Wave Patterns in Cell Membrane and Actin Cortex Uncoupled from Chemotactic Signals

Fluctuations of formin binding in the generation of membrane patterns

Circular actin waves that propagate on the substrate-attached membrane of Dictyostelium cells separate two distinct membrane domains from each other: an inner territory rich in phosphatidyl-(3,4,5) trisphosphate (PIP3) and an external area decorated with the PIP3-degrading 3-phosphatase PTEN. During wave propagation, the inner territory increases at the expense of the external area. Beyond a size limit, the inner territory becomes unstable, breaking into an inner and an external domain. The sharp boundary between these domains is demarcated by the insertion of an actin wave. During the conversion of inner territory to external area, the state of the membrane fluctuates, as visualized by dynamic landscapes of formin B binding. Here we analyze the formin B fluctuations in relation to three markers of the membrane state: activated Ras, PIP3, and PTEN.

Unified control of amoeboid pseudopod extension in multiple organisms by branched F-actin in the front and parallel F-actin/myosin in the cortex

The trajectory of moving eukaryotic cells depends on the kinetics and direction of extending pseudopods. The direction of pseudopods has been well studied to unravel mechanisms for chemotaxis, wound healing and inflammation. However, the kinetics of pseudopod extension-when and why do pseudopods start and stop- is equally important, but is largely unknown. Here the START and STOP of about 4000 pseudopods was determined in four different species, at four conditions and in nine mutants (fast amoeboids Dictyostelium and neutrophils, slow mesenchymal stem cells, and fungus B.d. chytrid with pseudopod and a flagellum). The START of a first pseudopod is a random event with a probability that is species-specific (23%/s for neutrophils). In all species and conditions, the START of a second pseudopod is strongly inhibited by the extending first pseudopod, which depends on parallel filamentous actin/myosin in the cell cortex. Pseudopods extend at a constant rate by polymerization of branched F-actin at the pseudopod tip, which requires the Scar complex. The STOP of pseudopod extension is induced by multiple inhibitory processes that evolve during pseudopod extension and mainly depend on the increasing size of the pseudopod. Surprisingly, no differences in pseudopod kinetics are detectable between polarized, unpolarized or chemotactic cells, and also not between different species except for small differences in numerical values. This suggests that the analysis has uncovered the fundament of cell movement with distinct roles for stimulatory branched F-actin in the protrusion and inhibitory parallel F-actin in the contractile cortex.

An Excitable Ras/PI3K/ERK Signaling Network Controls Migration and Oncogenic Transformation in Epithelial Cells

The Ras/PI3K/extracellular signal-regulated kinases (ERK) signaling network plays fundamental roles in cell growth, survival, and migration and is frequently activated in cancer. Here, we show that the activities of the signaling network propagate as coordinated waves, biased by growth factor, which drive actin-based protrusions in human epithelial cells. The network exhibits hallmarks of biochemical excitability: the annihilation of oppositely directed waves, all-or-none responsiveness, and refractoriness. Abrupt perturbations to Ras, PI(4,5)P2, PI(3,4)P2, ERK, and TORC2 alter the threshold, observations that define positive and negative feedback loops within the network. Oncogenic transformation dramatically increases the wave activity, the frequency of ERK pulses, and the sensitivity to EGF stimuli. Wave activity was progressively enhanced across a series of increasingly metastatic breast cancer cell lines. The view that oncogenic transformation is a shift to a lower threshold of excitable Ras/PI3K/ERK network, caused by various combinations of genetic insults, can facilitate the assessment of cancer severity and effectiveness of interventions.

Symmetry Breaking during Cell Movement in the Context of Excitability, Kinetic Fine-Tuning and Memory of Pseudopod Formation

The path of moving eukaryotic cells depends on the kinetics and direction of extending pseudopods. Amoeboid cells constantly change their shape with pseudopods extending in different directions. Detailed analysis has revealed that time, place and direction of pseudopod extension are not random, but highly ordered with strong prevalence for only one extending pseudopod, with defined life-times, and with reoccurring events in time and space indicative of memory. Important components are Ras activation and the formation of branched F-actin in the extending pseudopod and inhibition of pseudopod formation in the contractile cortex of parallel F-actin/myosin. In biology, order very often comes with symmetry. In this essay, I discuss cell movement and the dynamics of pseudopod extension from the perspective of symmetry and symmetry changes of Ras activation and the formation of branched F-actin in the extending pseudopod. Combining symmetry of Ras activation with kinetics and memory of pseudopod extension results in a refined model of amoeboid movement that appears to be largely conserved in the fast moving Dictyostelium and neutrophils, the slow moving mesenchymal stem cells and the fungus B.d. chytrid.

