Dictyostelium discoideum paxillin regulates actin-based processes

Reduced PaxillinB localization to cell-substrate adhesions promotes cell migration in Dictyostelium

Many cells adhere to extracellular matrix for efficient cell migration. This adhesion is mediated by focal adhesions, a protein complex linking the extracellular matrix to the intracellular cytoskeleton. Focal adhesions have been studied extensively in mesenchymal cells, but recent research in physiological contexts and amoeboid cells suggest focal adhesion regulation differs from the mesenchymal focal adhesion paradigm. We used Dictyostelium discoideum to uncover new mechanisms of focal adhesion regulation, as Dictyostelium are amoeboid cells that form focal adhesion-like structures for migration. We show that PaxillinB, the Dictyostelium homologue of Paxillin, localizes to dynamic focal adhesion-like structures during Dictyostelium migration. Unexpectedly, reduced PaxillinB recruitment to these structures increases Dictyostelium cell migration. Quantitative analysis of focal adhesion size and dynamics show that lack of PaxillinB recruitment to focal adhesions does not alter focal adhesion size, but rather increases focal adhesion turnover. These findings are in direct contrast to Paxillin function at focal adhesions during mesenchymal migration, challenging the established focal adhesion model.

Effects of wounds in the cell membrane on cell division

Cells are consistently subjected to wounding by physical or chemical damages from the external environment. We previously showed that a local wound of the cell membrane modulates the polarity of cell migration and the wounded cells escape from the wound site in Dictyostelium. Here, we examined effects of wounds on dividing cells. When the cell membrane at the cleavage furrow during cytokinesis was locally wounded using laserporation, furrow constriction was significantly accelerated. Neither myosin II nor cortexillins contributed to the acceleration, because the acceleration was not hindered in mutant cells deficient in these proteins. When the cell membrane outside the furrow was wounded, the furrow constriction was not accelerated. Instead, the wounded-daughter half became smaller and the unwounded half became larger, resulting in an asymmetrical cell division. These phenomena occurred independently of wound repair. When cells in anaphase were wounded at the presumptive polar region, about 30% of the wounded cells changed the orientation of the division axis. From these observations, we concluded that dividing cells also escape from the wound site. The wound experiments on dividing cells also provide new insights into the mechanism of cytokinesis and cell polarity establishment.

An altered transcriptome underlies cln5-deficiency phenotypes in Dictyostelium discoideum

Mutations in CLN5 cause a subtype of neuronal ceroid lipofuscinosis (NCL) called CLN5 disease. The NCLs, commonly referred to as Batten disease, are a family of neurodegenerative lysosomal storage diseases that affect all ages and ethnicities globally. Previous research showed that CLN5 participates in a variety of cellular processes. However, the precise function of CLN5 in the cell and the pathway(s) regulating its function are not well understood. In the model organism Dictyostelium discoideum, loss of the CLN5 homolog, cln5, impacts various cellular and developmental processes including cell proliferation, cytokinesis, aggregation, cell adhesion, and terminal differentiation. In this study, we used comparative transcriptomics to identify differentially expressed genes underlying cln5-deficiency phenotypes during growth and the early stages of multicellular development. During growth, genes associated with protein ubiquitination/deubiquitination, cell cycle progression, and proteasomal degradation were affected, while genes linked to protein and carbohydrate catabolism were affected during early development. We followed up this analysis by showing that loss of cln5 alters the intracellular and extracellular amounts of proliferation repressors during growth and increases the extracellular amount of conditioned medium factor, which regulates cAMP signalling during the early stages of development. Additionally, cln5- cells displayed increased intracellular and extracellular amounts of discoidin, which is involved in cell-substrate adhesion and migration. Previous work in mammalian models reported altered lysosomal enzyme activity due to mutation or loss of CLN5. Here, we detected altered intracellular activities of various carbohydrate enzymes and cathepsins during cln5- growth and starvation. Notably, cln5- cells displayed reduced β-hexosaminidase activity, which aligns with previous work showing that D. discoideum Cln5 and human CLN5 can cleave the substrate acted upon by β-hexosaminidase. Finally, consistent with the differential expression of genes associated with proteasomal degradation in cln5- cells, we also observed elevated amounts of a proteasome subunit and reduced proteasome 20S activity during cln5- growth and starvation. Overall, this study reveals the impact of cln5-deficiency on gene expression in D. discoideum, provides insight on the genes and proteins that play a role in regulating Cln5-dependent processes, and sheds light on the molecular mechanisms underlying CLN5 disease.

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.

