The developmental regulation of single-cell motility in Dictyostelium discoideum

Heterogeneous individual motility biases group composition in a model of aggregating cells

Aggregative life cycles are characterized by alternating phases of unicellular growth and multicellular development. Their multiple, independent evolutionary emergence suggests that they may have coopted pervasive properties of single-celled ancestors. Primitive multicellular aggregates, where coordination mechanisms were less efficient than in extant aggregative microbes, must have faced high levels of conflict between different co-aggregating populations. Such conflicts within a multicellular body manifest in the differential reproductive output of cells of different types. Here, we study how heterogeneity in cell motility affects the aggregation process and creates a mismatch between the composition of the population and that of self-organized groups of active adhesive particles. We model cells as self-propelled particles and describe aggregation in a plane starting from a dispersed configuration. Inspired by the life cycle of aggregative model organisms such as Dictyostelium discoideum or Myxococcus xanthus, whose cells interact for a fixed duration before the onset of chimeric multicellular development, we study finite-time configurations for identical particles and in binary mixes. We show that co-aggregation results in three different types of frequency-dependent biases, one of which is associated to evolutionarily stable coexistence of particles with different motility. We propose a heuristic explanation of such observations, based on the competition between delayed aggregation of slower particles and detachment of faster particles. Unexpectedly, despite the complexity and non-linearity of the system, biases can be largely predicted from the behavior of the two corresponding homogenous populations. This model points to differential motility as a possibly important factor in driving the evolutionary emergence of facultatively multicellular life-cycles.

Dual regulation of the actin cytoskeleton by CARMIL-GAP

Capping protein Arp2/3 myosin I linker (CARMIL) proteins are multi-domain scaffold proteins that regulate actin dynamics by regulating the activity of capping protein (CP). Here, we characterize CARMIL-GAP (GAP for GTPase-activating protein), a Dictyostelium CARMIL isoform that contains a ∼130 residue insert that, by homology, confers GTPase-activating properties for Rho-related GTPases. Consistent with this idea, this GAP domain binds Dictyostelium Rac1a and accelerates its rate of GTP hydrolysis. CARMIL-GAP concentrates with F-actin in phagocytic cups and at the leading edge of chemotaxing cells, and CARMIL-GAP-null cells exhibit pronounced defects in phagocytosis and chemotactic streaming. Importantly, these defects are fully rescued by expressing GFP-tagged CARMIL-GAP in CARMIL-GAP-null cells. Finally, rescue with versions of CARMIL-GAP that lack either GAP activity or the ability to regulate CP show that, although both activities contribute significantly to CARMIL-GAP function, the GAP activity plays the bigger role. Together, our results add to the growing evidence that CARMIL proteins influence actin dynamics by regulating signaling molecules as well as CP, and that the continuous cycling of the nucleotide state of Rho GTPases is often required to drive Rho-dependent biological processes.

Summary:Dictyostelium CARMIL-GAP supports phagocytosis and chemotaxis by regulating both capping protein and Rac1.

Mutant resources for functional genomics in Dictyostelium discoideum using REMI-seq technology

Background

Genomes can be sequenced with relative ease, but ascribing gene function remains a major challenge. Genetically tractable model systems are crucial to meet this challenge. One powerful model is the social amoeba Dictyostelium discoideum, a eukaryotic microbe widely used to study diverse questions in the cell, developmental and evolutionary biology.

Results

We describe REMI-seq, an adaptation of Tn-seq, which allows high throughput, en masse, and quantitative identification of the genomic site of insertion of a drug resistance marker after restriction enzyme-mediated integration. We use REMI-seq to develop tools which greatly enhance the efficiency with which the sequence, transcriptome or proteome variation can be linked to phenotype in D. discoideum. These comprise (1) a near genome-wide resource of individual mutants and (2) a defined pool of ‘barcoded’ mutants to allow large-scale parallel phenotypic analyses. These resources are freely available and easily accessible through the REMI-seq website that also provides comprehensive guidance and pipelines for data analysis. We demonstrate that integrating these resources allows novel regulators of cell migration, phagocytosis and macropinocytosis to be rapidly identified.

Conclusions

We present methods and resources, generated using REMI-seq, for high throughput gene function analysis in a key model system.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01108-y.

Cell and molecular transitions during efficient dedifferentiation

Dedifferentiation is a critical response to tissue damage, yet is not well understood, even at a basic phenomenological level. Developing Dictyostelium cells undergo highly efficient dedifferentiation, completed by most cells within 24 hr. We use this rapid response to investigate the control features of dedifferentiation, combining single cell imaging with high temporal resolution transcriptomics. Gene expression during dedifferentiation was predominantly a simple reversal of developmental changes, with expression changes not following this pattern primarily associated with ribosome biogenesis. Mutation of genes induced early in dedifferentiation did not strongly perturb the reversal of development. This apparent robustness may arise from adaptability of cells: the relative temporal ordering of cell and molecular events was not absolute, suggesting cell programmes reach the same end using different mechanisms. In addition, although cells start from different fates, they rapidly converged on a single expression trajectory. These regulatory features may contribute to dedifferentiation responses during regeneration.

A telomerase with novel non-canonical roles: TERT controls cellular aggregation and tissue size in Dictyostelium

Telomerase, particularly its main subunit, the reverse transcriptase, TERT, prevents DNA erosion during eukaryotic chromosomal replication, but also has poorly understood non-canonical functions. Here, in the model social amoeba Dictyostelium discoideum, we show that the protein encoded by tert has telomerase-like motifs, and regulates, non-canonically, important developmental processes. Expression levels of wild-type (WT) tert were biphasic, peaking at 8 and 12 h post-starvation, aligning with developmental events, such as the initiation of streaming (~7 h) and mound formation (~10 h). In tert KO mutants, however, aggregation was delayed until 16 h. Large, irregular streams formed, then broke up, forming small mounds. The mound-size defect was not induced when a KO mutant of countin (a master size-regulating gene) was treated with TERT inhibitors, but anti-countin antibodies did rescue size in the tert KO. Although, conditioned medium (CM) from countin mutants failed to rescue size in the tert KO, tert KO CM rescued the countin KO phenotype. These and additional observations indicate that TERT acts upstream of smlA/countin: (i) the observed expression levels of smlA and countin, being respectively lower and higher (than WT) in the tert KO; (ii) the levels of known size-regulation intermediates, glucose (low) and adenosine (high), in the tert mutant, and the size defect's rescue by supplemented glucose or the adenosine-antagonist, caffeine; (iii) the induction of the size defect in the WT by tert KO CM and TERT inhibitors. The tert KO's other defects (delayed aggregation, irregular streaming) were associated with changes to cAMP-regulated processes (e.g. chemotaxis, cAMP pulsing) and their regulatory factors (e.g. cAMP; acaA, carA expression). Overexpression of WT tert in the tert KO rescued these defects (and size), and restored a single cAMP signaling centre. Our results indicate that TERT acts in novel, non-canonical and upstream ways, regulating key developmental events in Dictyostelium.

Adhesion strategies of Dictyostelium discoideum- a force spectroscopy study

Biological adhesion is essential for all motile cells and generally limits locomotion to suitably functionalized substrates displaying a compatible surface chemistry. However, organisms that face vastly varying environmental challenges require a different strategy. The model organism Dictyostelium discoideum (D.d.), a slime mould dwelling in the soil, faces the challenge of overcoming variable chemistry by employing the fundamental forces of colloid science. To understand the origin of D.d. adhesion, we realized and modified a variety of conditions for the amoeba comprising the absence and presence of the specific adhesion protein Substrate Adhesion A (sadA), glycolytic degradation, ionic strength, surface hydrophobicity and strength of van der Waals interactions by generating tailored model substrates. By employing AFM-based single cell force spectroscopy we could show that experimental force curves upon retraction exhibit two regimes. The first part up to the critical adhesion force can be described in terms of a continuum model, while the second regime of the curve beyond the critical adhesion force is governed by stochastic unbinding of individual binding partners and bond clusters. We found that D.d. relies on adhesive interactions based on EDL-DLVO (Electrical Double Layer-Derjaguin-Landau-Verwey-Overbeek) forces and contributions from the glycocalix and specialized adhesion molecules like sadA. This versatile mechanism allows the cells to adhere to a large variety of natural surfaces under various conditions.

