Abstract 288: Glycolysis is enriched to propagating waves in cell cortex as a new mechanism for cancer progression

Tumors preferentially metabolize glucose anaerobically through glycolysis with lower ATP production efficiency rather than aerobically even when oxygen is available. This was reported by Otto Warburg 100 years ago, yet the mechanism is hitherto not well-understood. Glycolysis is canonically thought to occur only in cytosol; if and how it is regulated by the actin cytoskeletal network is controversial. I found that, in epithelial cells, 6 of the 9 glycolytic enzymes (HK, PFK, ALDO, GAPDH, ENO, and PK, others not tested) are enriched at newly formed LifeAct labeled waves and protrusions, where mitochondria are barely detected by Mito-Tracker. The application of glycolysis inhibitors but not oxidative phosphorylation inhibitors abolishes cell migration. These results indicate that cells rely on the local ATP production from glycolysis enriched in the cortical waves and protrusions to move. We visualized and measured glycolysis production in confocal and TIRF microscopes using a series of biosensors for ATP, NADH/NAD+ ratio, and pyruvate. We then found glycolysis was enhanced by perturbations that increase wave formation such as EGF/Insulin stimulation or recruiting ActA to membrane, and reduced by wave decrease from PI3K inhibition, hyper- and hypo-osmotic shock, or F-actin assembly inhibition. This suggests that enriching glycolytic enzymes on waves results in higher glycolysis production. We do not think the changes of glycolysis by wave perturbations are merely due to direct regulation on glycolytic enzymes by canonical signaling pathways (e.g., Ras-PI3K-AKT), since ActA recruitment or F-actin inhibition does not lead to acute changes in these signaling pathways but mainly causes the assembly or disassembly of the F-actin/glycolytic waves. These findings together lead to our new theory that energy production from glycolysis is enhanced by recruiting the glycolytic enzymes to the waves and protrusions on the cell cortex. This is potentially paradigm-shifting because for many decades glycolysis - one of the two major ways in a cell to produce ATP - has been thought to only occur in cytosol. Interestingly, we also found glycolytic enzymes enriched in F-actin labeled protrusions of Dictyostelium cells, which indicates that this can possibly be an evolutionally conserved mechanism. Additionally, we investigated non-cancer MCF-10A cells (M1) and a series of M1-derived cancer cell lines (M2 - M4) with increased metastatic index and cancer malignancy, and found a sequential increase in actin wave and glycolysis activities from M1 to M4 cells. Cancer cells such as M3 had a larger drop in glycolysis than non-cancer parental M1 cells upon wave inhibition. These results provide a new explanation for the Warburg effect that increased cortical waves in cancer cells will accelerate and improve glycolysis, which will not only greatly contribute to our understanding of cancer but also the design of new interventions.

Citation Format: Huiwang David Zhan, Jane Borleis, Chris Janetopoulos, Peter Devreotes. Glycolysis is enriched to propagating waves in cell cortex as a new mechanism for cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 288.

Electric signals counterbalanced posterior vs anterior PTEN signaling in directed migration of Dictyostelium

Background

Cells show directed migration response to electric signals, namely electrotaxis or galvanotaxis. PI3K and PTEN jointly play counterbalancing roles in this event via a bilateral regulation of PIP3 signaling. PI3K has been proved essential in anterior signaling of electrotaxing cells, whilst the role of PTEN remains elusive.

Methods

Dictyostelium cells with different genetic backgrounds were treated with direct current electric signals to investigate the genetic regulation of electrotaxis.

Results

We demonstrated that electric signals promoted PTEN phosphatase activity and asymmetrical translocation to the posterior plasma membrane of the electrotaxing cells. Electric stimulation produced a similar but delayed rear redistribution of myosin II, immediately before electrotaxis started. Actin polymerization is required for the asymmetric membrane translocation of PTEN and myosin. PTEN signaling is also responsible for the asymmetric anterior redistribution of PIP3/F-actin, and a biased redistribution of pseudopod protrusion in the forwarding direction of electrotaxing cells.

Conclusions

PTEN controls electrotaxis by coordinately regulating asymmetric redistribution of myosin to the posterior, and PIP3/F-actin to the anterior region of the directed migration cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00580-x.

Moving toward molecular mechanisms for chemotaxis in eukaryotic cells

It is a tremendous honor to receive the 2019 E.B. Wilson Award and be recognized for my work on chemotaxis in eukaryotic cells. In writing this essay, I hope to achieve three aims: 1) to tell the story of how people in my group made discoveries over the years; 2) to outline key principles we have learned about chemotaxis; and 3) to point to the most important outstanding questions.

A Tunable Brake for HECT Ubiquitin Ligases

The HECT E3 ligases ubiquitinate numerous transcription factors and signaling molecules, and their activity must be tightly controlled to prevent cancer, immune disorders, and other diseases. In this study, we have found unexpectedly that peptide linkers tethering WW domains in several HECT family members are key regulatory elements of their catalytic activities. Biochemical, structural, and cellular analyses have revealed that the linkers can lock the HECT domain in an inactive conformation and block the proposed allosteric ubiquitin binding site. Such linker-mediated autoinhibition of the HECT domain can be relieved by linker post-translational modifications, but complete removal of the brake can induce hyperactive autoubiquitination and E3 self destruction. These results clarify the mechanisms of several HECT protein cancer associated mutations and provide a new framework for understanding how HECT ubiquitin ligases must be finely tuned to ensure normal cellular behavior.

