Temperature-sensitive inhibition of development in Dictyostelium due to a point mutation in the piaA gene

The Dynamics of Aerotaxis in a Simple Eukaryotic Model

In aerobic organisms, oxygen is essential for efficient energy production, and it acts as the last acceptor of the mitochondrial electron transport chain and as regulator of gene expression. However, excessive oxygen can lead to production of deleterious reactive oxygen species. Therefore, the directed migration of single cells or cell clumps from hypoxic areas toward a region of optimal oxygen concentration, named aerotaxis, can be considered an adaptive mechanism that plays a major role in biological and pathological processes. One relevant example is the development of O₂ gradients when tumors grow beyond their vascular supply, leading frequently to metastasis. In higher eukaryotic organisms, aerotaxis has only recently begun to be explored, but genetically amenable model organisms suitable to dissect this process remain an unmet need. In this regard, we sought to assess whether Dictyostelium cells, which are an established model for chemotaxis and other motility processes, could sense oxygen gradients and move directionally in their response. By assessing different physical parameters, our findings indicate that both growing and starving Dictyostelium cells under hypoxic conditions migrate directionally toward regions of higher O₂ concentration. This migration is characterized by a specific pattern of cell arrangement. A thickened circular front of high cell density (corona) forms in the cell cluster and persistently moves following the oxygen gradient. Cells in the colony center, where hypoxia is more severe, are less motile and display a rounded shape. Aggregation-competent cells forming streams by chemotaxis, when confined under hypoxic conditions, undergo stream or aggregate fragmentation, giving rise to multiple small loose aggregates that coordinately move toward regions of higher O₂ concentration. By testing a panel of mutants defective in chemotactic signaling, and a catalase-deficient strain, we found that the latter and the pkbR1^null exhibited altered migration patterns. Our results suggest that in Dictyostelium, like in mammalian cells, an intracellular accumulation of hydrogen peroxide favors the migration toward optimal oxygen concentration. Furthermore, differently from chemotaxis, this oxygen-driven migration is a G protein-independent process.

Mammals and Dictyostelium rictor mutations swapping reveals two essential Gly residues for mTORC2 activity and integrity

mTORC2 regulates a variety of vital cellular processes, and its aberrant functioning is often associated with various diseases. Rictor is a peculiar and distinguishing mTORC2 component playing a pivotal role in controlling its assembly and activity. Among living organisms Rictor is conserved from unicellular eukaryotes to metazoan. We replaced two distinct, but conserved, glycines in both the Dictyostelium piaA gene and its human ortholog, rictor. The two conserved residues are spaced by approximately 50 aminoacids and both are embedded within a conserved region falling in between the Ras-GEFN2 and Rictor_V domains. The effects of point mutations on the mTORC2 activity and integrity were assessed by biochemical and functional assays.In both cases, the reciprocal exchange between mammals and Dictyostelium rictor and piaA gene point mutations impaired mTORC2 activity and integrity.Our data indicate that the two Gly residues are essential for the maintenance of mTORC2 activity and integrity in organisms that appear to be distantly related, suggesting a primeval role in the assembly and proper TOR complex 2 functioning.

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.

G-Protein Dependent Signal Transduction and Ubiquitination in Dictyostelium

Signal transduction through G-protein-coupled receptors (GPCRs) is central for the regulation of virtually all cellular functions, and it has been widely implicated in human diseases. These receptors activate a common molecular switch that is represented by the heterotrimeric G-protein generating a number of second messengers (cAMP, cGMP, DAG, IP3, Ca²⁺ etc.), leading to a plethora of diverse cellular responses. Spatiotemporal regulation of signals generated by a given GPCR is crucial for proper signalling and is accomplished by a series of biochemical modifications. Over the past few years, it has become evident that many signalling proteins also undergo ubiquitination, a posttranslational modification that typically leads to protein degradation, but also mediates processes such as protein-protein interaction and protein subcellular localization. The social amoeba Dictyostelium discoideum has proven to be an excellent model to investigate signal transduction triggered by GPCR activation, as cAMP signalling via GPCR is a major regulator of chemotaxis, cell differentiation, and multicellular morphogenesis. Ubiquitin ligases have been recently involved in these processes. In the present review, we will summarize the most significant pathways activated upon GPCRs stimulation and discuss the role played by ubiquitination in Dictyostelium cells.

