Leonard Bosgraaf

A novel cGMP signalling pathway mediating myosin phosphorylation and chemotaxis in Dictyostelium

Chemotactic stimulation of Dictyostelium cells results in a transient increase in cGMP levels, and transient phosphorylation of myosin II heavy and regulatory light chains. In Dictyostelium, two guanylyl cyclases and four candidate cGMP‐binding proteins (GbpA–GbpD) are implicated in cGMP signalling. GbpA and GbpB are homologous proteins with a Zn2+‐hydrolase domain. A double gbpA/gbpB gene disruption leads to a reduction of cGMP‐phosphodiesterase activity and a 10‐fold increase of basal and stimulated cGMP levels. Chemotaxis in gbpA−B− cells is associated with increased myosin II phosphorylation compared with wild‐type cells; formation of lateral pseudopodia is suppressed resulting in enhanced chemotaxis. GbpC is homologous to GbpD, and contains Ras, MAPKKK and Ras‐GEF domains. Inactivation of the gbp genes indicates that only GbpC harbours high affinity cGMP‐binding activity. Myosin phosphorylation, assembly of myosin in the cytoskeleton as well as chemotaxis are severely impaired in mutants lacking GbpC and GbpD, or mutants lacking both guanylyl cyclases. Thus, a novel cGMP signalling cascade is critical for chemotaxis in Dictyostelium, and plays a major role in myosin II regulation during this process.

A new set of small, extrachromosomal expression vectors for Dictyostelium discoideum

A new set of extrachromosomal Dictyostelium expression vectors is presented that can be modified according to the experimental needs with minimal cloning efforts. To achieve this, the vector consists of four functional regions that are separated by unique restriction sites, (1) an Escherichia coli replication region, and regions for (2) replication, (3) selection and (4) protein expression in Dictyostelium. Each region was trimmed down to its smallest possible size. A basic expression vector can be constructed from these modules with a size of only 6.8 kb. By exchanging modules, a large number of vectors with different properties can be constructed. The resulting set of vectors allows most basic expression needs, such as immuno blotting, protein purification, visualization of protein localization and identification of protein-protein interactions. In addition, two genes can be simultaneously expressed on one vector, which yields far more synchronous levels of expression than when expressing two genes on separate plasmids.

The Ordered Extension of Pseudopodia by Amoeboid Cells in the Absence of External Cues

Eukaryotic cells extend pseudopodia for movement. In the absence of external cues, cells move in random directions, but with a strong element of persistence that keeps them moving in the same direction Persistence allows cells to disperse over larger areas and is instrumental to enter new environments where spatial cues can lead the cell. Here we explore cell movement by analyzing the direction, size and timing of ∼2000 pseudopodia that are extended by Dictyostelium cells. The results show that pseudpopod are extended perpendicular to the surface curvature at the place where they emerge. The location of new pseudopods is not random but highly ordered. Two types of pseudopodia may be formed: frequent splitting of an existing pseudopod, or the occasional extension of a de novo pseudopod at regions devoid of recent pseudopod activity. Split-pseudopodia are extended at ∼60 degrees relative to the previous pseudopod, mostly as alternating Right/Left/Right steps leading to relatively straight zigzag runs. De novo pseudopodia are extended in nearly random directions thereby interrupting the zigzag runs. Persistence of cell movement is based on the ratio of split versus de novo pseudopodia. We identify PLA2 and cGMP signaling pathways that modulate this ratio of splitting and de novo pseudopodia, and thereby regulate the dispersal of cells. The observed ordered extension of pseudopodia in the absence of external cues provides a fundamental insight into the coordinated movement of cells, and might form the basis for movement that is directed by internal or external cues.

