Real-time measurements of cAMP production in live Dictyostelium cells

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

Visualization of c-di-GMP in multicellular Dictyostelium stages

The bacterial signaling molecule cyclic diguanosine monophosphate (c-di-GMP) is only synthesized and utilized by the cellular slime mold Dictyostelium discoideum among eukaryotes. Dictyostelium cells undergo a transition from a unicellular to a multicellular state, ultimately forming a stalk and spores. While Dictyostelium is known to employ c-di-GMP to induce differentiation into stalk cells, there have been no reports of direct observation of c-di-GMP using fluorescent probes. In this study, we used a fluorescent probe used in bacteria to visualize its localization within Dictyostelium multicellular bodies. Cytosolic c-di-GMP concentrations were significantly higher at the tip of the multicellular body during stalk formation.

cAMP Biosensors Based on Genetically Encoded Fluorescent/Luminescent Proteins

Cyclic adenosine monophosphate (cAMP) plays a key role in signal transduction pathways as a second messenger. Studies on the cAMP dynamics provided useful scientific insights for drug development and treatment of cAMP-related diseases such as some cancers and prefrontal cortex disorders. For example, modulation of cAMP-mediated intracellular signaling pathways by anti-tumor drugs could reduce tumor growth. However, most early stage tools used for measuring the cAMP level in living organisms require cell disruption, which is not appropriate for live cell imaging or animal imaging. Thus, in the last decades, tools were developed for real-time monitoring of cAMP distribution or signaling dynamics in a non-invasive manner. Genetically-encoded sensors based on fluorescent proteins and luciferases could be powerful tools to overcome these drawbacks. In this review, we discuss the recent genetically-encoded cAMP sensors advances, based on single fluorescent protein (FP), Föster resonance energy transfer (FRET), single luciferase, and bioluminescence resonance energy transfer (BRET) for real-time non-invasive imaging.

Analysis of barotactic and chemotactic guidance cues on directional decision-making of Dictyostelium discoideum cells in confined environments

Neutrophils and dendritic cells when migrating in confined environments have been shown to actuate a directional choice toward paths of least hydraulic resistance (barotaxis), in some cases overriding chemotactic responses. Here, we investigate whether this barotactic response is conserved in the more primitive model organism Dictyostelium discoideum using a microfluidic chip design. This design allowed us to monitor the behavior of single cells via live imaging when confronted with bifurcating microchannels, presenting different combinations of hydraulic and chemical stimuli. Under the conditions employed we find no evidence in support of a barotactic response; the cells base their directional choices on the chemotactic cues. When the cells are confronted by a microchannel bifurcation, they often split their leading edge and start moving into both channels, before a decision is made to move into one and retract from the other channel. Analysis of this decision-making process has shown that cells in steeper nonhydrolyzable adenosine- 3', 5'- cyclic monophosphorothioate, Sp- isomer (cAMPS) gradients move faster and split more readily. Furthermore, there exists a highly significant strong correlation between the velocity of the pseudopod moving up the cAMPS gradient to the total velocity of the pseudopods moving up and down the gradient over a large range of velocities. This suggests a role for a critical cortical tension gradient in the directional decision-making process.

Cytokinesis D is Mediated by Cortical Flow of Dividing Cells Instead of Chemotaxis

Cytokinesis D is known as the midwife mechanism in which neighboring cells facilitate cell division by crossing the cleavage furrow of dividing cells. Cytokinesis D is thought to be mediated by chemotaxis, where midwife cells migrate toward dividing cells by sensing an unknown chemoattractant secreted from the cleavage furrow. In this study, to validate this chemotaxis model, we aspirated the fluid from the vicinity of the cleavage furrow of a dividing Dictyostelium cell and discharged it onto a neighboring cell using a microcapillary. However, the neighboring cells did not show any chemotaxis toward the fluid. In addition, the cells did not manifest an increase in the levels of intracellular Ca²⁺, cAMP, or cGMP, which are expected to rise in chemotaxing cells. From several lines of our experiments, including these findings, we concluded that chemotaxis does not contribute to cytokinesis D. As an alternative, we propose a cortical-flow model, where a migrating cell attaches to a dividing cell by chance and is guided toward the furrow by the cortical flow on the dividing cell, and then physically assists the separation of the daughter cells.

