A Coronin7 homolog with functions in actin-driven processes

Adhesion of Dictyostelium Amoebae to Surfaces: A Brief History of Attachments

Dictyostelium amoebae adhere to extracellular material using similar mechanisms to metazoan cells. Notably, the cellular anchorage loci in Amoebozoa and Metazoa are both arranged in the form of discrete spots and incorporate a similar repertoire of intracellular proteins assembled into multicomponent complexes located on the inner side of the plasma membrane. Surprisingly, however, Dictyostelium lacks integrins, the canonical transmembrane heterodimeric receptors that dominantly mediate adhesion of cells to the extracellular matrix in multicellular animals. In this review article, we summarize the current knowledge about the cell-substratum adhesion in Dictyostelium, present an inventory of the involved proteins, and draw parallels with the situation in animal cells. The emerging picture indicates that, while retaining the basic molecular architecture common to their animal relatives, the adhesion complexes in free-living amoeboid cells have evolved to enable less specific interactions with diverse materials encountered in their natural habitat in the deciduous forest soil. Dissection of molecular mechanisms that underlay short lifetime of the cell-substratum attachments and high turnover rate of the adhesion complexes in Dictyostelium should provide insight into a similarly modified adhesion phenotype that accompanies the mesenchymal-amoeboid transition in tumor metastasis.

Macropinocytosis and Cell Migration: Don't Drink and Drive…

Macropinocytosis is a nonspecific mechanism by which cells compulsively "drink" the surrounding extracellular fluids in order to feed themselves or sample the molecules therein, hence gaining information about their environment. This process is cell-intrinsically incompatible with the migration of many cells, implying that the two functions are antagonistic. The migrating cell uses a molecular switch to stop and explore its surrounding fluid by macropinocytosis, after which it employs the same molecular machinery to start migrating again to examine another location. This cycle of migration/macropinocytosis allows cells to explore tissues, and it is key to a range of physiological processes. Evidence of this evolutionarily conserved antagonism between the two processes can be found in several cell types-immune cells, for example, being particularly adept-and ancient organisms (e.g., the social amoeba Dictyostelium discoideum). How macropinocytosis and migration are negatively coupled is the subject of this chapter.

Actin filament debranching regulates cell polarity during cell migration and asymmetric cell division

The formation of the branched actin networks is essential for cell polarity, but it remains unclear how the debranching activity of actin filaments contributes to this process. Here, we showed that an evolutionarily conserved coronin family protein, the Caenorhabditis elegans POD-1, debranched the Arp2/3-nucleated actin filaments in vitro. By fluorescence live imaging analysis of the endogenous POD-1 protein, we found that POD-1 colocalized with Arp2/3 at the leading edge of the migrating C. elegans neuroblasts. Conditional mutations of POD-1 in neuroblasts caused aberrant actin assembly, disrupted cell polarity, and impaired cell migration. In C. elegans one-cell-stage embryos, POD-1 and Arp2/3, moved together during cell polarity establishment, and inhibition of POD-1 blocked Arp2/3 motility and affected the polarized cortical flow, leading to symmetric segregation of cell fate determinants. Together, these results indicate that F-actin debranching organizes actin network and cell polarity in migrating neuroblasts and asymmetrically dividing embryos.

Regulation of the Actin Cytoskeleton via Rho GTPase Signalling in Dictyostelium and Mammalian Cells: A Parallel Slalom

Both Dictyostelium amoebae and mammalian cells are endowed with an elaborate actin cytoskeleton that enables them to perform a multitude of tasks essential for survival. Although these organisms diverged more than a billion years ago, their cells share the capability of chemotactic migration, large-scale endocytosis, binary division effected by actomyosin contraction, and various types of adhesions to other cells and to the extracellular environment. The composition and dynamics of the transient actin-based structures that are engaged in these processes are also astonishingly similar in these evolutionary distant organisms. The question arises whether this remarkable resemblance in the cellular motility hardware is accompanied by a similar correspondence in matching software, the signalling networks that govern the assembly of the actin cytoskeleton. Small GTPases from the Rho family play pivotal roles in the control of the actin cytoskeleton dynamics. Indicatively, Dictyostelium matches mammals in the number of these proteins. We give an overview of the Rho signalling pathways that regulate the actin dynamics in Dictyostelium and compare them with similar signalling networks in mammals. We also provide a phylogeny of Rho GTPases in Amoebozoa, which shows a variability of the Rho inventories across different clades found also in Metazoa.