Modeling random crawling, membrane deformation and intracellular polarity of motile amoeboid cells

Amoeboid movement is one of the most widespread forms of cell motility that plays a key role in numerous biological contexts. While many aspects of this process are well investigated, the large cell-to-cell variability in the motile characteristics of an otherwise uniform population remains an open question that was largely ignored by previous models. In this article, we present a mathematical model of amoeboid motility that combines noisy bistable kinetics with a dynamic phase field for the cell shape. To capture cell-to-cell variability, we introduce a single parameter for tuning the balance between polarity formation and intracellular noise. We compare numerical simulations of our model to experiments with the social amoeba Dictyostelium discoideum. Despite the simple structure of our model, we found close agreement with the experimental results for the center-of-mass motion as well as for the evolution of the cell shape and the overall intracellular patterns. We thus conjecture that the building blocks of our model capture essential features of amoeboid motility and may serve as a starting point for more detailed descriptions of cell motion in chemical gradients and confined environments.

V-1 regulates capping protein activity in vivo

Capping Protein (CP) plays a central role in the creation of the Arp2/3-generated branched actin networks comprising lamellipodia and pseudopodia by virtue of its ability to cap the actin filament barbed end, which promotes Arp2/3-dependent filament nucleation and optimal branching. The highly conserved protein V-1/Myotrophin binds CP tightly in vitro to render it incapable of binding the barbed end. Here we addressed the physiological significance of this CP antagonist in Dictyostelium, which expresses a V-1 homolog that we show is very similar biochemically to mouse V-1. Consistent with previous studies of CP knockdown, overexpression of V-1 in Dictyostelium reduced the size of pseudopodia and the cortical content of Arp2/3 and induced the formation of filopodia. Importantly, these effects scaled positively with the degree of V-1 overexpression and were not seen with a V-1 mutant that cannot bind CP. V-1 is present in molar excess over CP, suggesting that it suppresses CP activity in the cytoplasm at steady state. Consistently, cells devoid of V-1, like cells overexpressing CP described previously, exhibited a significant decrease in cellular F-actin content. Moreover, V-1-null cells exhibited pronounced defects in macropinocytosis and chemotactic aggregation that were rescued by V-1, but not by the V-1 mutant. Together, these observations demonstrate that V-1 exerts significant influence in vivo on major actin-based processes via its ability to sequester CP. Finally, we present evidence that V-1's ability to sequester CP is regulated by phosphorylation, suggesting that cells may manipulate the level of active CP to tune their "actin phenotype."

Quantitative Motion Analysis in Two and Three Dimensions

This chapter describes 2D quantitative methods for motion analysis as well as 3D motion analysis and reconstruction methods. Emphasis is placed on the analysis of dynamic cell shape changes that occur through extension and retraction of force generating structures such as pseudopodia and lamellipodia. Quantitative analysis of these structures is an underutilized tool in the field of cell migration. Our intent, therefore, is to present methods that we developed in an effort to elucidate mechanisms of basic cell motility, directed cell motion during chemotaxis, and metastasis. We hope to demonstrate how application of these methods can more clearly define alterations in motility that arise due to specific mutations or disease and hence, suggest mechanisms or pathways involved in normal cell crawling and treatment strategies in the case of disease. In addition, we present a 4D tumorigenesis model for high-resolution analysis of cancer cells from cell lines and human cancer tissue in a 3D matrix. Use of this model led to the discovery of the coalescence of cancer cell aggregates and unique cell behaviors not seen in normal cells or normal tissue. Graphic illustrations to visually display and quantify cell shape are presented along with algorithms and formulae for calculating select 2D and 3D motion analysis parameters.

PTEN redundancy: overexpressing lpten, a homolog of Dictyostelium discoideum ptenA, the ortholog of human PTEN, rescues all behavioral defects of the mutant ptenA-

Mutations in the tumor suppressor gene PTEN are associated with a significant proportion of human cancers. Because the human genome also contains several homologs of PTEN, we considered the hypothesis that if a homolog, functionally redundant with PTEN, can be overexpressed, it may rescue the defects of a PTEN mutant. We have performed an initial test of this hypothesis in the model system Dictyostelium discoideum, which contains an ortholog of human PTEN, ptenA. Deletion of ptenA results in defects in motility, chemotaxis, aggregation and multicellular morphogenesis. D. discoideum also contains lpten, a newly discovered homolog of ptenA. Overexpressing lpten completely rescues all developmental and behavioral defects of the D. discoideum mutant ptenA⁻. This hypothesis must now be tested in human cells.

Huntingtin regulates Ca(2+) chemotaxis and K(+)-facilitated cAMP chemotaxis, in conjunction with the monovalent cation/H(+) exchanger Nhe1, in a model developmental system: insights into its possible role in Huntington׳s disease

Huntington׳s disease is a neurodegenerative disorder, attributable to an expanded trinucleotide repeat in the coding region of the human HTT gene, which encodes the protein huntingtin. These mutations lead to huntingtin fragment inclusions in the striatum of the brain. However, the exact function of normal huntingtin and the defect causing the disease remain obscure. Because there are indications that huntingtin plays a role in Ca(2+) homeostasis, we studied the deletion mutant of the HTT ortholog in the model developmental system Dictyostelium discoideum, in which Ca(2+) plays a role in receptor-regulated behavior related to the aggregation process that leads to multicellular morphogenesis. The D. discoideum htt(-)-mutant failed to undergo both K(+)-facilitated chemotaxis in spatial gradients of the major chemoattractant cAMP, and chemotaxis up a spatial gradient of Ca(2+), but behaved normally in Ca(2+)-facilitated cAMP chemotaxis and Ca(2+)-dependent flow-directed motility. This was the same phenotypic profile of the null mutant of Nhel, a monovalent cation/H(+)exchanger. The htt(-)-mutant also failed to orient correctly during natural aggregation, as was the case for the Nhel mutant. Moreover, in a K(+)-based buffer the normal localization of actin was similarly defective in both htt(-) and nhe1(-) cells in a K(+)-based buffer, and the normal localization of Nhe1 was disrupted in the htt(-) mutant. These observations demonstrate that Htt and Nhel play roles in the same specific cation-facilitated behaviors and that Nhel localization is directly or indirectly regulated by Htt. Similar cation-dependent behaviors and a similar relationship between Htt and Nhe1 have not been reported for mammalian neurons and deserves investigation, especially as it may relate to Huntington׳s disease.

Innate non-specific cell substratum adhesion

Adhesion of motile cells to solid surfaces is necessary to transmit forces required for propulsion. Unlike mammalian cells, Dictyostelium cells do not make integrin mediated focal adhesions. Nevertheless, they can move rapidly on both hydrophobic and hydrophilic surfaces. We have found that adhesion to such surfaces can be inhibited by addition of sugars or amino acids to the buffer. Treating whole cells with αlpha-mannosidase to cleave surface oligosaccharides also reduces adhesion. The results indicate that adhesion of these cells is mediated by van der Waals attraction of their surface glycoproteins to the underlying substratum. Since glycoproteins are prevalent components of the surface of most cells, innate adhesion may be a common cellular property that has been overlooked.

Myosin heavy chain kinases play essential roles in Ca2+, but not cAMP, chemotaxis and the natural aggregation of Dictyostelium discoideum

Behavioral analyses of the deletion mutants of the four known myosin II heavy chain (Mhc) kinases of Dictyostelium discoideum revealed that all play a minor role in the efficiency of basic cell motility, but none play a role in chemotaxis in a spatial gradient of cAMP generated in vitro. However, the two kinases MhckA and MhckC were essential for chemotaxis in a spatial gradient of Ca(2+), shear-induced directed movement, and reorientation in the front of waves of cAMP during natural aggregation. The phenotypes of the mutants mhckA(-) and mhckC(-) were highly similar to that of the Ca(2+) channel/receptor mutant iplA(-) and the myosin II phosphorylation mutant 3XALA, which produces constitutively unphosphorylated myosin II. These results demonstrate that IplA, MhckA and MhckC play a selective role in chemotaxis in a spatial gradient of Ca(2+), but not cAMP, and suggest that Ca(2+) chemotaxis plays a role in the orientation of cells in the front of cAMP waves during natural aggregation.