Signaling networks that regulate cell migration

SUMMARY

Stimuli that promote cell migration, such as chemokines, cytokines, and growth factors in metazoans and cyclic AMP in Dictyostelium, activate signaling pathways that control organization of the actin cytoskeleton and adhesion complexes. The Rho-family GTPases are a key convergence point of these pathways. Their effectors include actin regulators such as formins, members of the WASP/WAVE family and the Arp2/3 complex, and the myosin II motor protein. Pathways that link to the Rho GTPases include Ras GTPases, TorC2, and PI3K. Many of the molecules involved form gradients within cells, which define the front and rear of migrating cells, and are also established in related cellular behaviors such as neuronal growth cone extension and cytokinesis. The signaling molecules that regulate migration can be integrated to provide a model of network function. The network displays biochemical excitability seen as spontaneous waves of activation that propagate along the cell cortex. These events coordinate cell movement and can be biased by external cues to bring about directed migration.

A unified nomenclature and amino acid numbering for human PTEN

The tumor suppressor PTEN is a major brake for cell transformation, mainly due to its phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] phosphatase activity that directly counteracts the oncogenicity of phosphoinositide 3-kinase (PI3K). PTEN mutations are frequent in tumors and in the germ line of patients with tumor predisposition or with neurological or cognitive disorders, which makes the PTEN gene and protein a major focus of interest in current biomedical research. After almost two decades of intense investigation on the 403-residue-long PTEN protein, a previously uncharacterized form of PTEN has been discovered that contains 173 amino-terminal extra amino acids, as a result of an alternate translation initiation site. To facilitate research in the field and to avoid ambiguities in the naming and identification of PTEN amino acids from publications and databases, we propose here a unifying nomenclature and amino acid numbering for this longer form of PTEN.

Phosphorylation-mediated PTEN conformational closure and deactivation revealed with protein semisynthesis

The tumor suppressor PIP₃ phosphatase PTEN is phosphorylated on four clustered Ser/Thr on its C-terminal tail (aa 380–385) and these phosphorylations are proposed to induce a reduction in PTEN’s plasma membrane recruitment. How these phosphorylations affect the structure and enzymatic function of PTEN is poorly understood. To gain insight into the mechanistic basis of PTEN regulation by phosphorylation, we generated semisynthetic site-specifically tetra-phosphorylated PTEN using expressed protein ligation. By employing a combination of biophysical and enzymatic approaches, we have found that purified tail-phosphorylated PTEN relative to its unphosphorylated counterpart shows reduced catalytic activity and membrane affinity and undergoes conformational compaction likely involving an intramolecular interaction between its C-tail and the C2 domain. Our results suggest that there is a competition between membrane phospholipids and PTEN phospho-tail for binding to the C2 domain. These findings reveal a key aspect of PTEN’s regulation and suggest pharmacologic approaches for direct PTEN activation.

DOI:http://dx.doi.org/10.7554/eLife.00691.001

PTEN is an enzyme that is found in almost every tissue in the body, and its job is to stop cells dividing. If it fails to perform this job, the uncontrolled proliferation of cells can lead to the growth of tumors. PTEN stops cells dividing by localizing at the plasma membrane of a cell and removing a phosphate group from a lipid called PIP₃: this sends a signal, via the PI3K pathway, that suppresses the replication and survival of cells.

Three regions of PTEN are thought to be central to its biological functions: one of these regions, the phosphatase domain, is directly responsible for removing a phosphate group from the lipid PIP₃; a second region, called the C2 domain, is known to be critical for PTEN binding to the cell membrane; however, the role of third region, called the C-terminal domain, is poorly understood.

Many proteins are regulated by the addition and removal of phosphate groups, and PTEN is no exception. In particular, it seems as if the addition of phosphate groups to four amino acid residues in the C-terminal domain can switch off the activity of PTEN, but the details of this process have been elusive.

Now, Bolduc et al. have employed a variety of biochemical and biophysical techniques to explore this process, finding that the addition of the phosphate groups reduced PTEN’s affinity for the plasma membrane. At the same time, interactions between the C-terminal and C2 domains of the PTEN cause the shape of the enzyme to change in a way that ‘buries’ the residues to which the phosphate groups have been added.

In addition to offering new insights into PTEN, the work of Bolduc et al. could help efforts to identify compounds with clinical anti-cancer potential.

DOI:http://dx.doi.org/10.7554/eLife.00691.002

Quantitative characterization of the lipid encapsulation of quantum dots for biomedical applications

The water solubilization of nanoparticles is key for many applications in biomedicine. Despite the importance of surface functionalization, progress has been largely empirical and very few systematic studies have been performed. Here we report on the water solubilization of quantum dots using lipid encapsulation. We systematically evaluate the monodispersity, zeta potential, stability, and quantum yield for quantum dots encapsulated with single and double acyl-chain lipids, pegylated double acyl-chain lipids, and single alkyl-chain surfactant molecules with charged head groups. We show that charged surfactants and pegylated lipids are important to obtain monodisperse suspensions with high yield and excellent long-term stability.

