Mechanisms of amoeboid chemotaxis: an evaluation of the cortical expansion model

The Roles of Signaling in Cytoskeletal Changes, Random Movement, Direction-Sensing and Polarization of Eukaryotic Cells

Movement of cells and tissues is essential at various stages during the lifetime of an organism, including morphogenesis in early development, in the immune response to pathogens, and during wound-healing and tissue regeneration. Individual cells are able to move in a variety of microenvironments (MEs) (A glossary of the acronyms used herein is given at the end) by suitably adapting both their shape and how they transmit force to the ME, but how cells translate environmental signals into the forces that shape them and enable them to move is poorly understood. While many of the networks involved in signal detection, transduction and movement have been characterized, how intracellular signals control re-building of the cyctoskeleton to enable movement is not understood. In this review we discuss recent advances in our understanding of signal transduction networks related to direction-sensing and movement, and some of the problems that remain to be solved.

Proteomic and Transcriptomic Profiling Identifies Early Developmentally Regulated Proteins in Dictyostelium Discoideum

Cyclic AMP acts as a secondary messenger involving different cellular functions in eukaryotes. Here, proteomic and transcriptomic profiling has been combined to identify novel early developmentally regulated proteins in eukaryote cells. These proteomic and transcriptomic experiments were performed in Dictyostelium discoideum given the unique advantages that this organism offers as a eukaryotic model for cell motility and as a nonmammalian model of human disease. By comparing whole-cell proteome analysis of developed (cAMP-pulsed) wild-type AX2 cells and an independent transcriptomic analysis of developed wild-type AX4 cells, our results show that up to 70% of the identified proteins overlap in the two independent studies. Among them, we have found 26 proteins previously related to cAMP signaling and identified 110 novel proteins involved in calcium signaling, adhesion, actin cytoskeleton, the ubiquitin-proteasome pathway, metabolism, and proteins that previously lacked any annotation. Our study validates previous findings, mostly for the canonical cAMP-pathway, and also generates further insight into the complexity of the transcriptomic changes during early development. This article also compares proteomic data between parental and cells lacking glkA, a GSK-3 kinase implicated in substrate adhesion and chemotaxis in Dictyostelium. This analysis reveals a set of proteins that show differences in expression in the two strains as well as overlapping protein level changes independent of GlkA.

Excitable Signal Transduction Networks in Directed Cell Migration

Although directed migration of eukaryotic cells may have evolved to escape nutrient depletion, it has been adopted for an extensive range of physiological events during development and in the adult organism. The subversion of these movements results in disease, such as cancer. Mechanisms of propulsion and sensing are extremely diverse, but most eukaryotic cells move by extending actin-filled protrusions termed macropinosomes, pseudopodia, or lamellipodia or by extension of blebs. In addition to motility, directed migration involves polarity and directional sensing. The hundreds of gene products involved in these processes are organized into networks of parallel and interconnected pathways. Many of these components are activated or inhibited coordinately with stimulation and on each spontaneously extended protrusion. Moreover, these networks display hallmarks of excitability, including all-or-nothing responsiveness and wave propagation. Cellular protrusions result from signal transduction waves that propagate outwardly from an origin and drive cytoskeletal activity. The range of the propagating waves and hence the size of the protrusions can be altered by lowering or raising the threshold for network activation, with larger and wider protrusions favoring gliding or oscillatory behavior over amoeboid migration. Here, we evaluate the variety of models of excitable networks controlling directed migration and outline critical tests. We also discuss the utility of this emerging view in producing cell migration and in integrating the various extrinsic cues that direct migration.

Role of Aquaporin 1 Signalling in Cancer Development and Progression

Cancer is a major health burden worldwide. Despite the advances in our understanding of its pathogenesis and continued improvement in cancer management and outcomes, there remains a strong clinical demand for more accurate and reliable biomarkers of metastatic progression and novel therapeutic targets to abrogate angiogenesis and tumour progression. Aquaporin 1 (AQP1) is a small hydrophobic integral transmembrane protein with a predominant role in trans-cellular water transport. Recently, over-expression of AQP1 has been associated with many types of cancer as a distinctive clinical prognostic factor. This has prompted researchers to evaluate the link between AQP1 and cancer biological functions. Available literature implicates the role of AQP1 in tumour cell migration, invasion and angiogenesis. This article reviews the current understanding of AQP1-facilitated tumour development and progression with a focus on regulatory mechanisms and downstream signalling pathways.

Progress and perspectives in signal transduction, actin dynamics, and movement at the cell and tissue level: lessons from Dictyostelium

Movement of cells and tissues is a basic biological process that is used in development, wound repair, the immune response to bacterial invasion, tumour formation and metastasis, and the search for food and mates. While some cell movement is random, directed movement stimulated by extracellular signals is our focus here. This involves a sequence of steps in which cells first detect extracellular chemical and/or mechanical signals via membrane receptors that activate signal transduction cascades and produce intracellular signals. These intracellular signals control the motile machinery of the cell and thereby determine the spatial localization of the sites of force generation needed to produce directed motion. Understanding how force generation within cells and mechanical interactions with their surroundings, including other cells, are controlled in space and time to produce cell-level movement is a major challenge, and involves many issues that are amenable to mathematical modelling.