A new HECT ubiquitin ligase regulating chemotaxis and development in Dictyostelium discoideum

Cyclic AMP (cAMP) binding to G-protein-coupled receptors (GPCRs) orchestrates chemotaxis and development in Dictyostelium. By activating the RasC-TORC2-PKB (PKB is also known as AKT in mammals) module, cAMP regulates cell polarization during chemotaxis. TORC2 also mediates GPCR-dependent stimulation of adenylyl cyclase A (ACA), enhancing cAMP relay and developmental gene expression. Thus, mutants defective in the TORC2 Pia subunit (also known as Rictor in mammals) are impaired in chemotaxis and development. Near-saturation mutagenesis of a Pia mutant by random gene disruption led to selection of two suppressor mutants in which spontaneous chemotaxis and development were restored. PKB phosphorylation and chemotactic cell polarization were rescued, whereas Pia-dependent ACA stimulation was not restored but bypassed, leading to cAMP-dependent developmental gene expression. Knocking out the gene encoding the adenylylcyclase B (ACB) in the parental strain showed ACB to be essential for this process. The gene tagged in the suppressor mutants encodes a newly unidentified HECT ubiquitin ligase that is homologous to mammalian HERC1, but harbours a pleckstrin homology domain. Expression of the isolated wild-type HECT domain, but not a mutant HECT C5185S form, from this protein was sufficient to reconstitute the parental phenotype. The new ubiquitin ligase appears to regulate cell sensitivity to cAMP signalling and TORC2-dependent PKB phosphorylation.

Ras proteins signaling in the early metazoan Dictyostelium discoideum

Since the discovery of Ras, Ras-mediated transforming activity has been the major investigative area of interest. Soon thereafter it has emerged that Ras family members regulate different biological processes, other than cell growth, like development and fine-tune the balance between cell death and survival. The lower metazoan Dictyostelium discoideum is a powerful and genetically accessible model organism that has been used to elucidate the roles played by different Ras members in some biological processes, such as cell motility and development. In the following chapter we describe some very basic techniques aiming to identify novel Ras signaling components, throughout insertional mutagenesis screening, and to investigate their role(s) in development and chemotaxis processes.

PIP3-independent activation of TorC2 and PKB at the cell's leading edge mediates chemotaxis

BACKGROUND

Studies show that high phosphotidylinositol 3,4,5-trisphosphate (PIP(3)) promotes cytoskeletal rearrangements and alters cell motility and chemotaxis, possibly through activation of protein kinase Bs (PKBs). However, chemotaxis can still occur in the absence of PIP(3), and the identities of the PIP(3)-independent pathways remain unknown.

RESULTS

Here, we outline a PIP(3)-independent pathway linking temporal and spatial activation of PKBs by Tor complex 2 (TorC2) to the chemotactic response. Within seconds of stimulating Dictyostelium cells with chemoattractant, two PKB homologs, PKBA and PKBR1, mediate transient phosphorylation of at least eight proteins, including Talin, PI4P 5-kinase, two Ras GEFs, and a RhoGap. Surprisingly, all of the substrates are phosphorylated with normal kinetics in cells lacking PI 3-kinase activity. Cells deficient in TorC2 or PKB activity show reduced phosphorylation of the endogenous substrates and are impaired in chemotaxis. The PKBs are activated through phosphorylation of their hydrophobic motifs via TorC2 and subsequent phosphorylation of their activation loops. These chemoattractant-inducible events are restricted to the cell's leading edge even in the absence of PIP(3). Activation of TorC2 depends on heterotrimeric G protein function and intermediate G proteins, including Ras GTPases.

CONCLUSIONS

The data lead to a model where cytosolic TorC2, encountering locally activated small G protein(s) at the leading edge of the cell, becomes activated and phosphorylates PKBs. These in turn phosphorylate a series of signaling and cytoskeletal proteins, thereby regulating directed migration.

SrfB, a member of the Serum Response Factor family of transcription factors, regulates starvation response and early development in Dictyostelium

The Serum Response Factor (SRF) is an important regulator of cell proliferation and differentiation. Dictyostelium discoideum srfB gene codes for an SRF homologue and is expressed in vegetative cells and during development under the control of three alternative promoters, which show different cell-type specific patterns of expression. The two more proximal promoters directed gene transcription in prestalk AB, stalk and lower-cup cells. The generation of a strain where the srfB gene has been interrupted (srfB⁻) has shown that this gene is required for regulation of actin–cytoskeleton-related functions, such as cytokinesis and macropinocytosis. The mutant failed to develop well in suspension, but could be rescued by cAMP pulsing, suggesting a defect in cAMP signaling. srfB⁻ cells showed impaired chemotaxis to cAMP and defective lateral pseudopodium inhibition. Nevertheless, srfB⁻ cells aggregated on agar plates and nitrocellulose filters 2 h earlier than wild type cells, and completed development, showing an increased tendency to form slug structures. Analysis of wild type and srfB⁻ strains detected significant differences in the regulation of gene expression upon starvation. Genes coding for lysosomal and ribosomal proteins, developmentally-regulated genes, and some genes coding for proteins involved in cytoskeleton regulation were deregulated during the first stages of development.