The IplA Ca2+ channel of Dictyostelium discoideum is necessary for chemotaxis mediated through Ca2+, but not through cAMP, and has a fundamental role in natural aggregation

During aggregation of Dictyostelium discoideum, nondissipating, symmetrical, outwardly moving waves of cAMP direct cells towards aggregation centers. It has been assumed that the spatial and temporal characteristics of the front and back of each cAMP wave regulate both chemokinesis and chemotaxis. However, during the period preceding aggregation, cells acquire not only the capacity to chemotax in a spatial gradient of cAMP, but also in a spatial gradient of Ca(2+). The null mutant of the putative IplA Ca(2+) channel gene, iplA(-), undergoes normal chemotaxis in spatial gradients of cAMP and normal chemokinetic responses to increasing temporal gradients of cAMP, both generated in vitro. However, iplA(-) cells lose the capacity to undergo chemotaxis in response to a spatial gradient of Ca(2+), suggesting that IplA is either the Ca(2+) chemotaxis receptor or an essential component of the Ca(2+) chemotaxis regulatory pathway. In response to natural chemotactic waves generated by wild-type cells, the chemokinetic response of iplA(-) cells to the temporal dynamics of the cAMP wave is intact, but the capacity to reorient in the direction of the aggregation center at the onset of each wave is lost. These results suggest that transient Ca(2+) gradients formed between cells at the onset of each natural cAMP wave augment reorientation towards the aggregation center. If this hypothesis proves correct, it will provide a more complex contextual framework for interpreting D. discoideum chemotaxis.

Nhe1 is essential for potassium but not calcium facilitation of cell motility and the monovalent cation requirement for chemotactic orientation in Dictyostelium discoideum

In Dictyostelium discoideum, extracellular K+ or Ca2+ at a concentration of 40 or 20 mM, respectively, facilitates motility in the absence or presence of a spatial gradient of chemoattractant. Facilitation results in maximum velocity, cellular elongation, persistent translocation, suppression of lateral pseudopod formation, and myosin II localization in the posterior cortex. A lower threshold concentration of 15 mM K+ or Na or 5 mM Ca2+ is required for chemotactic orientation. Although the common buffer solutions used by D. discoideum researchers to study chemotaxis contain sufficient concentrations of cations for chemotactic orientation, the majority contain insufficient levels to facilitate motility. Here it has been demonstrated that Nhe1, a plasma membrane protein, is required for K+ but not Ca2+ facilitation of cell motility and for the lower K+ but not Ca2+ requirement for chemotactic orientation.

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.

2D and 3D quantitative analysis of cell motility and cytoskeletal dynamics

2D- and 3D-Dynamic Image Analysis Systems (2D- and 3D-DIAS) for quantitative analysis of cell motility and chemotaxis are described. Particular attention is given to protocols that have proven useful in the quantitation of cell shape changes and pseudopod dynamics during basic cell motility (i.e. crawling in the absence of a chemotactic or other type of extracellular signal) and directed motion. In addition, methods provided, highlight the applicability of this approach to the accurate phenotypic characterizations of cytoskeletal mutations in Dictyostelium discoideum, cytoskeletal alterations in metastatic cells, and cytoskeletal defects in chemotactically defective polymorphonuclear neutrophils.

An unconventional myosin required for cell polarization and chemotaxis

MyTH/FERM (myosin tail homology 4/band 4.1, ezrin, radixin, and moesin) myosins have roles in cellular adhesion, extension of actin-filled projections such as filopodia and stereocilia, and directional migration. The amoeba Dictyostelium discoideum expresses a simple complement of MyTH/FERM myosins, a class VII (M7) myosin required for cell-substrate adhesion and a unique myosin named MyoG. Mutants lacking MyoG exhibit a wide range of normal actin-based behaviors, including chemotaxis to folic acid, but have a striking defect in polarization and chemotaxis to cAMP. Although the myoG mutants respond to cAMP stimulation by increasing persistence and weakly increasing levels of cortical F-actin, they do not polarize; instead, they maintain a round shape and move slowly and randomly when exposed to a chemotactic gradient. The mutants also fail to activate and localize PI3K to the membrane closest to the source of chemoattractant. These data reveal a role for a MyTH/FERM myosin in mediating early chemotactic signaling and suggest that MyTH/FERM proteins have conserved roles in signaling and the generation of cell polarity.

The effects of extracellular calcium on motility, pseudopod and uropod formation, chemotaxis, and the cortical localization of myosin II in Dictyostelium discoideum

Extracellular Ca(++), a ubiquitous cation in the soluble environment of cells both free living and within the human body, regulates most aspects of amoeboid cell motility, including shape, uropod formation, pseudopod formation, velocity, and turning in Dictyostelium discoideum. Hence it affects the efficiency of both basic motile behavior and chemotaxis. Extracellular Ca(++) is optimal at 10 mM. A gradient of the chemoattractant cAMP generated in the absence of added Ca(++) only affects turning, but in combination with extracellular Ca(++), enhances the effects of extracellular Ca(++). Potassium, at 40 mM, can partially substitute for Ca(++). Mg(++), Mn(++), Zn(++), and Na(+) cannot. Extracellular Ca(++), or K(+), also induce the cortical localization of myosin II in a polar fashion. The effects of Ca(++), K(+) or a cAMP gradient do not appear to be similarly mediated by an increase in the general pool of free cytosolic Ca(++). These results suggest a model, in which each agent functioning through different signaling systems, converge to affect the cortical localization of myosin II, which in turn effects the behavioral changes leading to efficient cell motility and chemotaxis. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Live imaging of theDictyosteliumcell cycle reveals widespread S phase during development, a G2 bias in spore differentiation and a premitotic checkpoint

The regulation of the Dictyostelium cell cycle has remained ambiguous owing to difficulties in long-term imaging of motile cells and a lack of markers for defining cell cycle phases. There is controversy over whether cells replicate their DNA during development, and whether spores are in G1 or G2 of the cell cycle. We have introduced a live-cell S-phase marker into Dictyostelium cells that allows us to precisely define cycle phase. We show that during multicellular development, a large proportion of cells undergo nuclear DNA synthesis. Germinating spores enter S phase only after their first mitosis, indicating that spores are in G2. In addition, we demonstrate that Dictyostelium heterochromatin is copied late in S phase and replicates via accumulation of replication factors, rather than recruitment of DNA to pre-existing factories. Analysis of variability in cycle times indicates that regulation of the cycle manifests at a single random transition in G2, and we present the first identified checkpoint in Dictyostelium, which operates at the G2-M transition in response to DNA damage.

PTEN plays a role in the suppression of lateral pseudopod formation during Dictyostelium motility and chemotaxis

It has been suggested that the phosphatydylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] phosphatase and tensin homolog PTEN plays a fundamental role in Dictyostelium discoideum chemotaxis. To identify that role, the behavior of a pten(-) mutant was quantitatively analyzed using two-dimensional and three-dimensional computer-assisted methods. pten(-) cells were capable of polarizing and translocating in the absence of attractant, and sensing and responding to spatial gradients, temporal gradients and natural waves of attractant. However, all of these responses were compromised (i.e. less efficient) because of the fundamental incapacity of pten(-) cells to suppress lateral pseudopod formation and turning. This defect was equally manifested in the absence, as well as presence, of attractant. PTEN, which is constitutively localized in the cortex of polarized cells, was found essential for the attractant-stimulated increase in cortical myosin II and F-actin that is responsible for the increased suppression of pseudopods during chemotaxis. PTEN, therefore, plays a fundamental role in the suppression of lateral pseudopod formation, a process essential for the efficiency of locomotion and chemotaxis, but not in directional sensing.