Measurement of cellular chemotaxis with ECIS/Taxis

Cellular movement in response to external stimuli is fundamental to many cellular processes including wound healing, inflammation and the response to infection. A common method to measure chemotaxis is the Boyden chamber assay, in which cells and chemoattractant are separated by a porous membrane. As cells migrate through the membrane toward the chemoattractant, they adhere to the underside of the membrane, or fall into the underlying media, and are subsequently stained and visually counted (1). In this method, cells are exposed to a steep and transient chemoattractant gradient, which is thought to be a poor representation of gradients found in tissues (2). Another assay system, the under-agarose chemotaxis assay, (3, 4) measures cell movement across a solid substrate in a thin aqueous film that forms under the agarose layer. The gradient that develops in the agarose is shallow and is thought to be an appropriate representation of naturally occurring gradients. Chemotaxis can be evaluated by microscopic imaging of the distance traveled. Both the Boyden chamber assay and the under-agarose assay are usually configured as endpoint assays. The automated ECIS/Taxis system combines the under-agarose approach with Electric Cell-substrate Impedance Sensing (ECIS) (5, 6). In this assay, target electrodes are located in each of 8 chambers. A large counter-electrode runs through each of the 8 chambers (Figure 2). Each chamber is filled with agarose and two small wells are the cut in the agarose on either side of the target electrode. One well is filled with the test cell population, while the other holds the sources of diffusing chemoattractant (Figure 3). Current passed through the system can be used to determine the change in resistance that occurs as cells pass over the target electrode. Cells on the target electrode increase the resistance of the system (6). In addition, rapid fluctuations in the resistance represent changes in the interactions of cells with the electrode surface and are indicative of ongoing cellular shape changes. The ECIS/Taxis system can measure movement of the cell population in real-time over extended periods of time, but is also sensitive enough to detect the arrival of a single cell at the target electrode. Dictyostelium discoidium is known to migrate in the presence of a folate gradient (7, 8) and its chemotactic response can be accurately measured by ECIS/Taxis (9). Leukocyte chemotaxis, in response to SDF1α and to chemotaxis antagonists has also been measured with ECIS/Taxis (10, 11). An example of the leukocyte response to SDF1α is shown in Figure 1.

The LRRK2-related Roco kinase Roco2 is regulated by Rab1A and controls the actin cytoskeleton

We identify a new pathway that is required for proper pseudopod formation. We show that Roco2, a leucine-rich repeat kinase 2 (LRRK2)-related Roco kinase, is activated in response to chemoattractant stimulation and helps mediate cell polarization and chemotaxis by regulating cortical F-actin polymerization and pseudopod extension in a pathway that requires Rab1A. We found that Roco2 binds the small GTPase Rab1A as well as the F-actin cross-linking protein filamin (actin-binding protein 120, abp120) in vivo. We show that active Rab1A (Rab1A-GTP) is required for and regulates Roco2 kinase activity in vivo and that filamin lies downstream from Roco2 and controls pseudopod extension during chemotaxis and random cell motility. Therefore our study uncovered a new signaling pathway that involves Rab1A and controls the actin cytoskeleton and pseudopod extension, and thereby, cell polarity and motility. These findings also may have implications in the regulation of other Roco kinases, including possibly LRRK2, in metazoans.

Ca2+ chemotaxis in Dictyostelium discoideum

Using a newly developed microfluidic chamber, we have demonstrated in vitro that Ca(2+) functions as a chemoattractant of aggregation-competent Dictyostelium discoideum amoebae, that parallel spatial gradients of cAMP and Ca(2+) are more effective than either alone, and that cAMP functions as a stronger chemoattractant than Ca(2+). Effective Ca(2+) gradients are extremely steep compared with effective cAMP gradients. This presents a paradox because there is no indication to date that steep Ca(2+) gradients are generated in aggregation territories. However, given that Ca(2+) chemotaxis is co-acquired with cAMP chemotaxis during development, we speculate on the role that Ca(2+) chemotaxis might have and the possibility that steep, transient Ca(2+) gradients are generated during natural aggregation in the interstitial regions between cells.

The N-terminus of Dictyostelium Scar interacts with Abi and HSPC300 and is essential for proper regulation and function

Scar/WAVE proteins, members of the conserved Wiskott-Aldrich syndrome (WAS) family, promote actin polymerization by activating the Arp2/3 complex. A number of proteins, including a complex containing Nap1, PIR121, Abi1/2, and HSPC300, interact with Scar/WAVE, though the role of this complex in regulating Scar function remains unclear. Here we identify a short N-terminal region of Dictyostelium Scar that is necessary and sufficient for interaction with HSPC300 and Abi in vitro. Cells expressing Scar lacking this N-terminal region show abnormalities in F-actin distribution, cell morphology, movement, and cytokinesis. This is true even in the presence of wild-type Scar. The data suggest that the first 96 amino acids of Scar are necessary for participation in a large-molecular-weight protein complex, and that this Scar-containing complex is responsible for the proper localization and regulation of Scar. The presence of mis-regulated or unregulated Scar has significant deleterious effects on cells and may explain the need to keep Scar activity tightly controlled in vivo either by assembly in a complex or by rapid degradation.

Taxis equations for amoeboid cells

The classical macroscopic chemotaxis equations have previously been derived from an individual-based description of the tactic response of cells that use a "run-and-tumble" strategy in response to environmental cues [17,18]. Here we derive macroscopic equations for the more complex type of behavioral response characteristic of crawling cells, which detect a signal, extract directional information from a scalar concentration field, and change their motile behavior accordingly. We present several models of increasing complexity for which the derivation of population-level equations is possible, and we show how experimentally measured statistics can be obtained from the transport equation formalism. We also show that amoeboid cells that do not adapt to constant signals can still aggregate in steady gradients, but not in response to periodic waves. This is in contrast to the case of cells that use a "run-and-tumble" strategy, where adaptation is essential.

Greatly impaired migration of implanted aquaporin-4-deficient astroglial cells in mouse brain toward a site of injury

We reported previously that astroglia cultured from aquaporin-4-deficient (AQP4-/-) mice migrate more slowly in vitro than those from wild-type (AQP4+/+) mice (J. Cell Sci. 2005;118, 5691-5698). Here, we investigate the migration of fluorescently labeled AQP4+/+ and AQP4-/- astroglia after implantation into mouse brains in which directional movement was stimulated by a planar stab wound 3 mm away from the axis of the injection needle. Two days after cell injection we determined the location, elongation ratio, and orientation of labeled cells. Migration of AQP4+/+ but not AQP4-/- cells toward the stab was greater than away from the stab. AQP4+/+ astroglia moved on average 1.5 mm toward the stab compared with 0.6 mm for AQP4-/- cells. More than 25% of the migrating AQP4+/+ cells but <3% of AQP4-/- cells appeared elongated (axial ratio>2.5). In transwell assays, AQP4+/+ astroglia migrated faster than AQP4-/- cells in a manner dependent on pore size. At 8 h, approximately 50% of AQP4+/+ cells migrated through 8-microm diameter pores, whereas equivalent migration of AQP4-/- cells was found for 12-microm diameter pores. These results provide in vivo evidence for AQP4-dependent astroglial migration and suggest that modulation of AQP4 expression or function might alter glial scarring.

