Identification of a Chemoattractant G-Protein-Coupled Receptor for Folic Acid that Controls Both Chemotaxis and Phagocytosis

Regulatory differences between atypical and typical MAP kinases in Dictyostelium discoideum

Within the large family of mitogen activated protein kinases (MAPKs), one outlier group referred to as atypical MAPKs is not regulated by conventional upstream MAPK kinases (MAP2Ks). This includes the Dictyostelium discoideum atypical MAPK Erk2, a protein kinase essential for chemotactic movement and development. The regulation and functional specificity of Erk2 was investigated through phenotypic analysis of chimeric and mutant MAPKs. Chimeric MAPKs containing regions of Erk2 were created using complementary regions of the more typical MAPK Erk1, that provides very different functions in this amoeba. The chimeric MAPKs were not phosphorylated at levels observed for wild-type MAPKs and none rescued wild-type MAPK function to erk1- or erk2- cells. Endogenous Erk1 and Erk2 MAPKs were destabilized in cells expressing chimeric MAPKs containing the same carboxyl terminus. A carboxyl terminal motif conserved among atypical MAPKs was important but not essential for Erk2 regulation and function and the motif did not confer atypical MAPK regulation when present in Erk1. A kinase-dead version of Erk2 was phosphorylated in response to folate or cAMP chemotactic stimulation, suggesting Erk2 is activated in vivo by an upstream protein kinase, contrary to previous predictions of autophosphorylation. This regulation implies a protein kinase distinct from the single conventional MAP2K in Dictyostelium regulates atypical MAPK signaling. A non-activatable form of Erk2 was not capable of rescuing Erk2 function in erk2- cells. These findings suggest that the regulation of atypical and typical MAPKs is substantially different and carried out by distinct upstream protein kinases.

A novel monomeric amyloid β-activated signaling pathway regulates brain development via inhibition of microglia

Amyloid β (Aβ) forms aggregates in the Alzheimer's disease brain and is well known for its pathological roles. Recent studies show that it also regulates neuronal physiology in the healthy brain. Whether Aβ also regulates glial physiology in the normal brain, however, has remained unclear. In this article, we describe the discovery of a novel signaling pathway activated by the monomeric form of Aβ in vitro that plays essential roles in the regulation of microglial activity and the assembly of neocortex during mouse development in vivo. We find that activation of this pathway depends on the function of amyloid precursor and the heterotrimeric G protein regulator Ric8a in microglia and inhibits microglial immune activation at transcriptional and post-transcriptional levels. Genetic disruption of this pathway during neocortical development results in microglial dysregulation and excessive matrix proteinase activation, leading to basement membrane degradation, neuronal ectopia, and laminar disruption. These results uncover a previously unknown function of Aβ as a negative regulator of brain microglia and substantially elucidate the underlying molecular mechanisms. Considering the prominence of Aβ and neuroinflammation in the pathology of Alzheimer's disease, they also highlight a potentially overlooked role of Aβ monomer depletion in the development of the disease.

Ras suppression potentiates rear actomyosin contractility-driven cell polarization and migration

Ras has been extensively studied as a promoter of cell proliferation, whereas few studies have explored its role in migration. To investigate the direct and immediate effects of Ras activity on cell motility or polarity, we focused on RasGAPs, C2GAPB in Dictyostelium amoebae and RASAL3 in HL-60 neutrophils and macrophages. In both cellular systems, optically recruiting the respective RasGAP to the cell front extinguished pre-existing protrusions and changed migration direction. However, when these respective RasGAPs were recruited uniformly to the membrane, cells polarized and moved more rapidly, whereas targeting to the back exaggerated these effects. These unexpected outcomes of attenuating Ras activity naturally had strong, context-dependent consequences for chemotaxis. The RasGAP-mediated polarization depended critically on myosin II activity and commenced with contraction at the cell rear, followed by sustained mTORC2-dependent actin polymerization at the front. These experimental results were captured by computational simulations in which Ras levels control front- and back-promoting feedback loops. The discovery that inhibiting Ras activity can produce counterintuitive effects on cell migration has important implications for future drug-design strategies targeting oncogenic Ras.

Plasma membrane abundance dictates phagocytic capacity and functional cross-talk in myeloid cells

Professional phagocytes like neutrophils and macrophages tightly control what they consume, how much they consume, and when they move after cargo uptake. We show that plasma membrane abundance is a key arbiter of these cellular behaviors. Neutrophils and macrophages lacking the G protein subunit Gβ4 exhibited profound plasma membrane expansion, accompanied by marked reduction in plasma membrane tension. These biophysical changes promoted the phagocytosis of bacteria, fungus, apoptotic corpses, and cancer cells. We also found that Gβ4-deficient neutrophils are defective in the normal inhibition of migration following cargo uptake. Sphingolipid synthesis played a central role in these phenotypes by driving plasma membrane accumulation in cells lacking Gβ4. In Gβ4 knockout mice, neutrophils not only exhibited enhanced phagocytosis of inhaled fungal conidia in the lung but also increased trafficking of engulfed pathogens to other organs. Together, these results reveal an unexpected, biophysical control mechanism central to myeloid functional decision-making.

Polycystin-2 Mediated Calcium Signalling in the Dictyostelium Model for Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) occurs when the proteins Polycystin-1 (PC1, PKD1) and Polycystin-2 (PC2, PKD2) contain mutations. PC1 is a large membrane receptor that can interact and form a complex with the calcium-permeable cation channel PC2. This complex localizes to the plasma membrane, primary cilia and ER. Dysregulated calcium signalling and consequential alterations in downstream signalling pathways in ADPKD are linked to cyst formation and expansion; however, it is not completely understood how PC1 and PC2 regulate calcium signalling. We have studied Polycystin-2 mediated calcium signalling in the model organism Dictyostelium discoideum by overexpressing and knocking down the expression of the endogenous Polycystin-2 homologue, Polycystin-2. Chemoattractant-stimulated cytosolic calcium response magnitudes increased and decreased in overexpression and knockdown strains, respectively, and analysis of the response kinetics indicates that Polycystin-2 is a significant contributor to the control of Ca2+ responses. Furthermore, basal cytosolic calcium levels were reduced in Polycystin-2 knockdown transformants. These alterations in Ca2+ signalling also impacted other downstream Ca2+-sensitive processes including growth rates, endocytosis, stalk cell differentiation and spore viability, indicating that Dictyostelium is a useful model to study Polycystin-2 mediated calcium signalling.

Transcriptomics reveal a unique phago-mixotrophic response to low nutrient concentrations in the prasinophyte Pterosperma cristatum

Constitutive mixoplankton-plastid-bearing microbial eukaryotes capable of both phototrophy and phagotrophy-are ubiquitous in marine ecosystems and facilitate carbon transfer to higher trophic levels within aquatic food webs, which supports enhanced sinking carbon flux. However, the regulation of the relative contribution of photosynthesis and prey consumption remains poorly characterized. We investigated the transcriptional dynamics behind this phenotypic plasticity in the prasinophyte green alga Pterosperma cristatum. Based on what is known of other mixoplankton species that cannot grow without photosynthesis (obligate phototrophs), we hypothesized that P. cristatum uses phagotrophy to circumvent the restrictions imposed on photosynthesis by nutrient depletion, to obtain nutrients from ingested prey, and to maintain photosynthetic carbon fixation. We observed an increase in feeding as a response to nutrient depletion, coinciding with an upregulation of expression for genes involved in essential steps of phagocytosis including prey recognition, adhesion and engulfment, transport and maturation of food vacuoles, and digestion. Unexpectedly, genes involved in the photosynthetic electron transfer chain, pigment biosynthesis, and carbon fixation were downregulated as feeding increased, implying an abatement of photosynthesis. Contrary to our original hypothesis, our results therefore suggest that depletion of inorganic nutrients triggered an alteration of trophic behavior from photosynthesis to phagotrophy in P. cristatum. While this behavior distinguishes P. cristatum from other groups of constitutive mixoplankton, its physiological response aligns with recent discoveries from natural microbial communities. These findings indicate that mixoplankton communities in nutrient-limited oceans can regulate photosynthesis against bacterivory based on nutrient availability.

