The regulation of cell motility and chemotaxis by phospholipid signaling

The group migration of Dictyostelium cells is regulated by extracellular chemoattractant degradation

Starvation of Dictyostelium induces a developmental program in which cells form an aggregate that eventually differentiates into a multicellular structure. The aggregate formation is mediated by directional migration of individual cells that quickly transition to group migration in which cells align in a head-to-tail manner to form streams. Cyclic AMP acts as a chemoattractant and its production, secretion, and degradation are highly regulated. A key protein is the extracellular phosphodiesterase PdsA. In this study we examine the role and localization of PdsA during chemotaxis and streaming. We find that pdsA(-) cells respond chemotactically to a narrower range of chemoattractant concentrations compared with wild-type (WT) cells. Moreover, unlike WT cells, pdsA(-) cells do not form streams at low cell densities and form unusual thick and transient streams at high cell densities. We find that the intracellular pool of PdsA is localized to the endoplasmic reticulum, which may provide a compartment for storage and secretion of PdsA. Because we find that cAMP synthesis is normal in cells lacking PdsA, we conclude that signal degradation regulates the external cAMP gradient field generation and that the group migration behavior of these cells is compromised even though their signaling machinery is intact.

Dictyostelium discoideum--a model for many reasons

The social amoeba or cellular slime mould Dictyostelium discoideum is a "professional" phagocyte that has long been recognized for its value as a biomedical model organism, particularly in studying the actomyosin cytoskeleton and chemotactic motility in non-muscle cells. The complete genome sequence of D. discoideum is known, it is genetically tractable, readily grown clonally as a eukaryotic microorganism and is highly accessible for biochemical, cell biological and physiological studies. These are the properties it shares with other microbial model organisms. However, Dictyostelium combines these with a unique life style, with motile unicellular and multicellular stages, and multiple cell types that offer for study an unparalleled variety of phenotypes and associated signalling pathways. These advantages have led to its recent emergence as a valuable model organism for studying the molecular pathogenesis and treatment of human disease, including a variety of infectious diseases caused by bacterial and fungal pathogens. Perhaps surprisingly, this organism, without neurons or brain, has begun to yield novel insights into the cytopathology of mitochondrial diseases as well as other genetic and idiopathic disorders affecting the central nervous system. Dictyostelium has also contributed significantly to our understanding of NDP kinase, as it was the Dictyostelium enzyme whose structure was first determined and related to enzymatic activity. The phenotypic richness and tractability of Dictyostelium should provide a fertile arena for future exploration of NDPK's cellular roles.

Activation of the PI3K/Akt pathway early during vaccinia and cowpox virus infections is required for both host survival and viral replication

Viral manipulation of the transduction pathways associated with key cellular functions such as actin remodeling, microtubule stabilization, and survival may favor a productive viral infection. Here we show that consistent with the vaccinia virus (VACV) and cowpox virus (CPXV) requirement for cytoskeleton alterations early during the infection cycle, PBK/Akt was phosphorylated at S473 [Akt(S473-P)], a modification associated with the mammalian target of rapamycin complex 2 (mTORC2), which was paralleled by phosphorylation at T308 [Akt(T308-P)] by PI3K/PDK1, which is required for host survival. Notably, while VACV stimulated Akt(S473-P/T308-P) at early (1 h postinfection [p.i.]) and late (24 h p.i.) times during the infective cycle, CPXV stimulated Akt at early times only. Pharmacological and genetic inhibition of PI3K (LY294002) or Akt (Akt-X and a dominant-negative form of Akt-K179M) resulted in a significant decline in virus yield (from 80% to >/=90%). This decline was secondary to the inhibition of late viral gene expression, which in turn led to an arrest of virion morphogenesis at the immature-virion stage of the viral growth cycle. Furthermore, the cleavage of both caspase-3 and poly(ADP-ribose) polymerase and terminal deoxynucleotidyl transferase-mediated deoxyuridine nick end labeling assays confirmed that permissive, spontaneously immortalized cells such as A31 cells and mouse embryonic fibroblasts (MEFs) underwent apoptosis upon orthopoxvirus infection plus LY294002 treatment. Thus, in A31 cells and MEFs, early viral receptor-mediated signals transmitted via the PI3K/Akt pathway are required and precede the expression of viral antiapoptotic genes. Additionally, the inhibition of these signals resulted in the apoptosis of the infected cells and a significant decline in viral titers.