EGFR-mediated expression of aquaporin-3 is involved in human skin fibroblast migration

AQP3 (aquaporin-3), known as an integral membrane channel in epidermal keratinocytes, facilitates water and glycerol movement into and out of the skin. Here, we demonstrate that AQP3 is also expressed in cultured human skin fibroblasts, which under normal wound healing processes migrate from surrounding tissues to close the wound. EGF (epidermal growth factor), which induced fibroblast migration, also induced AQP3 expression in a time- and dose-dependent manner. CuSO4 and NiCl2, previously known as AQP3 water transport inhibitors, as well as two other bivalent heavy metals Mn2+ and Co2+, inhibited EGF-induced cell migration in human skin fibroblasts. AQP3 knockdown by small interfering RNA inhibited EGF-induced AQP3 expression and cell migration. Furthermore, an EGFR (EGF receptor) kinase inhibitor, PD153035, blocked EGF-induced AQP3 expression and cell migration. MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase]/ERK inhibitor U0126 and PI3K (phosphoinositide 3-kinase) inhibitor LY294002 also inhibited EGF-induced AQP3 expression and cell migration. Collectively, our findings show for the first time that AQP3 is expressed in human skin fibroblasts and that EGF induces AQP3 expression via EGFR, PI3K and ERK signal transduction pathways. We have provided evidence for a novel role of AQP3 in human skin fibroblast cell migration, which occurs during normal wound healing.

Role of the Na/H exchanger NHE1 in cell migration

Cell migration plays a basic role in many physiological and pathophysiological processes such as embryogenesis, immune defence, wound healing or metastasis. The activity of the ubiquitously expressed NHE1 isoform of the plasma membrane Na+/H+ exchanger is one of the requirements for directed locomotion of migrating cells and also contributes to cell adhesion. The mechanisms by which NHE1 is involved in cell migration are multiple. NHE1 contributes to cell migration by affecting the cell volume, by regulating the intracellular pH and thereby the assembly and activity of cytoskeletal elements, by anchoring the cytoskeleton to the plasma membrane, by signalling, by regulating gene expression and by controlling cell adhesion. The present article gives a review of the different ways in which NHE1 is involved in and contributes to cell migration. These different mechanisms complement one another forming an intricate, integrative process.

Three distinct roles of aquaporin-4 in brain function revealed by knockout mice

Aquaporin-4 (AQP4) is expressed in astrocytes throughout the central nervous system, particularly at the blood-brain and brain-cerebrospinal fluid barriers. Phenotype analysis of transgenic mice lacking AQP4 has provided compelling evidence for involvement of AQP4 in cerebral water balance, astrocyte migration, and neural signal transduction. AQP4-null mice have reduced brain swelling and improved neurological outcome in models of (cellular) cytotoxic cerebral edema including water intoxication, focal cerebral ischemia, and bacterial meningitis. However, brain swelling and clinical outcome are worse in AQP4-null mice in models of vasogenic (fluid leak) edema including cortical freeze-injury, brain tumor, brain abscess and hydrocephalus, probably due to impaired AQP4-dependent brain water clearance. AQP4 deficiency or knock-down slows astrocyte migration in response to a chemotactic stimulus in vitro, and AQP4 deletion impairs glial scar progression following injury in vivo. AQP4-null mice also manifest reduced sound- and light-evoked potentials, and increased threshold and prolonged duration of induced seizures. Impaired K+ reuptake by astrocytes in AQP4 deficiency may account for the neural signal transduction phenotype. Based on these findings, we propose modulation of AQP4 expression or function as a novel therapeutic strategy for a variety of cerebral disorders including stroke, tumor, infection, hydrocephalus, epilepsy, and traumatic brain injury.

Involvement of aquaporin-4 in astroglial cell migration and glial scar formation

Aquaporin-4, the major water-selective channel in astroglia throughout the central nervous system, facilitates water movement into and out of the brain. Here, we identify a novel role for aquaporin-4 in astroglial cell migration, as occurs during glial scar formation. Astroglia cultured from the neocortex of aquaporin-4-null mice had similar morphology, proliferation and adhesion, but markedly impaired migration determined by Transwell migration efficiency (18+/-2 vs 58+/-4% of cells migrated towards 10% serum in 8 hours; P<0.001) and wound healing rate (4.6 vs 7.0 microm/hour speed of wound edge; P<0.001) compared with wild-type mice. Transwell migration was similarly impaired (25+/-4% migrated cells) in wild-type astroglia after approximately 90% reduction in aquaporin-4 protein expression by RNA inhibition. Aquaporin-4 was polarized to the leading edge of the plasma membrane in migrating wild-type astroglia, where rapid shape changes were seen by video microscopy. Astroglial cell migration was enhanced by a small extracellular osmotic gradient, suggesting that aquaporin-4 facilitates water influx across the leading edge of a migrating cell. In an in vivo model of reactive gliosis and astroglial cell migration produced by cortical stab injury, glial scar formation was remarkably impaired in aquaporin-4-null mice, with reduced migration of reactive astroglia towards the site of injury. Our findings provide evidence for the involvement of aquaporin-4 in astroglial cell migration, which occurs during glial scar formation in brain injury, stroke, tumor and focal abscess.

A developmentally regulated Na-H exchanger in Dictyostelium discoideum is necessary for cell polarity during chemotaxis

Increased intracellular H⁺ efflux is speculated to be an evolutionarily conserved mechanism necessary for rapid assembly of cytoskeletal filaments and for morphological polarity during cell motility. In Dictyostelium discoideum, increased intracellular pH through undefined transport mechanisms plays a key role in directed cell movement. We report that a developmentally regulated Na-H exchanger in Dictyostelium discoideum (DdNHE1) localizes to the leading edge of polarized cells and is necessary for intracellular pH homeostasis and for efficient chemotaxis. Starved DdNHE1-null cells (Ddnhe1⁻) differentiate, and in response to the chemoattractant cAMP they retain directional sensing; however, they cannot attain a polarized morphology, but instead extend mislocalized pseudopodia around the cell and exhibit decreased velocity. Consistent with impaired polarity, in response to chemoattractant, Ddnhe1⁻ cells lack a leading edge localization of F-actin and have significantly attenuated de novo F-actin polymerization but increased abundance of membrane-associated phosphatidylinositol 3,4,5-trisphosphate (PI_(3,4,5)P₃). These findings indicate that during chemotaxis DdNHE1 is necessary for establishing the kinetics of actin polymerization and PI_(3,4,5)P₃ production and for attaining a polarized phenotype.