Parallel evolution of the G protein-coupled receptor GrlG and the loss of fruiting body formation in the social amoeba Dictyostelium discoideum evolved under low relatedness

Aggregative multicellularity relies on cooperation among formerly independent cells to form a multicellular body. Previous work with Dictyostelium discoideum showed that experimental evolution under low relatedness profoundly decreased cooperation, as indicated by the loss of fruiting body formation in many clones and an increase of cheaters that contribute proportionally more to spores than to the dead stalk. Using whole-genome sequencing and variant analysis of these lines, we identified 38 single nucleotide polymorphisms in 29 genes. Each gene had 1 variant except for grlG (encoding a G protein-coupled receptor), which had 10 unique SNPs and 5 structural variants. Variants in the 5' half of grlG-the region encoding the signal peptide and the extracellular binding domain-were significantly associated with the loss of fruiting body formation; the association was not significant in the 3' half of the gene. These results suggest that the loss of grlG was adaptive under low relatedness and that at least the 5' half of the gene is important for cooperation and multicellular development. This is surprising given some previous evidence that grlG encodes a folate receptor involved in predation, which occurs only during the single-celled stage. However, non-fruiting mutants showed little increase in a parallel evolution experiment where the multicellular stage was prevented from happening. This shows that non-fruiting mutants are not generally selected by any predation advantage but rather by something-likely cheating-during the multicellular stage.

Atypical MAP kinases - new insights and directions from amoeba

Mitogen-activated protein kinases (MAPKs) have been the focus of many studies over the past several decades, but the understanding of one subgroup of MAPKs, orthologs of MAPK15, known as atypical MAPKs, has lagged behind others. In most organisms, specific activating signals or downstream responses of atypical MAPK signaling pathways have not yet been identified even though these MAPKs are associated with many eukaryotic processes, including cancer and embryonic development. In this Review, we discuss recent studies that are shedding new light on both the regulation and function of atypical MAPKs in different organisms. In particular, the analysis of the atypical MAPK in the amoeba Dictyostelium discoideum has revealed important roles in chemotactic responses and gene regulation. The rapid and transient phosphorylation of the atypical MAPK in these responses suggest a highly regulated activation mechanism in vivo despite the ability of atypical MAPKs to autophosphorylate in vitro. Atypical MAPK function can also impact the activation of other MAPKs in amoeba. These advances are providing new perspectives on possible MAPK roles in animals that have not been previously considered, and this might lead to the identification of potential targets for regulating cell movement in the treatment of diseases.

Plasma membrane abundance dictates phagocytic capacity and functional crosstalk in myeloid cells

Professional phagocytes like neutrophils and macrophages tightly control what they eat, how much they eat, and when they move after eating. We show that plasma membrane abundance is a key arbiter of these cellular behaviors. Neutrophils and macrophages lacking the G-protein subunit Gb4 exhibit profound plasma membrane expansion due to enhanced production of sphingolipids. This increased membrane allocation dramatically enhances phagocytosis of bacteria, fungus, apoptotic corpses, and cancer cells. Gb4 deficient neutrophils are also defective in the normal inhibition of migration following cargo uptake. In Gb4 knockout mice, myeloid cells exhibit enhanced phagocytosis of inhaled fungal conidia in the lung but also increased trafficking of engulfed pathogens to other organs. These results reveal an unexpected, biophysical control mechanism lying at the heart of myeloid functional decision-making.

The aerotaxis of Dictyostelium discoideum is independent of mitochondria, nitric oxide and oxidative stress

Spatial and temporal variations of oxygen environments affect the behaviors of various cells and are involved in physiological and pathological events. Our previous studies with Dictyostelium discoideum as a model of cell motility have demonstrated that aerotaxis toward an oxygen-rich region occurs below 2% O₂. However, while the aerotaxis of Dictyostelium seems to be an effective strategy to search for what is essential for survival, the mechanism underlying this phenomenon is still largely unclear. One hypothesis is that an oxygen concentration gradient generates a secondary oxidative stress gradient that would direct cell migration towards higher oxygen concentration. Such mechanism was inferred but not fully demonstrated to explain the aerotaxis of human tumor cells. Here, we investigated the role on aerotaxis of flavohemoglobins, proteins that can both act as potential oxygen sensors and modulators of nitric oxide and oxidative stress. The migratory behaviors of Dictyostelium cells were observed under both self-generated and imposed oxygen gradients. Furthermore, their changes by chemicals generating or preventing oxidative stress were tested. The trajectories of the cells were then analyzed through time-lapse phase-contrast microscopic images. The results indicate that both oxidative and nitrosative stresses are not involved in the aerotaxis of Dictyostelium but cause cytotoxic effects that are enhanced upon hypoxia.

Convergence of Ras- and Rac-regulated formin pathways is pivotal for phagosome formation and particle uptake in Dictyostelium

Macroendocytosis comprising phagocytosis and macropinocytosis is an actin-driven process regulated by small GTPases that depend on the dynamic reorganization of the membrane that protrudes and internalizes extracellular material by cup-shaped structures. To effectively capture, enwrap, and internalize their targets, these cups are arranged into a peripheral ring or ruffle of protruding actin sheets emerging from an actin-rich, nonprotrusive zone at its base. Despite extensive knowledge of the mechanism driving actin assembly of the branched network at the protrusive cup edge, which is initiated by the actin-related protein (Arp) 2/3 complex downstream of Rac signaling, our understanding of actin assembly in the base is still incomplete. In the Dictyostelium model system, the Ras-regulated formin ForG was previously shown to specifically contribute to actin assembly at the cup base. Loss of ForG is associated with a strongly impaired macroendocytosis and a 50% reduction in F-actin content at the base of phagocytic cups, in turn indicating the presence of additional factors that specifically contribute to actin formation at the base. Here, we show that ForG synergizes with the Rac-regulated formin ForB to form the bulk of linear filaments at the cup base. Consistently, combined loss of both formins virtually abolishes cup formation and leads to severe defects of macroendocytosis, emphasizing the relevance of converging Ras- and Rac-regulated formin pathways in assembly of linear filaments in the cup base, which apparently provide mechanical support to the entire structure. Remarkably, we finally show that active ForB, unlike ForG, additionally drives phagosome rocketing to aid particle internalization.