Transcriptional pulsing of a developmental gene

It has not been possible to view the transcriptional activity of a single gene within a living eukaryotic cell. It is therefore unclear how long and how frequently a gene is actively transcribed, how this is modulated during differentiation, and how transcriptional events are dynamically coordinated in cell populations. By means of an in vivo RNA detection technique , we have directly visualized transcription of an endogenous developmental gene. We found discrete "pulses" of gene activity that turn on and off at irregular intervals. Surprisingly, the length and height of these pulses were consistent throughout development. However, there was strong developmental variation in the proportion of cells recruited to the expressing pool. Cells were more likely to re-express than to initiate new expression, indicating that we directly observe a transcriptional memory. In addition, we used a clustering algorithm to reveal synchronous transcription initiation in neighboring cells. This study represents the first direct visualization of transcriptional pulsing in eukaryotes. Discontinuity of transcription may allow greater flexibility in the gene-expression decisions of a cell.

RasC plays a role in transduction of temporal gradient information in the cyclic-AMP wave of Dictyostelium discoideum

To define the role that RasC plays in motility and chemotaxis, the behavior of a rasC null mutant, rasC-, in buffer and in response to the individual spatial, temporal, and concentration components of a natural cyclic AMP (cAMP) wave was analyzed by using computer-assisted two-dimensional and three-dimensional motion analysis systems. These quantitative studies revealed that rasC- cells translocate at the same velocity and exhibit chemotaxis up spatial gradients of cAMP with the same efficiency as control cells. However, rasC- cells exhibit defects in maintaining anterior-posterior polarity along the substratum and a single anterior pseudopod when translocating in buffer in the absence of an attractant. rasC- cells also exhibit defects in their responses to both the increasing and decreasing temporal gradients of cAMP in the front and the back of a wave. These defects result in the inability of rasC- cells to exhibit chemotaxis in a natural wave of cAMP. The inability to respond normally to temporal gradients of cAMP results in defects in the organization of the cytoskeleton, most notably in the failure of both F actin and myosin II to exit the cortex in response to the decreasing temporal gradient of cAMP in the back of the wave. While the behavioral defect in the front of the wave is similar to that of the myoA-/myoF- myosin I double mutant, the behavioral and cytoskeletal defects in the back of the wave are similar to those of the S13A myosin II regulatory light-chain phosphorylation mutant. Expression array data support the premise that the behavioral defects exhibited by the rasC- mutant are the immediate result of the absence of RasC function.

Intracellular role of adenylyl cyclase in regulation of lateral pseudopod formation during Dictyostelium chemotaxis

Cyclic AMP (cAMP) functions as the extracellular chemoattractant in the aggregation phase of Dictyostelium development. There is some question, however, concerning what role, if any, it plays intracellularly in motility and chemotaxis. To test for such a role, the behavior of null mutants of acaA, the adenylyl cyclase gene that encodes the enzyme responsible for cAMP synthesis during aggregation, was analyzed in buffer and in response to experimentally generated spatial and temporal gradients of extracellular cAMP. acaA- cells were defective in suppressing lateral pseudopods in response to a spatial gradient of cAMP and to an increasing temporal gradient of cAMP. acaA- cells were incapable of chemotaxis in natural waves of cAMP generated by majority control cells in mixed cultures. These results indicate that intracellular cAMP and, hence, adenylyl cyclase play an intracellular role in the chemotactic response. The behavioral defects of acaA- cells were surprisingly similar to those of cells of null mutants of regA, which encodes the intracellular phosphodiesterase that hydrolyzes cAMP and, hence, functions opposite adenylyl cyclase A (ACA). This result is consistent with the hypothesis that ACA and RegA are components of a receptor-regulated intracellular circuit that controls protein kinase A activity. In this model, the suppression of lateral pseudopods in the front of a natural wave depends on a complete circuit. Hence, deletion of any component of the circuit (i.e., RegA or ACA) would result in the same chemotactic defect.

Computer-assisted analysis of filopod formation and the role of myosin II heavy chain phosphorylation in Dictyostelium

To investigate the role played by filopodia in the motility and chemotaxis of amoeboid cells, a computer-assisted 3D reconstruction and motion analysis system, DIAS 4.0, has been developed. Reconstruction at short time intervals of Dictyostelium amoebae migrating in buffer or in response to chemotactic signals, revealed that the great majority of filopodia form on pseudopodia, not on the cell body; that filopodia on the cell body originate primarily on pseudopodia and relocate; and that filopodia on the uropod are longer and more stable than those located on other portions of the cell. When adjusting direction through lateral pseudopod formation in a spatial gradient of chemoattractant, the temporal and spatial dynamics of lateral pseudopodia suggest that filopodia may be involved in stabilizing pseudopodia on the substratum while the decision is being made by a cell either to turn into a pseudopodium formed in the correct direction (up the gradient) or to retract a pseudopodium formed in the wrong direction (down the gradient). Experiments in which amoebae were treated with high concentrations of chemoattractant further revealed that receptor occupancy plays a role both in filopod formation and retraction. As phosphorylation-dephosphorylation of myosin II heavy chain (MHC) plays a role in lateral pseudopod formation, turning and chemotaxis, the temporal and spatial dynamics of filopod formation were analyzed in MHC phosphorylation mutants. These studies revealed that MHC phosphorylation-dephosphorylation plays a role in the regulation of filopod formation during cell migration in buffer and during chemotaxis. The computer-assisted technology described here for reconstructing filopodia at short time intervals in living cells, therefore provides a new tool for investigating the role filopodia play in the motility and chemotaxis of amoeboid cells.

Sphingosine-1-phosphate plays a role in the suppression of lateral pseudopod formation during Dictyostelium discoideum cell migration and chemotaxis

Sphingosine-1-phosphate (S-1-P) is a bioactive lipid that plays a role in diverse biological processes. It functions both as an extracellular ligand through a family of high-affinity G-protein-coupled receptors, and intracellularly as a second messenger. A growing body of evidence has implicated S-1-P in controlling cell movement and chemotaxis in cultured mammalian cells. Mutant D. discoideum cells, in which the gene encoding the S-1-P lyase had been specifically disrupted by homologous recombination, previously were shown to be defective in pseudopod formation, suggesting that a resulting defect might exist in motility and/or chemotaxis. To test this prediction, we analyzed the behavior of mutant cells in buffer, and in both spatial and temporal gradients of the chemoattractant cAMP, using computer-assisted 2-D and 3-D motion analysis systems. Under all conditions, S-1-P lyase null mutants were unable to suppress lateral pseudopod formation like wild-type control cells. This resulted in a reduction in velocity in buffer and spatial gradients of cAMP. Mutant cells exhibited positive chemotaxis in spatial gradients of cAMP, but did so with lowered efficiency, again because of their inability to suppress lateral pseudopod formation. Mutant cells responded normally to simulated temporal waves of cAMP but mimicked the temporal dynamics of natural chemotactic waves. The effect must be intracellular since no homologs of the S-1-P receptors have been identified in the Dictyostelium genome. The defects in the S-1-P lyase null mutants were similar to those seen in mutants lacking the genes for myosin IA, myosin IB, and clathrin, indicating that S-1-P signaling may play a role in modulating the activity or organization of these cytoskeletal elements in the regulation of lateral pseudopod formation.

