Direct measurement of vertical forces shows correlation between mechanical activity and proteolytic ability of invadopodia

Mechanobiology plays a prominent role in cancer invasion and metastasis. The ability of a cancer to degrade extracellular matrix (ECM) is likely connected to its invasiveness. Many cancer cells form invadopodia-micrometer-sized cellular protrusions that promote invasion through matrix degradation (proteolysis). Although it has been hypothesized that invadopodia exert mechanical force that is implicated in cancer invasion, direct measurements remain elusive. Here, we use a recently developed interferometric force imaging technique that provides piconewton resolution to quantify invadopodial forces in cells of head and neck squamous carcinoma and to monitor their temporal dynamics. We compare the force exerted by individual protrusions to their ability to degrade ECM and investigate the mechanical effects of inhibiting invadopodia through overexpression of microRNA-375. By connecting the biophysical and biochemical characteristics of invadopodia, our study provides a new perspective on cancer invasion that, in the future, may help to identify biomechanical targets for cancer therapy.

Autocrine HBEGF expression promotes breast cancer intravasation, metastasis and macrophage-independent invasion in vivo

Increased expression of HBEGF in ER negative breast tumors is correlated with enhanced metastasis to distant organ sites and more rapid disease recurrence upon removal of the primary tumor. Our previous work has demonstrated a paracrine loop between breast cancer cells and macrophages in which the tumor cells are capable of stimulating macrophages through the secretion of CSF-1 while the tumor associated macrophages (TAMs) in turn aid in tumor cell invasion by secreting EGF. To determine how the autocrine expression of EGFR ligands by carcinoma cells would affect this paracrine loop mechanism, and in particular whether tumor cell invasion depends on spatial ligand gradients generated by TAMs, we generated cell lines with increased HBEGF expression. We find that autocrine HBEGF expression enhanced in vivo intravasation and metastasis, and resulted in a novel phenomenon in which macrophages were no longer required for in vivo invasion of breast cancer cells. In vitro studies revealed that expression of HBEGF enhanced invadopodium formation, thus providing a mechanism for cell autonomous invasion. The increased invadopodium formation was directly dependent on EGFR signaling, as demonstrated by a rapid decrease in invadopodia upon inhibition of autocrine HBEGF/EGFR signaling as well as inhibition of signaling downstream of EGFR activation. HBEGF expression also resulted in enhanced invadopodium function via upregulation of MMP2 and MMP9 expression. We conclude that high levels of HBEGF expression can short-circuit the tumor cell/macrophage paracrine invasion loop, resulting in enhanced tumor invasion that is independent of macrophage signaling.

Chemotaxis assays for eukaryotic cells

Chemotaxis is a complex response of a cell to an external stimulus. It involves detecting and measuring the concentration of the chemoattractant, biochemical transmission of the information, and the motility and adhesive changes associated with the response. This unit describes a number of chemotaxis assays that can be used to identify chemoattractants individually and in large-scale screenings, to distinguish chemotaxis from chemokinesis, and to analyze cellular behavioral and biochemical responses. Some of these assays such as the filter, under agarose, and small population assays, can be used to monitor the behavior of large groups of cells; the bridge, pipet, and upshift assays can be used to analyze the responses of single cells.

N-terminal domains of the class ia phosphoinositide 3-kinase regulatory subunit play a role in cytoskeletal but not mitogenic signaling

Phosphoinositide (PI) 3-kinases are required for the acute regulation of the cytoskeleton by growth factors. We have shown previously that in the MTLn3 rat adenocarcinoma cells line, the p85/p110alpha PI 3-kinase is required for epidermal growth factor (EGF)-stimulated lamellipod extension and formation of new actin barbed ends at the leading edge of the cell. We have now examined the role of the p85alpha regulatory subunit in greater detail. Microinjection of recombinant p85alpha into MTLn3 cells blocked both EGF-stimulated mitogenic signaling and lamellipod extension. In contrast, a truncated p85(1-333), which lacks the SH2 and iSH2 domains and does not bind p110, had no effect on EGF-stimulated mitogenesis but still blocked EGF-stimulated lamellipod extension. Additional deletional analysis showed that the SH3 domain was not required for inhibition of lamellipod extension, as a construct containing only the proline-rich and breakpoint cluster region (BCR) homology domains was sufficient for inhibition. Although the BCR domain of p85 binds Rac, the effects of the p85 constructs were not because of a general inhibition of Rac signaling, because sorbitol-induced JNK activation in MTLn3 cells was not inhibited. These data show that the proline-rich and BCR homology domains of p85 are involved in the coupling of p85/p110 PI 3-kinases to regulation of the actin cytoskeleton. These data provide evidence of a distinct cellular function for the N-terminal domains of p85.

The F-actin side binding activity of the Arp2/3 complex is essential for actin nucleation and lamellipod extension

Most eukaryotic cells rely on localized actin polymerization to generate and sustain the protrusion activity necessary for cell movement [1, 2]. Such protrusions are often in the form of a flat lamellipod with a leading edge composed of a dense network of actin filaments [3, 4]. The Arp2/3 complex localizes within that network in vivo [3, 4] and nucleates actin polymerization and generates a branched network of actin filaments in vitro [5-7]. The complex has thus been proposed to generate the actin network at the leading edge of crawling cells in vivo [3, 4, 8]. However, the relative contributions of nucleation and branching to protrusive force are still unknown. We prepared antibodies to the p34 subunit of the Arp2/3 complex that selectively inhibit side binding of the complex to F-actin. We demonstrate that side binding is required for efficient nucleation and branching by the Arp2/3 complex in vitro. However, microinjection of these antibodies into cells specifically inhibits lamellipod extension without affecting the EGF-stimulated appearance of free barbed ends in situ. These results indicate that while the side binding activity of the Arp2/3 complex is required for nucleation in vitro and for protrusive force in vivo, it is not required for EGF-stimulated increases in free barbed ends in vivo. This suggests that the branching activity of the Arp2/3 complex is essential for lamellipod extension, while the generation of nucleation sites for actin polymerization is not sufficient.

