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The actin-rich cortex plays a fundamental role in many cellular processes. Its architecture and molecular composition vary across cell types and physiological states. The full complement of actin assembly factors driving cortex formation and how their activities are spatiotemporally regulated remain to be fully elucidated. Using Dictyostelium as a model for polarized and rapidly migrating cells, we show that GxcM, a RhoGEF localized specifically in the rear of migrating cells, functions together with F-BAR protein Fbp17, a small GTPase RacC, and the actin nucleation-promoting factor WASP to coordinately promote Arp2/3 complex-mediated cortical actin assembly. Overactivation of this signaling cascade leads to excessive actin polymerization in the rear cortex, whereas its disruption causes defects in cortical integrity and function. Therefore, apart from its well-defined role in the formation of the protrusions at the cell front, the Arp2/3 complex-based actin carries out a previously unappreciated function in building the rear cortical subcompartment in rapidly migrating cells., (© 2023 Li et al.)

Polarity is essential for diverse functions in many cell types. Establishing polarity requires targeting a network of specific signaling and cytoskeleton molecules to different subregions of the cell, yet the full complement of polarity regulators and how their activities are integrated over space and time to form morphologically and functionally distinct domains remain to be uncovered. Here, by using the model system Dictyostelium and exploiting the characteristic chemoattractant-stimulated translocation of polarly distributed molecules, we developed a proteomic screening approach, through which we identified a leucine-rich repeat domain-containing protein we named Leep1 as a novel polarity regulator. We combined imaging, biochemical, and phenotypic analyses to demonstrate that Leep1 localizes selectively at the leading edge of cells by binding to PIP3, where it modulates pseudopod and macropinocytic cup dynamics by negatively regulating the Scar/WAVE complex. The spatiotemporal coordination of PIP3 signaling, Leep1, and the Scar/WAVE complex provides a cellular mechanism for organizing protrusive structures at the leading edge., (© 2021 Yang et al.)

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., Competing Interests: The authors declare no conflict of interest., (Copyright © 2018 the Author(s). Published by PNAS.)

In many systems, including the social amoeba Dictyostelium discoideum, development is often marked by dynamic morphological and transcriptional changes orchestrated by key transcription factors. However, efforts to examine sequential genome-wide changes of gene regulation in developmental processes have been fairly limited. Here we report the developmental regulatory dynamics of GtaC, a GATA-type zinc-finger transcription factor, through the analyses of serial ChIP- and RNA-sequencing data. GtaC is essential for developmental progression, decoding extracellular cAMP pulses during early development and may play a role in mediating cell-type differentiation at later stages. We find that GtaC exhibits temporally distinctive DNA-binding patterns concordant with each developmental stage. We identify direct GtaC targets and observe cotemporaneous GtaC-binding and developmental expression regulation. Our results suggest that GtaC regulates multiple physiological processes as Dictyostelium transitions from a group of unicellular amoebae to an integrated multicellular organism.

Directional cell migration in an electric field, a phenomenon called galvanotaxis or electrotaxis, occurs in many types of cells, and may play an important role in wound healing and development. Small extracellular electric fields can guide the migration of amoeboid cells, and we established a large-scale screening approach to search for mutants with electrotaxis phenotypes from a collection of 563 Dictyostelium discoideum strains with morphological defects. We identified 28 strains that were defective in electrotaxis and 10 strains with a slightly higher directional response. Using plasmid rescue followed by gene disruption, we identified some of the mutated genes, including some previously implicated in chemotaxis. Among these, we studied PiaA, which encodes a critical component of TORC2, a kinase protein complex that transduces changes in motility by activating the kinase PKB (also known as Akt). Furthermore, we found that electrotaxis was decreased in mutants lacking gefA, rasC, rip3, lst8, or pkbR1, genes that encode other components of the TORC2-PKB pathway. Thus, we have developed a high-throughput screening technique that will be a useful tool to elucidate the molecular mechanisms of electrotaxis., (Copyright © 2015, American Association for the Advancement of Science.)

Biological oscillations are observed at many levels of cellular organization. In the social amoeba Dictyostelium discoideum, starvation-triggered multicellular development is organized by periodic cyclic adenosine 3',5'-monophosphate (cAMP) waves, which provide both chemoattractant gradients and developmental signals. We report that GtaC, a GATA transcription factor, exhibits rapid nucleocytoplasmic shuttling in response to cAMP waves. This behavior requires coordinated action of a nuclear localization signal and reversible G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor-mediated phosphorylation. Although both are required for developmental gene expression, receptor occupancy promotes nuclear exit of GtaC, which leads to a transient burst of transcription at each cAMP cycle. We demonstrate that this biological circuit filters out high-frequency signals and counts those admitted, thereby enabling cells to modulate gene expression according to the dynamic pattern of the external stimuli.