leukocyte chemotaxis

In response to a chemotactic gradient, leukocytes extravasate and chemotax toward the site of pathogen invasion. Although fundamental in the control of many leukocyte functions, the role of cytosolic free Ca2+ in chemotaxis is unclear and has been the subject of debate. Before becoming motile, the cell assumes a polarized morphology, as a result of modulation of the cytoskeleton by G protein and kinase activation. This morphology may be reinforced during chemotaxis by the intracellular redistribution of Ca2+ stores, cytoskeletal constituents, and chemoattractant receptors. Restricted subcellular distributions of signaling molecules, such as Ca2+, Ca2+/calmodulin, diacylglycerol, and protein kinase C, may also play a role in some types of leukocyte. Chemotaxis is an essential function of most cells at some stage during their development, and a deeper understanding of the molecular signaling and structural components involved will enable rational design of therapeutic strategies in a wide variety of diseases.