Dictyostelium PAKc is required for proper chemotaxis

We have identified a new Dictyostelium p21-activated protein kinase, PAKc, that we demonstrate to be required for proper chemotaxis. PAKc contains a Rac-GTPase binding (CRIB) and autoinhibitory domain, a PAK-related kinase domain, an N-terminal phosphatidylinositol binding domain, and a C-terminal extension related to the Gbetagamma binding domain of Saccharomyces cerevisiae Ste20, the latter two domains being required for PAKc transient localization to the plasma membrane. In response to chemoattractant stimulation, PAKc kinase activity is rapidly and transiently activated, with activity levels peaking at approximately 10 s. pakc null cells exhibit a loss of polarity and produce multiple lateral pseudopodia when placed in a chemoattractant gradient. PAKc preferentially binds the Dictyostelium Rac protein RacB, and point mutations in the conserved CRIB that abrogate this binding result in misregulated kinase activation and chemotaxis defects. We also demonstrate that a null mutation lacking the PAK family member myosin I heavy chain kinase (MIHCK) shows mild chemotaxis defects, including the formation of lateral pseudopodia. A null strain lacking both PAKc and the PAK family member MIHCK exhibits severe loss of cell movement, suggesting that PAKc and MIHCK may cooperate to regulate a common chemotaxis pathway.

Understanding the cortex of crawling cells: insights from Dictyostelium

All animal cells are believed to use the same basic molecular mechanisms for locomotion when crawling on a surface. Study of a wide range of crawling cells has tended to confirm this belief but has also led to a diversity of hypotheses for locomotion and a bewildering list of candidate effector proteins. The emergence of a powerful model system, Dictyostelium discoideum, for the study of crawling of cells makes definitive tests of hypotheses for locomotion a reality.

Two phases of actin polymerization display different dependencies on PI(3,4,5)P3 accumulation and have unique roles during chemotaxis

The directional movement of cells in chemoattractant gradients requires sophisticated control of the actin cytoskeleton. Uniform exposure of Dictyostelium discoideum amoebae as well as mammalian leukocytes to chemoattractant triggers two phases of actin polymerization. In the initial rapid phase, motility stops and the cell rounds up. During the second slow phase, pseudopodia are extended from local regions of the cell perimeter. These responses are highly correlated with temporal and spatial accumulations of PI(3,4,5)P3/PI(3,4)P2 reflected by the translocation of specific PH domains to the membrane. The slower phase of PI accumulation and actin polymerization is more prominent in less differentiated, unpolarized cells, is selectively increased by disruption of PTEN, and is relatively more sensitive to perturbations of PI3K. Optimal levels of the second responses allow the cell to respond rapidly to switches in gradient direction by extending lateral pseudopods. Consequently, PI3K inhibitors impair chemotaxis in wild-type cells but partially restore polarity and chemotactic response in pten- cells. Surprisingly, the fast phase of PI(3,4,5)P3 accumulation and actin polymerization, which is relatively resistant to PI3K inhibition, can support inefficient but reasonably accurate chemotaxis.

Reorganization of the actin cytoskeleton in the protruding lamellae of human fibroblasts

To investigate the mechanisms of protrusion in vertebrate cells, the primary event in cell motility, human fibroblasts were treated with neomycin, an inhibitor of the phosphatidylinositol cycle, to induce protrusion. Changes in cell motility and the cytoskeleton were examined by video, fluorescence, scanning electron, and confocal microscopy and by cytofluorometry. Protrusion in neomycin-treated human fibroblasts is correlated with a transient overall decrease in F-actin followed by an increase in F-actin at the leading edge of the protruding lamella. In growing lamellae, F-actin is organized in a marginal band at the leading edge. Although actin is present in the lamella behind the leading edge, very little of it is F-actin. Scanning electron microscopy of detergent-extracted cells reveals a band of dense filaments at the leading edge, corresponding to the marginal band of F-actin seen in fluorescently labeled cells, and a sparse population of short, fragmented filaments, in the rest of the lamella. Gelsolin is colocalized with F-actin in the marginal band and is also present in the lamella where F-actin is largely absent. The data support the hypothesis that the protrusion is initiated by the breakdown of cortical actin filaments, possibly mediated by gelsolin, whereas expansion of the protrusion requires de novo polymerization of actin filaments at the leading edge.

Spatiotemporal dynamics of actin concentration during cytokinesis and locomotion in Dictyostelium

To study the spatial and temporal regulation of the actin cytoskeleton, we have analyzed the actin concentration dynamics in live Dictyostelium. The relative actin concentration was analyzed with respect to cell behavior by fluorescence morphometry. We electroporated rhodamine-actin into Dictyostelium cells and acquired images with 200–300 millisecond temporal and approximately 250 nm spatial resolutions. To convert fluorescence intensity into actin concentration, the observation was made on nearly two-dimensional cells, and the actin signal was ratioed over a volume marker (FITC-BSA or GFP). Since the emission of FITC and GFP is pH-dependent, we first measured the cytoplasmic pH in live cells and determined that the pHi in pseudopods is same as that of general cytoplasm. During cytokinesis, the relative concentration of actin in the cleavage furrow was significantly higher than in the general cytoplasm. In migrating cells, actin was recruited surprisingly rapidly, particularly in the pseudopod. We found that the region of high actin concentration moves relative to the leading edge when a pseudopod projects or retracts. When the pseudopod retracts, the actin density dissipates within 5 seconds. We have also found that actin accumulates in developing pseudopods in an oscillatory manner, and this timing coordinates with advancement of the centroid. This is the first study to reveal the dynamic changes in relative concentration of actin in live cells and to quantitatively correlate these changes with the locomotive behavior of the amoeba.

