Measurement of chemotaxis in Dictyostelium

Actin-binding protein G (AbpG) participates in modulating the actin cytoskeleton and cell migration in Dictyostelium discoideum

Dictyostelium cells lacking actin-binding protein G (AbpG) migrate at a reduced speed and display elevated F-actin levels. AbpG is enriched in the cortical/lamellipodial regions and colocalizes with F-actin. A novel protein domain in AbpG mediates the interaction with F-actin and is required for the cellular function of AbpG.

Cell migration is involved in various physiological and pathogenic events, and the complex underlying molecular mechanisms have not been fully elucidated. The simple eukaryote Dictyostelium discoideum displays chemotactic locomotion in stages of its life cycle. By characterizing a Dictyostelium mutant defective in chemotactic responses, we identified a novel actin-binding protein serving to modulate cell migration and named it actin-binding protein G (AbpG); this 971–amino acid (aa) protein contains an N-terminal type 2 calponin homology (CH2) domain followed by two large coiled-coil regions. In chemoattractant gradients, abpG⁻ cells display normal directional persistence but migrate significantly more slowly than wild-type cells; expressing Flag-AbpG in mutant cells eliminates the motility defect. AbpG is enriched in cortical/lamellipodial regions and colocalizes well with F-actin; aa 401–600 and aa 501–550 fragments of AbpG show the same distribution as full-length AbpG. The aa 501–550 region of AbpG, which is essential for AbpG to localize to lamellipodia and to rescue the phenotype of abpG⁻ cells, is sufficient for binding to F-actin and represents a novel actin-binding protein domain. Compared with wild-type cells, abpG⁻ cells have significantly higher F-actin levels. Collectively our results suggest that AbpG may participate in modulating actin dynamics to optimize cell locomotion.

Single-molecule imaging technique to study the dynamic regulation of GPCR function at the plasma membrane

The lateral diffusion of a G-protein-coupled receptor (GPCR) in the plasma membrane determines its interaction capabilities with downstream signaling molecules and critically modulates its function. Mechanisms that control GPCR mobility, like compartmentalization, enable a cell to fine-tune its response through local changes in the rate, duration, and extent of signaling. These processes are known to be highly dynamic and tightly regulated in time and space, usually not completely synchronized in time. Therefore, bulk studies such as protein biochemistry or conventional confocal microscopy will only yield information on the average properties of the interactions and are compromised by poor time resolution. Single-particle tracking (SPT) in living cells is a key approach to directly monitor the function of a GPCR within its natural environment and to obtain unprecedented detailed information about receptor mobility, binding kinetics, aggregation states, and domain formation. This review provides a detailed description on how to perform single GPCR tracking experiments.

Disrupting microtubule network immobilizes amoeboid chemotactic receptor in the plasma membrane

Signaling cascades are initiated in the plasma membrane via activation of one molecule by another. The interaction depends on the mutual availability of the molecules to each other and this is determined by their localization and lateral diffusion in the cell membrane. The cytoskeleton plays a very important role in this process by enhancing or restricting the possibility of the signaling partners to meet in the plasma membrane. In this study we explored the mode of diffusion of the cAMP receptor, cAR1, in the plasma membrane of Dictyostelium discoideum cells and how this is regulated by the cytoskeleton. Single-particle tracking of fluorescently labeled cAR1 using Total Internal Reflection Microscopy showed that 70% of the cAR1 molecules were mobile. These receptors showed directed motion and we demonstrate that this is not because of tracking along the actin cytoskeleton. Instead, destabilization of the microtubules abolished cAR1 mobility in the plasma membrane and this was confirmed by Fluorescence Recovery after Photobleaching. As a result of microtubule stabilization, one of the first downstream signaling events, the jump of the PH domain of CRAC, was decreased. These results suggest a role for microtubules in cAR1 dynamics and in the ability of cAR1 molecules to interact with their signaling partners.