FROM THE CLINICAL EDITOR

This study reports on solubilization of nanoparticles in water, a key, but often neglected aspect for biomedical applications. The authors demonstrate that charged surfactants and PEGylated lipids are important to obtain monodisperse suspensions with high yield and long-term stability.

Disruption of PKB signaling restores polarity to cells lacking tumor suppressor PTEN

By limiting phosphotidylinositol 3,4,5-triphosphate (PIP(3)) levels, tumor suppressor PTEN not only controls cell growth but also maintains cell polarity required for cytokinesis and chemotaxis. To identify the critical targets of PIP(3) that link it to the cytoskeleton, we deleted secondary genes to reverse the deficiencies of pten- cells in Dictyostelium. The polarity defects in pten- cells correlate with elevated phosphorylations of PKB substrates. Deletion of AKT orthologue, PkbA, or a subunit of its activator TORC2, reduced the phosphorylations and suppressed the cytokinesis and chemotaxis defects in pten- cells. In these double mutants, the excessive PIP(3) levels and, presumably, activation of other PIP(3)-binding proteins had little or no effect on the cytoskeleton. In bands with increased phosphorylation in pten- cells, we found PKB substrates, PI5K, GefS, GacG, and PakA. Disruption of PakA in pten- cells restored a large fraction of the cells to normal behavior. Consistently, expression of phosphomimetic PakA in pten- cells exacerbated the defects but nonphosphorylatable PakA had no effect. Thus, among many putative PTEN- and PIP(3)-dependent events, phosphorylation of PKB substrates is the key downstream regulator of cell polarity.

Phosphoinositide signaling plays a key role in cytokinesis

To perform the vital functions of motility and division, cells must undergo dramatic shifts in cell polarity. Recent evidence suggests that polarized distributions of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate, which are clearly important for regulating cell morphology during migration, also play an important role during the final event in cell division, which is cytokinesis. Thus, there is a critical interplay between the membrane phosphoinositides and the cytoskeletal cortex that regulates the complex series of cell shape changes that accompany these two processes.

RacG regulates morphology, phagocytosis, and chemotaxis

RacG is an unusual member of the complex family of Rho GTPases in Dictyostelium. We have generated a knockout (KO) strain, as well as strains that overexpress wild-type (WT), constitutively active (V12), or dominant negative (N17) RacG. The protein is targeted to the plasma membrane, apparently in a nucleotide-dependent manner, and induces the formation of abundant actin-driven filopods. RacG is enriched at the rim of the progressing phagocytic cup, and overexpression of RacG-WT or RacG-V12 induced an increased rate of particle uptake. The positive effect of RacG on phagocytosis was abolished in the presence of 50 microM LY294002, a phosphoinositide 3-kinase inhibitor, indicating that generation of phosphatidylinositol 3,4,5-trisphosphate is required for activation of RacG. RacG-KO cells showed a moderate chemotaxis defect that was stronger in the RacG-V12 and RacG-N17 mutants, in part because of interference with signaling through Rac1. The in vivo effects of RacG-V12 could not be reproduced by a mutant lacking the Rho insert region, indicating that this region is essential for interaction with downstream components. Processes like growth, pinocytosis, exocytosis, cytokinesis, and development were unaffected in Rac-KO cells and in the overexpressor mutants. In a cell-free system, RacG induced actin polymerization upon GTPgammaS stimulation, and this response could be blocked by an Arp3 antibody. While the mild phenotype of RacG-KO cells indicates some overlap with one or more Dictyostelium Rho GTPases, like Rac1 and RacB, the significant changes found in overexpressors show that RacG plays important roles. We hypothesize that RacG interacts with a subset of effectors, in particular those concerned with shape, motility, and phagocytosis.

Regulation of PTEN function as a PIP3 gatekeeper through membrane interaction

PTEN, one of the most frequently mutated genes in human cancer, acts as a tumor suppressor by dephosphorylating the plasma membrane lipid second messenger phosphoinositide-3,4,5-trisphosphate (PIP3) generated by the action of PI3Kinases. PTEN activity to prevent elevated levels of PIP3 and tumorigenesis depends on its interaction with the lipid bilayer. PTEN binds dynamically to the plasma membrane through a complex mix of protein-lipid and protein-protein interactions and the translocation is regulated by several mechanisms including C-terminal tail phosphorylations. Here we have summarized our current view of the interaction of PTEN with the plasma membrane and what the implications are for cancer biology.

Dictyostelium RacH Regulates Endocytic Vesicular Trafficking and is Required for Localization of Vacuolin

Dictyostelium RacH localizes predominantly to membranes of the nuclear envelope, endoplasmic reticulum and Golgi apparatus. To investigate the role of this protein, we generated knockout and overexpressor strains. RacH-deficient cells displayed 50% reduced fluid-phase uptake and a moderate exocytosis defect, but phagocytosis was unaffected. Detailed examination of the endocytic pathway revealed defective acidification of early endosomes and reduced secretion of acid phosphatase in the presence of sucrose. The distribution of the post-lysosomal marker vacuolin was altered, with a high proportion of cells showing a diffuse vesicular pattern in contrast to the wild-type strain, where few intensely stained vacuoles predominate. Cytokinesis, cell motility, chemotaxis and development appeared largely unaffected. In a cell-free system, RacH stimulates actin polymerization, suggesting that this protein is involved in actin-based trafficking of vesicular compartments. We also investigated the determinants of subcellular localization of RacH by expression of green-fluorescent-protein-tagged chimeras in which the C-terminus of RacH and the plasma-membrane-targeted RacG were exchanged, the insert region was deleted or the net positive charge of the hypervariable region was increased. We show that several regions of the molecule, not only the hypervariable region, determine targeting of RacH. Overexpression of mistargeted RacH mutants did not recapitulate the phenotypes of a strain overexpressing nonmutated RacH, indicating that the function of this protein is in great part related to its subcellular localization.