Lamellipodia in invasion

In vivo imaging of GFP-labeled metastatic tumor cells reveals cell orientation towards blood vessels. Orientation of tumor cells during chemotactic responses to ligands such as EGF begins with lamellipod extension. Evaluation of some of the downstream events in lamellipod extension indicates: (1) plasma membrane distribution of the EGF receptor is uniform but internalized receptor accumulates on the side of the cell closest to the source of EGF; (2) the alpha p110 isoform of PI-3 kinase is required; and (3) protrusion of the lamellipod relies upon the combined actions of the Arp2/3 complex and cofilin for generation of filamentous actin.

Epidermal growth factor receptor distribution during chemotactic responses

To determine the distribution of the epidermal growth factor (EGF) receptor (EGFR) on the surface of cells responding to EGF as a chemoattractant, an EGFR-green fluorescent protein chimera was expressed in the MTLn3 mammary carcinoma cell line. The chimera was functional and easily visualized on the cell surface. In contrast to other studies indicating that the EGFR might be localized to certain regions of the plasma membrane, we found that the chimera is homogeneously distributed on the plasma membrane and becomes most concentrated in vesicles after endocytosis. In spatial gradients of EGF, endocytosed receptor accumulates on the upgradient side of the cell. Visualization of the binding of fluorescent EGF to cells reveals that the affinity properties of the receptor, together with its expression level on cells, can provide an initial amplification step in spatial gradient sensing.

The collection of the motile population of cells from a living tumor

In this study, we report that needles containing chemoattractants can be used to collect the subpopulation of motile and chemotactic tumor cells from a primary tumor in a live rat as a pure population suitable for further analysis. The most efficient cell collection requires the presence of chemotactic cytokines, such as epidermal growth factor and serum components, and occurs with 15-fold higher efficiency in metastatic tumors compared with nonmetastatic tumors. Although tumor cells of the nonmetastatic tumors show a motility response to serum, they were not collected with high efficiency into needles in vivo in response to serum, indicating that additional factors besides motility are required to explain differences in cell collection efficiencies between metastatic and nonmetastatic tumors. The results reported here indicate that needles filled with growth factors and matrigel, when inserted into the primary tumor, can faithfully mimic the environment that supports invasion and intravasation in vivo. Furthermore, the results indicate that the same cell behaviors that contribute to chemotaxis in vitro also contribute to invasion in vivo.

Imaging of cancer invasion and metastasis using green fluorescent protein

The use of green fluorescent protein to fluorescently tag tumour cells has allowed investigators to open the "black box" of metastasis in order to visualise the behaviour of tumour cells in living tissues. Analysis of cells leaving the primary tumour indicates that highly metastatic cells are able to polarise more effectively towards blood vessels while poorly metastatic cells fragment more often when interacting with blood. In addition, there appear to be greater numbers of host immune system cells interacting with metastatic tumours. After arresting in target organs such as the lungs or liver, most tumour cells become dormant or apoptose. A small fraction of the arrested cells form metastases. In some target organs, migration of tumour cells may enhance the ability to form metastases.

A critical step in metastasis: in vivo analysis of intravasation at the primary tumor

Detailed evaluation of all steps in tumor cell metastasis is critical for evaluating the cell mechanisms controlling metastasis. Using green fluorescent protein transfectants of metastatic (MTLn3) and nonmetastatic (MTC) cell lines derived from the rat mammary adenocarcinoma 13762 NF, we have measured tumor cell density in the blood, individual tumor cells in the lungs, and lung metastases. Correlation of blood burden with lung metastases indicates that entry into the circulation is a critical step for metastasis. To examine cell behavior during intravasation, we have used green fluorescent protein technology to view these cells in time lapse images within a single optical section using a confocal microscope. In vivo imaging of the primary tumors of MTLn3 and MTC cells indicates that both metastatic and nonmetastatic cells are motile and show protrusive activity. However, metastatic cells show greater orientation toward blood vessels and larger numbers of host cells within the primary tumor, whereas nonmetastatic cells fragment when interacting with vessels. These results demonstrate that a major difference in intravasation between metastatic and nonmetastatic cells is detected in the primary tumor and illustrate the value of a direct visualization of cell properties in vivo for dissection of the metastatic process.

Specific requirement for the p85-p110alpha phosphatidylinositol 3-kinase during epidermal growth factor-stimulated actin nucleation in breast cancer cells

We have studied the role of phosphatidylinositol 3-kinases (PI 3-kinases) in the regulation of the actin cytoskeleton in MTLn3 rat adenocarcinoma cells. Stimulation of MTLn3 cells with epidermal growth factor (EGF) induced a rapid increase in actin polymerization, with production of lamellipodia within 3 min. EGF-stimulated lamellipodia were blocked by 100 nM wortmannin, suggesting the involvement of a class Ia PI 3-kinase. MTLn3 cells contain equal amounts of p110alpha and p110beta, and do not contain p110delta. Injection of specific inhibitory antibodies to p110alpha induced cell rounding and blocked EGF-stimulated lamellipod extension, whereas control or anti-p110beta antibodies had no effect. In contrast, both antibodies inhibited EGF-stimulated DNA synthesis. An in situ assay for actin nucleation showed that EGF-stimulated formation of new barbed ends was blocked by injection of anti-p110alpha antibodies. In summary, the p110alpha isoform of PI 3-kinase is specifically required for EGF-stimulated actin nucleation during lamellipod extension in breast cancer cells.