Oscillatory Switches of Dorso-Ventral Polarity in Cells Confined between Two Surfaces

To maneuver in a three-dimensional space, migrating cells need to accommodate to multiple surfaces. In particular, phagocytes have to explore their environment in the search for particles to be ingested. To examine how cells decide between competing surfaces, we exposed single cells of Dictyostelium to a defined three-dimensional space by confining them between two planar surfaces: those of a cover glass and of a wedged microcantilever. These cells form propagating waves of filamentous actin and PIP3 on their ventral substrate-attached surface. The dynamics of wave formation in the confined cells was explored using two-focus fluorescence imaging. When waves formed on one substrate, wave formation on the other substrate was efficiently suppressed. The propensity for wave formation switched between the opposing cell surfaces with periods of 2-5 min by one of two modes: 1) a rolling mode involving the slipping of a wave along the nonattached plasma membrane and 2) de novo initiation of waves on the previously blank cell surface. These data provide evidence for a cell-autonomous oscillator that switches dorso-ventral polarity in a cell simultaneously exposed to multiple substrate surfaces.

The Architecture of Traveling Actin Waves Revealed by Cryo-Electron Tomography

Actin waves are dynamic supramolecular structures involved in cell migration, cytokinesis, adhesion, and neurogenesis. Although wave-like propagation of actin networks is a widespread phenomenon, the actin architecture underlying wave propagation remained unknown. In situ cryo-electron tomography of Dictyostelium cells unveils the wave architecture and provides evidence for wave progression by de novo actin nucleation. Subtomogram averaging reveals the structure of Arp2/3 complex-mediated branch junctions in their native state, and enables quantitative analysis of the 3D organization of branching within the waves. We find an excess of branches directed toward the substrate-attached membrane, and tent-like structures at sites of branch clustering. Fluorescence imaging shows that Arp2/3 clusters follow accumulation of the elongation factor VASP. We propose that filament growth toward the membrane lifts up the actin network as the wave propagates, until depolymerization of oblique filaments at the back causes the collapse of horizontal filaments into a compact layer.

Insight from the maximal activation of the signal transduction excitable network in Dictyostelium discoideum

Cell migration requires the coordination of an excitable signal transduction network involving Ras and PI3K pathways with cytoskeletal activity. We show that expressing activated Ras GTPase-family proteins in cells lacking PTEN or other mutations which increase cellular protrusiveness transforms cells into a persistently activated state. Leading- and trailing-edge markers were found exclusively at the cell perimeter and the cytosol, respectively, of the dramatically flattened cells. In addition, the lifetimes of dynamic actin puncta were increased where they overlapped with actin waves, suggesting a mechanism for the coupling between these two networks. All of these phenotypes could be reversed by inhibiting signal transduction. Strikingly, maintaining cells in this state of constant activation led to a form of cell death by catastrophic fragmentation. These findings provide insight into the feedback loops that control excitability of the signal transduction network, which drives migration.

Coupled excitable Ras and F-actin activation mediates spontaneous pseudopod formation and directed cell movement

Chemotaxis is the mechanism by which cells move in the direction of chemical gradients. The central circuit connecting basal movement and gradient sensing is unknown. Ras activation and F-actin form one coupled excitable system, which is the beating heart of cell movement in both the absence and presence of external cues.

Many eukaryotic cells regulate their mobility by external cues. Genetic studies have identified >100 components that participate in chemotaxis, which hinders the identification of the conceptual framework of how cells sense and respond to shallow chemical gradients. The activation of Ras occurs during basal locomotion and is an essential connector between receptor and cytoskeleton during chemotaxis. Using a sensitive assay for activated Ras, we show here that activation of Ras and F-actin forms two excitable systems that are coupled through mutual positive feedback and memory. This coupled excitable system leads to short-lived patches of activated Ras and associated F-actin that precede the extension of protrusions. In buffer, excitability starts frequently with Ras activation in the back/side of the cell or with F-actin in the front of the cell. In a shallow gradient of chemoattractant, local Ras activation triggers full excitation of Ras and subsequently F-actin at the side of the cell facing the chemoattractant, leading to directed pseudopod extension and chemotaxis. A computational model shows that the coupled excitable Ras/F-actin system forms the driving heart for the ordered-stochastic extension of pseudopods in buffer and for efficient directional extension of pseudopods in chemotactic gradients.