Navigating signaling networks: chemotaxis in Dictyostelium discoideum

Studies of chemotaxis in the social amoeba Dictyostelium discoideum have revealed numerous conserved signaling networks that are activated by chemoattractants. In the presence of a uniformly distributed stimulus, these pathways are transiently activated, but in a gradient they are activated persistently and can be localized to either the front or the back of the cell. Recent studies have begun to elucidate how chemoattractant signaling regulates the three main components of chemotaxis: directional sensing, pseudopod extension, and polarization.

Calcium mobilization stimulatesDictyostelium discoideumshear-flow-induced cell motility

Application of hydrodynamic mild shear stress to adherent Dictyostelium discoideum vegetative cells triggers active actin cytoskeleton remodeling resulting in net cell movement along the flow. The average cell speed is strongly stimulated by external calcium (Ca2+, K50%=22 μM), but the directionality of the movement is almost unaffected. This calcium concentration is ten times higher than the one promoting cell adhesion to glass surfaces (K50%=2 μM). Addition of the calcium chelator EGTA or the Ca2+-channel blocker gadolinium (Gd3+) transiently stops cell movement. Monitoring the evolution of cell-surface contact area with time reveals that calcium stimulates cell speed by increasing the amplitude of both protrusion and retraction events at the cell edge, but not the frequency. As a consequence, with saturating external calcium concentrations, cells are sensitive to very low shear forces (20 pN; σ=0.1 Pa). Moreover, a null-mutant lacking the unique Gβ subunit does not respond to external Ca2+ changes (K50%>1000 μM), although the directionality of the movement is comparable with that of wild-type cells. Furthermore, cells lacking the inositol 1,4,5-trisphosphate receptor (IP3-receptor) exhibit a markedly reduced Ca2+ sensitivity. Thus, calcium release from internal stores and calcium entry through the plasma membrane modulate cell speed in response to shear stress.

Temporal and Spatial Regulation of Phosphoinositide Signaling Mediates Cytokinesis

Polarity is a prominent feature of both chemotaxis and cytokinesis. In chemotaxis, polarity is established by local accumulation of PI(3,4,5)P3 at the cell's leading edge, achieved through temporal and spatial regulation of PI3 kinases and the tumor suppressor, PTEN. We find that as migrating D. discoideum cells round up to enter cytokinesis, PI(3,4,5)P3 signaling is uniformly suppressed. Then, as the spindle and cell elongate, PI3 kinases and PTEN move to and function at the poles and furrow, respectively. Cell lines lacking both of these enzymatic activities fail to modulate PI(3,4,5)P3 levels, are defective in cytokinesis, and cannot divide in suspension. The cells continue to grow and duplicate their nuclei, generating large multinucleate cells. Furrows that fail to ingress between nuclei are unable to stably accumulate myosin filaments or suppress actin-filled ruffles. We propose that phosphoinositide-linked circuits, similar to those that bring about asymmetry during cell migration, also regulate polarity in cytokinesis.

Local Excitation, Global Inhibition Mechanism for Gradient Sensing: An Interactive Applet

We have proposed a model in which cells detect gradients of chemoattractant by balancing a fast local excitation and a slower global inhibition. To illustrate this general mechanism, we have developed an interactive applet that mimics laboratory experiments in which either spatially homogeneous or heterogeneous stimuli of chemoattractant are applied.

Inositol pyrophosphates mediate chemotaxis in Dictyostelium via pleckstrin homology domain-PtdIns(3,4,5)P3 interactions

Inositol phosphates are well-known signaling molecules, whereas the inositol pyrophosphates, such as diphosphoinositol pentakisphosphate (InsP7/IP7) and bis-diphosphoinositol tetrakisphosphate (InsP8/IP8), are less well characterized. We demonstrate physiologic regulation of Dictyostelium chemotaxis by InsP7 mediated by its competition with PtdIns(3,4,5)P3 for binding pleckstrin homology (PH) domain-containing proteins. Chemoattractant stimulation triggers rapid and sustained elevations in InsP7/InsP8 levels. Depletion of InsP7 and InsP8 by deleting the gene for InsP6 kinase (InsP6K/IP6K), which converts inositol hexakisphosphate (InsP6/IP6) to InsP7, causes rapid aggregation of mutant cells and increased sensitivity to cAMP. Chemotaxis is mediated by membrane translocation of certain PH domain-containing proteins via specific binding to PtdIns(3,4,5)P3. InsP7 competes for PH domain binding with PtdIns(3,4,5)P3 both in vitro and in vivo. InsP7 depletion enhances PH domain membrane translocation and augments downstream chemotactic signaling activity.