CORO7 functions as a scaffold protein for the core kinase complex assembly of the Hippo pathway

The Hippo pathway controls organ size and tissue homeostasis through the regulation of cell proliferation and apoptosis. However, the exact molecular mechanisms underpinning Hippo pathway regulation are not fully understood. Here, we identify a new component of the Hippo pathway: coronin 7 (CORO7), a coronin protein family member that is involved in organization of the actin cytoskeleton. pod1, the Drosophila ortholog of CORO7, genetically interacts with key Hippo pathway genes in Drosophila. In mammalian cells, CORO7 is required for the activation of the Hippo pathway in response to cell-cell contact, serum deprivation, and cytoskeleton damage. CORO7 forms a complex with the core components of the pathway and functions as a scaffold for the Hippo core kinase complex. Collectively, these results demonstrate that CORO7 is a key scaffold controlling the Hippo pathway via modulating protein-protein interactions.

From Phagocytes to Immune Defense: Roles for Coronin Proteins in Dictyostelium and Mammalian Immunity

Microbes have interacted with eukaryotic cells for as long as they have been co-existing. While many of these interactions are beneficial for both the microbe as well as the eukaryotic cell, several microbes have evolved into pathogenic species. For some of these pathogens, host cell invasion results in irreparable damage and thus host cell destruction, whereas others use the host to avoid immune detection and elimination. One of the latter pathogens is Mycobacterium tuberculosis, arguably one of the most notorious pathogens on earth. In mammalian macrophages, M. tuberculosis manages to survive within infected macrophages by avoiding intracellular degradation in lysosomes using a number of different strategies. One of these is based on the recruitment and phagosomal retention of the host protein coronin 1, that is a member of the coronin protein family and a mammalian homolog of coronin A, a protein identified in Dictyostelium. Besides mediating mycobacterial survival in macrophages, coronin 1 is also an important regulator of naïve T cell homeostasis. How, exactly, coronin 1 mediates its activity in immune cells remains unclear. While in lower eukaryotes coronins are involved in cytoskeletal regulation, the functions of the seven coronin members in mammals are less clear. Dictyostelium coronins may have maintained multiple functions, whereas the mammalian coronins may have evolved from regulators of the cytoskeleton to modulators of signal transduction. In this minireview, we will discuss the different studies that have contributed to understand the molecular and cellular functions of coronin proteins in mammals and Dictyostelium.

Acanthamoeba and Dictyostelium as Cellular Models for Legionella Infection

Environmental bacteria of the genus Legionella naturally parasitize free-living amoebae. Upon inhalation of bacteria-laden aerosols, the opportunistic pathogens grow intracellularly in alveolar macrophages and can cause a life-threatening pneumonia termed Legionnaires' disease. Intracellular replication in amoebae and macrophages takes place in a unique membrane-bound compartment, the Legionella-containing vacuole (LCV). LCV formation requires the bacterial Icm/Dot type IV secretion system, which translocates literally hundreds of "effector" proteins into host cells, where they modulate crucial cellular processes for the pathogen's benefit. The mechanism of LCV formation appears to be evolutionarily conserved, and therefore, amoebae are not only ecologically significant niches for Legionella spp., but also useful cellular models for eukaryotic phagocytes. In particular, Acanthamoeba castellanii and Dictyostelium discoideum emerged over the last years as versatile and powerful models. Using genetic, biochemical and cell biological approaches, molecular interactions between amoebae and Legionella pneumophila have recently been investigated in detail with a focus on the role of phosphoinositide lipids, small and large GTPases, autophagy components and the retromer complex, as well as on bacterial effectors targeting these host factors.

A tripeptidyl peptidase 1 is a binding partner of the Golgi pH regulator (GPHR) in Dictyostelium

Mutations in tripeptidyl peptidase 1 (TPP1) have been associated with late infantile neuronal ceroid lipofuscinosis (NCL), a neurodegenerative disorder. TPP1 is a lysosomal serine protease, which removes tripeptides from the N-terminus of proteins and is composed of an N-terminal prodomain and a catalytic domain. It is conserved in mammals, amphibians, fish and the amoeba Dictyostelium discoideum. D. discoideum harbors at least six genes encoding TPP1, tpp1A to tpp1F. We identified TPP1F as binding partner of Dictyostelium GPHR (Golgi pH regulator), which is an evolutionarily highly conserved intracellular transmembrane protein. A region encompassing the DUF3735 (GPHR_N) domain of GPHR was responsible for the interaction. In TPP1F, the binding site is located in the prodomain of the protein. The tpp1F gene is transcribed throughout development and translated into a polypeptide of ∼65 kDa. TPP1 activity was demonstrated for TPP1F-GFP immunoprecipitated from D. discoideum cells. Its activity could be inhibited by addition of the recombinant DUF3735 domain of GPHR. Knockout tpp1F mutants did not display any particular phenotype, and TPP1 activity was not abrogated, presumably because tpp1B compensates as it has the highest expression level of all the TPP1 genes during growth. The GPHR interaction was not restricted to TPP1F but occurred also with TPP1B. As previous reports show that the majority of the TPP1 mutations in NCL resulted in reduction or loss of enzyme activity, we suggest that Dicyostelium could be used as a model system in which to test new reagents that could affect the activity of the protein and ameliorate the disease.