Phylogeny-wide analysis of G-protein coupled receptors in social amoebas and implications for the evolution of multicellularity

G-protein coupled receptors (GPCRs) are seven-transmembrane proteins and constitute the largest group of receptors within eukaryotes. The presence of a large set of GPCRs in the unicellular Amoebozoa was surprising and is indicative of the largely undiscovered environmental sensing capabilities in this group. Evolutionary transitions from unicellular to multicellular lifestyles, like we see in social amoebas, have occurred several times independently in the Amoebozoa, and GPCRs may have been co-opted for new functions in cell-cell communication. Methods We have analysed a set of GPCRs from fully sequenced Amoebozoan genomes by Bayesian inference, compared their phylogenetic distribution and domain composition, and analysed their temporal and spatial expression patterns in five species of dictyostelids. Results We found evidence that most GPCRs are conserved deeply in the Amoebozoa and are probably performing roles in general cell functions and complex environmental sensing. All families of GPCRs (apart from the family 4 fungal pheromone receptors) are present in dictyostelids with family 5 being the largest and family 2 the one with the fewest members. For the first time, we identify the presence of family 1 rhodopsin-like GPCRs in dictyostelids. Some GPCRs have been amplified in the dictyostelids and in specific lineages thereof and through changes in expression patterns may have been repurposed for signalling in multicellular development. Discussion Our phylogenetic analysis suggests that GPCR families 1, 2 and 6 already diverged early in the Amoebozoa, whereas families 3 and 5 expanded later within the dictyostelids. The family 6 cAMP receptors that have experimentally supported roles in multicellular development in dictyostelids ( carA-carD; tasA/B) originated at the root of all dictyostelids and only have weakly associated homologs in Physarum polycephalum. Our analysis identified candidate GPCRs which have evolved in the dictyostelids and could have been co-opted for multicellular development.

Folylpolyglutamate synthetase mRNA G-quadruplexes regulate its cell protrusion localization and enhance a cancer cell invasive phenotype upon folate repletion

BACKGROUND

Folates are crucial for the biosynthesis of nucleotides and amino acids, essential for cell proliferation and development. Folate deficiency induces DNA damage, developmental defects, and tumorigenicity. The obligatory enzyme folylpolyglutamate synthetase (FPGS) mediates intracellular folate retention via cytosolic and mitochondrial folate polyglutamylation. Our previous paper demonstrated the association of the cytosolic FPGS (cFPGS) with the cytoskeleton and various cell protrusion proteins. Based on these recent findings, the aim of the current study was to investigate the potential role of cFPGS at cell protrusions.

RESULTS

Here we uncovered a central role for two G-quadruplex (GQ) motifs in the 3'UTR of FPGS mediating the localization of cFPGS mRNA and protein at cell protrusions. Using the MBSV6-loop reporter system and fluorescence microscopy, we demonstrate that following folate deprivation, cFPGS mRNA is retained in the endoplasmic reticulum, whereas upon 15 min of folate repletion, this mRNA is rapidly translocated to cell protrusions in a 3'UTR- and actin-dependent manner. The actin dependency of this folate-induced mRNA translocation is shown by treatment with Latrunculin B and inhibitors of the Ras homolog family member A (RhoA) pathway. Upon folate repletion, the FPGS 3'UTR GQs induce an amoeboid/mesenchymal hybrid cell phenotype during migration and invasion through a collagen gel matrix. Targeted disruption of the 3'UTR GQ motifs by introducing point mutations or masking them by antisense oligonucleotides abrogated cell protrusion targeting of cFPGS mRNA.

CONCLUSIONS

Collectively, the GQ motifs within the 3'UTR of FPGS regulate its transcript and protein localization at cell protrusions in response to a folate cue, inducing cancer cell invasive phenotype. These novel findings suggest that the 3'UTR GQ motifs of FPGS constitute an attractive druggable target aimed at inhibition of cancer invasion and metastasis.

On the origin of the functional versatility of macrophages

Macrophages represent the most functionally versatile cells in the animal body. In addition to recognizing and destroying pathogens, macrophages remove senescent and exhausted cells, promote wound healing, and govern tissue and metabolic homeostasis. In addition, many specialized populations of tissue-resident macrophages exhibit highly specialized functions essential for the function of specific organs. Sometimes, however, macrophages cease to perform their protective function and their seemingly incomprehensible response to certain stimuli leads to pathology. In this study, we address the question of the origin of the functional versatility of macrophages. To this end, we have searched for the evolutionary origin of macrophages themselves and for the emergence of their characteristic properties. We hypothesize that many of the characteristic features of proinflammatory macrophages evolved in the unicellular ancestors of animals, and that the functional repertoire of macrophage-like amoebocytes further expanded with the evolution of multicellularity and the increasing complexity of tissues and organ systems. We suggest that the entire repertoire of macrophage functions evolved by repurposing and diversification of basic functions that evolved early in the evolution of metazoans under conditions barely comparable to that in tissues of multicellular organisms. We believe that by applying this perspective, we may find an explanation for the otherwise counterintuitive behavior of macrophages in many human pathologies.

C2GAP2 is a common regulator of Ras signaling for chemotaxis, phagocytosis, and macropinocytosis

Phagocytosis, macropinocytosis, and G protein coupled receptor-mediated chemotaxis are Ras-regulated and actin-driven processes. The common regulator for Ras activity in these three processes remains unknown. Here, we show that C2GAP2, a Ras GTPase activating protein, highly expressed in the vegetative growth state in model organism Dictyostelium. C2GAP2 localizes at the leading edge of chemotaxing cells, phagosomes during phagocytosis, and macropinosomes during micropinocytosis. c2gapB- cells lacking C2GAP2 displayed increased Ras activation upon folic acid stimulation and subsequent impaired chemotaxis in the folic acid gradient. In addition, c2gaB^- cells have elevated phagocytosis and macropinocytosis, which subsequently results in faster cell growth. C2GAP2 binds multiple phospholipids on the plasma membrane and the membrane recruitment of C2GAP2 requires calcium. Taken together, we show a shared negative regulator of Ras signaling that mediates Ras signaling for chemotaxis, phagocytosis, and macropinocytosis.

A meta-analysis indicates that the regulation of cell motility is a non-intrinsic function of chemoattractant receptors that is governed independently of directional sensing

Chemoattraction, defined as the migration of a cell toward a source of a chemical gradient, is controlled by chemoattractant receptors. Chemoattraction involves two basic activities, namely, directional sensing, a molecular mechanism that detects the direction of a source of chemoattractant, and actin-based motility, which allows the migration of a cell towards it. Current models assume first, that chemoattractant receptors govern both directional sensing and motility (most commonly inducing an increase in the migratory speed of the cells, i.e. chemokinesis), and, second, that the signaling pathways controlling both activities are intertwined. We performed a meta-analysis to reassess these two points. From this study emerge two main findings. First, although many chemoattractant receptors govern directional sensing, there are also receptors that do not regulate cell motility, suggesting that is the ability to control directional sensing, not motility, that best defines a chemoattractant receptor. Second, multiple experimental data suggest that receptor-controlled directional sensing and motility can be controlled independently. We hypothesize that this independence may be based on the existence of separated signalling modules that selectively govern directional sensing and motility in chemotactic cells. Together, the information gathered can be useful to update current models representing the signalling from chemoattractant receptors. The new models may facilitate the development of strategies for a more effective pharmacological modulation of chemoattractant receptor-controlled chemoattraction in health and disease.