Eukaryotic chemotaxis: distinctions between directional sensing and polarization

Directional sensing and polarization are fundamental cellular responses that play a central role in health and disease. In this review we define each process and evaluate a series of models previously proposed to explain these phenomena. New findings show that directional sensing by G protein-coupled receptors is localized at a discrete step in the signaling pathway downstream of G protein activation but upstream of the accumulation of PIP3. Local levels of PIP3, whether triggered by chemoattractants, particle binding, or spontaneous events, determine the sites of new actin-filled projections. Robust control of the temporal and spatial levels of PIP3 is achieved by reciprocal regulation of PI3K and PTEN. These observations suggest that a local excitation-global inhibition model can account for the localization of PI3K and PTEN and thereby explain directional sensing. However, elements of other models, including positive feedback and the reaction of the cytoskeleton, must be invoked to account for polarization.

Receptor-mediated regulation of PI3Ks confines PI(3,4,5)P3 to the leading edge of chemotaxing cells

Recent studies have demonstrated that PH domains specific for PI(3,4,5)P3 accumulate at the leading edge of a number of migrating cells and that PI3Ks and PTEN associate with the membrane at the front and back, respectively, of chemotaxing Dictyostelium discoideum cells. However, the dependence of chemoattractant induced changes in PI(3,4,5)P3 on PI3K and PTEN activities have not been defined. We find that bulk PI(3,4,5)P3 levels increase transiently upon chemoattractant stimulation, and the changes are greater and more prolonged in pten- cells. PI3K activation increases within 5 s of chemoattractant addition and then declines to a low level of activity identically in wild-type and pten- cells. Reconstitution of the PI3K activation profile can be achieved by mixing membranes from stimulated pi3k1-/pi3k2- cells with cytosolic PI3Ks from unstimulated cells. These studies show that significant control of chemotaxis occurs upstream of the PI3Ks and that regulation of the PI3Ks and PTEN cooperate to shape the temporal and spatial localization of PI(3,4,5)P3.

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

The Dictyostelium mutant HSB1 is temperature-sensitive for development, undergoing aggregation and fruiting body formation at temperatures below 18 degrees C but not above. In vivo G protein-linked adenylyl cyclase activation is defective in HSB1, and the enzyme is not stimulated in vitro by GTPgammaS; stimulation is restored upon addition of wild-type cytosol. Transfection with the gene encoding the cytosolic regulator PIA rescued the mutant. We excluded the possibility that HSB1 cells fail to express PIA and show that the HSB1 piaA gene harbors a point mutation, resulting in the amino acid exchange G(917)D. Both wild-type and HSB1 cells were also transfected with the HSB1 piaA gene. The piaA(HSB1) gene product displayed a partial inhibitory effect on wild-type cell development. We hypothesize that PIA couples the heterotrimeric G protein to adenylyl cyclase via two binding sites, one of which is altered in a temperature-sensitive way by the HSB1 mutation. When overexpressed in the wild-type background, PIA(HSB1) competes with wild-type PIA via the nonmutated binding site, resulting in dominant-negative inhibition of development. Expression of GFP-fused PIA shows that PIA is homogeneously distributed in the cytoplasm of chemotactically moving cells.

Temporal and spatial regulation of chemotaxis

The ability to sense and respond to shallow gradients of extracellular signals is remarkably similar in Dictyostelium discoideum amoebae and mammalian leukocytes. Chemoattractant receptors and G proteins are fairly evenly distributed along the cell surface. Receptor occupancy generates local excitatory and global inhibitory processes that balance to control the chemotactic response. Uniform stimuli transiently recruit PI3Ks to, and release PTEN from, the plasma membrane, while gradients of chemoattractant cause the two enzymes to bind to the membrane at the front and back of the cell, respectively. Interference with PI3Ks alters chemotaxis, and disruption of PTEN broadens PI localization and actin polymerization in parallel. Thus, counteracting signals from the upstream elements of the pathway converge to regulate the key enzymes of PI metabolism, localize these lipids, and direct pseudopod formation.

Monitoring receptor-mediated activation of heterotrimeric G-proteins by fluorescence resonance energy transfer

Green fluorescent protein (GFP)-centered fluorescence resonance energy transfer (FRET) relies on a distance-dependent transfer of energy from a donor fluorophore to an acceptor fluorophore and can be used to examine protein interactions in living cells. Here we describe a method to monitor the association and disassociation of heterotrimeric GTP-binding (G-proteins) from one another before and after stimulation of coupled receptors in living Dictyostelium discoideum cells. The Galpha(2)and Gbetagamma proteins were tagged with cyan and yellow fluorescent proteins and used to observe the state of the G-protein heterotrimer. Data from emission spectra were used to detect the FRET fluorescence and to determine kinetics and dose-response curves of bound ligand and analogs. Extending G-protein FRET to mammalian G-proteins should enable direct in situ mechanistic studies and applications such as drug screening and identifying ligands of new G-protein-coupled receptors.