Costars, a Dictyostelium protein similar to the C-terminal domain of STARS, regulates the actin cytoskeleton and motility

Through analysis of a chemotaxis mutant obtained from a genetic screen in Dictyostelium discoideum, we have identified a new gene involved in regulating cell migration and have named it costars (cosA). The 82 amino acid Costars protein sequence appears highly conserved among diverse species, and significantly resembles the C-terminal region of the striated muscle activator of Rho signaling (STARS), a mammalian protein that regulates the serum response factor transcriptional activity through actin binding and Rho GTPase activation. The cosA-null (cosA(-)) cells formed smooth plaques on bacterial lawns, produced abnormally small fruiting bodies when developed on the non-nutrient agar and displayed reduced migration towards the cAMP source in chemotactic assays. Analysis of cell motion in cAMP gradients revealed decreased speed but wild-type-like directional persistence of cosA(-) cells, suggesting a defect in the cellular machinery for motility rather than for chemotactic orientation. Consistent with this notion, cosA(-) cells exhibited changes in the actin cytoskeleton, showing aberrant distribution of F-actin in fluorescence cell staining and an increased amount of cytoskeleton-associated actin. Excessive pseudopod formation was also noted in cosA(-) cells facing chemoattractant gradients. Expressing cosA or its human counterpart mCostars eliminated abnormalities of cosA(-) cells. Together, our results highlight a role for Costars in modulating actin dynamics and cell motility.

Assays for chemotaxis and chemoattractant-stimulated TorC2 activation and PKB substrate phosphorylation in Dictyostelium

Chemotaxis is a highly coordinated biological system where chemoattractants trigger multiple signal transduction pathways which act in concert to bring about directed migration. A signaling pathway acting through PIP(3), which accumulates at the leading edge of the cell, has been extensively characterized. However, chemotaxis still remains in cells depleted of PIP(3), suggesting there are PIP(3)-independent pathways. We have identified a pathway involving TorC2-PKBR1 as well as another containing PLA2 activity that act in parallel with PIP(3). Activation of PKBR1, a myristoylated Protein Kinase B homolog, is dependent on TorC2 (Rapamycin-insensitive Tor complex 2) kinase but is completely independent of PIP(3). In response to chemoattractant, PKBs rapidly phosphorylate at least eight proteins, including Talin B, PI4P 5-kinase, two RasGefs, and a RhoGap. These studies help to link the signaling pathways to specific effectors and provide a more complete understanding of chemotaxis.

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.

Single-molecule analysis of chemotactic signaling in Dictyostelium cells

Single-molecule imaging techniques were used to reveal the binding of individual cyclic adenosine 3',5'-monophosphate molecules to heterotrimeric guanine nucleotide-binding protein coupled receptors on the surface of living Dictyostelium discoideum cells. The binding sites were uniformly distributed and diffused rapidly in the plane of the membrane. The probabilities of individual association and dissociation events were greater for receptors at the anterior end of the cell. Agonist-induced receptor phosphorylation had little effect on any of the monitored properties, whereas G protein coupling influenced the binding kinetics. These observations illustrate the dynamic properties of receptors involved in gradient sensing and suggest that these may be polarized in chemotactic cells.

Chemotactic responses of Dictyostelium discoideum amoebae to a cyclic AMP concentration gradient: evidence to support a spatial mechanism for sensing cyclic AMP

The motile responses of Dictyostelium discoideum amoebae to a cyclic AMP (cAMP) concentration gradient were examined using a novel assay system. In this system, a cAMP concentration gradient was generated, while the overall cAMP concentration could be either increased or decreased in a chamber containing amoebae. The chemotactic responses of amoebae were examined immediately after they had been subjected to the cAMP concentration gradient. Amoebae moving in random directions in a reference solution ascended a cAMP concentration gradient after they had been exposed to the gradient irrespective of whether there was an increase or a decrease in the overall cAMP concentration. This strongly supports the idea that D. discoideum amoebae can sense a spatial cAMP gradient around them and that this causes their chemoaccumulation behavior. Ascending locomotion became less conspicuous when the amoebae were treated with a homogeneous cAMP solution for approximately 8 min before exposure to a cAMP gradient. This cAMP pretreatment reduced the sensitivity of the amoeba to a cAMP concentration gradient. The cAMP concentration gradient could be reversed in less than 30 s in this assay system, allowing the generation of a cAMP wave by accumulating amoebae to be mimicked. The ascending amoebae continued to move in the same direction for 1–2 min after the gradient had been reversed. This is consistent with the well-known observation that reversal of a cAMP concentration gradient experienced by the amoebae passing through a cAMP wave does not negate their chemotactic movement towards the accumulation center.