Integration of Rac-dependent regulation of cyclin D1 transcription through a nuclear factor-kappaB-dependent pathway

The small GTP-binding protein Rac1, a member of the Ras superfamily, plays a fundamental role in cytoskeleton reorganization, cellular transformation, the induction of DNA synthesis, and superoxide production. Cyclin D1 abundance is rate-limiting in normal G(1) phase progression, and the abundance of cyclin D1 is induced by activating mutations of both Ras and Rac1. Nuclear factor-kappaB (NF-kappaB) proteins consist of cytoplasmic hetero- or homodimeric Rel-related proteins complexed to a member of the IkappaB family of inhibitor proteins. In the current studies, activating mutants of Rac1 (Rac(Leu-61), Rac(Val-12)) induced cyclin D1 expression and the cyclin D1 promoter in NIH 3T3 cells. Induction of cyclin D1 by Rac1 required both an NF-kappaB and an ATF-2 binding site. Inhibiting NF-kappaB by overexpression of an NF-kappaB trans-dominant inhibitor (nonphosphorylatable IkappaBalpha) reduced cyclin D1 promoter activation by the Rac1 mutants, placing NF-kappaB in a pathway of Rac1 activation of cyclin D1. Specific amino acid mutations in the amino-terminal effector domain of Rac(Leu-61) had comparable effects on NF-kappaB transcriptional activity and activation of the cyclin D1 promoter. The NF-kappaB factors Rel A (p65) and NF-kappaB(1) (p50) induced the cyclin D1 promoter, requiring both the NF-kappaB binding site and the ATF-2 site. Stable overexpression of Rac(Leu-61) increased binding of Rel A and NF-kappaB(1) to the cyclin D1 promoter NF-kappaB site. Activation of Rac1 in NIH 3T3 cells induces both NF-kappaB binding and activity and enhances expression of cyclin D1 through an NF-kappaB and ATF-2 site in the proximal promoter, suggesting a critical role for NF-kappaB in cell cycle regulation through cyclin D1 and Rac1.

Relationship between Arp2/3 complex and the barbed ends of actin filaments at the leading edge of carcinoma cells after epidermal growth factor stimulation

Using both light and high resolution electron microscopy, we analyzed the spatial and temporal relationships between the Arp2/3 complex and the nucleation activity that is required for lamellipod extension in mammary carcinoma cells after epidermal growth factor stimulation. A rapid two- to fourfold increase in filament barbed end number occurs transiently after stimulation and remains confined almost exclusively to the extreme outer edge of the extending lamellipod (within 100-200 nm of the plasma membrane). This is accompanied by an increase in filament density at the leading edge and a general decrease in filament length, with a specific loss of long filaments. Concomitantly, the Arp2/3 complex is recruited with a 1.5-fold increase throughout the entire cortical filament network extending 1-1.5 microm in depth from the membrane at the leading edge. The recruitment of the Arp2/3 complex at the membrane of the extending lamellipod indicates that Arp2/3 may be involved in initial generation of growing filaments. However, only a small subset of the complex present in the cortical network colocalizes near free barbed ends. This suggests that the 100-200-nm submembraneous compartment at the leading edge of the extending lamellipod constitutes a special biochemical microenvironment that favors the generation and maintenance of free barbed ends, possibly through the locally active Arp2/3 complex, severing or decreasing the on-rate of capping protein. Our results are inconsistent with the hypothesis suggesting uncapping is the dominant mechanism responsible for the generation of nucleation activity. However, they support the hypothesis of an Arp2/3-mediated capture of actin oligomers that formed close to the membrane by other mechanisms such as severing. They also support pointed-end capping by the Arp2/3 complex, accounting for its wide distribution at the leading edge.

Cell polarization: chemotaxis gets CRACKing

An early stage in the establishment of cell polarity during chemotaxis of Dictyostelium dicoideum has been identified by a recent study; the new results also show that the development of cell polarity does not rely upon cytoskeletal rearrangement, and may use a spatial sensing mechanism.

Chemoattractant-induced lamellipod extension

The mammary adenocarcinoma cell line MTLn3 is chemotactic towards epidermal growth factor (EGF), and this induced motility is thought to promote breast cancer invasion and metastasis. Stimulation of MTLn3 cells with EGF results in the extension of a flat, thin structure filled with filamentous actin and termed a lamellipod. Lamellipod extension is dependent on actin polymerization and is localized to the border of adherent cells. The structure of EGF-stimulated lamellipods in MTLn3 cells is well suited to analysis of chemoattractant-stimulated protrusion. Actin polymerization occurs within 200 nm of the extending edge of the lamellipod. Although extension of the lamellipod is not dependent upon interaction with the substratum, stabilization of the extended lamellipod is dependent on an adhesive substratum. Dorsal ruffling is suppressed during lamellipod extension. Tyrosine phosphorylation is reduced in preexisting focal contacts compared to new contacts induced by EGF stimulation. The coordination of turnover of focal contacts with lamellipod extension is proposed to result in polarized cell motility in response to gradients of chemoattractants.