Switching direction in electric-signal-induced cell migration by cyclic guanosine monophosphate and phosphatidylinositol signaling

Switching between attractive and repulsive migration in cell movement in response to extracellular guidance cues has been found in various cell types and is an important cellular function for translocation during cellular and developmental processes. Here we show that the preferential direction of migration during electrotaxis in Dictyostelium cells can be reversed by genetically modulating both guanylyl cyclases (GCases) and the cyclic guanosine monophosphate (cGMP)-binding protein C (GbpC) in combination with the inhibition of phosphatidylinositol-3-OH kinases (PI3Ks). The PI3K-dependent pathway is involved in cathode-directed migration under a direct-current electric field. The catalytic domains of soluble GCase (sGC) and GbpC also mediate cathode-directed signaling via cGMP, whereas the N-terminal domain of sGC mediates anode-directed signaling in conjunction with both the inhibition of PI3Ks and cGMP production. These observations provide an identification of the genes required for directional switching in electrotaxis and suggest that a parallel processing of electric signals, in which multiple-signaling pathways act to bias cell movement toward the cathode or anode, is used to determine the direction of migration.

Emerging findings from studies of phospholipase D in model organisms (and a short update on phosphatidic acid effectors)

Phospholipase D (PLD) catalyses the hydrolysis of phosphatidylcholine to generate phosphatidic acid and choline. Historically, much PLD work has been conducted in mammalian settings although genes encoding enzymes of this family have been identified in all eukaryotic organisms. Recently, important insights on PLD function are emerging from work in yeast, but much less is known about PLD in other organisms. In this review we will summarize what is known about phospholipase D in several model organisms, including C. elegans, D. discoideum, D. rerio and D. melanogaster. In the cases where knockouts are available (C. elegans, Dictyostelium and Drosophila) the PLD gene(s) appear not to be essential for viability, but several studies are beginning to identify pathways where this activity has a role. Given that the proteins in model organisms are very similar to their mammalian counterparts, we expect that future studies in model organisms will complement and extend ongoing work in mammalian settings. At the end of this review we will also provide a short update on phosphatidic acid targets, a topic last reviewed in 2006.

Isoform-selective phosphoinositide 3'-kinase inhibitors inhibit CXCR4 signaling and overcome stromal cell-mediated drug resistance in chronic lymphocytic leukemia: a novel therapeutic approach

Phosphoinositide 3-kinases (PI3Ks) are among the most frequently activated signaling pathways in cancer. In chronic lymphocytic leukemia (CLL), signals from the microenvironment are critical for expansion of the malignant B cells, and cause constitutive activation of PI3Ks. CXCR4 is a key receptor for CLL cell migration and adhesion to marrow stromal cells (MSCs). Because of the importance of CXCR4 and PI3Ks for CLL-microenvironment cross-talk, we investigated the activity of novel, isoform-selective PI3K inhibitors that target different isoforms of the p110-kDa subunit. Inhibition with p110alpha inhibitors (PIK-90 and PI-103) resulted in a significant reduction of chemotaxis and actin polymerization to CXCL12 and reduced migration beneath MSC (pseudoemperipolesis). Western blot and reverse phase protein array analyses consistently demonstrated that PIK-90 and PI-103 inhibited phosphorylation of Akt and S6, whereas p110delta or p110beta/p110delta inhibitors were less effective. In suspension and MSC cocultures, PI-103 and PIK-90 were potent inducers of CLL cell apoptosis. Moreover, these p110alpha inhibitors enhanced the cytotoxicity of fludarabine and reversed the protective effect of MSC on fludarabine-induced apoptosis. Collectively, our data demonstrate that p110alpha inhibitors antagonize stromal cell-derived migration, survival, and drug-resistance signals and therefore provide a rational to explore the therapeutic activity of these promising agents in CLL.