The role of myosin heavy chain phosphorylation in Dictyostelium motility, chemotaxis and F-actin localization

To assess the role of myosin II heavy chain (MHC) phosphorylation in basic motility and natural chemotaxis, the Dictyostelium mhcA null mutant mhcA(-), mhcA(-) cells rescued with a myosin II gene that mimics the constitutively unphosphorylated state (3XALA) and mhcA(-) cells rescued with a myosin II gene that mimics the constitutively phosphorylated state (3XASP), were analyzed in buffer and in response to the individual spatial, temporal and concentration components of a cAMP wave using computer-assisted methods. Each mutant strain exhibited unique defects in cell motility and chemotaxis. Although mhcA(-) cells could crawl with some polarity and showed chemotaxis with highly reduced efficiency in a spatial gradient of cAMP, they were very slow, far less polar and more three-dimensional than control cells. They were also incapable of responding to temporal gradients of cAMP, of chemotaxis in a natural wave of cAMP or streaming late in aggregation. 3XASP cells were faster and chemotactically more efficient than mhcA(-) cells, but still incapable of responding to temporal gradients of cAMP, chemotaxis in natural waves of cAMP or streaming late in aggregation. 3XALA cells were fast, were able to respond to temporal gradients of cAMP, and responded to natural waves of cAMP. However, they exhibited a 50% reduction in chemotactic efficiency, could not stream late in aggregation and could not enter the streams of control cells in mixed cultures. F-actin staining further revealed that while the presence of unphosphorylated MHC was essential for the increase in F-actin in the cytoplasm in response to the increasing temporal gradient of cAMP in the front of a natural wave, the actual dephosphorylation event was essential for the associated increase in cortical F-actin. The results of these studies indicate that MHC phosphorylation-dephosphorylation, like myosin II regulatory light chain phosphorylation-dephosphorylation, represents a potential downstream target of the regulatory cascades emanating from the different phases of the wave.

Pseudopodium dynamics and rapid cell movement in Dictyostelium Ras pathway mutants

Loss of either of the Ras pathway members RasS or GefB causes growing Dictyostelium cells to move aberrantly rapidly. In this study, we describe the changes in motility that underlie these phenotypes using computer-assisted 3D dynamic image analysis. Unexpectedly, the two mutants use different mechanisms to achieve rapid migration. The rasS(-) cells' motility is characterised by highly dynamic cell morphology, with rapidly extending and retracting pseudopodia. The gefB(-) cells do not have an unusually dynamic morphology, and achieve their efficient translocation by the continual remodelling of an existing dominant anterior pseudopodium. In spite of these dramatic changes in pseudopodium behaviour, the underlying motility cycle of both mutants remains normal. The levels of F-actin in both mutant cell lines are significantly elevated with respect to the wild-type parental cells, suggesting a possible biochemical basis for these emphatic phenotypes.

Phosphorylation of the myosin regulatory light chain plays a role in motility and polarity duringDictyosteliumchemotaxis

The myosin regulatory light chain (RLC) of Dictyostelium discoideum is phosphorylated at a single serine site in response to chemoattractant. To investigate the role of the phosphorylation of RLC in both motility and chemotaxis, mutants were generated in which the single phosphorylatable serine was replaced with a nonphosphorylatable alanine. Several independent clones expressing the mutant RLC in the RLC null mutant, mlcR-, were obtained. These S13A mutants were subjected to high resolution computer-assisted motion analysis to assess the basic motile behavior of cells in the absence of a chemotatic signal, and the chemotactic responsiveness of cells to the spatial, temporal and concentration components of natural cAMP waves. In the absence of a cAMP signal, mutant cells formed lateral pseudopods less frequently and crawled faster than wild-type cells. In a spatial gradient of cAMP, mutant cells chemotaxed more efficiently than wild-type cells. In the front of simulated temporal and natural waves of cAMP,mutant cells responded normally by suppressing lateral pseudopod formation. However, unlike wild-type cells, mutant cells did not lose cellular polarity at the peak and in the back of either wave. Since depolarization at the peak and in the descending phase of the natural wave is necessary for efficient chemotaxis, this deficiency resulted in a decrease in the capacity of S13A mutant cells to track natural cAMP waves relayed by wild-type cells, and in the fragmentation of streams late in mutant cell aggregation. These results reveal a regulatory pathway induced by the peak and back of the chemotactic wave that alters RLC phosphorylation and leads to cellular depolarization. We suggest that depolarization requires myosin II rearrangement in the cortex facilitated by RLC phosphorylation, which increases myosin motor function.

The Dictyostelium CARMIL protein links capping protein and the Arp2/3 complex to type I myosins through their SH3 domains

Fusion proteins containing the Src homology (SH)3 domains of Dictyostelium myosin IB (myoB) and IC (myoC) bind a 116-kD protein (p116), plus nine other proteins identified as the seven member Arp2/3 complex, and the alpha and beta subunits of capping protein. Immunoprecipitation reactions indicate that myoB and myoC form a complex with p116, Arp2/3, and capping protein in vivo, that the myosins bind to p116 through their SH3 domains, and that capping protein and the Arp2/3 complex in turn bind to p116. Cloning of p116 reveals a protein dominated by leucine-rich repeats and proline-rich sequences, and indicates that it is a homologue of Acan 125. Studies using p116 fusion proteins confirm the location of the myosin I SH3 domain binding site, implicate NH(2)-terminal sequences in binding capping protein, and show that a region containing a short sequence found in several G-actin binding proteins, as well as an acidic stretch, can activate Arp2/3-dependent actin nucleation. p116 localizes along with the Arp2/3 complex, myoB, and myoC in dynamic actin-rich cellular extensions, including the leading edge of cells undergoing chemotactic migration, and dorsal, cup-like, macropinocytic extensions. Cells lacking p116 exhibit a striking defect in the formation of these macropinocytic structures, a concomitant reduction in the rate of fluid phase pinocytosis, a significant decrease in the efficiency of chemotactic aggregation, and a decrease in cellular F-actin content. These results identify a complex that links key players in the nucleation and termination of actin filament assembly with a ubiquitous barbed end-directed motor, indicate that the protein responsible for the formation of this complex is physiologically important, and suggest that previously reported myosin I mutant phenotypes in Dictyostelium may be due, at least in part, to defects in the assembly state of actin. We propose that p116 and Acan 125, along with homologues identified in Caenorhabditis elegans, Drosophila, mouse, and man, be named CARMIL proteins, for capping protein, Arp2/3, and myosin I linker.

Recruitment of a specific amoeboid myosin I isoform to the plasma membrane in chemotactic Dictyostelium cells

The Dictyostelium class I myosins, MyoA, -B, -C, and -D, participate in plasma membrane-based cellular processes such as pseudopod extension and macropinocytosis. Given the existence of a high affinity membrane-binding site in the C-terminal tail domain of these motor proteins and their localized site of action at the cortical membrane-cytoskeleton, it was of interest to determine whether each myosin I was directly associated with the plasma membrane. The membrane association of a myosin I heavy chain kinase that regulates the activity of one of the class I myosins, MyoD was also examined. Cellular fractionation experiments revealed that the majority of the Dicyostelium MyoA, -B, -C and -D heavy chains and the kinase are cytosolic. However, a small, but significant, fraction (appr. 7. -15%) of each myosin I and the kinase was associated with the plasma membrane. The level of plasma membrane-associated MyoB, but neither that of MyoC nor MyoD, increases up to 2-fold in highly motile, streaming cells. These results indicate that Dictyostelium specifically recruits myoB to the plasma membrane during directed cell migration, consistent with its known role in pseudopod formation.

A novel Dictyostelium RasGEF is required for normal endocytosis, cell motility and multicellular development

BACKGROUND

Dictyostelium possesses a surprisingly large number of Ras proteins and little is known about their activators, the guanine nucleotide exchange factors (GEFs). It is also unclear, in Dictyostelium or in higher eukaryotes, whether Ras pathways are linear, with each Ras controlled by its own GEF, or networked, with multiple GEFs acting on multiple Ras proteins.

RESULTS

We have identified the Dictyostelium gene that encodes RasGEFB, a protein with homology to known RasGEFs such as the Son-of-sevenless (Sos) protein. Dictyostelium cells in which the gene for RasGEFB was disrupted moved unusually rapidly, but lost the ability to perform macropinocytosis and therefore to grow in liquid medium. Crowns, the sites of macropinocytosis, were replaced by polarised lamellipodia. Mutant cells were also profoundly defective in early development, although they eventually formed tiny but normally proportioned fruiting bodies. This defect correlated with loss of discoidin Igamma mRNA, a starvation-induced gene, although other genes required for development were expressed normally or even precociously. RasGEFB was able to rescue a Saccharomyces CDC25 mutant, indicating that it is a genuine GEF for Ras proteins.