Suppression of ruffling by the EGF receptor in chemotactic cells

To clarify the relationship between ruffling and lamellipod extension in growth factor-stimulated chemotactic responses, we utilized cell lines derived from the rat 13762 NF mammary adenocarcinoma. Nonmetastatic MTC cells expressing the human EGF receptor (termed MTC HER cells) demonstrated chemotactic responses to TGF-alpha, an EGF receptor ligand typically present in mammary tissue. In microchemotaxis chambers, peak chemotactic responses occurred in response to 5 nM TGF-alpha. MTC HER cells showed dramatic ruffling edges in the absence of external stimuli, and addition of 5 nM TGF-alpha led to a transient reduction in ruffling concomitant with lamellipod extension. Lamellipod extension correlated with an overall increase in actin polymerization. These responses were blocked by the PI 3 kinase inhibitor wortmannin but not by the MAP kinase inhibitors PD98059 and SB203580. We conclude that the initial chemotactic response to TGF-alpha involves lamellipod extension and that ruffling reflects a dynamic turnover of lamellipodia that is arrested during lamellipod extension. By regulating the dissolution of ruffles and extension of lamellipods, a chemotactic response can be achieved, which may contribute to the metastatic process.

Regulation of protrusion shape and adhesion to the substratum during chemotactic responses of mammalian carcinoma cells

We report here the first direct observation of chemotaxis to EGF by rat mammary carcinoma cells. When exposed to a gradient of EGF diffusing from a micropipette, MTLn3 cells displayed typical ameboid chemotaxis, extending a lamellipod-like protrusion and moving toward the pipette. Using a homogeneous upshift in EGF to model stimulated lamellipod extension (J. E. Segall et al., 1996, Clin. Exp. Metastasis 14, 61-72), we analyzed the relationship between adhesion and chemoattractant-stimulated protrusion. Exposure to EGF led to a rapid remodeling of the adhesive contacts on adherent cells, in synchrony with extension of a flat lamellipod over the substratum. EGF-stimulated lamellipods still extended in the presence of adhesion-blocking peptides or over nonadhesive surfaces. They were, however, slightly shorter and retracted rapidly under those conditions. The major protrusive structure observed on well-spread, adherent cells, after EGF stimulation was a flat broad lamellipod, whether or not in contact with the substratum, while cells in suspension showed transient protrusive activity over the entire cell surface. We conclude that the initial adhesive status of the cell conditions the shape of the outcoming protrusion. Altogether our results suggest that, although adhesive contacts are not necessary for lamellipod extension, they play a role in stabilizing the protrusion as well as in the control of its final shape and amplitude.

Cell motility of tumor cells visualized in living intact primary tumors using green fluorescent protein

Metastasis is the leading cause of death in cancer patients. Cell motility is believed to be a necessary step in the metastatic process (L. Liotta and W. G. Stetler-Stevenson, In: Cancer: Principles and Practice of Oncology, pp. 134-149, 1993). Currently, most methods available to study the behavior of metastatic tumor cells are indirect, e.g., cell motility is examined in vitro and the results are correlated with metastatic capability (A. W. Partin, et al., Cancer Treat. Res., 59: 121-130, 1992). We have developed a model that directly examines the motility of metastatic primary tumor cells in situ. A metastatic rat breast cancer cell line was established that constitutively expresses green fluorescent protein. Upon s.c. injection of these cells into the mammary fat pad of female Fischer 344 rats, primary and metastatic tumors form that fluoresce when they are excited with FITC-filtered light. Animations of metastatic tumor cells moving in live rats were generated by intravital imaging of the primary tumor in situ on a laser scanning confocal microscope. With this model, the behavioral phenotype of metastatic and nonmetastatic tumor cells can be described and determined. This information will allow the effects of genetic manipulations or therapeutic treatments on this phenotype to be determined (D. R. Soll, Int. Rev. Cytol., 163: 43-104, 1995). This is the first time that living primary tumor cells in a live animal have been visualized as part of a clinically relevant model.

The Dictyostelium MAP kinase DdERK2 functions as a cytosolic protein in complexes with its potential substrates in chemotactic signal transduction

A polyclonal antibody against a MAP kinase (DdERK2) in Dictyostelium has been made and used to study DdERK2 activation and localization. The activation of DdERK2 by the chemoattractants cAMP and folate is rapid and transient. Its activity peaks between 15 and 60 seconds after cAMP stimulation and declines to basal levels after 5 minutes. In parallel with the DdERK2 activation is the appearance of a higher mobility band on Western blots. An antibody specific for activated MAP kinase shows that only the shifted band is tyrosine phosphorylated, suggesting that it is the active form. Both unstimulated and stimulated DdERK2 are soluble. In vitro phosphorylation with cell lysate supernatants or immunoprecipitates demonstrates the presence of several potential substrates, as identified by SDS-PAGE with mobility corresponding to molecular weights of 150, 25, and 19 kDa. Furthermore, immunoprecipitation studies suggest that these substrates are in a complex with DdERK2. These data suggest that DdERK2 works via cytoplasmic proteins to mediate signaling responses in Dictyostelium.

MAP kinase function in amoeboid chemotaxis

Mutants lacking the MAP kinase DdERK2 show reduced chemotactic responses to folate and cAMP. Analysis of cAMP chemotaxis shows that Dderk2- cells are defective in chemotaxis to high concentrations of cAMP. This defect is due to an inability to repolarize in the continued presence of high concentrations of cAMP. Under these conditions, the speed of movement of mutant cells remains low. Instead of generating a leading pseudopod, mutant cells generate transient crown-like structures over multiple regions of the cell surface. These structures differ from pseudopods in that they contain myosin II as well as F actin and coronin. These studies identify a role for MAP kinases in coordinating the formation of cell projections generated in response to chemoattractants.