Summary: Interaction of Dictyostelium tripeptidyl peptidase 1 with GPHR could be relevant for studies of the human enzyme, which is associated with a neurodegenerative disorder.

Cul3-KLHL20 ubiquitin ligase: physiological functions, stress responses, and disease implications

Cullin-RING ubiquitin ligases are the largest Ubiquitin ligase family in eukaryotes and are multi-protein complexes. In these complexes, the Cullin protein serves as a scaffold to connect two functional modules of the ligases, the catalytic subunit and substrate-binding subunit. KLHL20 is a substrate-binding subunit of Cullin3 (Cul3) ubiquitin ligase. Recent studies have identified a number of substrates of KLHL20-based ubiquitin ligase. Through ubiquitination of these substrates, KLHL20 elicits diverse cellular functions, some of which are associated with human diseases. Furthermore, the functions, subcellular localizations, and expression of KLHL20 are regulated by several physiological and stressed signals, which allow KLHL20 to preferentially act on certain substrates to response to these signals. Here, we provide a summary of the functions and regulations of KLHL20 in several physiological processes and stress responses and its disease implications.

Identification of regions within the Legionella pneumophila VipA effector protein involved in actin binding and polymerization and in interference with eukaryotic organelle trafficking

The Legionella pneumophila effector protein VipA is an actin nucleator that co‐localizes with actin filaments and early endosomes in infected macrophages and which interferes with organelle trafficking when expressed in yeast. To identify the regions of VipA involved in its subcellular localization and functions, we ectopically expressed specific VipA mutant proteins in eukaryotic cells. This indicated that the characteristic punctate distribution of VipA depends on its NH₂‐terminal (amino acid residues 1–133) and central coiled‐coil (amino acid residues 133–206) regions, and suggested a role for the COOH‐terminal (amino acid residues 206–339) region in association with actin filaments and for the NH₂‐terminal in co‐localization with early endosomes. Co‐immunoprecipitation and in vitro assays showed that the COOH‐terminal region of VipA is necessary and sufficient to mediate actin binding, and is essential but insufficient to induce microfilament formation. Assays in yeast revealed that the NH₂ and the COOH‐terminal regions, and possibly an NPY motif within the NH₂ region of VipA, are necessary for interference with organelle trafficking. Overall, this suggests that subversion of eukaryotic vesicular trafficking by VipA involves both its ability to associate with early endosomes via its NH₂‐terminal region and its capacity to bind and polymerize actin through its COOH‐terminal region.

Coronin7 regulates WASP and SCAR through CRIB mediated interaction with Rac proteins

Coronin7 (CRN7) stabilizes F-actin and is a regulator of processes associated with the actin cytoskeleton. Its loss leads to defects in phagocytosis, motility and development. It harbors a CRIB (Cdc42- and Rac-interactive binding) domain in each of its WD repeat domains which bind to Rac GTPases preferably in their GDP-loaded forms. Expression of wild type CRN7 in CRN7 deficient cells rescued these defects, whereas proteins with mutations in the CRIB motifs which were associated with altered Rac binding were effective to varying degrees. The presence of one functional CRIB was sufficient to reestablish phagocytosis, cell motility and development. Furthermore, by molecular modeling and mutational analysis we identified the contact regions between CRN7 and the GTPases. We also identified WASP, SCAR and PAKa as downstream effectors in phagocytosis, development and cell surface adhesion, respectively, since ectopic expression rescued these functions.

Actin dynamics: cell migration takes a new turn with arpin

Accurate cell migration requires intricate control over the actin cytoskeleton. Recent work has identified an Arp2/3-interacting protein called Arpin, which restricts the rate of actin polymerization and is the latest component in the steadily expanding protein repertoire that controls cell migration.

