GFP-fusion proteins as fluorescent reporters to study organelle and cytoskeleton dynamics in chemotaxis and phagocytosis

How Phagocytes Acquired the Capability of Hunting and Removing Pathogens From a Human Body: Lessons Learned From Chemotaxis and Phagocytosis of Dictyostelium discoideum (Review)

How phagocytes find invading microorganisms and eliminate pathogenic ones from human bodies is a fundamental question in the study of infectious diseases. About 2.5 billion years ago, eukaryotic unicellular organisms-protozoans-appeared and started to interact with various bacteria. Less than 1 billion years ago, multicellular animals-metazoans-appeared and acquired the ability to distinguish self from non-self and to remove harmful organisms from their bodies. Since then, animals have developed innate immunity in which specialized white-blood cells phagocytes- patrol the body to kill pathogenic bacteria. The social amoebae Dictyostelium discoideum are prototypical phagocytes that chase various bacteria via chemotaxis and consume them as food via phagocytosis. Studies of this genetically amendable organism have revealed evolutionarily conserved mechanisms underlying chemotaxis and phagocytosis and shed light on studies of phagocytes in mammals. In this review, we briefly summarize important studies that contribute to our current understanding of how phagocytes effectively find and kill pathogens via chemotaxis and phagocytosis.

Integration of linear and dendritic actin nucleation in Nck-induced actin comets

The role of the Nck adaptor protein in balancing linear versus branched actin nucleation in comet tails is evaluated. Nck recruits both linear and branched nucleation-promoting factors, both of which are necessary for the formation of actin comets. The findings highlight a novel role for Nck in pathogen-like actin motility.

The Nck adaptor protein recruits cytosolic effectors such as N-WASP that induce localized actin polymerization. Experimental aggregation of Nck SH3 domains at the membrane induces actin comet tails—dynamic, elongated filamentous actin structures similar to those that drive the movement of microbial pathogens such as vaccinia virus. Here we show that experimental manipulation of the balance between unbranched/branched nucleation altered the morphology and dynamics of Nck-induced actin comets. Inhibition of linear, formin-based nucleation with the small-molecule inhibitor SMIFH2 or overexpression of the formin FH1 domain resulted in formation of predominantly circular-shaped actin structures with low mobility (actin blobs). These results indicate that formin-based linear actin polymerization is critical for the formation and maintenance of Nck-dependent actin comet tails. Consistent with this, aggregation of an exclusively branched nucleation-promoting factor (the VCA domain of N-WASP), with density and turnover similar to those of N-WASP in Nck comets, did not reconstitute dynamic, elongated actin comets. Furthermore, enhancement of branched Arp2/3-mediated nucleation by N-WASP overexpression caused loss of the typical actin comet tail shape induced by Nck aggregation. Thus the ratio of linear to dendritic nucleation activity may serve to distinguish the properties of actin structures induced by various viral and bacterial pathogens.

Live-cell imaging of phosphoinositide dynamics and membrane architecture during Legionella infection

The causative agent of Legionnaires’ disease, Legionella pneumophila, replicates in amoebae and macrophages in a distinct membrane-bound compartment, the Legionella-containing vacuole (LCV). LCV formation is governed by the bacterial Icm/Dot type IV secretion system that translocates ~300 different “effector” proteins into host cells. Some of the translocated effectors anchor to the LCV membrane via phosphoinositide (PI) lipids. Here, we use the soil amoeba Dictyostelium discoideum, producing fluorescent PI probes, to analyze the LCV PI dynamics by live-cell imaging. Upon uptake of wild-type or Icm/Dot-deficient L. pneumophila, PtdIns(3,4,5)P₃ transiently accumulated for an average of 40 s on early phagosomes, which acquired PtdIns(3)P within 1 min after uptake. Whereas phagosomes containing ΔicmT mutant bacteria remained decorated with PtdIns(3)P, more than 80% of wild-type LCVs gradually lost this PI within 2 h. The process was accompanied by a major rearrangement of PtdIns(3)P-positive membranes condensing to the cell center. PtdIns(4)P transiently localized to early phagosomes harboring wild-type or ΔicmT L. pneumophila and was cleared within minutes after uptake. During the following 2 h, PtdIns(4)P steadily accumulated only on wild-type LCVs, which maintained a discrete PtdIns(4)P identity spatially separated from calnexin-positive endoplasmic reticulum (ER) for at least 8 h. The separation of PtdIns(4)P-positive and ER membranes was even more pronounced for LCVs harboring ΔsidC-sdcA mutant bacteria defective for ER recruitment, without affecting initial bacterial replication in the pathogen vacuole. These findings elucidate the temporal and spatial dynamics of PI lipids implicated in LCV formation and provide insight into host cell membrane and effector protein interactions.