EGF stimulates an increase in actin nucleation and filament number at the leading edge of the lamellipod in mammary adenocarcinoma cells

Stimulation of metastatic MTLn3 cells with EGF causes the rapid extension of lamellipods, which contain a zone of F-actin at the leading edge. In order to establish the mechanism for accumulation of F-actin at the leading edge and its relationship to lamellipod extension in response to EGF, we have studied the kinetics and location of EGF-induced actin nucleation activity in MTLn3 cells and characterized the actin dynamics at the leading edge by measuring the changes at the pointed and barbed ends of actin filaments upon EGF stimulation of MTLn3 cells. The major result of this study is that stimulation of MTLn3 cells with EGF causes a transient increase in actin nucleation activity resulting from the appearance of free barbed ends very close to the leading edge of extending lamellipods. In addition, cytochalasin D causes a significant decrease in the total F-actin content in EGF-stimulated cells, indicating that both actin polymerization and depolymerization are stimulated by EGF. Pointed end incorporation of rhodamine-labeled actin by the EGF stimulated cells is 2.12+/−0.47 times higher than that of control cells. Since EGF stimulation causes an increase in both barbed and pointed end incorporation of rhodamine-labeled actin in the same location, the EGF-stimulated nucleation sites are more likely due either to severing of pre-existing filaments or de novo nucleation of filaments at the leading edge thereby creating new barbed and pointed ends. The timing and location of EGF-induced actin nucleation activity in MTLn3 cells can account for the observed accumulation of F-actin at the leading edge and demonstrate that this F-actin rich zone is the primary actin polymerization zone after stimulation.

Functional characterization of the Cdc42p binding domain of yeast Ste20p protein kinase

Ste20p from Saccharomyces cerevisiae belongs to the Ste20p/p65PAK family of protein kinases which are highly conserved from yeast to man and regulate conserved mitogen-activated protein kinase pathways. Ste20p fulfills multiple roles in pheromone signaling, morphological switching and vegetative growth and binds Cdc42p, a Rho-like small GTP binding protein required for polarized morphogenesis. We have analyzed the functional consequences of mutations that prevent binding of Cdc42p to Ste20p. The complete amino-terminal, non-catalytic half of Ste20p, including the conserved Cdc42p binding domain, was dispensable for heterotrimeric G-protein-mediated pheromone signaling. However, the Cdc42p binding domain was necessary for filamentous growth in response to nitrogen starvation and for an essential function that Ste20p shares with its isoform Cla4p during vegetative growth. Moreover, the Cdc42p binding domain was required for cell-cell adhesion during conjugation. Subcellular localization of wild-type and mutant Ste20p fused to green fluorescent protein showed that the Cdc42p binding domain is needed to direct localization of Ste20p to regions of polarized growth. These results suggest that Ste20p is regulated in different developmental pathways by different mechanisms which involve heterotrimeric and small GTP binding proteins.

Dual role of cAMP and involvement of both G-proteins and ras in regulation of ERK2 in Dictyostelium discoideum

Dictyostelium discoideum expresses two Extracellular signal Regulated Kinases, ERK1 and ERK2, which are involved in growth, multicellular development and regulation of adenylyl cyclase. Binding of extracellular cAMP to cAMP receptor 1, a G-protein coupled cell surface receptor, transiently stimulates phosphorylation, activation and nuclear translocation of ERK2. Activation of ERK2 by cAMP is dependent on heterotrimeric G-proteins, since activation of ERK2 is absent in cells lacking the Galpha4 subunit. The small G-protein rasD also activates ERK2. In cells overexpressing a mutated, constitutively active rasD, ERK2 activity is elevated prior to cAMP stimulation. Intracellular cAMP and cAMP-dependent protein kinase (PKA) are essential for adaptation of the ERK2 response. This report shows that multiple signalling pathways are involved in regulation of ERK2 activity in D.discoideum.

The Dictyostelium MAP kinase ERK2 regulates multiple, independent developmental pathways

We showed previously that the MAP kinase ERK2 is essential for aggregation. erk2 null cells lack cAMP stimulation of adenylyl cyclase and thus cannot relay the cAMP chemotactic signal, although the cells chemotax to cAMP (Segall et al. 1995). In this paper we have examined the role of ERK2 in controlling developmental gene expression and morphogenesis during the multicellular stages, making use of a temperature-sensitive ERK2 mutation. Using suspension assays, we show that ERK2 is not essential for aggregation-stage, cAMP pulse-induced gene expression, or for the expression of postaggregative genes, which are induced at the onset of mound formation in response to cAMP in wild-type cells. In contrast, the prespore-specific gene SP60 is not induced and the prestalk-specific gene ecmA is induced but at a significantly reduced level. Chimeric organisms, comprised of wild-type and erk2 null cells expressing the prestalk-specific ecmA/lacZ reporter, show an abnormal spatial patterning, in which Erk2ts/erk2 cells are excluded from the very anterior prestalk A region. To further examine the function of ERK2 during the multicellular stages, we bypassed the requirement of ERK2 for aggregation by creating an ERK2 temperature-sensitive mutant. erk2 null cells expressing the ERK2ts mutant develop normally at 20 degrees C and express cell-type-specific genes but do not aggregate at temperatures above 25 degrees C. Using temperature shift experiments, we showed that ERK2 is essential for proper morphogenesis and for the induction and maintenance of prespore but not prestalk gene expression. Our results indicate that ERK2 functions at independent stages during Dictyostelium development to control distinct developmental programs: during aggregation, ERK2 is required for the activation of adenylyl cyclase and during multicellular development, ERK2 is essential for morphogenesis and cell-type-specific gene expression. Analysis of these results and other supports the conclusion that the requirement of ERK2 for cell-type differentiation is independent of its role in the activation of adenylyl cyclase.