Oscillatory cAMP cell-cell signalling persists during multicellular Dictyostelium development

Propagating waves of cAMP, periodically initiated in the aggregation centre, are known to guide the chemotactic aggregation of hundreds of thousands of starving individual Dictyostelium discoideum cells into multicellular aggregates. Propagating optical density waves, reflecting cell periodic movement, have previously been shown to exist in streaming aggregates, mounds and migrating slugs. Using a highly sensitive cAMP-FRET reporter, we have now been able to measure periodically propagating cAMP waves directly in these multicellular structures. In slugs cAMP waves are periodically initiated in the tip and propagate backward through the prespore zone. Altered cAMP signalling dynamics in mutants with developmental defects strongly support a key functional role for cAMP waves in multicellular Dictyostelium morphogenesis. These findings thus show that propagating cAMP not only control the initial aggregation process but continue to be the long range cell-cell communication mechanism guiding cell movement during multicellular Dictyostelium morphogenesis at the mound and slugs stages.

Extracellular vesicles direct migration by synthesizing and releasing chemotactic signals

Chemotactic signals are relayed to neighboring cells through the secretion of additional chemoattractants. We previously showed in Dictyostelium discoideum that the adenylyl cyclase A, which synthesizes the chemoattractant cyclic adenosine monophosphate (cAMP), is present in the intraluminal vesicles of multivesicular bodies (MVBs) that coalesce at the back of cells. Using ultrastructural reconstructions, we now show that ACA-containing MVBs release their contents to attract neighboring cells. We show that the released vesicles are capable of directing migration and streaming and are central to chemotactic signal relay. We demonstrate that the released vesicles not only contain cAMP but also can actively synthesize and release cAMP to promote chemotaxis. Through proteomic, pharmacological, and genetic approaches, we determined that the vesicular cAMP is released via the ABCC8 transporter. Together, our findings show that extracellular vesicles released by Ddiscoideum cells are functional entities that mediate signal relay during chemotaxis and streaming.

Studying Chemoattractant Signal Transduction Dynamics in Dictyostelium by BRET

Understanding the dynamics of chemoattractant signaling is key to our understanding of the mechanisms underlying the directed migration of cells, including that of neutrophils to sites of infections and of cancer cells during metastasis. A model frequently used for deciphering chemoattractant signal transduction is the social amoeba Dictyostelium discoideum. However, the methods available to quantitatively measure chemotactic signaling are limited. Here, we describe a protocol to quantitatively study chemoattractant signal transduction in Dictyostelium by monitoring protein-protein interactions and conformational changes using Bioluminescence Resonance Energy Transfer (BRET).

Protein kinase A regulates the Ras, Rap1 and TORC2 pathways in response to the chemoattractant cAMP in Dictyostelium

Efficient directed migration requires tight regulation of chemoattractant signal transduction pathways in both space and time, but the mechanisms involved in such regulation are not well understood. Here, we investigated the role of protein kinase A (PKA) in controlling signaling of the chemoattractant cAMP in Dictyostelium discoideum We found that cells lacking PKA display severe chemotaxis defects, including impaired directional sensing. Although PKA is an important regulator of developmental gene expression, including the cAMP receptor cAR1, our studies using exogenously expressed cAR1 in cells lacking PKA, cells lacking adenylyl cyclase A (ACA) and cells treated with the PKA-selective pharmacological inhibitor H89, suggest that PKA controls chemoattractant signal transduction, in part, through the regulation of RasG, Rap1 and TORC2. As these pathways control the ACA-mediated production of intracellular cAMP, they lie upstream of PKA in this chemoattractant signaling network. Consequently, we propose that the PKA-mediated regulation of the upstream RasG, Rap1 and TORC2 signaling pathways is part of a negative feedback mechanism controlling chemoattractant signal transduction during Dictyostelium chemotaxis.