An atypical MAPK regulates translocation of a GATA transcription factor in response to chemoattractant stimulation

The Dictyostelium atypical mitogen-activated protein kinase (MAPK) Erk2 is required for chemotactic responses to cAMP as amoeba undergo multicellular development. In this study, Erk2 was found to be essential for the cAMP-stimulated translocation of the GATA transcription factor GtaC as indicated by the distribution of a GFP-GtaC reporter. Erk2 was also found to be essential for the translocation of GtaC in response to external folate, a foraging signal that directs the chemotaxis of amoeba to bacteria. Erk1, the only other Dictyostelium MAPK, was not required for the GtaC translocation to either chemoattractant, indicating that GFP-GtaC is a kinase translocation reporter specific for atypical MAPKs. The translocation of GFP-GtaC in response to folate was absent in mutants lacking the folate receptor Far1 or the coupled G-protein subunit Gα4. Loss of GtaC function resulted in enhanced chemotactic movement to folate, suggesting that GtaC suppresses responses to folate. The alteration of four Erk2-preferred phosphorylation sites in GtaC impacted the translocation of GFP-GtaC in response to folate and the GFP-GtaC-mediated rescue of aggregation and development of gtaC- cells. The ability of different chemoattractants to stimulate Erk2-regulated GtaC translocation suggests that atypical MAPK-mediated regulation of transcription factors can contribute to different cell fates.

Centrosome Positioning in Migrating Dictyostelium Cells

Directional cell migration and the establishment of polarity play an important role in development, wound healing, and host cell defense. While actin polymerization provides the driving force at the cell front, the microtubule network assumes a regulatory function, in coordinating front protrusion and rear retraction. By using Dictyostelium discoideum cells as a model for amoeboid movement in different 2D and 3D environments, the position of the centrosome relative to the nucleus was analyzed using live-cell microscopy. Our results showed that the centrosome was preferentially located rearward of the nucleus under all conditions tested for directed migration, while the nucleus was oriented toward the expanding front. When cells are hindered from straight movement by obstacles, the centrosome is displaced temporarily from its rearward location to the side of the nucleus, but is reoriented within seconds. This relocalization is supported by the presence of intact microtubules and their contact with the cortex. The data suggest that the centrosome is responsible for coordinating microtubules with respect to the nucleus. In summary, we have analyzed the orientation of the centrosome during different modes of migration in an amoeboid model and present evidence that the basic principles of centrosome positioning and movement are conserved between Dictyostelium and human leukocytes.

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.

Under-Agarose Chemotaxis and Migration Assays for Dictyostelium

Chemotaxis-directional cell movement steered by chemical gradients-involved in many biological processes including embryonic morphogenesis and immune cell function. Eukaryotic cells, in response to external gradients of attractants, use conserved mechanisms to achieve chemotaxis by regulating the actin cytoskeleton at their fronts and myosin II at their rears. Dictyostelium discoideum, an amoeba that is widely used to study chemotaxis, uses chemotaxis to move up gradients of folate to identify and locate its bacterial prey. Similarly, when starved, Dictyostelium cells synthesize and secrete cyclic AMP (cAMP) while simultaneously expressing cAMP receptors. This allows them to chemotax toward their neighbors and aggregate together. The chemotactic behavior of cells can be studied using several techniques. One such, under-agarose chemotaxis, is a robust, easy, and inexpensive assay that allows direct quantification of chemotactic parameters such as speed and directionality. With the use of high-resolution imaging, for example confocal microscopy, detailed examination of the distribution of actin and membrane proteins in migrating wild type and mutant cells can be performed. In this chapter, we describe simple and optimized methods for studying folate and cAMP chemotaxis in Dictyostelium cells under agarose.

Chemotaxis: Under Agarose Assay

The unicellular eukaryotic amoeba, Dictyostelium discoideum, represents a superb model for examining the molecular mechanism of chemotaxis. Under vegetative conditions, the amoebae are chemotactically responsive to pterins, such as folic acid. Under starved conditions, they lose their sensitivity to pterins and become chemotactically responsive to cAMP. As an NIH model system, Dictyostelium offers a variety of advantages in studying chemotaxis, including ease of growth, genetic tractability, and the conservation of mammalian signaling pathways. In this chapter, we describe the use of the under-agarose chemotaxis assay to understand the signaling pathways controlling directional sensing and motility in Dictyostelium discoideum. Given the similarities between Dictyostelium and mammalian cells, this allows us to dissect conserved pathways involved in eukaryotic chemotaxis.

Integrative Pan-Cancer Analysis Confirmed that FCGR3A is a Candidate Biomarker Associated With Tumor Immunity

Background: Fc gamma receptor 3A (FCGR3A) encodes a receptor for the Fc portion of immunoglobulin G, which plays a significant role in the immune response. However, the role of FCGR3A in cancers remains unclear. This study aimed to visualize the prognostic landscape of FCGR3A in pan-cancer and investigate the relationship between FCGR3A expression and tumor microenvironment. Method: Based on the TCGA database, GTEx database, and GDSC database, we analyzed the expression of FCGR3A in pan-cancers and adjacent normal tissues and its relationship with prognosis, immune cells infiltration, immune-related genes, DNA mismatch repair (MMR) genes, DNA methylation, and drugs sensitivity. The gene alteration frequency of FCGR3A was acquired on the cBioportal website. Moreover, we constructed PPI networks, performed GO and KEGG analysis to illustrate the function, and signaling pathways of FCGR3A-related genes, and conducted gene set enrichment analysis (GSEA) of FCGR3A to further explore its potential biological functions. Result: The differential analysis results of the publicly available databases showed that FCGR3A was generally highly expressed in pan-cancer. Survival analysis revealed that FCGR3A predominated as a risk prognostic factor in most cancers. Additionally, the expression of FCGR3A was confirmed to be associated with several immune cells infiltration, multiple immune checkpoint genes, and DNA mismatch repair genes expression in generalized carcinoma. We also identified a negative correlation between FCGR3A and DNA methylation levels. Through GO/KEGG and GESA, we found that FCGR3A was involved in many pathologic and physiological processes, and was most closely related to tumor immune-related pathways. Drug sensitivity analysis showed that higher FCGR3A expression predicts a low IC50 value for the vast majority of drugs. Conclusions: FCGR3A may be an immune-oncogenic molecule that correlates with tumor immune infiltration levels and affects drug sensitivity, thus it can be served as a promising biomarker for cancer detection, prognosis, therapeutic design, and follow-up.

MAPK docking motif in the Dictyostelium Gα2 subunit is required for aggregation and transcription factor translocation

Some G protein alpha subunits contain a mitogen-activated protein kinase (MAPK) docking motif (D-motif) near the amino terminus that can impact cellular responses to external signals. The Dictyostelium Gα2 G protein subunit is required for chemotaxis to cAMP during the onset of multicellular development and this subunit contains a putative D-motif near the amino terminus. The Gα2 subunit D-motif was altered to examine its potential role in chemotaxis and multicellular development. In gα2^- cells the expression of the D-motif mutant (Gα2^D-) or wild-type subunit from high copy number vectors rescued cell aggregation but blocked the transition of mounds into slugs. This phenotype was also observed in parental strains with a wild-type gα2 locus indicating that the heterologous Gα2 subunit expression interferes with multicellular morphogenesis. Expression of the Gα2^D- subunit from a low copy number vectors in gα2^- cells did not rescue aggregation whereas the wild-type Gα2 subunit rescued aggregation efficiently and allowed wild-type morphological development. The Gα2^D- and Gα2 subunit were both capable of restoring comparable levels of cAMP stimulated motility and the ability to co-aggregate with wild-type cells implying that the aggregation defect of Gα2^D- expressing cells is due to insufficient intercellular signaling. Expression of the Gα2 subunit but not the Gα2^D- subunit fully restored the ability of cAMP to stimulate the translocation of the GtaC transcription factor suggesting the D-motif is important for transcription factor regulation. These results suggest that the D-motif of Gα2 plays a role in aggregation and other developmental responses involved with cAMP signaling.