Cell biology of the movement of breast cancer cells: intracellular signalling and the actin cytoskeleton

Cell motility is a critical step in cancer invasion and metastasis that must be unravelled to gain an appropriate understanding of the behaviour of cancer cells. A broad spectrum of motility mechanisms that facilitate invasion of extramammary tissues and metastasis exists in breast cancer cells (e.g. reorganization of the actin cytoskeleton, regulation of focal adhesion, changes in response to a different microenvironment, epithelial mesenchymal transition, and control of membrane proteins through endocytosis). These cellular responses are tightly regulated by intracellular signalling pathways evoked by humoral factors that include growth factors, chemokines, and cytokines. Learning more about the cellular and molecular basis of these different motility programmes will aid in the development of treatments for breast cancer invasion and metastasis. This review of recent literature focuses on aspects of cell biology related to motility and metastasis, and suggests some directions for future breast cancer research.

Single-molecule fluorescence studies of a PH domain: new insights into the membrane docking reaction

Proteins containing membrane targeting domains play essential roles in many cellular signaling pathways. However, important features of the membrane-bound state are invisible to bulk methods, thereby hindering mechanistic analysis of membrane targeting reactions. Here we use total internal reflection fluorescence microscopy (TIRFM), combined with single particle tracking, to probe the membrane docking mechanism of a representative pleckstrin homology (PH) domain isolated from the general receptor for phosphoinositides, isoform 1 (GRP1). The findings show three previously undescribed features of GRP1 PH domain docking to membranes containing its rare target lipid, phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P(3)]. First, analysis of surface diffusion kinetics on supported lipid bilayers shows that in the absence of other anionic lipids, the PI(3,4,5)P(3)-bound protein exhibits the same diffusion constant as a single lipid molecule. Second, the binding of the anionic lipid phosphatidylserine to a previously unidentified secondary binding site slows both diffusion and dissociation kinetics. Third, TIRFM enables direct observation of rare events in which dissociation from the membrane surface is followed by transient diffusion through solution and rapid rebinding to a nearby, membrane-associated target lipid. Overall, this study shows that in vitro single-molecule TIRFM provides a new window into the molecular mechanisms of membrane docking reactions.

Mechanisms of chemokine and antigen-dependent T-lymphocyte navigation

T-lymphocyte trafficking is targeted to specific organs by selective molecular interactions depending on their differentiation and functional properties. Specific chemokine receptors have been associated with organ-specific trafficking of memory and effector T-cells, as well as the recirculation of naïve T-cells to secondary lymphoid organs. In addition to the acquisition of tissue-selective integrins and chemokine receptors, an additional level of specificity for T-cell trafficking into the tissue is provided by specific recognition of antigen displayed by the endothelium involving the TCRs (T-cell antigen receptors) and co-stimulatory receptors. Activation of PI3K (phosphoinositide 3-kinase) is a robust signalling event shared by most chemokine receptors as well as the TCR and co-stimulatory receptors, contributing to several aspects of T-lymphocyte homing as well as actin reorganization and other components of the general migratory machinery. Accordingly, inhibition of PI3K has been considered seriously as a potential therapeutic strategy by which to combat various T-lymphocyte-dependent pathologies, including autoimmune and inflammatory diseases, as well as to prevent transplant rejection. However, there is substantial evidence for PI3K-independent mechanisms that facilitate T-lymphocyte migration. In this regard, several other signalling-pathway components, including small GTPases, PLC (phospholipase C) and PKC (protein kinase C) isoforms, have also been implicated in T-lymphocyte migration in response to chemokine stimulation. The present review will therefore examine the PI3K-dependent and -independent signal-transduction pathways involved in T-cell migration during distinct modes of T-cell trafficking in response to either chemokines or the TCR and co-stimulatory molecules.

Great expectations for PIP: phosphoinositides as regulators of signaling during development and disease

Phosphoinositides function as signaling precursors as well as regulators and scaffolds of signaling molecules required for important cellular processes such as membrane trafficking. Although a picture of the biochemical and cell biological functions of phosphoinositides is emerging, less is known about how these functions impact signaling on a broader scale during development. This review summarizes recent work on the role of phosphoinositides in developmental signaling and in a number of diseases and developmental disorders.