From intracellular signaling to population oscillations: bridging size- and time-scales in collective behavior

Collective behavior in cellular populations is coordinated by biochemical signaling networks within individual cells. Connecting the dynamics of these intracellular networks to the population phenomena they control poses a considerable challenge because of network complexity and our limited knowledge of kinetic parameters. However, from physical systems, we know that behavioral changes in the individual constituents of a collectively behaving system occur in a limited number of well-defined classes, and these can be described using simple models. Here, we apply such an approach to the emergence of collective oscillations in cellular populations of the social amoeba Dictyostelium discoideum. Through direct tests of our model with quantitative in vivo measurements of single-cell and population signaling dynamics, we show how a simple model can effectively describe a complex molecular signaling network at multiple size and temporal scales. The model predicts novel noise-driven single-cell and population-level signaling phenomena that we then experimentally observe. Our results suggest that like physical systems, collective behavior in biology may be universal and described using simple mathematical models.

A fluorescent nucleic acid nanodevice quantitatively images elevated cyclic adenosine monophosphate in membrane-bound compartments

cAMPhor: In the presence of cAMP, cAMPhor folds into a structure that binds DFHBI (green), increasing its fluorescence, while Alexa 647 (red) functions as a normalizing dye. It can thus be used to spatially image cAMP quantitatively in membrane-bound compartments.

Distinct cell shapes determine accurate chemotaxis

The behaviour of an organism often reflects a strategy for coping with its environment. Such behaviour in higher organisms can often be reduced to a few stereotyped modes of movement due to physiological limitations, but finding such modes in amoeboid cells is more difficult as they lack these constraints. Here, we examine cell shape and movement in starved Dictyostelium amoebae during migration toward a chemoattractant in a microfluidic chamber. We show that the incredible variety in amoeboid shape across a population can be reduced to a few modes of variation. Interestingly, cells use distinct modes depending on the applied chemical gradient, with specific cell shapes associated with shallow, difficult-to-sense gradients. Modelling and drug treatment reveals that these behaviours are intrinsically linked with accurate sensing at the physical limit. Since similar behaviours are observed in a diverse range of cell types, we propose that cell shape and behaviour are conserved traits.

Luminescent proteins for high-speed single-cell and whole-body imaging

The use of fluorescent proteins has revolutionized our understanding of biological processes. However, the requirement for external illumination precludes their universal application to the study of biological processes in all tissues. Although light can be created by chemiluminescence, light emission from existing chemiluminescent probes is too weak to use this imaging modality in situations when fluorescence cannot be used. Here we report the development of the brightest luminescent protein to date, Nano-lantern, which is a chimera of enhanced Renilla luciferase and Venus, a fluorescent protein with high bioluminescence resonance energy transfer efficiency. Nano-lantern allows real-time imaging of intracellular structures in living cells with spatial resolution equivalent to fluorescence and sensitive tumour detection in freely moving unshaved mice. We also create functional indicators based on Nano-lantern that can image Ca(2+), cyclic adenosine monophosphate and adenosine 5'-triphosphate dynamics in environments where the use of fluorescent indicators is not feasible. These luminescent proteins allow visualization of biological phenomena at previously unseen single-cell, organ and whole-body level in animals and plants.

Modeling and measuring signal relay in noisy directed migration of cell groups

We develop a coarse-grained stochastic model for the influence of signal relay on the collective behavior of migrating Dictyostelium discoideum cells. In the experiment, cells display a range of collective migration patterns, including uncorrelated motion, formation of partially localized streams, and clumping, depending on the type of cell and the strength of the external, linear concentration gradient of the signaling molecule cyclic adenosine monophosphate (cAMP). From our model, we find that the pattern of migration can be quantitatively described by the competition of two processes, the secretion rate of cAMP by the cells and the degradation rate of cAMP in the gradient chamber. Model simulations are compared to experiments for a wide range of strengths of an external linear-gradient signal. With degradation, the model secreting cells form streams and efficiently transverse the gradient, but without degradation, we find that model secreting cells form clumps without streaming. This indicates that the observed effective collective migration in streams requires not only signal relay but also degradation of the signal. In addition, our model allows us to detect and quantify precursors of correlated motion, even when cells do not exhibit obvious streaming.