The environmental bacterium Legionella pneumophila is the causative agent of Legionnaires’ pneumonia. The bacteria form in free-living amoebae and mammalian immune cells a replication-permissive compartment, the Legionella-containing vacuole (LCV). To subvert host cell processes, the bacteria secrete the amazing number of ~300 different proteins into host cells. Some of these proteins bind phosphoinositide (PI) lipids to decorate the LCV. PI lipids are crucial factors involved in host cell membrane dynamics and LCV formation. Using Dictyostelium amoebae producing one or two distinct fluorescent probes, we elucidated the dynamic LCV PI pattern in high temporal and spatial resolution. Notably, the endocytic PI lipid PtdIns(3)P was slowly cleared from LCVs, thus incapacitating the host cell’s digestive machinery, while PtdIns(4)P gradually accumulated on the LCV, enabling critical interactions with host organelles. The LCV PI pattern underlies the spatiotemporal configuration of bacterial effector proteins and therefore represents a crucial aspect of LCV formation.

Evolutionary crossroads in developmental biology: Dictyostelium discoideum

Dictyostelium discoideum belongs to a group of multicellular life forms that can also exist for long periods as single cells. This ability to shift between uni- and multicellularity makes the group ideal for studying the genetic changes that occurred at the crossroads between uni- and multicellular life. In this Primer, I discuss the mechanisms that control multicellular development in Dictyostelium discoideum and reconstruct how some of these mechanisms evolved from a stress response in the unicellular ancestor.

Competition between targeting signals in hybrid proteins provides information on their relative in vivo affinities for subcellular compartments

After their translation and folding in the cytoplasm, proteins may be imported into an organelle, associate with a membrane, or rather become part of large, highly localised cytoplasmic structures such as the cytoskeleton. The localisation of a protein is governed by the strength of binding to its immediate target, such as an import receptor for an organelle or a major component of the cytoskeleton, e.g. actin. We have experimentally provided a set of actin-binding proteins with competing targeting information and expressed them at various concentrations to analyse the strength of the signal that governs their subcellular localisation. Our microscopic observations indicate that organellar sorting signals override the targeting preference of most cytoskeletal proteins. Among these signals, the nuclear localisation signal of SV40 is strongest, followed by the oligomerised PHB domain that targets vacuolin to the endosomal surface, and finally the tripeptide SKL mediating transport into the peroxisome. The actin-associated protein coronin, however, can only be misled by the nuclear localisation signal. Interestingly, the targeting behaviour of this model set of hybrid proteins in living Dictyostelium amoebae correlates surprisingly well with the affinities of their constituent signals derived from in vitro experiments conducted in various other organisms. Accordingly, this approach allows estimating the in vivo affinity of a protein to its target even if the latter is not known, as in the case of vacuolin.

Recent advances using green and red fluorescent protein variants

Fluorescent proteins have proven to be excellent tools for live-cell imaging. In addition to green fluorescent protein (GFP) and its variants, recent progress has led to the development of monomeric red fluorescent proteins (mRFPs) that show improved properties with respect to maturation, brightness, and the monomeric state. This review considers green and red spectral variants, their paired use for live-cell imaging in vivo, in vitro, and in fluorescence resonance energy transfer (FRET) studies, in addition to other recent "two-color" advances including photoswitching and bimolecular fluorescence complementation (BiFC). It will be seen that green and red fluorescent proteins now exist with nearly ideal properties for dual-color microscopy and FRET.

Monomeric red fluorescent protein variants used for imaging studies in different species

Fluorescent proteins have proven to be excellent tools for live-cell imaging studies. In addition to green fluorescent protein (GFP) and its variants, recent progress was achieved in the development of monomeric red fluorescent proteins (mRFPs) that show improved properties in respect to maturation and intracellular fluorescence. mRFPmars, a red fluorescent protein designed especially for the use in Dictyostelium, has been employed to tag different proteins for live-cell investigations in Dictyostelium. mRFPruby, which differs in sequence from mRFPmars in four amino acids, has a codon usage optimised for the application in mammalian cells. Here, we show that both mRFP variants can also be applied for localisation studies in other organisms. mRFPmars was expressed in Hydra and fused to the Bcl-2 family protein Bax. mRFPruby in combination with histone 2B was expressed in Drosophila S2 cells to monitor mitosis. Using mouse cell lines, mRFPruby fused to beta-actin was assayed with high spatial resolution to study details of actin cytoskeleton dynamics. In addition, we demonstrate that both mRFP variants are also suitable for dual-colour microscopy in the different species.