Chemotaxis: signalling modules join hands at front and tail

Chemotaxis is the result of a refined interplay among various intracellular molecules that process spatial and temporal information. Here we present a modular scheme of the complex interactions between the front and the back of cells that allows them to navigate. First, at the front of the cell, activated Rho‐type GTPases induce actin polymerization and pseudopod formation. Second, phosphatidylinositol‐3,4,5‐trisphosphate (PtdIns(3,4,5)P3) is produced in a patch at the leading edge, where it binds pleckstrin‐homology‐domain‐containing proteins, which enhance actin polymerization and translocation of the pseudopod. Third, in Dictyostelium amoebae, a cyclic‐GMP‐signalling cascade has been identified that regulates myosin filament formation in the posterior of the cell, thereby inhibiting the formation of lateral pseudopodia that could misdirect the cell.

Reduced Protein Diffusion Rate by Cytoskeleton in Vegetative and Polarized Dictyostelium Cells

Fluorescence recovery after photobleaching measurements with high spatial resolution are performed to elucidate the impact of the actin cytoskeleton on translational mobility of green fluorescent protein (GFP) in aqueous domains of Dictyostelium discoideum amoebae. In vegetative Dictyostelium cells, GFP molecules experience a 3.6-fold reduction of their translational mobility relative to dilute aqueous solutions. In disrupting the actin filamentous network using latrunculin-A, the intact actin cytoskeletal network is shown to contribute an effective viscosity of 1.36 cP, which accounts for 53% of the restrained molecular diffusion of GFP. The remaining 47% of hindered protein motions is ascribed to other mechanical barriers and the viscosity of the cell liquid. A direct correlation between the density of the actin network and its limiting action on protein diffusion is furthermore established from measurements under different osmotic conditions. In highly locomotive polarized cells, the obstructing effect of the actin filamentous network is seen to decline to 0.46 cP in the non-cortical regions of the cell. Our results indicate that the meshwork of actin filaments constitutes the primary mechanical barrier for protein diffusion and that any noticeable reorganization of the network is accompanied by altered intracellular protein mobility.

Identification of four candidate cGMP targets in Dictyostelium

In Dictyostelium, a transient increase in intracellular cGMP is important for cytoskeletal rearrangements during chemotaxis. There must be cGMP-binding proteins in Dictyostelium that regulate key cytoskeletal components after treatment with chemoattractants, but to date, no such proteins have been identified. Using a bioinformatics approach, we have found four candidate cGMP-binding proteins (GbpA–D). GbpA and -B have two tandem cGMP-binding sites downstream of a metallo β-lactamase domain, a superfamily that includes cAMP phosphodiesterases. GbpC contains the following nine domains (in order): leucine-rich repeats, Ras, MEK kinase, Ras guanine nucleotide exchange factor N-terminal (RasGEF-N), DEP, RasGEF, cGMP-binding, GRAM, and a second cGMP-binding domain. GbpD is related to GbpC, but is much shorter; it begins with the RasGEF-N domain, and lacks the DEP domain. Disruption of the gbpC gene results in loss of all high-affinity cGMP-binding activity present in the soluble cellular fraction. GbpC mRNA levels increase dramatically 8 h after starvation is initiated. GbpA, -B, and -D mRNA levels show less dramatic changes, with gbpA mRNA levels highest 4 h into starvation, gbpB mRNA levels highest in vegetative cells, and gbpD levels highest at 8 h. The identification of these genes is the first step in a molecular approach to studying downstream effects of cGMP signaling in Dictyostelium.

Navigation of chemotactic cells by parallel signaling to pseudopod persistence and orientation.

The mechanism of chemotaxis is one of the most interesting issues in modern cell biology. Recent work shows that shallow chemoattractant gradients do not induce the generation of pseudopods, as has been predicted in many models. This poses the question of how else cells can steer towards chemoattractants. Here we use a new computational algorithm to analyze the extension of pseudopods by Dictyostelium cells. We show that a shallow gradient of cAMP induces a small bias in the direction of pseudopod extension, without significantly affecting parameters such as pseudopod frequency or size. Persistent movement, caused by alternating left/right splitting of existing pseudopodia, amplifies the effects of this bias by up to 5-fold. Known players in chemotactic pathways play contrasting parts in this mechanism; PLA2 and cGMP signal to the cytoskeleton to regulate the splitting process, while PI 3-kinase and soluble guanylyl cyclase mediate the directional bias. The coordinated regulation of pseudopod generation, orientation and persistence by multiple signaling pathways allows eukaryotic cells to detect extremely shallow gradients.