Hypoxia triggers collective aerotactic migration in Dictyostelium discoideum

Using a self-generated hypoxic assay, we show that the amoeba Dictyostelium discoideum displays a remarkable collective aerotactic behavior. When a cell colony is covered, cells quickly consume the available oxygen (O₂) and form a dense ring moving outwards at constant speed and density. To decipher this collective process, we combined two technological developments: porphyrin-based O₂ -sensing films and microfluidic O₂ gradient generators. We showed that Dictyostelium cells exhibit aerotactic and aerokinetic response in a low range of O₂ concentration indicative of a very efficient detection mechanism. Cell behaviors under self-generated or imposed O₂ gradients were modeled using an in silico cellular Potts model built on experimental observations. This computational model was complemented with a parsimonious ‘Go or Grow’ partial differential equation (PDE) model. In both models, we found that the collective migration of a dense ring can be explained by the interplay between cell division and the modulation of aerotaxis.

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.

Aberrant Autophagy Impacts Growth and Multicellular Development in a Dictyostelium Knockout Model of CLN5 Disease

Mutations in CLN5 cause a subtype of neuronal ceroid lipofuscinosis (NCL) called CLN5 disease. While the precise role of CLN5 in NCL pathogenesis is not known, recent work revealed that the protein has glycoside hydrolase activity. Previous work on the Dictyostelium discoideum homolog of human CLN5, Cln5, revealed its secretion during the early stages of development and its role in regulating cell adhesion and cAMP-mediated chemotaxis. Here, we used Dictyostelium to examine the effect of cln5-deficiency on various growth and developmental processes during the life cycle. During growth, cln5^– cells displayed reduced cell proliferation, cytokinesis, viability, and folic acid-mediated chemotaxis. In addition, the growth of cln5^– cells was severely impaired in nutrient-limiting media. Based on these findings, we assessed autophagic flux in growth-phase cells and observed that loss of cln5 increased the number of autophagosomes suggesting that the basal level of autophagy was increased in cln5^– cells. Similarly, loss of cln5 increased the amounts of ubiquitin-positive proteins. During the early stages of multicellular development, the aggregation of cln5^– cells was delayed and loss of the autophagy genes, atg1 and atg9, reduced the extracellular amount of Cln5. We also observed an increased amount of intracellular Cln5 in cells lacking the Dictyostelium homolog of the human glycoside hydrolase, hexosaminidase A (HEXA), further supporting the glycoside hydrolase activity of Cln5. This observation was also supported by our finding that CLN5 and HEXA expression are highly correlated in human tissues. Following mound formation, cln5^– development was precocious and loss of cln5 affected spore morphology, germination, and viability. When cln5^– cells were developed in the presence of the autophagy inhibitor ammonium chloride, the formation of multicellular structures was impaired, and the size of cln5^– slugs was reduced relative to WT slugs. These results, coupled with the aberrant autophagic flux observed in cln5^– cells during growth, support a role for Cln5 in autophagy during the Dictyostelium life cycle. In total, this study highlights the multifaceted role of Cln5 in Dictyostelium and provides insight into the pathological mechanisms that may underlie CLN5 disease.

Neutrophils self-limit swarming to contain bacterial growth in vivo

Neutrophils communicate with each other to form swarms in infected organs. Coordination of this population response is critical for the elimination of bacteria and fungi. Using transgenic mice, we found that neutrophils have evolved an intrinsic mechanism to self-limit swarming and avoid uncontrolled aggregation during inflammation. G protein-coupled receptor (GPCR) desensitization acts as a negative feedback control to stop migration of neutrophils when they sense high concentrations of self-secreted attractants that initially amplify swarming. Interference with this process allows neutrophils to scan larger tissue areas for microbes. Unexpectedly, this does not benefit bacterial clearance as containment of proliferating bacteria by neutrophil clusters becomes impeded. Our data reveal how autosignaling stops self-organized swarming behavior and how the finely tuned balance of neutrophil chemotaxis and arrest counteracts bacterial escape.

Origins of eukaryotic excitability

All living cells interact dynamically with a constantly changing world. Eukaryotes, in particular, evolved radically new ways to sense and react to their environment. These advances enabled new and more complex forms of cellular behaviour in eukaryotes, including directional movement, active feeding, mating, and responses to predation. But what are the key events and innovations during eukaryogenesis that made all of this possible? Here we describe the ancestral repertoire of eukaryotic excitability and discuss five major cellular innovations that enabled its evolutionary origin. The innovations include a vastly expanded repertoire of ion channels, the emergence of cilia and pseudopodia, endomembranes as intracellular capacitors, a flexible plasma membrane and the relocation of chemiosmotic ATP synthesis to mitochondria, which liberated the plasma membrane for more complex electrical signalling involved in sensing and reacting. We conjecture that together with an increase in cell size, these new forms of excitability greatly amplified the degrees of freedom associated with cellular responses, allowing eukaryotes to vastly outperform prokaryotes in terms of both speed and accuracy. This comprehensive new perspective on the evolution of excitability enriches our view of eukaryogenesis and emphasizes behaviour and sensing as major contributors to the success of eukaryotes. This article is part of the theme issue 'Basal cognition: conceptual tools and the view from the single cell'.

Role of LrrkA in the Control of Phagocytosis and Cell Motility in Dictyostelium discoideum

LrrkA is a Dictyostelium discoideum kinase with leucine-rich repeats. LrrkA stimulates Kil2 and intra-phagosomal killing of ingested bacteria in response to folate. In this study, we show that genetic inactivation of lrrkA also causes a previously unnoticed phenotype: lrrkA KO cells exhibit enhanced phagocytosis and cell motility compared to parental cells. This phenotype is cell autonomous, is reversible upon re-expression of LrrkA, and is not due to an abnormal response to inhibitory quorum-sensing factors secreted by D. discoideum in its medium. In addition, folate increases motility in parental D. discoideum cells, but not in lrrkA KO cells, suggesting that LrrkA plays a pivotal role in the cellular response to folate. On the contrary, lrrkA KO cells regulate gene transcription in response to folate in a manner indistinguishable from parental cells. Overall, based on analysis of mutant phenotypes, we identify gene products that participate in the control of intracellular killing, cell motility, and gene transcription in response to folate. These observations reveal a mechanism by which D. discoideum encountering bacterially-secreted folate can migrate, engulf, and kill bacteria more efficiently.

Imaging GPCR-Mediated Signal Events Leading to Chemotaxis and Phagocytosis

Eukaryotic phagocytes locate microorganisms via chemotaxis and consume them through phagocytosis. The social amoeba Dictyostelium discoideum is a stereotypical phagocyte and a well-established model to study both processes. Recent studies show that a G-protein-coupled receptor (fAR1) mediate a signaling network to control reorganization of the actin cytoskeleton leading both the directional cell movement and the engulfment of bacteria. Many live cell imaging methods have been developed and applied to monitor these signaling events. In this chapter, we will introduce how to measure GPCR-mediated signaling events for cell migration and phagocytosis in Dictyostelium.