EGF stimulates lamellipod extension in metastatic mammary adenocarcinoma cells by an actin-dependent mechanism

Changes in lamellipod extension and chemotaxis in response to EGF were analysed for MTLn3 cells (a metastatic cell line derived from the 13762NF rat mammary adenocarcinoma). Addition of EGF produced a cessation of ruffling followed by extension of hyaline lamellipods containing increased amounts of F-actin at the growing edge. A non-metastatic cell line (MTC) derived from the same tumor did not show such responses. Lamellipod extension was maximal within 5 min, followed by retraction and resumption of ruffling. Maximal area increases due to lamellipod extension occurred at about 5 nM EGF. Chemotactic and chemokinetic responses, measured using a microchemotaxis chamber, were also greatest at 5 nM. Cytochalasin D inhibited EGF-stimulated responses including lamellipod extension, increases in F-actin in lamellipods, and chemotaxis. Nocodazole affected chemotaxis at higher concentrations but not EGF-induced lamellipod extension. We conclude that polymerization of F-actin at the leading edges of lamellipods is necessary for extension of lamellipods and chemotaxis of MTLn3 cells in response to EGF. The motility and chemotaxis responses of this metastatic cell line have strong similarities to those seen in well-characterized chemotactic cells such as Dictyostelium and neutrophils.

A MAP kinase necessary for receptor-mediated activation of adenylyl cyclase in Dictyostelium

Analysis of a developmental mutant in Dictyostelium discoideum which is unable to initiate morphogenesis has shown that a protein kinase of the MAP kinase/ERK family affects relay of the cAMP chemotactic signal and cell differentiation. Strains in which the locus encoding ERK2 is disrupted respond to a pulse of cAMP by synthesizing cGMP normally but show little synthesis of cAMP. Since mutant cells lacking ERK2 contain normal levels of both the cytosolic regulator of adenylyl cyclase (CRAC) and manganese-activatable adenylyl cyclase, it appears that this kinase is important for receptor-mediated activation of adenylyl cyclase.

Polarization of yeast cells in spatial gradients of alpha mating factor

The process of cell fusion during mating of the yeast Saccharomyces cerevisiae is mediated by factors secreted by the mating partners. Spatial gradients of one of these mating factors, alpha-factor, polarized the growth of projections by MATa cells. The site of previous budding did not affect the direction of polarization, and subsequent budding was also polarized if mating factor was removed. Orientation occurred in the presence of nocodazole, suggesting that microtubules were not critical. At extremely low concentrations of alpha-factor, sst2-mutants (which in genetic studies do not discriminate between partners producing different amounts of alpha-factor) were able to polarize their projections. The sensitivity of this spatial sensing mechanism in wild-type cells is such that differences in receptor occupancy estimated to be about 1% are sufficient for orientation.

Behavioral responses of streamer F mutants of Dictyostelium discoideum: effects of cyclic GMP on cell motility

Streamer F (stmF) mutants have a prolonged increase in intracellular cGMP in response to addition of the chemoattractant cAMP. The speed of movement and area of stmF cells were quantitated as the cells were stimulated with a rapid, uniform increase in extracellular cAMP. The speed of stmF cells rapidly drops as does that of the wild-type, but then requires about 300 seconds to recover. In contrast, the speed of the parental strain, XP55, recovers within 60–70 seconds. This prolonged drop in speed correlates with the time during which intracellular cGMP remains high, suggesting that intracellular cGMP induces this prolonged reduction in speed. Mutants from other streamer complementation groups do not show this altered response. Area measurements indicate that stmF cells do not cringe or round up as XP55 does, but spread with the same kinetics as XP55. Chemotactic orientation of the stmF cells in stable spatial gradients is similar to or slightly greater than that of the wild-type. Tracking of cells moving during aggregation indicates that the stmF cells show large drops in speed between pulses, resulting in the banding pattern seen in streams. The cells can still respond to new pulses, resulting in an aggregation time that is similar to that of XP55.

Chemotaxis of metastatic tumor cells: clues to mechanisms from the Dictyostelium paradigm

Amoeboid movement, and in some cases, amoeboid chemotaxis, is a key step in tumor metastasis. The high degree of conservation in signal transduction pathways and motile machinery in eukaryotic cells suggests that insights and molecular probes developed from the study of these processes in easily manipulated experimental model systems will be applicable directly to experimentally intractable tumor cells. One such model system, Dictyostelium discoideum, is discussed in terms of the molecular events involved in amoeboid chemotaxis. The application of insights and assays developed with Dictyostelium to early events in the chemotaxis of Lewis lung carcinoma cells is reviewed.

cDNA sequence of cyclophilin from Dictyostelium discoideum

A cDNA encoding a protein homologous to cyclophilins from other species has been isolated from a Dictyostelium discoideum cDNA library. From the deduced amino acid sequence a protein with a molecular mass of 19 kD and 64% identity with human cyclophilin is predicted. Southern blot analysis indicates that there is one cyclophilin gene in the D. discoideum genome. The mRNA is present in all developmental stages.