Amoebae as a tool to isolate new bacterial species, to discover new virulence factors and to study the host-pathogen interactions

Amoebae are unicellular protozoan present worldwide in several environments mainly feeding on bacteria. Some of them, the amoebae-resistant bacteria (ARBs), have evolved mechanisms to survive and replicate inside amoebal species. These mainly include legionella, mycobacteria and Chlamydia-related bacteria. Amoebae can provide a replicative niche, can act as reservoir for bacteria whereas the cystic form can protect the internalized bacteria. Moreover, the amoebae represent a Trojan horse for ARBs to infect animals. The long interaction between amoebae and bacteria has likely selected for bacterial virulence traits leading to the adaptation towards an intracellular lifestyle, and some ARBs have acquired the ability to infect mammals. This review intends to highlight the important uses of amoebae in several fields in microbiology by describing the main tools developed using amoebal cells. First, amoebae such as Acanthamoeba are used to isolate and discover new intracellular bacterial species by two main techniques: the amoebal co-culture and the amoebal enrichment. In the second part, taking Waddlia chondrophila as example, we summarize some important recent applications of amoebae to discover new bacterial virulence factors, in particular thanks to the amoebal plaque assay. Finally, the genetically tractable Dictyostelium discoideum is used as a model organism to study host-pathogen interactions, in particular with the development of several approaches to manipulate its genome that allowed the creation of a wide range of mutated strains largely shared within the Dictyostelium community.

Fluorescent reporters and methods to analyze fluorescent signals

The use of fluorescent reporters and the development of new imaging technologies have revolutionized studies in cell biology. During recent years the number of fluorescent proteins offering the ability to visualize the distribution of proteins, organelles, and cells has increased tremendously. In parallel, the imaging tools available were refined rapidly enabling now the use of a huge spectrum of specialized methods to explore the cellular and subcellular localization and dynamics of fluorescently tagged markers. This chapter presents an overview of fluorescent reporters and methods available, and describes a selection of those that are routinely applicable in imaging studies using Dictyostelium discoideum.

Dictyostelium host response to legionella infection: strategies and assays

The professional phagocyte Dictyostelium discoideum is a simple eukaryotic microorganism, whose natural habitat is deciduous forest soil and decaying leaves, where the amoebae feed on bacteria and grow as separate, independent, single cells. In the last decade, the organism has been successfully used as a host for several human pathogens, including Legionella pneumophila, Mycobacterium avium and Mycobacterium marinum,Pseudomonas aeruginosa, Klebsiella pneumoniae, Cryptococcus neoformans, and Salmonella typhimurium. To dissect the complex cross-talk between host and pathogen Dictyostelium offers easy cultivation, a high quality genome sequence and excellent molecular genetic and biochemical tools. Dictyostelium cells are also extremely suitable for cell biological studies, which in combination with in vivo expression of fluorescence-tagged proteins allow investigating the dynamics of bacterial uptake and infection. Inactivation of genes by homologous recombination as well as gene rescue and overexpression are well established and a large mutant collection is available at the Dictyostelium stock center, favoring identification of host resistance or susceptibility genes. Here, we briefly introduce the organism, address the value of Dictyostelium as model host, describe strategies to identify host cell factors important for infection followed by protocols for cell culture and storage, uptake and infection, and confocal microscopy of infected cells.

RpkA, a highly conserved GPCR with a lipid kinase domain, has a role in phagocytosis and anti-bacterial defense

RpkA (Receptor phosphatidylinositol kinase A) is an unusual seven-helix transmembrane protein of Dictyostelium discoideum with a G protein coupled receptor (GPCR) signature and a C-terminal lipid kinase domain (GPCR-PIPK) predicted as a phosphatidylinositol-4-phosphate 5-kinase. RpkA-homologs are present in all so far sequenced Dictyostelidae as well as in several other lower eukaryotes like the oomycete Phytophthora, and in the Legionella host Acanthamoeba castellani. Here we show by immunofluorescence that RpkA localizes to endosomal membranes and is specifically recruited to phagosomes. RpkA interacts with the phagosomal protein complex V-ATPase as proteins of this complex co-precipitate with RpkA-GFP as well as with the GST-tagged PIPK domain of RpkA. Loss of RpkA leads to a defect in phagocytosis as measured by yeast particle uptake. The uptake of the pathogenic bacterium Legionella pneumophila was however unaltered whereas its intra-cellular replication was significantly enhanced in rpkA^-. The difference between wild type and rpkA^- was even more prominent when L. hackeliae was used. When we investigated the reason for the enhanced susceptibility for L. pneumophila of rpkA^- we could not detect a difference in endosomal pH but rpkA^- showed depletion of phosphoinositides (PIP and PIP₂) when we compared metabolically labeled phosphoinositides from wild type and rpkA^-. Furthermore rpkA^- exhibited reduced nitrogen starvation tolerance, an indicator for a reduced autophagy rate. Our results indicate that RpkA is a component of the defense system of D. discoideum as well as other lower eukaryotes.