Phosphorylation of Nephrin Triggers Ca2+ Signaling by Recruitment and Activation of Phospholipase C-{gamma}1

A specialized intercellular junction between podocytes, known as the slit diaphragm (SD), forms the essential structural frame-work for glomerular filtration in the kidney. In addition, mounting evidence demonstrates that the SD also plays a crucial role as a signaling platform in physiological and pathological states. Nephrin, the major component of the SD, is tyrosine-phosphorylated by a Src family tyrosine kinase, Fyn, in developing or injured podocytes, recruiting Nck to Nephrin via its Src homology 2 domain to regulate dynamic actin remodeling. Dysregulated Ca(2+) homeostasis has also been implicated in podocyte damage, but the mechanism of how podocytes respond to injury is largely unknown. Here we have identified phospholipase C-gamma1 (PLC-gamma1) as a novel phospho-Nephrin-binding protein. When HEK293T cells expressing a chimeric protein consisting of CD8 and Nephrin cytoplasmic domain (CD) were treated with anti-CD8 and anti-mouse antibodies, clustering of Nephrin and phosphorylation of Nephrin-CD were induced. Upon this clustering, PLC-gamma1 was bound to phosphorylated Nephrin Tyr-1204, which induced translocation of PLC-gamma1 from cytoplasm to the CD8/Nephrin cluster on the plasma membrane. The recruitment of PLC-gamma1 to Nephrin activated PLC-gamma1, as detected by phosphorylation of PLC-gamma1 Tyr-783 and increase in inositol 1,4,5-trisphosphate level. We also found that Nephrin Tyr-1204 phosphorylation triggers the Ca(2+) response in a PLC-gamma1-dependent fashion. Furthermore, PLC-gamma1 is significantly phosphorylated in injured podocytes in vivo. Given the profound effect of PLC-gamma in diverse cellular functions, regulation of the Ca(2+) signaling by Nephrin may be important in modulating the glomerular filtration barrier function.

Reversible binding and rapid diffusion of proteins in complex with inositol lipids serves to coordinate free movement with spatial information

Polyphosphoinositol lipids convey spatial information partly by their interactions with cellular proteins within defined domains. However, these interactions are prevented when the lipids' head groups are masked by the recruitment of cytosolic effector proteins, whereas these effectors must also have sufficient mobility to maximize functional interactions. To investigate quantitatively how these conflicting functional needs are optimized, we used different fluorescence recovery after photobleaching techniques to investigate inositol lipid–effector protein kinetics in terms of the real-time dissociation from, and diffusion within, the plasma membrane. We find that the protein–lipid complexes retain a relatively rapid (∼0.1–1 µm²/s) diffusion coefficient in the membrane, likely dominated by protein–protein interactions, but the limited time scale (seconds) of these complexes, dictated principally by lipid–protein interactions, limits their range of action to a few microns. Moreover, our data reveal that GAP1^IP4BP, a protein that binds PtdIns(4,5)P₂ and PtdIns(3,4,5)P₃ in vitro with similar affinity, is able to “read” PtdIns(3,4,5)P₃ signals in terms of an elongated residence time at the membrane.

Polarity proteins in migration and invasion

Cancer is the result of the deregulation of cell proliferation and cell migration. In advanced tumors, cells invade the surrounding tissue and eventually form metastases. This is particularly evident in carcinomas in which epithelial cells have undergone epithelial-mesenchymal transition. Increased cell migration often correlates with a weakening of intercellular interactions. Junctions between neighboring epithelial cells are required to establish and maintain baso-apical polarity, suggesting that not only loss of cell-cell adhesion but also alteration of cell polarity is involved during invasion. Accordingly, perturbation of cell polarity is an important hallmark of advanced invasive tumors. Cell polarity is also essential for cell migration. Indeed, a front-rear polarity axis has first to be generated to allow a cell to migrate. Because cells migrate during invasion, cell polarity is not completely lost. Instead, polarity is modified. From a nonmigrating baso-apically polarized epithelial phenotype, cells acquire a polarized migrating mesenchymal phenotype. The aim of this review is to highlight the molecular relationship between the control of cell polarity and the regulation of cell motility during oncogenesis.