Collective cell migration is observed in various biological processes including angiogenesis, gastrulation, fruiting body formation, and wound healing. Dictyostelium discoideum, for example, exhibits highly dynamic patterns such as streams and clumps during its early phases of collective motion and has served as a model organism for the study of collective migration. In this study, facilitated by experiments, we develop a conceptual, minimalistic, computational model to analyze the dynamical processes leading to the emergence of collective patterns and the associated dependence on the external injection of a cAMP signal, the intercellular cAMP secretion rate, and the cAMP degradation rate. We demonstrate that degradation is necessary to reproduce the experimentally observed collective migration patterns, and show how our model can be utilized to uncover basic dependences of migration modes on cell characteristics. Our numerical observations elucidate the different possible types of motion and quantify the onset of collective motion. Thus, the model allows us to distinguish noisy motion guided by the external signal from weakly correlated motion.

Measuring CREB activation using bioluminescent probes that detect KID-KIX interaction in living cells

The cyclic adenosine monophosphate response element-binding protein (CREB) is a transcription factor that contributes to memory formation. The transcriptional activity of CREB is induced by its phosphorylation at Ser-133 and subsequent interaction with the CREB-binding protein (CBP)/p300. We designed and optimized firefly split luciferase probe proteins that detect the interaction of the kinase-inducible domain (KID) of CREB and the KIX domain of CBP/p300. The increase in the light intensity of the probe proteins results from the phosphorylation of the responsible serine corresponding to Ser-133 of CREB. Because these proteins have a high signal-to-noise ratio and are nontoxic, it has become possible for the first time to carry out long-term measurement of KID-KIX interaction in living cells. Furthermore, we examined the usefulness of the probe proteins for future high-throughput cell-based drug screening and found several herbal extracts that activated CREB.

Collective oscillations in developing cells: insights from simple systems

From hormonal secretion to gene expression, multicellular dynamics are rich in oscillatory regulation. When organized in space and time, periodic cell-cell signaling can give rise to long-range coordination of gene expression and cell movement in tissues. Lack of synchrony of the oscillations on the other hand can serve as a source of initial divergence of cell fate in stem cells. How properties of individual cells can account for collective rhythmic behaviors at the tissue level remains elusive in most cases. Recently, studies in chemical reactions, synthetic gene circuits, yeast and social amoeba Dictyostelium have greatly enhanced our view of collective oscillations in cell populations. From these relatively simple systems, a unified view of how excitable and oscillatory regulations could be tuned and coupled to give rise to tissue-level oscillations is emerging. The review focuses on recent progress in cyclic adenosine monophosphate oscillations in Dictyostelium and highlights similarities and differences with other systems. We will see that the autonomy of single-cell level oscillations and different ways in which cells are coupled influence how group-level information can be encoded in collective oscillations.

Direct biochemical measurements of signal relay during Dictyostelium development

Upon starvation, individual Dictyostelium discoideum cells enter a developmental program that leads to collective migration and the formation of a multicellular organism. The process is mediated by extracellular cAMP binding to the G protein-coupled cAMP receptor 1, which initiates a signaling cascade leading to the activation of adenylyl cyclase A (ACA), the synthesis and secretion of additional cAMP, and an autocrine and paracrine activation loop. The release of cAMP allows neighboring cells to polarize and migrate directionally and form characteristic chains of cells called streams. We now report that cAMP relay can be measured biochemically by assessing ACA, ERK2, and TORC2 activities at successive time points in development after stimulating cells with subsaturating concentrations of cAMP. We also find that the activation profiles of ACA, ERK2, and TORC2 change in the course of development, with later developed cells showing a loss of sensitivity to the relayed signal. We examined mutants in PKA activity that have been associated with precocious development and find that this loss in responsiveness occurs earlier in these mutants. Remarkably, we show that this loss in sensitivity correlates with a switch in migration patterns as cells transition from streams to aggregates. We propose that as cells proceed through development, the cAMP-induced desensitization and down-regulation of cAMP receptor 1 impacts the sensitivities of chemotactic signaling cascades leading to changes in migration patterns.