The actomyosin cytoskeleton of amoebae of the cellular slime molds acrasis rosea and protostelium mycophaga: structure, biochemical properties, and function

In amoebae of the cellular slime molds (mycetozoans) Acrasis rosea and Protostelium mycophaga, bundles of F-actin radiate from the endoplasm-ectoplasm interface into the pseudopodia, where G-actin is also located. We conclude that these actin bundles form a core scaffold driving pseudopod extension which is subsequently completed by filling with a more loosely organized meshwork of F-actin. Some bipolar, elongate amoebae of A. rosea also contained long bundles of F-actin that traverse the cells lengthwise and remotely resemble stress fibers. Rodlets of F-actin were scattered in the body of amoebae of A. rosea or formed star-shaped or polygonal complexes near or around contractile vacuoles, where they may play a role in contraction. In total protein extracts analyzed by SDS-PAGE and immunoblots the actins migrated like the rabbit skeletal muscle control. The relative proportion of actin in total protein extracts was 7.9% for A. rosea and 34.5% for P. mycophaga. We detected four or five isoactins in extracts of both species and we determined that the genome of each species contains approximately six actin genes. Whether they are all expressed or if posttranslational modifications occur remains to be determined. Myosin II was enriched in actomyosin extracts; its Mr was 187.8 kDa for A. rosea and 220.7 kDa for P. mycophaga. Cell models ("ghosts") contracted upon the addition of ATP. We conclude that amoebae of A. rosea and P. mycophaga, although behaving differently from those of Dictyostelium discoideum, contain the basic repertoire of molecules that enable pseudopod extension by actin polymerization and ATP-induced contraction of the cell cortex. Copyright 1998 Academic Press.

Changes in actin filament organization during pseudopod formation

Fluorescent phalloidin has been introduced into Dicytostelium amoebae in order to visualize dynamic changes in the localization of F-actin during pseudopod extension. Phalloidin was initially localized to the peripheral cortex of the cell. Newly formed pseudopods were not fluorescent, indicating that phalloidin was tightly bound to existing F-actin filaments and could not rapidly relocalize to newly formed filaments. As pseudopod extension proceeded, the fluorescent signal disappeared from the region directly underlying the expansion zone, leaving a gap in the actin cortex. Similar results were obtained in both wild-type cells and those lacking myosin II heavy chain. The disappearance of the fluorescent signal from the cortical region underlying the new pseudopod is presumed to be due to breakdown of the actin cortex and dispersion of the remnants. These results suggest that new pseudopods are not built upon the existing actin cortex but rather that the cortex is locally solated as part of the construction of the new actin network.

Re-expression of ABP-120 rescues cytoskeletal, motility, and phagocytosis defects of ABP-120- Dictyostelium mutants

The actin binding protein ABP-120 has been proposed to cross-link actin filaments in nascent pseudopods, in a step required for normal pseudopod extension in motile Dictyostelium amoebae. To test this hypothesis, cell lines that lack ABP-120 were created independently either by chemical mutagenesis or homologous recombination. Different phenotypes were reported in these two studies. The chemical mutant shows only a subtle defect in actin cross-linking, while the homologous recombinant mutants show profound defects in actin cross-linking, cytoskeletal structure, pseudopod number and size, cell motility and chemotaxis and, as shown here, phagocytosis. To resolve the controversy as to what the ABP-120- phenotype is, ABP-120 was re-expressed in an ABP-120- cell line created by homologous recombination. Two independently "rescued" cell lines that express wild-type levels of ABP-120 were analyzed. In both rescued cell lines, actin incorporation into the cytoskeleton, pseudopod formation, cell morphology, instantaneous velocity, phagocytosis, and chemotaxis were restored to wild-type levels. There is no alteration in the expression levels of several related actin binding proteins in either the original ABP-120- cell line or in the rescued cell lines, leading to the conclusion that neither the aberrant phenotype observed in ABP-120- cells nor the normal phenotype reasserted in rescued cells can be attributed to alterations in the levels of other abundant and related actin binding proteins. Re-expression of ABP-120 in ABP-120- cells reestablishes normal structural and behavioral parameters, demonstrating that the severity and properties of the structural and behavioral defects of ABP-120- cell lines produced by homologous recombination are the direct result of the absence of ABP-120.

Responses of tumor cell pseudopod protrusion to changes in medium osmolality

The potential involvement of osmotically generated force in protrusion of tumor cell pseudopods was examined during a micropipette assay. Experiments were performed on single A2058 melanoma cells activated by a micropipette filled with soluble type IV collagen. Previous observations suggested that tumor cell pseudopod protrusion induced by type IV collagen took place in distinct, separable phases: an initial bleb (first phase) caused by localized Ca2+-activated actin filament severing resulting in an osmotic flux followed by an extension with an irregular shape (second phase) which required G protein-mediated actin polymerization (Dong et al., 1994, Microvasc. Res., 47:55-67). Presently we studied cell pseudopod protrusion in response to the changes in chemoattractant osmolality. Reduction of attractant osmolality by 20-25% from its baseline value (297 mmol/ kg) resulted in an increase in pseudopod length by 50% apparent in the initial phase. Increases in attractant osmolality by 25-30% from the baseline value arrested pseudopod protrusion significantly during both initial and later phases. Using a dual-pipette method, such osmotic influence on the cell pseudopod protrusion was shown to be only a local effect in a small region where the cell surface was stimulated by the micropipette. While forces derived from actin polymerization and osmotic pressure have been proposed to cause protrusion in general, our results suggested that osmotically generated force is more apparent in the initial phase of the pseudopod formation.