A CRISPR-based method for testing the essentiality of a gene

The CRISPR/Cas9 system is a powerful method of editing genes by randomly introducing errors into the target sites. Here, we describe a CRISPR-based test for gene essentiality (CRISPR-E test) that allows the identification of essential genes. Specifically, we use sgRNA-mediated CRISPR/Cas9 to target the open reading frame of a gene in the genome and analyze the in-frame (3n) and frameshift (3n + 1 and 3n + 2) mutations in the targeted region of the gene in surviving cells. If the gene is non-essential, the cells would carry both in-frame (3n) and frameshift (3n + 1 and 3n + 2) mutations. In contrast, the cells would carry only in-frame (3n) mutations if the targeted gene is essential, and this selective elimination of frameshift (3n + 1 and 3n + 2) mutations of the gene indicate its essentiality. As a proof of concept, we have used this CRISPR-E test in the model organism Dictyostelium discoideum to demonstrate that Dync1li1 is an essential gene while KIF1A and fAR1 are not. We further propose a simple method for quantifying the essentiality of a gene using the CRISPR-E test.

Extracellular signaling in Dictyostelium

In the last few decades, we have learned a considerable amount about how eukaryotic cells communicate with each other, and what it is the cells are telling each other. The simplicity of Dictyostelium discoideum, and the wide variety of available tools to study this organism, makes it the equivalent of a hydrogen atom for cell and developmental biology. Studies using Dictyostelium have pioneered a good deal of our understanding of eukaryotic cell communication. In this review, we will present a brief overview of how Dictyostelium cells use extracellular signals to attract each other, repel each other, sense their local cell density, sense whether the nearby cells are starving or stressed, count themselves to organize the formation of structures containing a regulated number of cells, sense the volume they are in, and organize their multicellular development. Although we are probably just beginning to learn what the cells are telling each other, the elucidation of Dictyostelium extracellular signals has already led to the development of possible therapeutics for human diseases.

The excitable signal transduction networks: movers and shapers of eukaryotic cell migration

In response to a variety of external cues, eukaryotic cells display varied migratory modes to perform their physiological functions during development and in the adult. Aberrations in cell migration result in embryonic defects and cancer metastasis. The molecular components involved in cell migration are remarkably conserved between the social amoeba Dictyostelium and mammalian cells. This makes the amoeba an excellent model system for studies of eukaryotic cell migration. These migration-associated components can be grouped into three networks: input, signal transduction and cytoskeletal. In migrating cells, signal transduction events such as Ras or PI3K activity occur at the protrusion tips, referred to as 'front', whereas events such as dissociation of PTEN from these regions are referred to as 'back'. Asymmetric distribution of such front and back events is crucial for establishing polarity and guiding cell migration. The triggering of these signaling events displays properties of biochemical excitability including all-or-nothing responsiveness to suprathreshold stimuli, refractoriness, and wave propagation. These signal transduction waves originate from a point and propagate towards the edge of the cell, thereby driving cytoskeletal activity and cellular protrusions. Any change in the threshold for network activation alters the range of the propagating waves and the size of cellular protrusions which gives rise to various migratory modes in cells. Thus, this review highlights excitable signal transduction networks as key players for coordinating cytoskeletal activities to drive cell migration in all eukaryotes.

Calmodulin-mediated events during the life cycle of the amoebozoan Dictyostelium discoideum

This review focusses on the functions of intracellular and extracellular calmodulin, its target proteins and their binding proteins during the asexual life cycle of Dictyostelium discoideum. Calmodulin is a primary regulatory protein of calcium signal transduction that functions throughout all stages. During growth, it mediates autophagy, the cell cycle, folic acid chemotaxis, phagocytosis, and other functions. During mitosis, specific calmodulin-binding proteins translocate to alternative locations. Translocation of at least one cell adhesion protein is calmodulin dependent. When starved, cells undergo calmodulin-dependent chemotaxis to cyclic AMP generating a multicellular pseudoplasmodium. Calmodulin-dependent signalling within the slug sets up a defined pattern and polarity that sets the stage for the final events of morphogenesis and cell differentiation. Transected slugs undergo calmodulin-dependent transdifferentiation to re-establish the disrupted pattern and polarity. Calmodulin function is critical for stalk cell differentiation but also functions in spore formation, events that begin in the pseudoplasmodium. The asexual life cycle restarts with the calmodulin-dependent germination of spores. Specific calmodulin-binding proteins as well as some of their binding partners have been linked to each of these events. The functions of extracellular calmodulin during growth and development are also discussed. This overview brings to the forefront the central role of calmodulin, working through its numerous binding proteins, as a primary downstream regulator of the critical calcium signalling pathways that have been well established in this model eukaryote. This is the first time the function of calmodulin and its target proteins have been documented through the complete life cycle of any eukaryote.

Interactome and F-Actin Interaction Analysis of Dictyostelium discoideum Coronin A

Coronin proteins are evolutionary conserved WD repeat containing proteins that have been proposed to carry out different functions. In Dictyostelium, the short coronin isoform, coronin A, has been implicated in cytoskeletal reorganization, chemotaxis, phagocytosis and the initiation of multicellular development. Generally thought of as modulators of F-actin, coronin A and its mammalian homologs have also been shown to mediate cellular processes in an F-actin-independent manner. Therefore, it remains unclear whether or not coronin A carries out its functions through its capacity to interact with F-actin. Moreover, the interacting partners of coronin A are not known. Here, we analyzed the interactome of coronin A as well as its interaction with F-actin within cells and in vitro. Interactome analysis showed the association with a diverse set of interaction partners, including fimbrin, talin and myosin subunits, with only a transient interaction with the minor actin10 isoform, but not the major form of actin, actin8, which was consistent with the absence of a coronin A-actin interaction as analyzed by co-sedimentation from cells and lysates. In vitro, however, purified coronin A co-precipitated with rabbit muscle F-actin in a coiled-coil-dependent manner. Our results suggest that an in vitro interaction of coronin A and rabbit muscle actin may not reflect the cellular interaction state of coronin A with actin, and that coronin A interacts with diverse proteins in a time-dependent manner.

DPF is a cell-density sensing factor, with cell-autonomous and non-autonomous functions during Dictyostelium growth and development

Background

Cellular functions can be regulated by cell-cell interactions that are influenced by extra-cellular, density-dependent signaling factors. Dictyostelium grow as individual cells in nutrient-rich sources, but, as nutrients become depleted, they initiate a multi-cell developmental program that is dependent upon a cell-density threshold. We hypothesized that novel secreted proteins may serve as density-sensing factors to promote multi-cell developmental fate decisions at a specific cell-density threshold, and use Dictyostelium in the identification of such a factor.