Tumor suppressor PTEN mediates sensing of chemoattractant gradients

Shallow gradients of chemoattractants, sensed by G protein-linked signaling pathways, elicit localized binding of PH domains specific for PI(3,4,5)P3 at sites on the membrane where rearrangements of the cytoskeleton and pseudopod extension occur. Disruption of the PI 3-phosphatase, PTEN, in Dictyostelium discoideum dramatically prolonged and broadened the PH domain relocation and actin polymerization responses, causing the cells lacking PTEN to follow a circuitous route toward the attractant. Exogenously expressed PTEN-GFP localized to the surface membrane at the rear of the cell. Membrane localization required a putative PI(4,5)P2 binding motif and was required for chemotaxis. These results suggest that specific phosphoinositides direct actin polymerization to the cell's leading edge and regulation of PTEN through a feedback loop plays a critical role in gradient sensing and directional migration.

Single-molecule analysis of chemotactic signaling in Dictyostelium cells

Single-molecule imaging techniques were used to reveal the binding of individual cyclic adenosine 3',5'-monophosphate molecules to heterotrimeric guanine nucleotide-binding protein coupled receptors on the surface of living Dictyostelium discoideum cells. The binding sites were uniformly distributed and diffused rapidly in the plane of the membrane. The probabilities of individual association and dissociation events were greater for receptors at the anterior end of the cell. Agonist-induced receptor phosphorylation had little effect on any of the monitored properties, whereas G protein coupling influenced the binding kinetics. These observations illustrate the dynamic properties of receptors involved in gradient sensing and suggest that these may be polarized in chemotactic cells.

Receptor-mediated activation of heterotrimeric G-proteins in living cells

Receptor-mediated activation of heterotrimeric GTP-binding proteins (G-proteins) was visualized in living Dictyostelium discoideum cells by monitoring fluorescence resonance energy transfer (FRET) between alpha- and beta- subunits fused to cyan and yellow fluorescent proteins. The G-protein heterotrimer rapidly dissociated and reassociated upon addition and removal of chemoattractant. During continuous stimulation, G-protein activation reached a dose-dependent steady-state level. Even though physiological responses subsided, the activation did not decline. Thus, adaptation occurs at another point in the signaling pathway, and occupied receptors, whether or not they are phosphorylated, catalyze the G-protein cycle. Construction of similar energy-transfer pairs of mammalian G-proteins should enable direct in situ mechanistic studies and applications such as drug screening and identifying ligands of newly found G-protein-coupled receptors.

Desensitization of G-protein-coupled receptors. agonist-induced phosphorylation of the chemoattractant receptor cAR1 lowers its intrinsic affinity for cAMP

Agonist-induced phosphorylation of G-protein-coupled receptors has been shown to facilitate the desensitization processes, such as receptor internalization, decreased efficiency of coupling to G-proteins, or decreased ligand affinity. The lowered affinity may be an intrinsic property of the phosphorylated receptor or it may be the result of altered interactions between the modified receptor and downstream components such as G-proteins or arrestins. To address this issue, we purified cAR1, the major chemoattractant receptor of Dictyostelium discoideum by a strategy that is independent of the ligand binding capacity of the receptor. To our knowledge, this represents the first successful purification of a chemoattractant receptor. The hexyl-histidine-tagged receptor was solubilized from a highly enriched plasma membrane preparation and purified by Ni2+-chelating chromatography. The protocol offers a simple way to purify 100-500 micrograms of a G-protein coupled receptor that can be targeted to the plasma membrane of D. discoideum. The Kd value for the purified cAR1 was about 200 nM, consistent with that of receptors that are not coupled to G-proteins in intact cells. In contrast, the affinity of phosphorylated cAR1, purified from desensitized cells, was about three times lower. Treatment of the phosphorylated receptor with protein phosphatases caused dephosphorylation and parallel restoration of higher affinity. We propose that ligand-induced phosphorylation of G-protein-coupled receptors causes a decrease in intrinsic affinity and may be useful in maintaining the receptor's sensitivity at high agonist levels. This affinity decrease may precede other processes such as receptor internalization or uncoupling from G-proteins.

G protein beta subunit-null mutants are impaired in phagocytosis and chemotaxis due to inappropriate regulation of the actin cytoskeleton

Chemotaxis and phagocytosis are basically similar in cells of the immune system and in Dictyostelium amebae. Deletion of the unique G protein beta subunit in D. discoideum impaired phagocytosis but had little effect on fluid-phase endocytosis, cytokinesis, or random motility. Constitutive expression of wild-type beta subunit restored phagocytosis and normal development. Chemoattractants released by cells or bacteria trigger typical transient actin polymerization responses in wild-type cells. In beta subunit-null cells, and in a series of beta subunit point mutants, these responses were impaired to a degree that correlated with the defect in phagocytosis. Image analysis of green fluorescent protein-actin transfected cells showed that beta subunit- null cells were defective in reshaping the actin network into a phagocytic cup, and eventually a phagosome, in response to particle attachment. Our results indicate that signaling through heterotrimeric G proteins is required for regulating the actin cytoskeleton during phagocytic uptake, as previously shown for chemotaxis. Inhibitors of phospholipase C and intracellular Ca2+ mobilization inhibited phagocytosis, suggesting the possible involvement of these effectors in the process.