mTORC2 regulates neutrophil chemotaxis in a cAMP- and RhoA-dependent fashion

We studied the role of the target of rapamycin complex 2 (mTORC2) during neutrophil chemotaxis, a process that is mediated through the polarization of actin and myosin filament networks. We show that inhibition of mTORC2 activity, achieved via knock down (KD) of Rictor, severely inhibits neutrophil polarization and directed migration induced by chemoattractants, independently of Akt. Rictor KD also abolishes the ability of chemoattractants to induce cAMP production, a process mediated through the activation of the adenylyl cyclase 9 (AC9). Cells with either reduced or higher AC9 levels also exhibit specific and severe tail retraction defects that are mediated through RhoA. We further show that cAMP is excluded from extending pseudopods and remains restricted to the cell body of migrating neutrophils. We propose that the mTORC2-dependent regulation of MyoII occurs through a cAMP/RhoA-signaling axis, independently of actin reorganization during neutrophil chemotaxis.

Approaching the molecular origins of collective dynamics in oscillating cell populations

From flocking birds, to organ generation, to swarming bacterial colonies, biological systems often exhibit collective behaviors. Here, we review recent advances in our understanding of collective dynamics in cell populations. We argue that understanding population-level oscillations requires examining the system under consideration at three different levels of complexity: at the level of isolated cells, homogenous populations, and spatially structured populations. We discuss the experimental and theoretical challenges this poses and highlight how new experimental techniques, when combined with conceptual tools adapted from physics, may help us overcome these challenges.

The exocytic gene secA is required for Dictyostelium cell motility and osmoregulation

We investigated the link between cell movement and plasma membrane recycling using a fast-acting, temperature-sensitive mutant of the Dictyostelium SecA exocytic protein. Strikingly, most mutant cells become almost paralysed within minutes at the restrictive temperature. However, they can still sense cyclic-AMP (cAMP) gradients and polymerise actin up-gradient, but form only abortive pseudopodia, which cannot expand. They also relay a cAMP signal normally, suggesting that cAMP is released by a non-exocytic mechanism. To investigate why SecA is required for motility, we examined membrane trafficking in the mutant. Plasma membrane circulation is rapidly inhibited at the restrictive temperature and the cells acquire a prominent vesicle. Organelle-specific markers show that this is an undischarged contractile vacuole, and we found the cells are correspondingly osmo-sensitive. Electron microscopy shows that many smaller vesicles, probably originating from the plasma membrane, also accumulate at the restrictive temperature. Consistent with this, the surface area of mutant cells shrinks. We suggest that SecA mutant cells cannot move at the restrictive temperature because their block in exocytosis results in a net uptake of plasma membrane, reducing its area, and so restricting pseudopodial expansion. This demonstrates the importance of proper surface area regulation in cell movement.

Live cell imaging of mechanotransduction

Mechanical forces play important roles in the regulation of cellular functions, including polarization, migration and stem cell differentiation. Tremendous advancement in our understanding of mechanotransduction has been achieved with the recent development of imaging technologies and molecular biosensors. In particular, genetically encoded biosensors based on fluorescence resonance energy transfer (FRET) technology have been widely developed and applied in the field of mechanobiology. In this article, we will provide an overview of the recent progress of FRET application in mechanobiology, specifically mechanotransduction. We first introduce fluorescent proteins and FRET technology. We then discuss the mechanotransduction processes in different cells including stem cells, with a special emphasis on the important signalling molecules involved in mechanotransduction. Finally, we discuss methods that can allow the integration of simultaneous FRET imaging and mechanical stimulation to trigger signalling transduction. In summary, FRET technology has provided a powerful tool for the study of mechanotransduction to advance our systematic understanding of the molecular mechanisms by which cells respond to mechanical stimulation.