CONCLUSIONS

RasGEFB appears to be the principal activator of the RasS protein, which regulates macropinocytosis and cell speed, but it also appears to regulate one or more other Ras proteins.

Phosphorylation of the Dictyostelium myosin II heavy chain is necessary for maintaining cellular polarity and suppressing turning during chemotaxis

Conversion of the three mapped threonine phosphorylation sites in the myosin II heavy chain tail to alanines results in a mutant (3XALA) in Dictyostelium discoideum, which displays constitutive myosin overassembly in the cytoskeleton and increased cortical tension. To assess the importance of myosin phosphorylation in cellular translocation and chemotaxis, 3XALA mutant cells have been analyzed by 2D and 3D computer-assisted methods in buffer, in a spatial gradient of cAMP, and after the rapid addition of cAMP. 3XALA cells crawling in buffer exhibit distinct abnormalities in cellular shape, the maintenance of polarity and the complexity of the pseudopod perimeter. 3XALA cells crawling in buffer also exhibit a decrease in directionality. In a spatial gradient of cAMP, the behavioral defects are accentuated. In a spatial gradient, 3XALA cells exhibit a repeating 1- to 2-min behavior cycle in which the shape of each cell changes abnormally from elongate to extremely wide with lateral, opposing pseudopods. At the end of each cycle, 3XALA cells turn 90 degrees into the left or right lateral pseudopod, resulting in a dramatic depression in chemotactic efficiency, even though 3XALA cells are chemotactically responsive to cAMP. These results demonstrate that the phosphorylation of myosin II heavy chain plays a critical role in the maintenance of cell shape and in persistent translocation in a spatial gradient of chemoattractant.

A computer-assisted system for reconstructing and interpreting the dynamic three-dimensional relationships of the outer surface, nucleus and pseudopods of crawling cells

Newly developed software additions to the three-dimensional dynamic image analysis system, 3D-DIAS, are described for simultaneously reconstructing and motion analyzing in three dimensions the outer surface, nucleus and pseudopods of living, crawling cells. This new system is then used to describe for the first time a nuclear behavior cycle in translocating Dictyostelium discoideum amoebae and to investigate the role of pseudopod extension in this process. The nuclear behavior cycle is tuned to the two phases of the general cell behavior cycle [Wessels et al., 1994], and includes nuclear migration both in the z- and in the x,y-axes from the proximal border of the prior anterior pseudopod to the proximal border of a newly expanding anterior pseudopod. Nuclear migration is cued by pseudopod-substratum contact, achieves velocities in excess of 50 microm/min, and is accompanied by characteristic changes in nuclear shape. The rules and characteristics of nuclear behavior are demonstrated to be intact in two mutants affecting pseudopod formation, a myosin IB null mutant (myoB-) and a myosin II heavy chain phosphorylation mutant (3XALA). The rules and characteristics of nuclear migration, however, are disrupted upon dissolution of microtubules by colcemid. Together the above results demonstrate that the newly developed 3D-DIAS system can be used to gain new insights into the dynamic changes in the intracellular 3D architecture associated with cellular translocation.

The Dictyostelium RasS protein is required for macropinocytosis, phagocytosis and the control of cell movement

Endocytosis and cell migration both require transient localised remodelling of the cell cortex. Several lines of evidence suggest a key regulatory role in these activities for members of the Ras family of small GTPases. We have generated Dictyostelium cells lacking one member of this family, RasS, and the mutant cells are perturbed in endocytosis and cell migration. Mutant amoebae are defective in phagocytosis and fluid-phase endocytosis and are impaired in growth. Conversely, the rasS(-)cells show an enhanced rate of cell migration, moving three times faster than wild-type controls. The mutant cells display an aberrant morphology, are highly polarised, carry many elongated actin protrusions and show a concomitant decrease in formation of pinocytic crowns on the cell surface. These morphological aberrations are paralleled by changes in the actin cytoskeleton, with a significant proportion of the cortical F-actin relocalised to prominent pseudopodia. Rapid migration and endocytosis appear to be mutually incompatible and it is likely that RasS protein is required to maintain the normal balance between these two actin-dependent processes.

Clathrin plays a novel role in the regulation of cell polarity, pseudopod formation, uropod stability and motility in Dictyostelium

Although the traditional role of clathrin has been in vesicle trafficking and the internalization of receptors, a novel role in cytokinesis was recently revealed in an analysis of a clathrin-minus Dictyostelium mutant (chc(-)). chc(-) cells grown in suspension were demonstrated to be defective in assembling myosin II into a normal contractile ring. To test whether this defect reflected a more general one of cytoskeletal dysfunction, chc(-) cells were analyzed for cell polarity, pseudopod formation, uropod stability, cell locomotion, chemotaxis, cytoskeletal organization and vesicle movement. chc(-) cells crawled, chemotaxed, localized F-actin in pseudopods, organized their microtubule cytoskeleton in a relatively normal fashion and exhibited normal vesicle dynamics. Although chc(-) cells extended pseudopods from the anterior half of the cell with the same frequency as normal chc(+) cells, they extended pseudopods at twice the normal frequency from the posterior half of the cell. The uropods of chc(-) cells also exhibited spatial instability. These defects resulted in an increase in roundness, a reduction in polarity, a reduction in velocity, a dramatic increase in turning, a high frequency of 180 degrees direction reversals and a decrease in the efficiency of chemotaxis. All defects were reversed in a rescued strain. These results are the first to suggest a novel role for clathrin in cell polarity, pseudopod formation, uropod stability and locomotion. It is hypothesized that clathrin functions to suppress pseudopod formation and to stabilize the uropod in the posterior half of a crawling cell, two behavioral characteristics that are essential for the maintenance of cellular polarity, efficient locomotion and efficient chemotaxis.

Novel cellular tracks of migrating Dictyostelium cells

After Dictyostelium cells were settled on a coverslip and allowed to migrate freely on the surface, they were stained with fluorescently labeled Concanavalin A. Tracks with distinct patterns that consist of dots and short fibers were observed behind the cells. In this study, we refer to these tracks as "cellular tracks", CTs for short. We characterized the biological effect of CTs on cell behavior and development. CTs decreased the strength of cell-substratum adhesion, increased the velocity of cell migration, but did not affect growth of cells. CTs also promoted cell aggregation. When pre-aggregation cells touched the CTs of other cells, they avoided or orthogonally crossed them, but did not migrate along them. These observations suggest that the CTs of pre-aggregation cells prompts cells to disperse uniformly on substratum and may enable cells to sense cell density. On the other hand, when aggregation-competent cells touched the CTs of other aggregation-competent cells, a half of them migrated along the CTs. Pre-aggregation cells did not migrate along the CTs of aggregation-competent cells. The CTs of aggregation-competent cells may help the cells to aggregate toward the aggregation center.

Induction of optical density waves and chemotactic cell movement in Dictyostelium discoideum by microinjection of cAMP pulses

The development of most multicellular organisms involves coordinated cell movement. The early aggregation of Dictyostelium cells has been shown to be mediated by chemotactic movement to propagating waves of cAMP. We have proposed that propagating waves of a chemoattractant, most likely cAMP, also control the movement of cells in mounds and slugs. We have now used periodic pressure injection of pulses of cAMP in the extracellular space of aggregation streams, mounds, and slugs to investigate whether these signals can be relayed and control cell movement, using quantitative digital time-lapse microscopy. Our major findings are (1) short (0.1 s) pulses of cAMP (10(7) molecules) were able to elicit optical density (OD) waves in fields of aggregating amoebae. They propagate from the micropipet outward and interact with endogenous OD waves. (2) Periodic injection of cAMP pulses into aggregation streams blocked the pulses coming from the center and led to the rapid accumulation of cells downstream of the pipet around the pipet. (3) Injection of pulses of cAMP into mounds elicited OD waves, which propagated from the pipet outward and interacted with the endogenous waves, indicating that the same propagator carries them. (4) Periodic microinjection of cAMP in the prespore zone of slugs led to accumulation of anterior-like cells around the micropipet followed by tip formation. Furthermore, the cAMP signal could control the spacing of the endogenous sorting pattern. These results strongly support the hypothesis that the optical density waves observed during early development up to the mound stage represent propagating cAMP waves. They suggest furthermore that cAMP is the morphogen that controls cell movements in slugs.