RasGEF-containing proteins GbpC and GbpD have differential effects on cell polarity and chemotaxis in Dictyostelium

The regulation of cell polarity plays an important role in chemotaxis. Previously, two proteins termed GbpC and GbpD were identified in Dictyostelium, which contain RasGEF and cyclic nucleotide binding domains. Here we show that gbpC-null cells display strongly reduced chemotaxis, because they are unable to polarise effectively in a chemotactic gradient. However, gbpD-null mutants exhibit the opposite phenotype: cells display improved chemotaxis and appear hyperpolar, because cells make very few lateral pseudopodia, whereas the leading edge is continuously remodelled. Overexpression of GbpD protein results in severely reduced chemotaxis. Cells extend many bifurcated and lateral pseudopodia, resulting in the absence of a leading edge. Furthermore, cells are flat and adhesive owing to an increased number of substrate-attached pseudopodia. This GbpD phenotype is not dependent on intracellular cGMP or cAMP, like its mammalian homolog PDZ-GEF. Previously we showed that GbpC is a high-affinity cGMP-binding protein that acts via myosin II. We conclude that cGMP activates GbpC, mediating the chemoattractant-induced establishment of cell polarity through myosin. GbpD induces the formation of substrate-attached pseudopodia, resulting in increased attachment and suppression of polarity.

Paxillin is required for cell-substrate adhesion, cell sorting and slug migration during Dictyostelium development

Paxillin is a key regulatory component of focal adhesion sites, implicated in controlling cell-substrate interactions and cell movement. We analyse the function of aDictyostelium discoideumpaxillin homologue, PaxB, which contains four highly conserved LD and four LIM domains, but lacks two characteristic tyrosine residues, that form the core of vertebrate SH2-binding domains. PaxB is expressed during growth and all stages of development, but expression peaks during slug formation. Using apaxB-gfpknockin strain we show the existence of focal adhesions and characterise their dynamics. During multicellular development PaxB is not only found in focal adhesions at the cell-substrate interface, but also in the tips of filopodial structures predominantly located at the trailing ends of cells.paxB–strains are less adhesive to the substrate, they can aggregate but multicellular development from the mound stage onwards is severely impeded.paxB– strains are defective in proper cell type proportioning, cell sorting, slug migration and form-defective fruiting bodies. Mutation of a conserved JNK phosphorylation site, implicated in the control of cell migration, does not have any major effects on cell sorting, slug migration or morphogenesis inDictyostelium. PaxB does not appear to function redundantly with its closest relative Lim2 (paxA), which when deleted also results in a mound arrest phenotype. However, analysis ofpaxA–andpaxB–single and double null mutants suggest that PaxB may act upstream of Lim2.

Analysis of cell movement by simultaneous quantification of local membrane displacement and fluorescent intensities using Quimp2

The use of fluorescent markers in living cells has increased dramatically in the recent years. The quantitative analysis of the images requires specific analysis software. Previously, the program Quimp was launched for quantitating fluorescent intensities at the membrane or the cortex of the cell. However, Quimp is not well suited to quantitate local membrane displacement. Here we present Quimp2 that is capable of tracking membrane subregions in time, which enables the simultaneous quantification of fluorescent intensities and membrane movement. Quimp2 has two new tools, (i) conversion filters to analyze movies obtained with fluorescent, DIC and phase contrast different microscopes, and (ii) a macro that calculates the local membrane displacement and provides various options to display the results. Quimp2 is used here to investigate the molecular mechanism of cell movement by correlating the dynamics of local membrane movement with the local concentration of myosin and F-actin.