Analysis of cell movement during the culmination phase of Dictyostelium development

Co-ordinated cell movement of tens of thousands of cells and periodic signals characterise the multicellular development of the cellular slime mould Dictyostelium discoideum. We investigated cell movement by analysing time-lapse video recordings made during the slug stage and the culmination phase of Dictyostelium development. Slugs viewed from the side showed an even, straight forward movement with the tip slightly raised in the air. Slugs that had migrated for a prolonged period of time either culminated or showed a behaviour best described as abortive culmination. Culmination is initiated by a local aggregation of anterior-like cells at the base of the slug at the prestalk-prespore boundary, where they form a stationary mass of cells. Prespore cells continue to move forward over this stationary pile and, as a result, are lifted into the air. The stationary group of anterior-like cells thereby end up to the back of the slug. At this point the slug either falls back on the agar surface or continues culmination. If the slug continues to migrate these cells regain motility, move forward to the prespore-prestalk boundary and form a new pile again. In the case of culmination the neutral red stained cells in the pile move to the back of the slug and form a second signalling centre beside the tip. Both centres are characterised by vigorous rotational cell movement. The cells belonging to the basal centre will form the basal disc and the lower cup in the fruiting body. The upper cup will be formed by the prestalk cells rotating most vigorously at the prestalk-prespore boundary. The remaining neutral red stained anterior-like cells in the prespore zone sort either to the upper or lower organising centre in the fruiting body.

Intracellular pressure is a motive force for cell motion in Amoeba proteus

The cortical filament layer of free-living amoebae contains concentrated actomyosin, suggesting that it can contract and produce an internal hydrostatic pressure. We report here on direct and dynamic intracellular pressure (P(ic)) measurements in Amoeba proteus made using the servo-null technique. In resting apolar A. proteus, P(ic) increased while the cells remained immobile and at apparently constant volume. P(ic) then decreased approximately coincident with pseudopod formation. There was a positive correlation between P(ic) at the onset of movement and the rate of pseudopod formation. These results are the first direct evidence that hydrostatic pressure may be a motive force for cell motion. We postulate that contractile elements in the amoeba's cortical layer contract and increase P(ic) and that this P(ic) is utilized to overcome the viscous flow resistance of the intracellular contents during pseudopod formation.

The Dictyostelium cytoskeleton

New avenues of cytoskeleton research in Dictyostelium discoideum have opened up with the cloning of the alpha- and beta-tubulin genes and the characterization of kinesins and cytoplasmic dynein. Much research, however, continues to focus on the actin cytoskeleton and its dynamics during chemotaxis, morphogenesis, and other motile processes. New actin-associated proteins are being identified and characterized by biochemical means and through isolation of mutants lacking individual components. This work is shedding light on the roles of specific actin assemblies in various biological processes.

Patterns of spontaneous motility in videomicrographs of human epidermal keratinocytes (HEK)

The subject of our observations was the spontaneous behaviour of normal and transfected human epidermal keratinocytes. Cell movements were recorded on video micrographs and analyzed by a mathematical approach, using new methods of image processing and statistical correlation analysis. Protrusive activity of single lamellae was examined using one-dimensional analysis of phase-contrast image sequences along section lines transversal to the cell edge. This method revealed high periodicity and correlation in the motility patterns of lamellae and ruffles. Two-dimensional correlation analysis of automatically digitized cell outlines was applied to detect spatiotemporal patterns and coordination of lamellar extension and retraction. Most cells showed regularly alternating pulsations of lamellar protrusions. In some extreme cases, extension waves rotating around the cell periphery were observed. The results were compared with computer simulations of two simple models for lamellar dynamics and shape deformation, based on few assumptions about chemical kinetics of F-actin and cytomechanical properties of the actin network, neglecting regulatory effects of actin-associated proteins or extracellular stimulations. The simulation results reproduced the main dynamical features of the observed real cells, indicating the possibility that the basic universal mechanism for lateral coordination of lamellipodial protrusion is the interplay between hydrostatic pressure and viscocontractile tension in the cortical F-actin-plasma membrane complex.

Intracellular calcium levels correlate with speed and persistent forward motion in migrating neutrophils

The relationship between cytosolic free calcium concentration ([Ca2+]i) and human neutrophil motility was studied by video microscopy. Neutrophils stimulated by a uniform concentration of an N-formylated peptide chemoattractant (f-Met-Leu-Phe) were tracked during chemokinetic migration on albumin, fibronectin, and vitronectin. [Ca2+]i buffering with quin2 resulted in significant decreases in mean speed on albumin. To further characterize the relationship between [Ca2+]i changes and motility we carried out a cross-correlation analysis of [Ca2+]i with several motility parameters. Cross-correlations between [Ca2+]i and each cell's speed, angle changes, turn strength, and persistent forward motion revealed (i) a positive correlation between [Ca2+]i and cell speed (p < 0.05), (ii) no significant correlation between turns and calcium spikes, and (iii) the occurrence of turns during periods of low speed. Significant negative correlations between [Ca2+]i and angle change were noted on the high adhesion substrates vitronectin and fibronectin but not on the low adhesion substrate albumin. These data imply that there is a general temporal relationship between [Ca2+]i, speed, and persistent motion. However, the correlations are not sufficiently strong to imply that changes in [Ca2+]i are required proximal signals for velocity changes.

Genetic deletion of ABP-120 alters the three-dimensional organization of actin filaments in Dictyostelium pseudopods

This study extends the observations on the defects in pseudopod formation of ABP-120+ and ABP-120- cells by a detailed morphological and biochemical analysis of the actin based cytoskeleton. Both ABP-120+ and ABP-120- cells polymerize the same amount of F-actin in response to stimulation with cAMP. However, unlike ABP-120+ cells, ABP-120- cells do not incorporate actin into the Triton X-100-insoluble cytoskeleton at 30-50 s, the time when ABP-120 is incorporated into the cytoskeleton and when pseudopods are extended after cAMP stimulation in wild-type cells. By confocal and electron microscopy, pseudopods extended by ABP-120- cells are not as large or thick as those produced by ABP-120+ cells and in the electron microscope, an altered filament network is found in pseudopods of ABP-120- cells when compared to pseudopods of ABP-120+ cells. The actin filaments found in areas of pseudopods in ABP-120+ cells either before or after stimulation were long, straight, and arranged into space filling orthogonal networks. Protrusions of ABP-120- cells are less three-dimensional, denser, and filled with multiple foci of aggregated filaments consistent with collapse of the filament network due to the absence of ABP-120-mediated cross-linking activity. The different organization of actin filaments may account for the diminished size of protrusions observed in living and fixed ABP-120- cells compared to ABP-120+ cells and is consistent with the role of ABP-120 in regulating pseudopod extension through its cross-linking of actin filaments.