Possible cooperation of differential adhesion and chemotaxis in mound formation of Dictyostelium

In the mound stage of Dictyostelium discoideum, pre-stalk cells sort and form a tip at the apex. How this pattern forms is as yet unknown. A cellular level model allows us to simulate both differential cell adhesion and chemotaxis, to show that with differential adhesion only, pre-stalk cells move to the surface of the mound but form no tip. With chemotaxis driven by an outgoing circular wave only, a tip forms but contains both pre-stalk and pre-spore cells. Only for a narrow range of relative strengths between differential adhesion and chemotaxis can both mechanisms work in concert to form a tip containing only pre-stalk cells. The simulations provide a method to determine the processes necessary for patterning and suggest a series of further experiments.

Chemotaxis to cAMP and slug migration in Dictyostelium both depend on migA, a BTB protein

Chemotaxis in natural aggregation territories and in a chamber with an imposed gradient of cyclic AMP (cAMP) was found to be defective in a mutant strain of Dictyostelium discoideum that forms slugs unable to migrate. This strain was selected from a population of cells mutagenized by random insertion of plasmids facilitated by introduction of restriction enzyme (a method termed restriction enzyme-mediated integration). We picked this strain because it formed small misshapen fruiting bodies. After isolation of portions of the gene as regions flanking the inserted plasmid, we were able to regenerate the original genetic defect in a fresh host and show that it is responsible for the developmental defects. Transformation of this recapitulated mutant strain with a construct carrying the full-length migA gene and its upstream regulatory region rescued the defects. The sequence of the full-length gene revealed that it encodes a novel protein with a BTB domain near the N terminus that may be involved in protein-protein interactions. The migA gene is expressed at low levels in all cells during aggregation and then appears to be restricted to prestalk cells as a consequence of rapid turnover in prespore cells. Although migA- cells have a dramatically reduced chemotactic index to cAMP and an abnormal pattern of aggregation in natural waves of cAMP, they are completely normal in size, shape, and ability to translocate in the absence of any chemotactic signal. They respond behaviorally to the rapid addition of high levels of cAMP in a manner indicative of intact circuitry connecting receptor occupancy to restructuring of the cytoskeleton. Actin polymerization in response to cAMP is also normal in the mutant cells. The defects at both the aggregation and slug stage are cell autonomous. The MigA protein therefore is necessary for efficiently assessing chemical gradients, and its absence results in defective chemotaxis and slug migration.

Myosin I overexpression impairs cell migration

Dictyostelium myoB, a member of the myosin I family of motor proteins, is important for controlling the formation and retraction of membrane projections by the cell's actin cortex (Novak, K.D., M.D. Peterson, M.C. Reedy, and M.A. Titus. 1995. J. Cell Biol. 131:1205-1221). Mutants that express a three- to sevenfold excess of myoB (myoB+ cells) were generated to further analyze the role of myosin I in these processes. The myoB+ cells move with an instantaneous velocity that is 35% of the wild-type rate and exhibit a 6-8-h delay in initiation of aggregation when placed under starvation conditions. The myoB+ cells complete the developmental cycle after an extended period of time, but they form fewer fruiting bodies that appear to be small and abnormal. The myoB+ cells are also deficient in their ability both to form distinct F-actin filled projections such as crowns and to become elongate and polarized. This defect can be attributed to the presence of at least threefold more myoB at the cortex of the myoB+ cells. In contrast, threefold overexpression of a truncated myoB that lacks the src homology 3 (SH3) domain (myoB/SH3- cells) or myoB in which the consensus heavy chain phosphorylation site was mutated to an alanine (S332A-myoB) does not disturb normal cellular function. However, there is an increased concentration of myoB in the cortex of the myoB/SH3- and S332A-myoB cells comparable to that found in the myoB+ cells. These results suggest that excess full-length cortical myoB prevents the formation of the actin-filled extensions required for locomotion by increasing the tension of the F-actin cytoskeleton and/or retracting projections before they can fully extend. They also demonstrate a role for the phosphorylation site and SH3 domain in mediating the in vivo activity of myosin I.

Dictyostelium mutants lacking multiple classic myosin I isoforms reveal combinations of shared and distinct functions

Dictyostelium cells that lack the myoB isoform were previously shown to exhibit reduced efficiencies of phagocytosis and chemotactic aggregation ("streaming") and to crawl at about half the speed of wild-type cells. Of the four other Dictyostelium myosin I isoforms identified to date, myoC and myoD are the most similar to myoB in terms of tail domain sequence. Furthermore, we show here that myoC, like myoB and myoD, is concentrated in actin-rich cortical regions like the leading edge of migrating cells. To look for evidence of functional overlap between these isoforms, we analyzed myoB, myoC, and myoD single mutants, myoB/myoD double mutants, and myoB/myoC/myoD triple mutants, which were created using a combination of gene targeting techniques and constitutive expression of antisense RNA. With regard to the speed of locomoting, aggregation-stage cells, of the three single mutants, only the myoB mutant was significantly slower. Moreover, double and triple mutants were only slightly slower than the myoB single mutant. Consistent with this, the protein level of myoB alone rises dramatically during early development, suggesting that a special demand is placed on this one isoform when cells become highly motile. We also found, however, that the absolute amount of myoB protein in aggregation-stage cells is much higher than that for myoC and myoD, suggesting that what appears to be a case of nonoverlapping function could be the result of large differences in the amounts of functionally overlapping isoforms. Streaming assays also suggest that myoC plays a significant role in some aspect of motility other than cell speed. With regard to phagocytosis, both myoB and myoC single mutants exhibited significant reductions in initial rate, suggesting that these two isoforms perform nonredundant roles in supporting the phagocytic process. In triple mutants these defects were not additive, however. Finally, because double and triple mutants exhibited significant and progressive decreases in doubling times, we also measured the kinetics of fluid phase endocytic flux (uptake, transit time, efflux). Not only do all three isoforms contribute to this process, but their contributions are synergistic. While these results, when taken together, refute the simple notion that these three "classic" myosin I isoforms perform exclusively identical functions, they do reveal that all three share in supporting at least one cellular process (endocytosis), and they identify several other processes (motility, streaming, and phagocytosis) that are supported to a significant extent by either individual isoforms or various combinations of them.

A Dictyostelium myosin I plays a crucial role in regulating the frequency of pseudopods formed on the substratum

Analysis of the motile behavior of a strain of Dictyostelium lacking a myosin I, myoA, revealed that this mutant strain formed pseudopods and turned twice as frequently as wild type cells [Titus et al., 1993: Mol. Biol. Cell 4:233-246]. The basis for this aberrant behavior has been explored using three-dimensional reconstructions of translocating cells. Wild type cells form approximately 40% of pseudopods on the substratum and 60% off the substratum. The majority of pseudopods formed on the substratum initiate sharp turns while the majority of pseudopods formed off the substratum are retracted. Although myoA- cells form pseudopods at roughly twice the frequency of wild type cells, the increase in frequency is specific for only those pseudopods formed on the substratum. This increase is the basis for the aberrant increase in turning in myoA- cells. The selective increase in the frequency of pseudopods formed on the substratum correlates with a number of additional abnormalities in myoA- pseuodpod formation. First, myoA- cells can simultaneously extend more than one pseudopod, whereas wild type cells extend only one pseudopod at a time. Second, although wild type and myoA- pseudopods achieve the same final volumes, myoA- pseudopods grow at half the rate of wild type pseudopods and, therefore, take longer to achieve final volume. Third, while a wild type pseudopod grows in a continuous fashion, a myoA- pseudopod grows in a discontinuous fashion. Together, these results demonstrate that myoA plays a fundamental role in controlling the frequency of only those pseudopods formed on the substratum, and that maintenance of the normal frequency of pseudopod formation appears to be necessary for the normal velocity of cellular translocation, the normal frequency of turning, the normal rate of average pseudopod growth, and the high efficiency of chemotaxis. These results in turn indicate that pseudopod formation is precisely coordinated in space and time, and actin-associated proteins like myoA play key roles in coordination.