Selection of gbeta subunits with point mutations that fail to activate specific signaling pathways in vivo: dissecting cellular responses mediated by a heterotrimeric G protein in Dictyostelium discoideum

In Dictyostelium discoideum, a unique Gbeta subunit is required for a G protein-coupled receptor system that mediates a variety of cellular responses. Binding of cAMP to cAR1, the receptor linked to the G protein G2, triggers a cascade of responses, including activation of adenylyl cyclase, gene induction, actin polymerization, and chemotaxis. Null mutations of the cAR1, Galpha2, and Gbeta genes completely impair all these responses. To dissect specificity in Gbetagamma signaling to downstream effectors in living cells, we screened a randomly mutagenized library of Gbeta genes and isolated Gbeta alleles that lacked the capacity to activate some effectors but retained the ability to regulate others. These mutant Gbeta subunits were able to link cAR1 to G2, to support gene expression, and to mediate cAMP-induced actin polymerization, and some were able to mediate to chemotaxis toward cAMP. None was able to activate adenylyl cyclase, and some did not support chemotaxis. Thus, we separated in vivo functions of Gbetagamma by making point mutations on Gbeta. Using the structure of the heterotrimeric G protein displayed in the computer program CHAIN, we examined the positions and the molecular interactions of the amino acids substituted in each of the mutant Gbetas and analyzed the possible effects of each replacement. We identified several residues that are crucial for activation of the adenylyl cyclase. These residues formed an area that overlaps but is not identical to regions where bovine Gtbetagamma interacts with its regulators, Galpha and phosducin.

Dictyostelium TRFA homologous to yeast Ssn6 is required for normal growth and early development

The TPR (tetratricopeptide repeat) family became widespread during evolution, having been found from bacteria to mammals. By means of restriction enzyme-mediated integration, we have identified a Dictyostelium gene (trfA) highly homologous to a Saccharomyces cerevisiae gene encoding a TPR protein, Ssn6 (Cyc8), which functions as a global transcriptional repressor for diverse genes. The deduced amino acid sequence of the Dictyostelium gene product, TRFA, contains 10 consecutive TPR units as well as Gln repeats, Asn repeats, and a region rich in Glu, Lys, Ser, and Thr. The sequences of some of the 10 TPR units in TRFA are more than 70% identical to the corresponding units in Ssn6. The trfA- cells produced smooth plaques on a bacterial lawn and failed to aggregate normally when starved on a plain agar plate. Individual trfA- cells also failed to correctly respond to cAMP, although the adenylyl cyclase of trfA- cells was expressed upon starvation and activated by stimulation with cAMP as in the wild-type cells. When cultured in a rich medium in suspension, they grew more slowly and stopped growing at a lower density than the wild-type cells. Furthermore, they divided into cells of various sizes and tended to be much smaller than the wild-type cells. These pleiotropic defects of the trfA- cells suggest the possibility that Dictyostelium TRFA may regulate the transcription of diverse genes required for normal growth and early development.

Identification and analysis of a gene that is essential for morphogenesis and prespore cell differentiation in Dictyostelium

We have identified a gene (PslA) that is expressed throughout Dictyostelium development and encodes a novel protein that is required for proper aggregation and subsequent cell-type differentiation and morphogenesis. pslA null (pslA-) cells produce large aggregation streams under conditions in which wild-type cells form discrete aggregates. Tips form along the stream, elongate to produce a finger, and eventually form a terminal structure that lacks a true sorus (spore head). More than half of the cells remain as a mass at the base of the developing fingers. The primary defect in the pslA- strain is the inability to induce prespore cell differentiation. Analyses of gene expression show a complete lack of prespore-specific gene expression and no mature spores are produced. In chimeras with wild-type cells, pslA- cells form the prestalk domain and normal, properly proportioned fruiting bodies can be produced. This indicates that pslA- cells are able to interact with wild-type cells and regulate patterning, even though pslA- cells are unable to express prespore cell-type-specific genes, do not participate in prespore cell differentiation and do not produce pslA- spores in the chimeras. While pslA- cells produce mature, vacuolated stalk cells during multicellular development, pslA- cells are unable to do so in vitro in response to exogenous DIF (a morphogen required for prestalk and stalk cell differentiation). These results indicate that pslA- cells exhibit a defect in the prestalk/stalk cell pathways under these experimental conditions. Our results suggest that PslA's primary function is to regulate prespore cell determination very early in the prespore pathway via a cell-autonomous mechanism, possibly at the time of the initial prestalk/prespore cell-fate decision. Indirect immunofluorescence of myc-tagged PslA localizes the protein to the nucleus, suggesting that PslA may function to control the prespore pathway at the level of transcription.