Mutants lacking myosin II cannot resist forces generated during multicellular morphogenesis

We have used fluorescent labeling, confocal microscopy and computer-assisted motion analysis to observe and quantify individual wild-type and myosin II mutant cell behavior during early multicellular development in Dictyostelium discoideum. When cultured with an excess of unlabeled wild-type cells, labeled control cells are randomly distributed within aggregation streams, while myosin II mutant cells are found primarily at the lateral edges of streams. Wild-type cells move at average rates of 8.5 +/- 4.9 microns/min within aggregation streams and can exhibit regular periodic movement at 3.5 minute intervals; half as long as the 7 minute period reported previously for isolated cells. Myosin II mutants under the same conditions move at 5.0 +/- 4.8 microns/min, twice as fast as reported previously for isolated myosin II mutant cells, and fail to display regular periodic movement. When removed from aggregation streams myosin II mutant cells move at only 2.5 +/- 2.0 microns/min, while wild-type cells under these conditions move at 5.9 +/- 4.5 microns/min. Analysis of cell morphology further reveals that myosin II mutant cells are grossly and dynamically deformed within wild-type aggregation streams but not when removed from streams and examined in isolation. These data reveal that the loss of myosin II has dramatic consequences for cells undergoing multicellular development. The segregation of mutant cells to aggregation stream edges demonstrates that myosin II mutants are unable to penetrate a multicellular mass of wild-type cells, while the observed distortion of myosin II mutant cells suggests that the cortex of such cells is too flacid to resist forces generated during movement. The increased rate of mutant cell movement and distortion of mutant cell morphology seen within wild-type aggregation streams further argues both that movement of wild-type cells within a multicellular mass can generate traction forces on neighboring cells and that mutant cell morphology and behavior can be altered by these forces. In addition, the distortion of myosin II mutant cells within wild-type aggregation streams indicates that myosin is not required for the formation of cell-cell contacts. Finally, the consequences of the loss of myosin II for cells during multicellular development are much more severe than has been previously revealed for isolated cells. The techniques used here to analyze the behavior of individual cells within multicellular aggregates provide a more sensitive assay of mutant cell phenotype than has been previously available and will be generally applicable to the study of motility and cytoskeletal mutants in Dictyostelium.

Regulatory role of the G alpha 1 subunit in controlling cellular morphogenesis in Dictyostelium

To determine the function of the Dictyostelium G alpha 1 subunit during aggregation and multicellular development, we analyzed the phenotypes of g alpha 1 null cells and strains overexpressing either wild-type G alpha 1 or two putative constitutively active mutations of G alpha 1. Strains overexpressing the wild-type or mutant G alpha 1 proteins showed very abnormal culmination with an aberrant stalk differentiation. The similarity of the phenotypes between G alpha 1 overexpression and expression of a putative constitutively active G alpha 1 subunit suggests that these phenotypes are due to increased G alpha 1 activity rather than resulting from a non-specific interference of other pathways. In contrast, g alpha 1 null strains showed normal morphogenesis except that the stalks were thinner and longer than those of wild-type culminants. Analysis of cell-type-specific gene expression using lacZ reporter constructs indicated that strains overexpressing G alpha 1 show a loss of ecmB expression in the central core of anterior prestalk AB cells. However, expression of ecmB in anterior-like cells and the expression of prestalk A-specific gene ecmA and the prespore-specific gene SP60/cotC appeared normal. Using a G alpha 1/lacZ reporter construct, we show that G alpha 1 expression is cell-type-specific during the multicellular stages, with a pattern of expression similar to ecmB, being preferentially expressed in the anterior prestalk AB cells and anterior-like cells. The developmental and molecular phenotypes of G alpha 1 overexpression and the cell-type-specific expression of G alpha 1 suggest that G alpha 1-mediated signaling pathways play an essential role in regulating multicellular development by controlling prestalk morphogenesis, possibly by acting as a negative regulator of prestalk AB cell differentiation. During the aggregation phase of development, g alpha 1 null cells display a delayed peak in cAMP-stimulated accumulation of cGMP compared to wild-type cells, while G alpha 1 overexpressors and dominant activating mutants show parallel kinetics of activation but decreased levels of cGMP accumulation compared to that seen in wild-type cells. These data suggest that G alpha 1 plays a role in the regulation of the activation and/or adaptation of the guanylyl cyclase pathway. In contrast, the activation of adenylyl cyclase, another pathway activated by cAMP stimulation, was unaffected in g alpha 1 null cells and cell lines overexpressing wild-type G alpha 1 or the G alpha 1 (Q206L) putative dominant activating mutation.(ABSTRACT TRUNCATED AT 400 WORDS)

A gradient method for the quantitative analysis of cell movement and tissue flow and its application to the analysis of multicellular Dictyostelium development

We describe the application of a novel image processing method, which allows quantitative analysis of cell and tissue movement in a series of digitized video images. The result is a vector velocity field showing average direction and velocity of movement for every pixel in the frame. We apply this method to the analysis of cell movement during different stages of the Dictyostelium developmental cycle. We analysed time-lapse video recordings of cell movement in single cells, mounds and slugs. The program can correctly assess the speed and direction of movement of either unlabelled or labelled cells in a time series of video images depending on the illumination conditions. Our analysis of cell movement during multicellular development shows that the entire morphogenesis of Dictyostelium is characterized by rotational cell movement. The analysis of cell and tissue movement by the velocity field method should be applicable to the analysis of morphogenetic processes in other systems such as gastrulation and neurulation in vertebrate embryos.

Regulation of leukocyte locomotion by Ca2+

Neutrophils migrate towards sites of inflammation and infection by chemotaxis. Their motility is dependent on the actin cytoskeleton and on adhesion to extracellular substrates, but how these are regulated in response to stimuli is not clear. This review focuses on the potential role of Ca(2+) as a second messenger in neutrophil motility. Several effects of Ca(2+) and Ca(2+)-binding proteins on the stability and crosslinking of actin polymers have been demonstrated in vitro. Nevertheless, the complex mechanism by which Ca(2+) regulates actin in neutrophils is not fully understood. In addition, intracellular Ca(2+) regulates the intergin-mediated adhesion of neutrophils to extracellular matrix.