Ponticulin plays a role in the positional stabilization of pseudopods

Ponticulin is a 17-kD glycoprotein that represents a major high affinity link between the plasma membrane and the cortical actin network of Dictyostelium. To assess the role of ponticulin in pseudopod extension and retraction, the motile behavior of two independently generated mutants lacking ponticulin was analyzed using computer-assisted two- and three-dimensional motion analysis systems. More than half of the lateral pseudopods formed off the substratum by ponticulin-minus cells slipped relative to the substratum during extension and retraction. In contrast, all pseudopods formed off the substratum by wild-type cells were positionally fixed in relation to the substratum. Ponticulin-minus cells also formed a greater proportion of both anterior and lateral pseudopods off the substratum and absorbed a greater proportion of lateral pseudopods into the uropod than wild-type cells. In a spatial gradient of cAMP, ponticulin-minus cells were less efficient in tracking the source of chemoattractant. Since ponticulin-minus cells extend and retract pseudopods with the same time course as wild-type cells, these behavioral defects in ponticulin-minus cells appear to be the consequence of pseudopod slippage. These results demonstrate that pseudopods formed off the substratum by wild-type cells are positionally fixed in relation to the substratum, that ponticulin is required for positional stabilization, and that the loss of ponticulin and the concomitant loss of positional stability of pseudopods correlate with a decrease in the efficiency of chemotaxis.

Myosin IB null mutants of Dictyostelium exhibit abnormalities in motility

Cellular and intracellular motility are compared between normal Dictyostelium amoebae and amoebae lacking myosin IB (DMIB-). DMIB- cells generate elongated cell shapes, form particulate-free pseudopodia filled with F-actin, and exhibit an anterior bias in pseudopod extension in a fashion similar to normal amoebae. DMIB- cells also exhibit a normal response to the addition of the chemoattractant cAMP, including a depression in cellular and intracellular particle velocity, depolymerization of F-actin in pseudopodia, and a concomitant increase in cortical F-actin. DMIB- cells do, however, form lateral pseudopodia roughly three times as frequently as normal cells, turn more often, and exhibit depressed average instantaneous cell velocity. DMIB- cells also exhibit a decrease in the average instantaneous velocity of intracellular particle movement and an increase in the degree of randomness in particle direction. These findings indicate that if there is functional substitution for myosin IB by other myosin I isoforms, it is at best only partial, with myosin IB being necessary for maintenance of the normal rate and persistence of cellular translocation, suppression of lateral pseudopod formation and subsequent turning, rapid intracellular particle motility, and the normal anterograde bias of intracellular particle movement. Furthermore, it is likely that the behavioral abnormalities observed here for DMIB- cells underlie the delay in the onset of chemotactic aggregation, the increase in the time required to complete streaming, and the abnormalities in morphogenesis exhibited by DMIB- cells.

Intracellular vesicle movement, cAMP and myosin II in Dictyostelium

Dictyostelium amoebae were analyzed before and after rapid addition of 10(-6) M cAMP for cellular motility, dynamic shape changes, and intracellular particle movement. Before cAMP addition, amoebae moved in a persistent anterior fashion and were elongate with F-actin localized predominantly in the anterior pseudopod. Intracellular particles moved rapidly and anteriorly. Within seconds after 10(-6) M cAMP addition, cells stopped translocating, pseudopod formation ceased, intracellular particle movement was depressed, and F-actin was lost from the pseudopod and concomitantly relocalized in the cell cortex. After 10 seconds, expansion zones reappeared but were small and no longer anteriorly localized. Vesicle movement partially rebounded but was no longer anteriorly directed. The myosin II null mutant HS2215 exhibited both depressed cellular translocation and vesicle movement. The addition of cAMP to HS2215 cells did not result in any detectable change in the random, depressed movement of particles. The results with HS2215 suggest that myosin II is essential for (1) rapid cellular translocation, (2) cellular polarity, (3) rapid particle movement, (4) anteriorly directed particle movement, and (5) the cAMP response. Electron micrographs suggest that at least half of the particles examined in this study contain in turn smaller membrane bound vesicles or multilamellar membrane bodies. The possible role of these vesicles is discussed.

cAMP-mediated inhibition of intracellular particle movement and actin reorganization in Dictyostelium

Before addition of cAMP, Dictyostelum amoebae rapidly translocating in buffer are elongate, exhibit expansion zones primarily at the anterior end and filamentous actin (F-actin) localization primarily in the anterior pseudopodia. Intracellular particle movement is primarily in the anterior direction, and the average rate of particle movement is roughly five times the rate of cellular translocation. Within seconds after the addition of 10(-6)M cAMP, there is a dramatic suppression of cellular translocation, an inhibition of pseudopod formation, a freeze in cellular morphology, a dramatic depression in intracellular particle movement, loss of F-actin localization in pseudopodia concomitant with relocalization of F-actin in the general cytoplasmic cortex under the plasma membrane, and a doubling of F-actin content. After 10 s, expansion zones are again visible at the cell perimeter, but they no longer are localized in the original anterior portion of the cell. There is a slight rebound in particle movement after 10 s, but particles with persistent tracks now show no directionality towards the original anterior portion of the cell, as they did before cAMP addition. Finally, in parallel with the resumption of peripheral expansion and the small rebound in particle movement, there is a decrease in total cellular F-actin to the untreated level. The pattern of microtubule organization is unaffected by the addition of cAMP.

Quantitative analysis of cell motility and chemotaxis in Dictyostelium discoideum by using an image processing system and a novel chemotaxis chamber providing stationary chemical gradients

An image processing system was programmed to automatically track and digitize the movement of amebae under phase-contrast microscopy. The amebae moved in a novel chemotaxis chamber designed to provide stable linear attractant gradients in a thin agarose gel. The gradients were established by pumping attractant and buffer solutions through semipermeable hollow fibers embedded in the agarose gel. Gradients were established within 30 min and shown to be stable for at least a further 90 min. By using this system it is possible to collect detailed data on the movement of large numbers of individual amebae in defined attractant gradients. We used the system to study motility and chemotaxis by a score of Dictyostelium discoideum wild-type and mutant strains, including "streamer" mutants which are generally regarded as being altered in chemotaxis. None of the mutants were altered in chemotaxis in the optimal cAMP gradient of 25 nM/mm, with a midpoint of 25 nM. The dependence of chemotaxis on cAMP concentration, gradient steepness, and temporal changes in the gradient were investigated. We also analyzed the relationship between turning behavior and the direction of travel during chemotaxis in stable gradients. The results suggest that during chemotaxis D. discoideum amebae spatially integrate information about local increases in cAMP concentration at various points on the cell surface.

Evidence that a glycolipid tail anchors antigen 117 to the plasma membrane of Dictyostelium discoideum cells

We describe the biochemical features of the putative cell cohesion molecule antigen 117, indicating that it is anchored to the plasma membrane by a glycolipid tail. Antigen 117 can be radiolabeled with [3H]myristate, [3H]palmitate, and [14C]ethanolamine. The fatty acid label is removed by periodate oxidation and nitrous acid deamination, indicating that the fatty acid is attached to the protein by a structure containing carbohydrate and an unsubstituted glucosamine. As cells develop aggregation competence, the antigen is released from the cell surface in a soluble form that can still be radiolabeled with [14C]ethanolamine but not with [3H]myristate or [3H]palmitate. The molecular weight of the released antigen is similar to that found in the plasma membrane, but it preferentially partitions in Triton X-114 as a hydrophilic, as opposed to a hydrophobic, protein. Plasma membranes contain the enzyme activity responsible for the release of the antigen in a soluble form.