Differential Regulation of Adhesion and Phagocytosis of Resting and Activated Microglia by Dopamine

Microglia, the immune competent cells of the central nervous system (CNS), normally exist in a resting state characterized by a ramified morphology with many processes, and become activated to amoeboid morphology in response to brain injury, infection, and a variety of neuroinflammatory stimuli. Many studies focused on how neurotransmitters affect microglia activation in pathophysiological circumstances. In this study, we tried to gain mechanistic insights on how dopamine (DA) released from neurons modulates cellular functions of resting and activated microglia. DA induced the reduction of the number of cellular processes, the increase of cell adhesion/spreading, and the increase of vimentin filaments in resting primary and BV₂ microglia. In contrast to resting cells, DA downregulated the cell spreading and phagocytosis of microglia activated by LPS. DA also significantly downregulated ERK1/2 phosphorylation in activated microglia, but not in resting microglia. Downregulation of ERK1/2 by DA in activated microglia required receptor signaling. In contrast, we found a significant increase of p38MAPK activity by DA treatment in resting, but not in activated microglia. These latter effects required the uptake of DA through the high-affinity transporter but did not require receptor signaling. Activation of p38MAPK resulted in the increase of focal adhesion number via phosphorylation of paxillin at Ser⁸³. These results indicate that DA might have a differential, depending upon the activation stage of microglia, impact on cellular functions such as adhesion and phagocytosis.

Paxillin facilitates timely neurite initiation on soft-substrate environments by interacting with the endocytic machinery

Neurite initiation is the first step in neuronal development and occurs spontaneously in soft tissue environments. Although the mechanisms regulating the morphology of migratory cells on rigid substrates in cell culture are widely known, how soft environments modulate neurite initiation remains elusive. Using hydrogel cultures, pharmacologic inhibition, and genetic approaches, we reveal that paxillin-linked endocytosis and adhesion are components of a bistable switch controlling neurite initiation in a substrate modulus-dependent manner. On soft substrates, most paxillin binds to endocytic factors and facilitates vesicle invagination, elevating neuritogenic Rac1 activity and expression of genes encoding the endocytic machinery. By contrast, on rigid substrates, cells develop extensive adhesions, increase RhoA activity and sequester paxillin from the endocytic machinery, thereby delaying neurite initiation. Our results highlight paxillin as a core molecule in substrate modulus-controlled morphogenesis and define a mechanism whereby neuronal cells respond to environments exhibiting varying mechanical properties.

Functional interaction of PkcA and PldB regulate aggregation and development in Dictyostelium discoideum

Multicellular development in Dictyostelium discoideum involves tightly regulated signaling events controlling the entry into development, initiation of aggregation and chemotaxis, and cellular differentiation. Here we show that PkcA, a Dictyostelium discoideum Protein Kinase C-orthologue, is involved in quorum sensing and the initiation of development, as well as cAMP sensing during chemotaxis. Additionally, by epistasis analysis we provide evidence that PkcA and PldB (a Phospholipase D-orthologue) functionally interact to regulate aggregation, differentiation, and cell-cell adhesion during development. Finally, we show that PkcA acts as a positive regulator of intracellular PLD-activity during development. Taken together, our results suggest that PkcA act through PldB, by regulating PLD-activity, in order to control events during development.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

The HCMV gH/gL/UL128-131 complex triggers the specific cellular activation required for efficient viral internalization into target monocytes

We have established that HCMV acts as a specific ligand engaging and activating cellular integrins on monocytes. As a result, integrin signaling via Src activation leads to the functional activation of paxillin required for efficient viral entry and for the biological changes in monocytes needed for viral dissemination. These biological/molecular changes allow HCMV to use monocytes as "vehicles" for systemic spread and the establishment of lifelong persistence. However, it remains unresolved how HCMV specifically induces this observed monocyte activation. It was previously demonstrated that the HCMV gH/gL/UL128-131 glycoprotein complex facilitates viral entry into biologically relevant cell types. Nevertheless, the mechanism by which the gH/gL/UL128-131 complex promotes this process is unknown. We now show that only HCMV virions possessing the gH/gL/UL128-131 complex are capable of activating integrin/Src/paxillin-signaling in monocytes. In fibroblasts, this signaling is reversed, such that virus lacking the gH/gL/UL128-131 complex is the only virus able to induce the paxillin activation cascade. The presence of the gH/gL/UL128-131 complex also may have an inhibitory effect on integrin-mediated signaling pathway in fibroblasts. Furthermore, we demonstrate that the presence of the gH/gL/UL128-131 complex on the viral envelope, through its activation of the integrin/Src/paxillin pathway, is necessary for efficient HCMV internalization into monocytes and that appropriate actin and dynamin regulation is critical for this entry process. Importantly, productive infection in monocyte-derived macrophages was seen only in cells exposed to HCMV expressing the gH/gL/UL128-131 complex. From our data, the HCMV gH/gL/U128-131 complex emerges as the specific ligand driving the activation of the receptor-mediated signaling required for the regulation of the actin cytoskeleton and, consequently, for efficient and productive internalization of HCMV into monocytes. To our knowledge, our studies demonstrate a possible molecular mechanism for why the gH/gL/UL128-131 complex dictates HCMV tropism and why the complex is lost as clinical isolates are passaged in the laboratory.