Phosphoinositide signaling pathways: promising role as builders of epithelial cell polarity

Polarity is a prerequisite for proper development and function of epithelia in metazoa. The major feature of polarized epithelial cells is the presence of specialized domains with asymmetric distribution of macromolecular contents including proteins and lipids. The apical domain is involved in exchange with the organ lumen, and the basolateral membrane maintains contact with neighboring cells and the underlying extracellular matrix. The two domains are separated by tight junctions, which act as a diffusion barrier to prevent free mixing of domain-specific proteins and lipids. Extensive studies have shed light on the numerous protein families involved in cell polarization. However, many questions still remain regarding the molecular mechanisms of polarity regulation and in particular very little is known about the role of lipids in building polarity. In this chapter, essential determinants of epithelial polarity will be reviewed with a particular focus on metabolism and function of phosphoinositides.

Signal-activated phospholipase regulation of leukocyte chemotaxis

Signal-activated phospholipases are a recent focus of the rapidly growing field of lipid signaling. The extent of their impact on the pathways regulating diverse cell functions is beginning to be appreciated. A critical step in inflammation is the attraction of leukocytes to injured or diseased tissue. Chemotaxis of leukocytes, a requisite process for monocyte and neutrophil extravasation from the blood into tissues, is a critical step for initiating and maintaining inflammation in both acute and chronic settings. Recent studies have identified new important and required roles for two signal-activated phospholipases A2 (PLA2) in regulating chemotaxis. The two intracellular phospholipases, cPLA2alpha (Group IVA) and iPLA2beta (Group VIA), act in parallel to provide distinct lipid mediators at different intracellular sites that are both required for leukocytes to migrate toward the chemokine monocyte chemoattractant protein-1. This review will summarize the separate roles of these phospholipases as well as what is currently known about the influence of two other classes of intracellular signal-activated phospholipases, phospholipase C and phospholipase D, in regulating chemotaxis in eukaryotic cells, but particularly in human monocytes. The contributions of these phospholipases to chemotaxis both in vitro and in vivo will be highlighted.

Effects of cell wall synthesis on cell polarity in the red alga Porphyra yezoensis

Polarity is a fundamental cell property essential for differentiation, proliferation and morphogenesis in unicellular and multicellular organisms. We have recently demonstrated that phosphatidylinositol 3-kinase (PI3K) activity is required for the establishment of anterior-posterior axis, leading to asymmetrical localization of F-actin in migrating monospores of the red alga Porphyra yezoensis. We also showed that the formation of the apical-basal axis via adhesion of monospores to the substratum after the cessation of migration requires newly synthesized proteins and does not depend on PI3K activity. However, little is known about the mechanism and regulation of axis conversion during development of monospores. In this addendum, we report our investigation as to the role of the cell wall in axis conversion. Our results indicate that inhibition of cell wall synthesis prevented the development of germlings. Also, defects in the cell wall disrupted the asymmetrical distribution of F-actin and inhibited the adhesion to the substratum that is required for establishment of apical-basal axis. Hence, we conclude that the cell wall is critical for the maintenance of cell polarity in migrating cells, which is indirectly involved in axis conversion via enabling monospores to adhere to the substratum.

Involvement of extracellular Cu/Zn superoxide dismutase in cotton fiber primary and secondary cell wall biosynthesis

Extracellular Cu/Zn superoxide dismutases (CSDs) that catalyze the conversion of superoxide to hydrogen peroxide have been suggested to be involved in lignification of secondary walls in spinach, pine and aspen. In cotton fibers, hydrogen peroxide was proposed to be involved in the induction of secondary cell wall biosynthesis. Recently, we identified extracellular CSDs from developing cotton fibers using both immunological and epitope tagging techniques. Since cotton fibers are not lignified, we suggested that extracellular CSDs may be involved in plant cell wall growth and development processes other than lignification. In this addendum, we have further characterized the extracellular CSD in cotton fiber. Immunoblots, enzyme activity assays, and transcript levels show that an extracellular CSD is present in elongating primary walls as well as thickening secondary walls of cotton fibers. Our working model proposes that extracellular hydrogen peroxide levels, regulated by redox status-related enzymes including extracellular CSDs and peroxidases, may affect the processes of wall loosening and wall tightening.