Imaging of cell migration

Cell migration is an essential process during many phases of development and adult life. Cells can either migrate as individuals or move in the context of tissues. Movement is controlled by internal and external signals, which activate complex signal transduction cascades resulting in highly dynamic and localised remodelling of the cytoskeleton, cell-cell and cell-substrate interactions. To understand these processes, it will be necessary to identify the critical structural cytoskeletal components, their spatio-temporal dynamics as well as those of the signalling pathways that control them. Imaging plays an increasingly important and powerful role in the analysis of these spatio-temporal dynamics. We will highlight a variety of imaging techniques and their use in the investigation of various aspects of cell motility, and illustrate their role in the characterisation of chemotaxis in Dictyostelium and cell movement during gastrulation in chick embryos in more detail.

Is there a pilot in a pseudopod?

The concept of pilot pseudopodia is reconsidered 30 years after its inauguration (Gerisch, G., Hülser, D., Malchow, D., Wick, U., 1975. Cell communication by periodic cyclic-AMP pulses. Phil. Trans. R. Soc. Lond. B 272, 181-192). The original hypothesis stated that protruding pseudopodia serve as dynamic sensory organelles that aid a cell in perceiving variations of chemoattractant concentration and, consequently, in navigation during chemotaxis. This influential idea is reevaluated in the light of recent findings about the mechanisms governing chemotactic cell motility, morphology and dynamics of pseudopodia, and about molecular constituents and regulators of pseudopod extension and retraction. It is proposed that stimulation by a chemoattractant modulates speed of pseudopod protrusion and thereby increases cell elongation. Elongation further enhances chemotactic sensitivity of the cell to shallow chemoattractant gradients, reinforces cell polarization, and finally leads to suppression of lateral pseudopodia and continuation of cell migration in the gradient direction.

The Dictyostelium genome: the private life of a social model revealed?

The complete genome sequence of the social amoeba Dictyostelium reveals unexpected complexities in genome structure, and cell motility and signaling.

The complete genome sequence of Dictyostelium, a widely studied social amoeba, reveals unexpected complexities in genome structure, and cell motility and signaling, most notably the presence of a large number of G-protein-coupled receptors not previously found outside animals and the absence of receptor tyrosine kinases.

Guenther Gerisch and Dictyostelium, the microbial model for ameboid motility and multicellular morphogenesis

Beginning in 1960 and continuing to this day, Guenther Gerisch's work on the social ameba Dictyostelium discoideum has helped to make it the model organism of choice for studies of cellular activities that depend upon the actomyosin cytoskeleton. Gerisch has brought insight and quantitative rigor to cell biology by developing novel assays and by applying advanced genetic, biochemical and microscopic techniques to topics as varied as cell-cell adhesion, chemotaxis, motility, endocytosis and cytokinesis.

A brilliant monomeric red fluorescent protein to visualize cytoskeleton dynamics in Dictyostelium

Red fluorescent proteins (RFPs) combined with GFP are attractive probes for double-fluorescence labeling of proteins in live cells. However, the application of these proteins is restrained by stable oligomer formation and by their weak fluorescence in vivo. Previous attempts to eliminate these problems by mutagenesis of RFP from Discosoma (DsRed) resulted in the monomeric mRFP1 and in the tetrameric RedStar RFP, which is distinguished by its enhanced fluorescence in vivo. Based on these mutations, we have generated an enhanced monomeric RFP, mRFPmars, and report its spectral properties. Together with green fluorescent labels, we used mRFPmars to visualize filamentous actin structures and microtubules in Dictyostelium cells. This enhanced RFP proved to be suitable to monitor the dynamics of cytoskeletal proteins in cell motility, mitosis, and endocytosis using dual-wavelength fluorescence microscopy.