A Dictyostelium mutant lacking an F-actin cross-linking protein, the 120-kD gelation factor

Actin-binding proteins are known to regulate in vitro the assembly of actin into supramolecular structures, but evidence for their activities in living nonmuscle cells is scarce. Amebae of Dictyostelium discoideum are nonmuscle cells in which mutants defective in several actin-binding proteins have been described. Here we characterize a mutant deficient in the 120-kD gelation factor, one of the most abundant F-actin cross-linking proteins of D. discoideum cells. No F-actin cross-linking activity attributable to the 120-kD protein was detected in mutant cell extracts, and antibodies recognizing different epitopes on the polypeptide showed the entire protein was lacking. Under the conditions used, elimination of the gelation factor did not substantially alter growth, shape, motility, or chemotactic orientation of the cells towards a cAMP source. Aggregates of the mutant developed into fruiting bodies consisting of normally differentiated spores and stalk cells. In cytoskeleton preparations a dense network of actin filaments as typical of the cell cortex, and bundles as they extend along the axis of filopods, were recognized. A significant alteration found was an enhanced accumulation of actin in cytoskeletons of the mutant when cells were stimulated with cyclic AMP. Our results indicate that control of cell shape and motility does not require the fine-tuned interactions of all proteins that have been identified as actin-binding proteins by in vitro assays.

A Dictyostelium mutant deficient in severin, an F-actin fragmenting protein, shows normal motility and chemotaxis

A severin deficient mutant of Dictyostelium discoideum has been isolated by the use of colony immunoblotting after chemical mutagenesis. In homogenates of wild-type cells, severin is easily detected as a very active F-actin fragmenting protein. Tests for severin in the mutant, HG1132, included viscometry for the assay of F-actin fragmentation in fractions from DEAE-cellulose columns, labeling of blots with monoclonal and polyclonal antibodies, and immunofluorescent-labeling of cryosections. Severin could not be detected in the mutant using these methods. The mutation in HG1132 is recessive and has been mapped to linkage group VII. The mutant failed to produce the normal severin mRNA, but small amounts of a transcript that was approximately 100 bases larger than the wild-type mRNA were detected in the mutant throughout all stages of development. On the DNA level a new Mbo II restriction site was found in the mutant within the coding region of the severin gene. The severin deficient mutant cells grew at an approximately normal rate, aggregated and formed fruiting bodies with viable spores. By the use of an image processing system, speed of cell movement, turning rates, and precision of chemotactic orientation in a stable gradient of cyclic AMP were quantitated, and no significant differences between wild-type and mutant cells were found. Thus, under the culture conditions used, severin proved to be neither essential for growth of D. discoideum nor for any cell function that is important for aggregation or later development.

Analysis of a Dictyostelium chemotaxis mutant with altered chemoattractant binding

A Dictyostelium discoideum mutant defective in folate chemotaxis has been analysed using biochemical, behavioural, and genetic methods. A subset of the cell-surface folate binding sites appeared to be locked in a high-affinity state from which folate dissociated extremely slowly. Changes in cell area and motility induced by step increases in folate required 10- to 100-fold higher concentrations than in the wild type. Folate-stimulated cyclic GMP production was also altered. Chemotactic responses to cyclic AMP as well as cyclic AMP-stimulated cyclic GMP production were normal. The mutation responsible for the chemotaxis defect, termed folA1000, was localized to linkage group IV. The alterations in folate binding and sensitivity to folate co-localized with the folA1000 mutation. We conclude that the folA1000 mutation arrests the folate chemotaxis receptor in a high affinity state that can only poorly transduce folate binding into chemotactic responses.

Quantification of motility and area changes of Dictyostelium discoideum amoebae in response to chemoattractants

This report presents quantitative measurements of cell area and motility on the time scale of seconds. The response of Dictyostelium discoideum amoebae to step changes in chemoattractant concentration were followed using an image-processing system. Parameters reflecting total area and motility of several hundred to thousand cells were measured with a time resolution of 2.5 s. Responses of growth phase cells to folate and of starved cells to cAMP were similar. An increase in chemoattractant concentration produced a brief increase in motility followed by a longer-lasting decrease that returned to initial values in 90 s. At high cAMP concentrations the motility remained depressed. Area also increased transiently. Half-maximal responses were produced by 2 nM folate or 2 nM cAMP. Removal of chemoattractant produced a temporary increase in motility and decrease in area. These responses support a model in which antagonistic signals are used to orient cell movement.

Selection of chemotaxis mutants of Dictyostelium discoideum

A method has been developed for the efficient selection of chemotaxis mutants of Dictyostelium discoideum. Mutants defective in the chemotactic response to folate could be enriched up to 30-fold in one round of selection using a chamber in which a compartment that contained the chemoattractant was separated by a sandwich of four nitrocellulose filters from a compartment that contained buffer. Mutagenized cells were placed in the center of the filter layer and exposed to the attractant gradient built up between the compartments for a period of 3-4 h. While wild-type cells moved through the filters in a wave towards the compartment that contained attractant, mutant cells remained in the filter to which they were applied. After several repetitions of the selection procedure, mutants defective in chemotaxis made up 10% of the total cell population retained in that filter. Mutants exhibiting three types of alterations were collected: motility mutants with either reduced speed of movement, or altered rates of turning; a single mutant defective in production of the attractant-degrading enzyme, folate deaminase; and mutants with normal motility but reduced chemotactic responsiveness. One mutant showed drastically reduced sensitivity in folate-induced cGMP production. Morphogenetic alterations of mutants defective in folate chemotaxis are described.