CRAC, a cytosolic protein containing a pleckstrin homology domain, is required for receptor and G protein-mediated activation of adenylyl cyclase in Dictyostelium

Adenylyl cyclase in Dictyostelium, as in higher eukaryotes, is activated through G protein-coupled receptors. Insertional mutagenesis into a gene designated dagA resulted in cells that cannot activate adenylyl cyclase, but have otherwise normal responses to exogenous cAMP. Neither cAMP treatment of intact cells nor GTP gamma S treatment of lysates stimulates adenylyl cyclase activity in dagA mutants. A cytosolic protein that activates adenylyl cyclase, CRAC, has been previously identified. We trace the signaling defect in dagA- cells to the absence of CRAC, and we demonstrate that dagA is the structural gene for CRAC. The 3.2-kb dagA mRNA encodes a predicted 78.5-kD product containing a pleckstrin homology domain, in agreement with the postulated interaction of CRAC with activated G proteins. Although dagA expression is tightly developmentally regulated, the cDNA restores normal development when constitutively expressed in transformed mutant cells. In addition, the megabase region surrounding the dagA locus was mapped. We hypothesize that CRAC acts to connect free G protein beta gamma subunits to adenylyl cyclase activation. If so, it may be the first member of an important class of coupling proteins.

Localization of ligand-induced phosphorylation sites to serine clusters in the C-terminal domain of the Dictyostelium cAMP receptor, cAR1

When Dictyostelium cells are stimulated with cyclic adenosine 3',5'-monophosphate (cAMP), the major surface cAMP receptor expressed in early development, cAR1, undergoes a rapid phosphorylation and parallel decrease in electrophoretic mobility which may serve to regulate the activity of this G protein-coupled receptor. Biochemical analyses indicate the electrophoretic mobility shift is caused by phosphorylation of serine residues within the C-terminal cytoplasmic domain. The 18 serines of this domain are grouped in four clusters, designated 1 to 4 (in N- to C-terminal order). Two approaches were taken to determine the distribution of phosphorylation sites among the serine clusters. First, a proteolytic analysis of the C-terminal domain was performed. Second, mutants lacking various combinations of the serine clusters were created by site-directed mutagenesis and their abilities to undergo ligand-induced modification were determined. Both approaches yielded corroborative results consistent with the following model: the stimulus induces the addition of approximately two phosphates to cluster 1 and one to cluster 2; basal phosphorylation occurs predominantly in cluster 3 and to a lesser extent in cluster 2; and cluster 4 is not phosphorylated. The phosphorylation-deficient receptor mutants should be useful for establishing the role of ligand-induced phosphorylation of cAR1 in chemotaxis, cell-cell signaling, and gene expression.

Dictyostelium discoideum: a model system for cell-cell interactions in development

The cellular slime mold Dictyostelium discoideum undergoes a transition from single-celled amoebae to a multicellular organism as a natural part of its life cycle. A method of cell-cell signaling that controls chemotaxis, morphogenesis, and gene expression has developed in this organism, and a detailed understanding of this signaling system provides clues to mechanisms of intercellular communication in the development of metazoans.

Cell-cell interactions in Dictyostelium development

The development of an organism requires extensive cell-cell communication; however, little is known about the signals transmitted among differentiating cells. Observations of Dictyostelium amoebae reveal that transmembrane signaling systems have been highly conserved in evolution. The signals that cause these cells to differentiate are processed by mechanisms similar to those that process sensory and hormonal stimuli in higher animals.

Purification of the surface cAMP receptor in Dictyostelium

We have previously identified and demonstrated reversible ligand-induced modification of the major cell surface cAMP receptor in Dictyostelium discoideum. The receptor, or a subunit of it, has been purified to homogeneity by hydroxylapatite chromatography followed by two-dimensional preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purification was monitored by following 32Pi incorporated by photoaffinity labeling with 8-azido-[32P]cAMP or by in vivo labeling with 32Pi. Two interconvertible forms of the receptor, designated R (Mr 40,000) and D (Mr 43,000), co-purified. Two-dimensional peptide maps of independently purified and 125I-iodinated R and D forms of the receptor were nearly identical but did have several distinct peptides. The estimated 6000-fold purification required is consistent with the number of cell surface binding sites assuming there are not multiple binding sites/polypeptide. In the accompanying article we report the generation of a monospecific polyclonal antiserum which has helped to further elucidate the physical properties and developmental regulation of the cAMP receptor.

The surface cyclic AMP receptor in Dictyostelium. Levels of ligand-induced phosphorylation, solubilization, identification of primary transcript, and developmental regulation of expression

A monospecific polyclonal antiserum to the surface cAMP receptor of Dictyostelium has been developed by immunization with purified receptor immobilized on particles of polyacrylamide and on nitrocellulose paper. In Western blots, the antiserum displays high affinity and specificity for both the R (Mr 40,000) and D (Mr 43,000) forms of the receptor previously identified by photoaffinity labeling with 8-azido-[32P] cAMP. These bands, labeled with the photoaffinity label or with 32 Pi, were quantitatively and specifically immunoprecipitated, supporting co-purification data that all represent the same polypeptide. The R form, found in unstimulated cells, contained at least 0.2 mol of phosphate/mol of receptor. The D form, generated by cAMP stimulation of intact cells, contained at least 4 mol of phosphate/mol of receptor. In the absence of detergents, the receptor was exclusively located on membranes. The receptor was solubilized effectively in Triton X-100 and sedimented as a broad peak of 5-7 S on sucrose velocity gradients. Western blots of membranes isolated at different times after starvation indicate that the appearance of cell surface cAMP binding sites during the aggregation stage of development (5-6 h) is due to de novo synthesis of receptor protein. Pulse labeling with [35S]methionine indicated that the receptor is most rapidly synthesized during the preaggregation stage of development (1-3 h), prior to its maximal accumulation in membranes. The serum specifically immunoprecipitates a polypeptide of Mr 37,000 from an in vitro translation reaction using RNA isolated from preaggregation stage cells. The time course of expression of the mRNA coding for the Mr 37,000 polypeptide parallels the rate of receptor synthesis in vivo.