Dictyostelium Dock180-related RacGEFs regulate the actin cytoskeleton during cell motility

Cell motility of amoeboid cells is mediated by localized F-actin polymerization that drives the extension of membrane protrusions to promote forward movements. We show that deletion of either of two members of the Dictyostelium Dock180 family of RacGEFs, DockA and DockD, causes decreased speed of chemotaxing cells. The phenotype is enhanced in the double mutant and expression of DockA or DockD complements the reduced speed of randomly moving DockD null cells' phenotype, suggesting that DockA and DockD are likely to act redundantly and to have similar functions in regulating cell movement. In this regard, we find that overexpressing DockD causes increased cell speed by enhancing F-actin polymerization at the sites of pseudopod extension. DockD localizes to the cell cortex upon chemoattractant stimulation and at the leading edge of migrating cells and this localization is dependent on PI3K activity, suggesting that DockD might be part of the pathway that links PtdIns(3,4,5)P(3) production to F-actin polymerization. Using a proteomic approach, we found that DdELMO1 is associated with DockD and that Rac1A and RacC are possible in vivo DockD substrates. In conclusion, our work provides a further understanding of how cell motility is controlled and provides evidence that the molecular mechanism underlying Dock180-related protein function is evolutionarily conserved.

PI3K pathway alterations in cancer: variations on a theme

The high frequency of phosphoinositide 3-kinase (PI3K) pathway alterations in cancer has led to a surge in the development of PI3K inhibitors. Many of these targeted therapies are currently in clinical trials and show great promise for the treatment of PI3K-addicted tumors. These recent developments call for a re-evaluation of the oncogenic mechanisms behind PI3K pathway alterations. This pathway is unique in that every major node is frequently mutated or amplified in a wide variety of solid tumors. Receptor tyrosine kinases upstream of PI3K, the p110 alpha catalytic subunit of PI3K, the downstream kinase, AKT, and the negative regulator, PTEN, are all frequently altered in cancer. In this review, we will examine the oncogenic properties of these genetic alterations to understand whether they are redundant or distinct and propose treatment strategies tailored for these genetic lesions.

Class 1A PI3K regulates vessel integrity during development and tumorigenesis

PI3K is important in the regulation of growth, proliferation, and survival of tumor cells. We show that class 1A PI3K is also critical in the tumor microenvironment by regulating the integrity of the tumor vasculature. Using Tie2Cre-mediated deletion of the PI3K regulatory subunits (p85alpha, p55alpha, p50alpha, and p85beta), we generated mice with endothelial cell-specific loss of class 1A PI3K. Complete loss of all subunits caused acute embryonic lethality at E11.5 due to hemorrhaging, whereas retention of a single p85alpha allele yielded viable mice that survived to adulthood. These heterozygous mice exhibited no vascular defects until challenged with a pathological insult, such as tumor cells or high levels of VEGF. Under these pathological conditions, heterozygous mice exhibited localized vascular abnormalities, including vessel leakage and the inability to maintain large vessels, which caused a deceleration of tumorigenesis. Furthermore, we show that a PI3K inhibitor can mimic the effects of class 1A PI3K loss, which suggests that targeting class 1A PI3K may be a promising therapy for blocking tumor angiogenesis.

Steering in quadruplet: the complex signaling pathways directing chemotaxis

Studies in the social amoeba Dictyostelium discoideum reveal that signaling cascades coordinating chemotactic directional sensing and migration are complex, with redundant pathways emerging as cells differentiate. Lack of accumulation of the leading-edge marker phosphatidylinositol-3,4,5-trisphosphate can be compensated by a pathway containing phospholipase A2 (PLA2) in early developed cells and guanylyl cyclase (GC) in later developed, polarized cells. Because numerous signaling networks operational during Dictyostelium chemotaxis are conserved in mammalian cells, PLA2 and GC pathways may also be effective in higher eukaryotes, providing avenues for future research.