Results

We show that multi-cell developmental aggregation in Dictyostelium is lost upon minimal (2-fold) reduction in local cell density. Remarkably, developmental aggregation response at non-permissive cell densities is rescued by addition of conditioned media from high-density, developmentally competent cells. Using rescued aggregation of low-density cells as an assay, we purified a single, 150-kDa extra-cellular protein with density aggregation activity. MS/MS peptide sequence analysis identified the gene sequence, and cells that overexpress the full-length protein accumulate higher levels of a development promoting factor (DPF) activity than parental cells, allowing cells to aggregate at lower cell densities; cells deficient for this DPF gene lack density-dependent developmental aggregation activity and require higher cell density for cell aggregation compared to WT. Density aggregation activity co-purifies with tagged versions of DPF and tag-affinity-purified DPF possesses density aggregation activity. In mixed development with WT, cells that overexpress DPF preferentially localize at centers for multi-cell aggregation and define cell-fate choice during cytodifferentiation. Finally, we show that DPF is synthesized as a larger precursor, single-pass transmembrane protein, with the p150 fragment released by proteolytic cleavage and ectodomain shedding. The TM/cytoplasmic domain of DPF possesses cell-autonomous activity for cell-substratum adhesion and for cellular growth.

Conclusions

We have purified a novel secreted protein, DPF, that acts as a density-sensing factor for development and functions to define local collective thresholds for Dictyostelium development and to facilitate cell-cell communication and multi-cell formation. Regions of high DPF expression are enriched at centers for cell-cell signal-response, multi-cell formation, and cell-fate determination. Additionally, DPF has separate cell-autonomous functions for regulation of cellular adhesion and growth.

LrrkA, a kinase with leucine-rich repeats, links folate sensing with Kil2 activity and intracellular killing

Phagocytic cells ingest bacteria by phagocytosis and kill them efficiently inside phagolysosomes. The molecular mechanisms involved in intracellular killing and their regulation are complex and still incompletely understood. Dictyostelium discoideum has been used as a model to discover and to study new gene products involved in intracellular killing of ingested bacteria. In this study, we performed random mutagenesis of Dictyostelium cells and isolated a mutant defective for growth on bacteria. This mutant is characterized by the genetic inactivation of the lrrkA gene, which encodes a protein with a kinase domain and leucine-rich repeats. LrrkA knockout (KO) cells kill ingested Klebsiella pneumoniae bacteria inefficiently. This defect is not additive to the killing defect observed in kil2 KO cells, suggesting that the function of Kil2 is partially controlled by LrrkA. Indeed, lrrkA KO cells exhibit a phenotype similar to that of kil2 KO cells: Intraphagosomal proteolysis is inefficient, and both intraphagosomal killing and proteolysis are restored upon exogenous supplementation with magnesium ions. Bacterially secreted folate stimulates intracellular killing in Dictyostelium cells, but this stimulation is lost in cells with genetic inactivation of kil2, lrrkA, or far1. Together, these results indicate that the stimulation of intracellular killing by folate involves Far1 (the cell surface receptor for folate), LrrkA, and Kil2. This study is the first identification of a signalling pathway regulating intraphagosomal bacterial killing in Dictyostelium cells.

The Atypical MAP Kinase ErkB Transmits Distinct Chemotactic Signals through a Core Signaling Module

Signaling from chemoattractant receptors activates the cytoskeleton of crawling cells for chemotaxis. We show using phosphoproteomics that different chemoattractants cause phosphorylation of the same core set of around 80 proteins in Dictyostelium cells. Strikingly, the majority of these are phosphorylated at an [S/T]PR motif by the atypical MAP kinase ErkB. Unlike most chemotactic responses, ErkB phosphorylations are persistent and do not adapt to sustained stimulation with chemoattractant. ErkB integrates dynamic autophosphorylation with chemotactic signaling through G-protein-coupled receptors. Downstream, our phosphoproteomics data define a broad panel of regulators of chemotaxis. Surprisingly, targets are almost exclusively other signaling proteins, rather than cytoskeletal components, revealing ErkB as a regulator of regulators rather than acting directly on the motility machinery. ErkB null cells migrate slowly and orientate poorly over broad dynamic ranges of chemoattractant. Our data indicate a central role for ErkB and its substrates in directing chemotaxis.

The origins and evolution of macropinocytosis

In macropinocytosis, cells take up micrometre-sized droplets of medium into internal vesicles. These vesicles are acidified and fused to lysosomes, their contents digested and useful compounds extracted. Indigestible contents can be exocytosed. Macropinocytosis has been known for approaching 100 years and is described in both metazoa and amoebae, but not in plants or fungi. Its evolutionary origin goes back to at least the common ancestor of the amoebozoa and opisthokonts, with apparent secondary loss from fungi. The primary function of macropinocytosis in amoebae and some cancer cells is feeding, but the conserved processing pathway for macropinosomes, which involves shrinkage and the retrieval of membrane to the cell surface, has been adapted in immune cells for antigen presentation. Macropinocytic cups are large actin-driven processes, closely related to phagocytic cups and pseudopods and appear to be organized around a conserved signalling patch of PIP3, active Ras and active Rac that directs actin polymerization to its periphery. Patches can form spontaneously and must be sustained by excitable kinetics with strong cooperation from the actin cytoskeleton. Growth-factor signalling shares core components with macropinocytosis, based around phosphatidylinositol 3-kinase (PI3-kinase), and we suggest that it evolved to take control of ancient feeding structures through a coupled growth factor receptor. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

G-protein-coupled formyl peptide receptors play a dual role in neutrophil chemotaxis and bacterial phagocytosis

A dogma of innate immunity is that neutrophils use G-protein–coupled receptors (GPCRs) for chemoattractant to chase bacteria through chemotaxis and then use phagocytic receptors coupled with tyrosine kinases to destroy opsonized bacteria via phagocytosis. Our current work showed that G-protein–coupled formyl peptide receptors (FPRs) directly mediate neutrophil phagocytosis. Mouse neutrophils lacking formyl peptide receptors (Fpr1/2^–/–) are defective in the phagocytosis of Escherichia coli and the chemoattractant N-formyl-Met-Leu-Phe (fMLP)-coated beads. fMLP immobilized onto the surface of a bead interacts with FPRs, which trigger a Ca²⁺ response and induce actin polymerization to form a phagocytic cup for engulfment of the bead. This chemoattractant GPCR/Gi signaling works independently of phagocytic receptor/tyrosine kinase signaling to promote phagocytosis. Thus, in addition to phagocytic receptor-mediated phagocytosis, neutrophils also utilize the chemoattractant GPCR/Gi signaling to mediate phagocytosis to fight against invading bacteria.

G protein subunit phosphorylation as a regulatory mechanism in heterotrimeric G protein signaling in mammals, yeast, and plants

Heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits are vital eukaryotic signaling elements that convey information from ligand-regulated G protein-coupled receptors (GPCRs) to cellular effectors. Heterotrimeric G protein-based signaling pathways are fundamental to human health [Biochimica et Biophysica Acta (2007) 1768, 994-1005] and are the target of >30% of pharmaceuticals in clinical use [Biotechnology Advances (2013) 31, 1676-1694; Nature Reviews Drug Discovery (2017) 16, 829-842]. This review focuses on phosphorylation of G protein subunits as a regulatory mechanism in mammals, budding yeast, and plants. This is a re-emerging field, as evidence for phosphoregulation of mammalian G protein subunits from biochemical studies in the early 1990s can now be complemented with contemporary phosphoproteomics and genetic approaches applied to a diversity of model systems. In addition, new evidence implicates a family of plant kinases, the receptor-like kinases, which are monophyletic with the interleukin-1 receptor-associated kinase/Pelle kinases of metazoans, as possible GPCRs that signal via subunit phosphorylation. We describe early and modern observations on G protein subunit phosphorylation and its functional consequences in these three classes of organisms, and suggest future research directions.