A Lim protein involved in the progression of cytokinesis and regulation of the mitotic spindle

DdLimE regulates cell motility and cytokinesis in Dictyostelium. To specify its function, we generated knock-out mutants and analyzed mitosis by marking the mitotic apparatus with GFP-alpha-tubulin. Characteristic of DdLimE-null cells is a late reversal of cytokinesis caused by backward movement of the incipient daughter cells. This process of "retro-cytokinesis" is accompanied by a delay in disassembly of the mitotic spindle. The length of interphase microtubules is increased and their depolymerization at prophase is impaired. These data indicate that DdLimE links the cortical actin network, where it is located, to the microtubule system, whose dynamics it regulates.

Cell polarity and Dictyostelium development

Cell polarity is essential for unicellular and multicellular stages of Dictyostelium development. Chemotaxis during early development requires each cell to rapidly reorganize its cytoskeleton to point towards a source of cAMP. This involves a balance between local induction of F-actin polymerization and suppression of pseudopods that point in other directions. Both the lipid phosphatidylinositol (3,4,5) trisphosphate and the soluble signal cGMP have been implicated in these processes, in addition to conserved and novel proteins. During later development cells adopt newly discovered, alternative modes of movement and interact through adhesion molecules. Finally, cells polarize secretion to particular regions of their surface.

Inducible clustering of membrane-targeted SH3 domains of the adaptor protein Nck triggers localized actin polymerization

BACKGROUND

SH2/SH3 adaptor proteins play a critical role in tyrosine kinase signaling pathways, regulating essential cell functions by increasing the local concentration or altering the subcellular localization of downstream effectors. The SH2 domain of the Nck adaptor can bind tyrosine-phosphorylated proteins, while its SH3 domains can modulate actin polymerization by interacting with effectors such as WASp/Scar family proteins. Although several studies have implicated Nck in regulating actin polymerization, its role in living cells is not well understood.

RESULTS

We used an antibody-based system to experimentally modulate the local concentration of Nck SH3 domains on the plasma membrane of living cells. Clustering of fusion proteins containing all three Nck SH3 domains induced localized polymerization of actin, including the formation of actin tails and spots, accompanied by general cytoskeletal rearrangements. All three Nck SH3 domains were required, as clustering of individual SH3 domains or a combination of the two N-terminal Nck SH3 domains failed to promote significant local polymerization of actin in vivo. Changes in actin dynamics induced by Nck SH3 domain clustering required the recruitment of N-WASp, but not WAVE1, and were unaffected by downregulation of Cdc42.

CONCLUSIONS

We show that high local concentrations of Nck SH3 domains are sufficient to stimulate localized, Cdc42-independent actin polymerization in living cells. This study provides strong evidence of a pivotal role for Nck in directly coupling ligand-induced tyrosine phosphorylation at the plasma membrane to localized changes in organization of the actin cytoskeleton through a signaling pathway that requires N-WASp.

Dynamic organization of the actin system in the motile cells of Dictyostelium

The actin system forms a supramolecular, membrane-associated network that serves multiple functions in Dictyostelium cells, including cell motility controlled by chemoattractant, phagocytosis, macropinocytosis, and cytokinesis. In executing these functions the monomeric G-actin polymerizes reversibly, and the actin filaments are assembled into membrane-anchored networks together with other proteins involved in shaping the networks and controlling their dynamics. Most impressive is the speed at which actin-based structures are built, reorganized, or disassembled. We used GFP-tagged coronin and Arp3, an intrinsic constituent of the Arp2/3 complex, as examples of proteins that are recruited to highly dynamic actin-filament networks. By fluorescence recovery after photobleaching (FRAP), average exchange rates of cell-cortex bound coronin were estimated. A nominal value of 5 s for half-maximal incorporation of coronin into the cortex, and a value of 7 s for half-maximal dissociation from cortical binding sites has been obtained. Actin dynamics implies also flow of F-actin from sites of polymerization to sites of depolymerization, i.e. to the tail of a migrating cell, the base of a phagocytic cup, and the cleavage furrow in a mitotic cell. To monitor this flow, we expressed in Dictyostelium cells a GFP-tagged actin-binding fragment of talin. This fragment (GFP-TalC63) translocates from the front to the tail during cell migration and from the polar regions to the cleavage furrow during mitotic cell division. The intrinsic dynamics of the actin system can be manipulated in vivo by drugs or other probes that act either as inhibitors of actin polymerization or as stabilizers of filamentous actin. In order to investigate structure-function relationships in the actin system, a technique of reliably arresting transient network structures is in demand. We discuss the potential of electron tomography of vitrified cells to visualize actin networks in their native association with membranes.