The professional phagocyte Dictyostelium discoideum as a model host for bacterial pathogens

The use of simple hosts such as Dictyostelium discoideum in the study of host pathogen interactions offers a number of advantages and has steadily increased in recent years. Infection-specific genes can often only be studied in a very limited way in man and even in the mouse model their analysis is usually expensive, time consuming and technically challenging or sometimes even impossible. In contrast, their functional analysis in D. discoideum and other simple model organisms is often easier, faster and cheaper. Because host-pathogen interactions necessarily involve two organisms, it is desirable to be able to genetically manipulate both the pathogen and its host. Particularly suited are those hosts, like D. discoideum, whose genome sequence is known and annotated and for which excellent genetic and cell biological tools are available in order to dissect the complex crosstalk between host and pathogen. The review focusses on host-pathogen interactions of D. discoideum with Legionella pneumophila, mycobacteria, and Salmonella typhimurium which replicate intracellularly.

Unraveling the enigma: progress towards understanding the coronin family of actin regulators

Coronins are a conserved family of actin cytoskeleton regulators that promote cell motility and modulate other actin-dependent processes. Although these proteins have been known for 20 years, substantial progress has been made in the past 5 years towards their understanding. In this review, we examine this progress, place it into the context of what was already known, and pose several questions that remain to be addressed. In particular, we cover the emerging consensus about the role of Type I coronins in coordinating the function of Arp2/3 complex and ADF/cofilin proteins. This coordination plays an important role in leading-edge actin dynamics and overall cell motility. Finally, we discuss the roles played by the more exotic coronins of the Type II and III classes in cellular processes away from the leading edge.

Redundant and unique roles of coronin proteins in Dictyostelium

Dictyostelium discoideum harbors a short (CRN12) and a long coronin (CRN7) composed of one and two beta-propellers, respectively. They are primarily present in the cell cortex and cells lacking CRN12 (corA⁻) or CRN7 (corB⁻) have defects in actin driven processes. We compared the characteristics of a mutant cell line (corA⁻/corB⁻) lacking CRN12 and CRN7 with the single mutants focusing on cytokinesis, phagocytosis, chemotaxis and development. Cytokinesis, uptake of small particles, and developmental defects were not enhanced in the corA⁻/corB⁻ strain as compared to the single mutants, whereas motility and phagocytosis of yeast particles were more severely impaired. It appears that although both proteins affect the same processes they do not act in a redundant manner. Rather, they often act antagonistically, which is in accordance with their proposed roles in the actin cytoskeleton where CRN12 acts in actin disassembly whereas CRN7 stabilizes actin filaments and protects them from disassembly.

Genetic evidence for concerted control of actin dynamics in cytokinesis, endocytic traffic, and cell motility by coronin and Aip1

Coronin and actin-interacting protein 1 (Aip1) are actin-binding proteins that by different mechanisms inhibit actin polymerization or enhance the disassembly of actin filaments. Cells of Dictyostelium discoideum lacking both proteins are retarded in growth and early development and often fail to proceed to fruiting body formation. Coronin/Aip1-null cells show numerous surface protrusions enriched in filamentous actin and cofilin. We show that the double-null cells are characterized by an increase in filamentous actin that causes a thickening of the cell cortex. This imbalance has severe consequences for processes that rely on the dynamic reorganization of the actin cytoskeleton, such as cell motility, cytokinesis and endocytosis. Although cell motility is considerably slowed down, the double-mutant cells are still capable of orientating in a gradient of chemoattractant. The cytokinesis defect is caused by the lack of proper cleavage furrow formation, a defect that is partially rescued by low concentrations of latrunculin A, an inhibitor of actin polymerization. Furthermore, we demonstrate that the disassembly of the actin coat after phagocytic or macropinocytic uptake is significantly delayed in the double-mutant cells. Our results prove that coronin and Aip1 are important effectors that act together in maintaining the balance of actin polymerization and depolymerization in living cells.

Pathogen-host interactions in Dictyostelium, Legionella, Mycobacterium and other pathogens

Dictyostelium discoideum is a haploid social soil amoeba that is an established host model for several human pathogens. The research areas presently pursued include the use of D. discoideum to identify genetic host factors determining the outcome of infections and the use as screening system for identifying bacterial virulence factors. Here we report about the Legionella pneumophila directed phagosome biogenesis and the cell-to-cell spread of Mycobacterium species. Moreover, we highlight recent insights from the host-pathogen cross-talk between D. discoideum and the pathogens Salmonella typhimurium, Klebsiella pneumoniae, Yersinia pseudotuberculosis, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Burkholderia cenocepacia, Vibrio cholerae and Neisseria meningitidis.