Ribosome-inactivating proteins (RIPs) of wild Oregon cucumber (Marah oreganus)

Two type 1 RIPs, designated as MOR-I and MOR-II, have been isolated from Marah oreganus (manroot) seed extract. They are similar but not identical to trichosanthin, a type 1 RIP in the same family. MOR-I and MOR-II are monomeric proteins with molecular weights of 27989.0 and 27632.8 respectively and have pI values greater than 8.8. MOR-I and MOR-II inhibit cell-free protein synthesis with IC50s of 0.063 and 0.071 nM, respectively, and are relatively stable with respect to temperature and pH variations. They share a conserved N-terminal amino acid sequence (D-SF-LS) and cross-react with goat anti-trichosanthin polyclonal serum.

A novel cytosolic regulator, Pianissimo, is required for chemoattractant receptor and G protein-mediated activation of the 12 transmembrane domain adenylyl cyclase in Dictyostelium

Genetic analysis was applied to identify novel genes involved in G protein-linked pathways controlling development. Using restriction enzyme-mediated integration (REMI), we have identified a new gene, Pianissimo (PiaA), involved in cAMP signaling in Dictyostelium discoideum. PiaA encodes a 130-kD cytosolic protein required for chemoattractant receptor and G protein-mediated activation of the 12 transmembrane domain adenylyl cyclase. In piaA- null mutants, neither chemoattractant stimulation of intact cells nor GTPgammaS treatment of lysates activates the enzyme; constitutive expression of PiaA reverses these defects. Cytosols of wild-type cells that contain Pia protein reconstitute the GTPgammaS stimulation of adenylyl cyclase activity in piaA- lysates, indicating that Pia is directly involved in the activation. Pia and CRAC, a previously identified cytosolic regulator, are both essential for activation of the enzyme as lysates of crac- piaA- double mutants require both proteins for reconstitution. Homologs of PiaA are found in Saccharomyces cerevisiae and Schizosaccaromyces pombe; disruption of the S. cerevisiae homolog results in lethality. We propose that homologs of Pia and similar modes of regulation of these ubiquitous G protein-linked pathways are likely to exist in higher eukaryotes.

Centrosome positioning and directionality of cell movements

In several cell types, an intriguing correlation exists between the position of the centrosome and the direction of cell movement: the centrosome is located behind the leading edge, suggesting that it serves as a steering device for directional movement. A logical extension of this suggestion is that a change in the direction of cell movement is preceded by a reorientation, or shift, of the centrosome in the intended direction of movement. We have used a fusion protein of green fluorescent protein (GFP) and gamma-tubulin to label the centrosome in migrating amoebae of Dictyostelium discoideum, allowing us to determine the relationship of centrosome positioning and the direction of cell movement with high spatial and temporal resolution in living cells. We find that the extension of a new pseudopod in a migrating cell precedes centrosome repositioning. An average of 12 sec elapses between the initiation of pseudopod extension and reorientation of the centrosome. If no reorientation occurs within approximately 30 sec, the pseudopod is retracted. Thus the centrosome does not direct a cell's migration. However, its repositioning stabilizes a chosen direction of movement, most probably by means of the microtubule system.

Chemoattractant-elicited translocation of myosin in motile Dictyostelium

The distribution of myosin was studied in amebae of the Ax-3 and NC-4 strains of Dictyostelium migrating at room temperature, using indirect immunofluorescence of aggregation-competent amebae and the agar-overlay technique. Amebae were fixed in methanol-formaldehyde or absolute acetone at -15 degrees C before or after stimulation with micromolar cyclic AMP at room temperature (20-25 degrees C). Myosin was detected by monoclonal antibodies to Dictyostelium myosin heavy chain followed by a fluorescent secondary antibody that had been preabsorbed to remove nonspecific staining. In both strains there was a striking increase in intensity of anti-myosin immunofluorescence in the cortex where it appeared as a continuous ring 30 seconds after addition of cyclic AMP. This correlated with a rounding up of the cell body. Sixty seconds after stimulation there was a clear reduction of cytoplasmic myosin rods in conjunction with the increased cortical localization. At this time extensions of largely hyaline cytoplasm were observed that extended beyond the cortical shell of myosin. Two minutes after the stimulus the immunofluorescence remained as a distinct line at the cortex, but the cells began to resume in elongated shape. By 3 minutes (NC-4 strain) or 5 minutes (Ax-3 strain) the amebae had largely returned to the control shape, and myosin had returned to its control distribution. Counts of the treated cells at different time points substantiated the observations of individual cells. The time course of translocation of myosin in the Ax-3 strain parallels the time course of myosin phosphorylation reported in previous studies. The results are interpreted in terms of a working hypothesis for the mechanism of translocation.