On the crawling of animal cells

Cells crawl in response to external stimuli by extending and remodeling peripheral elastic lamellae in the direction of locomotion. The remodeling requires vectorial assembly of actin subunits into linear polymers at the lamella's leading edge and the crosslinking of the filaments by bifunctional gelation proteins. The disassembly of the crosslinked filaments into short fragments or monomeric subunits away from the leading edge supplies components for the actin assembly reactions that drive protrusion. Cellular proteins that respond to lipid and ionic signals elicited by sensory cues escort actin through this cycle in which filaments are assembled, crosslinked, and disassembled. One class of myosin molecules may contribute to crawling by guiding sensory receptors to the cell surface, and another class may contribute by imposing contractile forces on actin networks in the lamellae.

Relative distribution of actin, myosin I, and myosin II during the wound healing response of fibroblasts

Myosin I is present in Swiss 3T3 fibroblasts and its localization reflects a possible involvement in the extension and/or retraction of protrusions at the leading edge of locomoting cells and the transport of vesicles, but not in the contraction of stress fibers or transverse fibers. An affinity-purified polyclonal antibody to brush border myosin I colocalizes with a polypeptide of 120 kD in fibroblast extracts. Within initial protrusions of polarized, migrating fibroblasts, myosin I exhibits a punctate distribution, whereas actin is diffuse and myosin II is absent. Myosin I also exists in linear arrays parallel to the direction of migration in filopodia and microspikes, established protrusions, and within the leading lamellae of migrating cells. Myosin II and actin colocalize along transverse fibers in the lamellae of migrating cells, while myosin I displays no definitive organization along these fibers. During contractions of actin-based fibers, myosin II is concentrated in the center of the cell, while the distribution of myosin I does not change. Thus, myosin I is found at the correct location and time to be involved in the extension and/or retraction of protrusions and the transport of vesicles. Myosin II-based contractions in more posterior cellular regions could generate forces to separate cells, maintain a polarized cell shape, maintain the direction of locomotion, maximize the rate of locomotion, and/or aid in the delivery of cytoskeletal/contractile subunits to the leading edge.

The integral membrane protein, ponticulin, acts as a monomer in nucleating actin assembly

Ponticulin, an F-actin binding transmembrane glycoprotein in Dictyostelium plasma membranes, was isolated by detergent extraction from cytoskeletons and purified to homogeneity. Ponticulin is an abundant membrane protein, averaging approximately 10(6) copies/cell, with an estimated surface density of approximately 300 per microns2. Ponticulin solubilized in octylglucoside exhibited hydrodynamic properties consistent with a ponticulin monomer in a spherical or slightly ellipsoidal detergent micelle with a total molecular mass of 56 +/- 6 kD. Purified ponticulin nucleated actin polymerization when reconstituted into Dictyostelium lipid vesicles, but not when a number of commercially available lipids and lipid mixtures were substituted for the endogenous lipid. The specific activity was consistent with that expected for a protein comprising 0.7 +/- 0.4%, by mass, of the plasma membrane protein. Ponticulin in octylglucoside micelles bound F-actin but did not nucleate actin assembly. Thus, ponticulin-mediated nucleation activity was sensitive to the lipid environment, a result frequently observed with transmembrane proteins. At most concentrations of Dictyostelium lipid, nucleation activity increased linearly with increasing amounts of ponticulin, suggesting that the nucleating species is a ponticulin monomer. Consistent with previous observations of lateral interactions between actin filaments and Dictyostelium plasma membranes, both ends of ponticulin-nucleated actin filaments appeared to be free for monomer assembly and disassembly. Our results indicate that ponticulin is a major membrane protein in Dictyostelium and that, in the proper lipid matrix, it is sufficient for lateral nucleation of actin assembly. To date, ponticulin is the only integral membrane protein known to directly nucleate actin polymerization.

Tyrosine phosphorylation of actin in Dictyostelium associated with cell-shape changes

When Dictyostelium cells that have initiated their developmental program upon starvation are returned to growth medium, there is a rapid and transient de novo tyrosine phosphorylation of a 43-kilodalton protein. This protein was found to be actin. Most of the phosphorylation occurred in a single, minor acidic isoform of actin. Developing cells that had been returned to growth medium lost their pseudopod extensions, became round, and had reduced adhesion to the substratum. These effects occurred with kinetics that matched the increase in tyrosine phosphorylation of actin. In mutant cell lines in which the gene for the phosphotyrosine phosphatase PTP1 had been disrupted, tyrosine phosphorylation of actin was rapid and more prolonged. These cells responded with proportionally accelerated kinetics of cell rounding. Cell lines overexpressing PTP1 had diminished amplitude and duration of actin tyrosine phosphorylation and exhibited diminished cell-shape change and an accelerated return to the extended cell-shape morphology seen in starved cells.

Cytoskeletal agents inhibit motility and adherence of human tumor cells

Cytoskeletal agents have been demonstrated to inhibit stimulated motility and substrate adherence by the human tumor cell line, A2058. cis-tubulozole, taxol, and cytochalasin D were tested for their effects on chemotaxis in response to a tumor cytokine, autocrine motility factor, and on adherence to several substrata: laminin- and gelatin-coated dishes as well as tissue culture plastic. Cytochalasin D, which inhibits microfilament polymerization, abolished stimulated motility. Taxol, which stabilizes microtubules, decreased stimulated motility to a greater degree than cis-tubulozole, which inhibits microtubular polymerization. In contrast, cis-tubulozole had the greatest inhibitory effect on adherence with a gelatin substratum more affected (100% inhibition) than tissue culture plastic (90%) or laminin substratum (52%). Taxol affected adherence in the same order but less than cis-tubulozole. Cytochalasin D had no significant effect on adherence to laminin with moderate inhibition of adherence to tissue culture plastic or gelatin. These data suggest that, in these tumor cells, microfilaments are more crucial for motility than adherence, but the dynamic polymerization and depolymerization of microtubules are required for both types of cellular activities.