Calcium influx mediates the chemoattractant-induced translocation of the arrestin-related protein AdcC in Dictyostelium

Arrestins are key adaptor proteins that control the fate of cell-surface membrane proteins and modulate downstream signaling cascades. The Dictyostelium discoideum genome encodes six arrestin-related proteins, harboring additional modules besides the arrestin domain. Here, we studied AdcB and AdcC, two homologs that contain C2 and SAM domains. We showed that AdcC - in contrast to AdcB - responds to various stimuli (such as the chemoattractants cAMP and folate) known to induce an increase in cytosolic calcium by transiently translocating to the plasma membrane, and that calcium is a direct regulator of AdcC localization. This response requires the calcium-dependent membrane-targeting C2 domain and the double SAM domain involved in AdcC oligomerization, revealing a mode of membrane targeting and regulation unique among members of the arrestin clan. AdcB shares several biochemical properties with AdcC, including in vitro binding to anionic lipids in a calcium-dependent manner and auto-assembly as large homo-oligomers. AdcB can interact with AdcC; however, its intracellular localization is insensitive to calcium. Therefore, despite their high degree of homology and common characteristics, AdcB and AdcC are likely to fulfill distinct functions in amoebae.

Functions of the Dictyostelium LIMP-2 and CD36 homologues in bacteria uptake, phagolysosome biogenesis and host cell defence

Phagocytic cells take up, kill and digest microbes by a process called phagocytosis. To this end, these cells bind the particle, rearrange their actin cytoskeleton, and orchestrate transport of digestive factors to the particle-containing phagosome. The mammalian lysosomal membrane protein LIMP-2 (also known as SCARB2) and CD36, members of the class B of scavenger receptors, play a crucial role in lysosomal enzyme trafficking and uptake of mycobacteria, respectively, and generally in host cell defences against intracellular pathogens. Here, we show that the Dictyostelium discoideum LIMP-2 homologue LmpA regulates phagocytosis and phagolysosome biogenesis. The lmpA knockdown mutant is highly affected in actin-dependent processes, such as particle uptake, cellular spreading and motility. Additionally, the cells are severely impaired in phagosomal acidification and proteolysis, likely explaining the higher susceptibility to infection with the pathogenic bacterium Mycobacterium marinum, a close cousin of the human pathogen Mycobacterium tuberculosis Furthermore, we bring evidence that LmpB is a functional homologue of CD36 and specifically mediates uptake of mycobacteria. Altogether, these data indicate a role for LmpA and LmpB, ancestors of the family of which LIMP-2 and CD36 are members, in lysosome biogenesis and host cell defence.

Decoding the chemotactic signal

From an individual bacterium to the cells that compose the human immune system, cellular chemotaxis plays a fundamental role in allowing cells to navigate, interpret, and respond to their environments. While many features of cellular chemotaxis are shared among systems as diverse as bacteria and human immune cells, the machinery that guides the migration of these model organisms varies widely. In this article, we review current literature on the diversity of chemoattractant ligands, the cell surface receptors that detect and process chemotactic gradients, and the link between signal recognition and the regulation of cellular machinery that allow for efficient directed cellular movement. These facets of cellular chemotaxis are compared among E. coli, Dictyostelium discoideum, and mammalian neutrophils to derive organizational principles by which diverse cell systems sense and respond to chemotactic gradients to initiate cellular migration.

A G-protein-coupled chemoattractant receptor recognizes lipopolysaccharide for bacterial phagocytosis

Phagocytes locate microorganisms via chemotaxis and then consume them using phagocytosis. Dictyostelium amoebas are stereotypical phagocytes that prey on diverse bacteria using both processes. However, as typical phagocytic receptors, such as complement receptors or Fcγ receptors, have not been found in Dictyostelium, it remains mysterious how these cells recognize bacteria. Here, we show that a single G-protein-coupled receptor (GPCR), folic acid receptor 1 (fAR1), simultaneously recognizes the chemoattractant folate and the phagocytic cue lipopolysaccharide (LPS), a major component of bacterial surfaces. Cells lacking fAR1 or its cognate G-proteins are defective in chemotaxis toward folate and phagocytosis of Klebsiella aerogenes. Computational simulations combined with experiments show that responses associated with chemotaxis can also promote engulfment of particles coated with chemoattractants. Finally, the extracellular Venus-Flytrap (VFT) domain of fAR1 acts as the binding site for both folate and LPS. Thus, fAR1 represents a new member of the pattern recognition receptors (PRRs) and mediates signaling from both bacterial surfaces and diffusible chemoattractants to reorganize actin for chemotaxis and phagocytosis.

The physiological regulation of macropinocytosis during Dictyostelium growth and development

Macropinocytosis is a conserved endocytic process used by Dictyostelium amoebae for feeding on liquid medium. To further Dictyostelium as a model for macropinocytosis, we developed a high-throughput flow cytometry assay to measure macropinocytosis, and used it to identify inhibitors and investigate the physiological regulation of macropinocytosis. Dictyostelium has two feeding states: phagocytic and macropinocytic. When cells are switched from phagocytic growth on bacteria to liquid media, the rate of macropinocytosis slowly increases, due to increased size and frequency of macropinosomes. Upregulation is triggered by a minimal medium containing three amino acids plus glucose and likely depends on macropinocytosis itself. The presence of bacteria suppresses macropinocytosis while their product, folate, partially suppresses upregulation of macropinocytosis. Starvation, which initiates development, does not of itself suppress macropinocytosis: this can continue in isolated cells, but is shut down by a conditioned-medium factor or activation of PKA signalling. Thus macropinocytosis is a facultative ability of Dictyostelium cells, regulated by environmental conditions that are identified here.This article has an associated First Person interview with the first author of the paper.

Dictyostelium Erk2 is an atypical MAPK required for chemotaxis

The Dictyostelium genome encodes only two MAPKs, Erk1 and Erk2, and both are expressed during growth and development. Reduced levels of Erk2 expression have been shown previously to restrict cAMP production during development but still allow for chemotactic movement. In this study the erk2 gene was disrupted to eliminate Erk2 function. The absence of Erk2 resulted in a complete loss of folate and cAMP chemotaxis suggesting that this MAPK plays an integral role in the signaling mechanisms involved with this cellular response. However, folate stimulation of early chemotactic responses, such as Ras and PI3K activation and rapid actin filament formation, were not affected by the loss of Erk2 function. The erk2^- cells had a severe defect in growth on bacterial lawns but assays of bacterial cell engulfment displayed only subtle changes in the rate of bacterial engulfment. Only cells with no MAPK function, erk1^-erk2^- double mutants, displayed a severe proliferation defect in axenic medium. Loss of Erk2 impaired the phosphorylation of Erk1 in secondary responses to folate stimulation indicating that Erk2 has a role in the regulation of Erk1 activation during chemotaxis. Loss of the only known Dictyostelium MAPK kinase, MekA, prevented the phosphorylation of Erk1 but not Erk2 in response to folate and cAMP confirming that Erk2 is not regulated by a conventional MAP2K. This lack of MAP2K phosphorylation of Erk2 and the sequence similarity of Erk2 to mammalian MAPK15 (Erk8) suggest that the Dictyostelium Erk2 belongs to a group of atypical MAPKs. MAPK activation has been observed in chemotactic responses in a wide range of organisms but this study demonstrates an essential role for MAPK function in chemotactic movement. This study also confirms that MAPKs provide critical contributions to cell proliferation.