The adhesion modulation protein, AmpA localizes to an endocytic compartment and influences substrate adhesion, actin polymerization and endocytosis in vegetative Dictyostelium cells

Background

AmpA is a secreted 24Kd protein that has pleiotropic effects on Dictyostelium development. Null mutants delay development at the mound stage with cells adhering too tightly to the substrate. Prestalk cells initially specify as prespore cells and are delayed in their migration to the mound apex. Extracellular AmpA can rescue these defects, but AmpA is also necessary in a cell autonomous manner for anterior like cells (ALCs) to migrate to the upper cup. The ALCs are only 10% of the developing cell population making it difficult to study the cell autonomous effect of AmpA on the migration of these cells. AmpA is also expressed in growing cells, but, while it contains a hydrophobic leader sequence that is cleaved, it is not secreted from growing cells. This makes growing cells an attractive system for studying the cell autonomous function of AmpA.

Results

In growing cells AmpA plays an environment dependent role in cell migration. Excess AmpA facilitates migration on soft, adhesive surfaces but hinders migration on less adhesive surfaces. AmpA also effects the level of actin polymerization. Knockout cells polymerize less actin while over expressing cells polymerize more actin than wild type. Overexpression of AmpA also causes an increase in endocytosis that is traced to repeated formation of multiple endocytic cups at the same site on the membrane. Immunofluorescence analysis shows that AmpA is found in the Golgi and colocalizes with calnexin and the slow endosomal recycling compartment marker, p25, in a perinuclear compartment. AmpA is found on the cell periphery and is endocytically recycled to the perinuclear compartment.

Conclusion

AmpA is processed through the secretory pathway and traffics to the cell periphery where it is endocytosed and localizes to what has been defined as a slow endosomal recycling compartment. AmpA plays a role in actin polymerization and cell substrate adhesion. Additionally AmpA influences cell migration in an environment dependent manner. Wild type cells show very little variation in migration rates under the different conditions examined here, but either loss or over expression of AmpA cause significant substrate and environment dependent changes in migration.

Paxillin and phospholipase D interact to regulate actin-based processes in Dictyostelium discoideum

The actin cytoskeleton forms a membrane-associated network whose proper regulation is essential for numerous processes, including cell differentiation, proliferation, adhesion, chemotaxis, endocytosis, exocytosis, and multicellular development. In this report, we show that in Dictyostelium discoideum, paxillin (PaxB) and phospholipase D (PldB) colocalize and coimmunoprecipitate, suggesting that they interact physically. Additionally, the phenotypes observed during development, cell sorting, and several actin-required processes, including cyclic AMP (cAMP) chemotaxis, cell-substrate adhesion, actin polymerization, phagocytosis, and exocytosis, reveal a genetic interaction between paxB and pldB, suggesting a functional interaction between their gene products. Taken together, our data point to PldB being a required binding partner of PaxB during processes involving actin reorganization.

The role of cell contraction and adhesion in dictyostelium motility

The crawling motion of Dictyostelium discoideum on substrata involves a number of coordinated events including cell contractions and cell protrusions. The mechanical forces exerted on the substratum during these contractions have recently been quantified using traction force experiments. Based on the results from these experiments, we present a biomechanical model of the contraction phase of Dictyostelium discoideum motility with an emphasis on the adhesive properties of the cell-substratum contact. Our model assumes that the cell contracts at a constant rate and is bound to the substratum by adhesive bridges that are modeled as elastic springs. These bridges are established at a spatially uniform rate while detachment occurs at a spatially varying, load-dependent rate. Using Monte Carlo simulations and assuming a rigid substratum, we find that the cell speed depends only weakly on the detachment kinetics of the cell-substratum interface, in agreement with experimental data. By varying the parameters that control the adhesive and contractile properties of the cell, we are able to make testable predictions. We also extend our model to include a flexible substrate and show that our model is able to produce substratum deformations and force patterns that are quantitatively and qualitatively in agreement with experimental data.