Cloning of Dictyostelium eIF6 (p27BBP) and mapping its nucle(ol)ar localization subdomains

Eukaryotic translation initiation factor 6 (eIF6), also termed p27BBP, is an evolutionary conserved regulator of ribosomal function. The protein is involved in maturation and/or export from the nucleus of the 60S ribosomal subunit. Regulated binding to and release from the 60S subunit also regulates formation of 80S ribosomes, and thus translation. The protein is also found in hemidesmosomes of epithelial cells expressing beta4 integrin and is assumed to regulate cross-talk between beta4 integrin, intermediate filaments and ribosomes. In the present study we show that the Dictyostelium eIF6 (also called p27BBP) gene is expressed during growth, down-regulated during the first hours of starvation, and up-regulated again at the end of aggregation. Phagocytosis, and to a lesser extent pinocytic uptake of axenic medium, stimulate gene expression in starving cells. The eIF6 gene is present in single copy and its ablation is lethal. We utilized the green fluorescent protein (GFT) as fusion protein marker to investigate sequences responsible for eIF6 subcellular localization. The protein is found both in cytoplasm and nucleus, and is enriched in nucleoli. Deletion sequence analysis shows that nucle(ol)ar localization sequences are located within the N- and C-terminal subdomains of the protein.

A novel Dictyostelium RasGEF required for chemotaxis and development

BACKGROUND

Ras proteins are guanine-nucleotide-binding enzymes that couple cell surface receptors to intracellular signaling pathways controlling cell proliferation and differentiation, both in lower and higher eukaryotes. They act as molecular switches by cycling between active GTP and inactive GDP-bound states, through the action of two classes of regulatory proteins: a) guanine nucleotide exchange factor (GEFs) and b) GTP-ase activating proteins (GAPs). Genome wide analysis of the lower eukaryote Dictyostelium discoideum revealed a surprisingly large number of Ras Guanine Nucleotide Exchange Factors (RasGEFs). RasGEFs promote the activation of Ras proteins by catalyzing the exchange of GDP for GTP, thus conferring to RasGEFs the role of main activator of Ras proteins. Up to date only four RasGEFs, which are all non-redundant either for growth or development, have been characterized in Dictyostelium. We report here the identification and characterization of a fifth non-redundant GEF, RasGEFM.

RESULTS

RasGEFM is a multi-domain protein containing six poly-proline stretches, a DEP, RasGEFN and RasGEF catalytic domain. The rasGEFM gene is differentially expressed during growth and development. Inactivation of the gene results in cells that form small, flat aggregates and fail to develop further. Expression of genes required for aggregation is delayed. Chemotaxis towards cAMP is impaired in the mutant, due to inability to inhibit lateral pseudopods. Endogenous cAMP accumulates during early development to a much lower extent than in wild type cells. Adenylyl cyclase activation in response to cAMP pulses is strongly reduced, by contrast guanylyl cyclase is stimulated to higher levels than in the wild type. The actin polymerization response to cAMP is also altered in the mutant. Cyclic AMP pulsing for several hours partially rescues the mutant. In vitro experiments suggest that RasGEFM acts downstream of the cAMP receptor but upstream of the G protein.

CONCLUSION

The data indicate that RasGEFM is involved in the establishment of the cAMP relay system. We propose that RasGEFM is a component of a Ras regulated pathway, which integrate signals acting as positive regulator for adenylyl cyclase and negative regulator for guanylyl cyclase. Altered guanylyl cyclase, combined with defective regulation of actin polymerization, results in altered chemotaxis.

Chemoattractant signaling in dictyostelium discoideum

Dictyostelium is an accessible organism for studies of signaling via chemoattractant receptors. Chemoattractant-mediated signaling events and components are reviewed and presented as a series of connected modules, including excitation, inhibition, G protein-independent responses, early gene expression, inositol lipids, PH domain-containing proteins, cyclic AMP signaling, polarization acquisition, actin polymerization, and cortical myosin. The network incorporates information from biochemical, genetic, and cell biological experiments carried out on living cells. The modules and connections represent current understanding, and future information is expected to modify and build upon this structure.