Temporal comparisons in bacterial chemotaxis

Responses of tethered cells of Escherichia coli to impulse, step, exponential-ramp or exponentiated sine-wave stimuli are internally consistent, provided that allowance is made for the nonlinear effect of thresholds. This result confirms that wild-type cells exposed to stimuli in the physiological range make short-term temporal comparisons extending 4 sec into the past: the past second is given a positive weighting, the previous 3 sec are given a negative weighting, and the cells respond to the difference. cheRcheB mutants (defective in methylation and demethylation) weight the past second in a manner similar to the wild type, but they do not make short-term temporal comparisons. When exposed to small steps delivered iontophoretically, they fail to adapt over periods of up to 12 sec; when exposed to longer steps in a flow cell, they partially adapt, but with a decay time of greater than 30 sec. cheZ mutants use a weighting that extends at least 40 sec into the past. The gain of the chemotactic system is large: the change in occupancy of one receptor molecule produces a significant response.

Chemotactic signaling in filamentous cells of Escherichia coli

Video techniques were used to record chemotactic responses of filamentous cells of Escherichia coli stimulated iontophoretically with aspartate. Long, nonseptate cells were produced from polyhook strains either by introducing a cell division mutation or by growth in the presence of cephalexin. Markers indicating rotation of flagellar motors were attached with anti-hook antibodies. Aspartate was applied by iontophoretic ejection from a micropipette, and the effects on the direction of rotation of the markers were measured. Motors near the pipette responded, whereas those sufficiently far away did not, even when the pipette was near the cell surface. The response of a given motor decreased as the pipette was moved away, but it did so less steeply when the pipette remained near the cell surface than when it was moved out into the external medium. This shows that there is an internal signal, but its range is short, only a few micrometers. These experiments rule out signaling by changes in membrane potential, by simple release or binding of a small molecule, or by diffusion of the receptor-attractant complex. A likely candidate for the signal is a protein or ligand that is activated by the receptor and inactivated as it diffuses through the cytoplasm. The range of the signal was found to be substantially longer in a cheZ mutant, suggesting that the product of the cheZ gene contributes to this inactivation.

Coordination of flagella on filamentous cells of Escherichia coli

Video techniques were used to study the coordination of different flagella on single filamentous cells of Escherichia coli. Filamentous, nonseptate cells were produced by introducing a cell division mutation into a strain that was polyhook but otherwise wild type for chemotaxis. Markers for its flagellar motors (ordinary polyhook cells that had been fixed with glutaraldehyde) were attached with antihook antibodies. The markers were driven alternately clockwise and counterclockwise, at angular velocities comparable to those observed when wild-type cells are tethered to glass. The directions of rotation of different markers on the same cell were not correlated; reversals of the flagellar motors occurred asynchronously. The bias of the motors (the fraction of time spent spinning counterclockwise) changed with time. Variations in bias were correlated, provided that the motors were within a few micrometers of one another. Thus, although the directions of rotation of flagellar motors are not controlled by a common intracellular signal, their biases are. This signal appears to have a limited range.

Adaptation kinetics in bacterial chemotaxis

Cells of Escherichia coli, tethered to glass by a single flagellum, were subjected to constant flow of a medium containing the attractant alpha-methyl-DL-aspartate. The concentration of this chemical was varied with a programmable mixing apparatus over a range spanning the dissociation constant of the chemoreceptor at rates comparable to those experienced by cells swimming in spatial gradients. When an exponentially increasing ramp was turned on (a ramp that increases the chemoreceptor occupancy linearly), the rotational bias of the cells (the fraction of time spent spinning counterclockwise) changed rapidly to a higher stable level, which persisted for the duration of the ramp. The change in bias increased with ramp rate, i.e., with the time rate of change of chemoreceptor occupancy. This behavior can be accounted for by a model for adaptation involving proportional control, in which the flagellar motors respond to an error signal proportional to the difference between the current occupancy and the occupancy averaged over the recent past. Distributions of clockwise and counterclockwise rotation intervals were found to be exponential. This result cannot be explained by a response regular model in which transitions between rotational states are generated by threshold crossings of a regular subject to statistical fluctuation; this mechanism generates distributions with far too many long events. However, the data can be fit by a model in which transitions between rotational states are governed by first-order rate constants. The error signal acts as a bias regulator, controlling the values of these constants.

Impulse responses in bacterial chemotaxis

The chemotactic behavior of Escherichia coli has been studied by exposing cells tethered by a single flagellum to pulses of chemicals delivered iontophoretically. Normally, wild-type cells spin alternately clockwise and counterclockwise, changing their direction on the average approximately once per second. When cells were exposed to a very brief diffusive wave of attractant, the probability of spinning counterclockwise quickly peaked, then fell below the prestimulus value, returning to baseline within a few seconds; repellent responses were similar but inverted. The width of the response indicates that cells integrate sensory inputs over a period of seconds, while the biphasic character implies that they also take time derivatives of these inputs. The sensory system is maximally tuned to concentration changes that occur over a span of approximately 2 sec, an interval over which changes normally occur when cells swim in spatial gradients; it is optimized to extract information from signals subject to statistical fluctuation. Impulse responses of cells defective in methylation were similar to those of wild-type cells, but did not fall as far below the baseline, indicating a partial defect in adaptation. Impulse responses of cheZ mutants were aberrant, indicating a serious defect in excitation.