Phenotypic changes induced by a mutated ras gene during the development of Dictyostelium transformants

The ras proto-oncogene, found in all eukaryotes so far examined, encode s a protein with guanine nucleotide-binding and GTPase activity. Gene disruption experiments in yeast indicate that ras is essential for cell growth. Anit-sense mutagenesis approaches suggest that this is also true for Dictyostelium. Most mutations causing an amino-acid substitution for Gly 12 result in decreased GTPase activity and produce a transforming phenotype. In yeast, a Gly 19---- Val 19, missense mutation (Gly 19 is similar to Gly 12 in mammalian and Dictyostelium ras proteins) causes a series of dominant phenotypes, including elevated adenylate cyclase activity. In mammalian cells there is no evidence that ras activates adenylate cyclase activity. D. discoideum contains a single ras gene (Dd-ras) that encodes a protein very similar to the mammalian ras protein and identical to c-ras at the potentially transforming positions. Dd-ras is expressed in vegetative cells and later in development in prestalk cells whereas ras protein is found in vegetative and developing cells. In the migrating pseudoplasmodium, ras protein is found in prestalk but not prespore cells, suggesting it is involved in the function and/or differentiation of the anteriorly localized prestalk cells. In this report we examine the effects of expression of a Dd-ras gene carrying a Gly-12----Thr 12 missense mutation.

The specificity of the cAMP receptor mediating activation of adenylate cyclase in Dictyostelium discoideum

In Dictyostelium discoideum amoebae, binding of cyclic AMP (cAMP) to surface receptors elicits numerous responses including chemotaxis, cyclic GMP (cGMP) accumulation, and activation of adenylate cyclase. The specificity of the surface cAMP receptor which mediates activation of adenylate cyclase and cAMP secretion was determined by testing the relative effectiveness of a series of 10 cAMP analogs. Each of the 10 analogs elicited cAMP secretion, chemotaxis, and cGMP accumulation in the same dose range. The order of potency for eliciting these responses (cAMP greater than 2'-H-cAMP greater than N1-O-cAMP greater than cAMPS(Sp) greater than 6-Cl-cAMP greater than cAMPN(CH3)2(Sp) greater than 3'-NH-cAMP greater than 8-Br-cAMP greater than cAMPS(Rp) greater than cAMPN(CH3)2(Rp] matches that for binding to the major cell surface cAMP binding sites and differs from that of the cell surface phosphodiesterase and the major intracellular cAMP binding protein.

Cell-cell interactions in the development of Dictyostelium

We have described in D. discoideum a highly organized cell aggregation that is mediated by cAMP. After suitable differentiation induced by starvation, the cells develop the capacity to orient in gradients of cAMP and to secrete cAMP in response to cAMP. This signaling response sets up the cell-cell relay of cAMP waves that transiently orients the cells toward the center. Both the signaling response and the chemotactic response, measured in isolated cells, adapt. The kinetics and properties of adaptation of the two responses are similar and may be due to the same mechanism. The mechanism does not involve protein synthesis, a change in the number or affinity of surface receptors, or the activation of adenylate cyclase. Adaptation of signaling is essential for the oscillatory production of cAMP at the aggregation centers and ensures that the cAMP waves move steadily toward the edge of the aggregation territories. Adaptation of the chemotactic response also ensures that cells do not reorient away from the center in the gradient presented by the trailing edge of the wave. We have demonstrated that both chemotaxis and cAMP signaling are mediated by the same surface receptor. The polypeptide containing the binding site of the receptor has been identified by photoaffinity labeling with [32P]-8-N3-cAMP as a diffuse band of 41,000-45,000 Mr. The receptor and adenylate cyclase copurify on a homogeneous class of vesicles resistant to extraction by nonionic detergents. A GTP-binding protein that is a substrate for cholera toxin-catalyzed ADP ribosylation is found in supernatants and membranes and may be similar to the Gs regulatory protein of adenylate cyclase in higher organisms. The mechanism of activation of the adenylate cyclase and chemotactic machinery is unknown. We have been able to inhibit the activation of the adenylate cyclase selectively and rapidly with agents acting to crosslink cell surface components, which may give a clue to the activation mechanism. The elaborate mechanisms of cell-cell communication occurring in D. discoideum are without precedent in biological literature, although models of oscillatory wave propagation have been proposed to account for pattern formation. Although it is unlikely that extracellular cAMP would be involved, it is not inconceivable that such mechanisms occur during the development of more evolutionarily advanced organisms. The organized communication system in D. discoideum is only apparent when cells are plated uniformly on a flat surface; such organized movements occurring in a three-dimensional structure such as an embryo would be very difficult to discern.