Osmotic stress activates Rac and Cdc42 in neutrophils: role in hypertonicity-induced actin polymerization

Hypertonicity inhibits a variety of neutrophil functions through poorly defined mechanisms. Our earlier studies suggest that osmotically induced actin polymerization and cytoskeleton remodeling is a key component in the hypertonic block of exocytosis and cell movement. To gain insight into the signaling mechanisms underlying the hyperosmotic F-actin response, we investigated whether hypertonicity stimulates Rac and Cdc42 and, if so, whether their activation contributes to the hypertonic rise in F-actin. Using a recently developed pull-down assay that specifically captures the active forms of these small GTPases, we found that hypertonicity caused an ∼2.5- and ∼7.2-fold activation of Rac and Cdc42, respectively. This response was rapid and sustained. Small GTPase activation was not mediated by the osmotic stimulation of Src kinases, heterotrimeric G proteins, or phosphatidylinositol 3-kinase. Interestingly, an increase in intracellular ionic strength was sufficient to activate Rac even in the absence of cell shrinkage. Inhibition of Rac and Cdc42 by Clostridium difficile toxin B substantially reduced but did not abolish the hypertonicity-induced F-actin response. Thus hypertonicity is a potent activator of Rac and Cdc42, and this effect seems to play an important but not exclusive role in the hyperosmolarity-triggered cytoskeleton remodeling.

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Localized Ras signaling at the leading edge regulates PI3K, cell polarity, and directional cell movement

The Journal of Cell Biology ◽

10.1083/jcb.200406177 ◽

2004 ◽

Vol 167 (3) ◽

pp. 505-518 ◽

Cited By ~ 242

Author(s):

Atsuo T. Sasaki ◽

Cheryl Chun ◽

Kosuke Takeda ◽

Richard A. Firtel

Keyword(s):

Actin Polymerization ◽

Cell Movement ◽

Leading Edge ◽

Ras Signaling ◽

Phosphatidylinositol 3 Kinase ◽

Ras Activation ◽

Directional Movement ◽

Ras Effectors ◽

Chemotaxis Receptors ◽

Intermediate Event

During chemotaxis, receptors and heterotrimeric G-protein subunits are distributed and activated almost uniformly along the cell membrane, whereas PI(3,4,5)P3, the product of phosphatidylinositol 3-kinase (PI3K), accumulates locally at the leading edge. The key intermediate event that creates this strong PI(3,4,5)P3 asymmetry remains unclear. Here, we show that Ras is rapidly and transiently activated in response to chemoattractant stimulation and regulates PI3K activity. Ras activation occurs at the leading edge of chemotaxing cells, and this local activation is independent of the F-actin cytoskeleton, whereas PI3K localization is dependent on F-actin polymerization. Inhibition of Ras results in severe defects in directional movement, indicating that Ras is an upstream component of the cell's compass. These results support a mechanism by which localized Ras activation mediates leading edge formation through activation of basal PI3K present on the plasma membrane and other Ras effectors required for chemotaxis. A feedback loop, mediated through localized F-actin polymerization, recruits cytosolic PI3K to the leading edge to amplify the signal.

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Conformational resolution of nucleotide cycling and effector interactions for multiple small GTPases in parallel

10.1101/577437 ◽

2019 ◽

Author(s):

Ryan C. Killoran ◽

Matthew J. Smith

Keyword(s):

Binding Pocket ◽

Small Gtpases ◽

Small Gtpase ◽

Nucleotide Exchange ◽

Cellular Processes ◽

Selective Labelling ◽

Activation Potential ◽

Single Complex ◽

Gtpase Activation ◽

Ras Isoforms

AbstractSmall GTPase proteins alternatively bind GDP/GTP guanine nucleotides to gate signaling pathways that direct most cellular processes. Numerous GTPases are implicated in oncogenesis, particularly three RAS isoforms HRAS, KRAS and NRAS, and the RHO family GTPase RAC1. Signaling networks comprising small GTPases are highly connected, and there is evidence of direct biochemical crosstalk between the functional G-domains of these proteins. The activation potential of a given GTPase is contingent on a co-dependent interaction with nucleotide and a Mg2+ ion, which bind to individual variants via distinct affinities coordinated by residues in the nucleotide binding pocket. Here, we utilize a selective-labelling strategy coupled with real-time nuclear magnetic resonance (NMR) spectroscopy to monitor nucleotide exchange, GTP hydrolysis and effector interactions of multiple small GTPases in a single complex system. We provide new insight on nucleotide preference and the role of Mg2+ in activating both wild-type and oncogenic mutant enzymes. Multiplexing reveals GEF, GAP and effector binding specificity in mixtures of GTPases and establishes the complete biochemical equivalence of the three related RAS isoforms. This work establishes that direct quantitation of the nucleotide-bound conformation is required to accurately resolve GTPase activation potential, as GTPases such as RALA or the G12C mutant of KRAS demonstrate fast exchange kinetics but have a high affinity for GDP. Further, we propose that the G-domains of small GTPases behave autonomously in solution and nucleotide cycling proceeds independent of protein concentration but is highly impacted by Mg2+ abundance.

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Big roles for small GTPases in the control of directed cell movement

Biochemical Journal ◽

10.1042/bj20061432 ◽

2006 ◽

Vol 401 (2) ◽

pp. 377-390 ◽

Cited By ~ 144

Author(s):

Pascale G. Charest ◽

Richard A. Firtel

Keyword(s):

Molecular Mechanisms ◽

Cell Movement ◽

Small Gtpases ◽

Cell Polarization ◽

Small Gtpase ◽

Cellular Growth ◽

Cellular Motility ◽

Gradient Sensing ◽

Directed Cell Migration ◽

Shed Light

Small GTPases are involved in the control of diverse cellular behaviours, including cellular growth, differentiation and motility. In addition, recent studies have revealed new roles for small GTPases in the regulation of eukaryotic chemotaxis. Efficient chemotaxis results from co-ordinated chemoattractant gradient sensing, cell polarization and cellular motility, and accumulating data suggest that small GTPase signalling plays a central role in each of these processes as well as in signal relay. The present review summarizes these recent findings, which shed light on the molecular mechanisms by which small GTPases control directed cell migration.

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Force measurements in E-cadherin–mediated cell doublets reveal rapid adhesion strengthened by actin cytoskeleton remodeling through Rac and Cdc42

The Journal of Cell Biology ◽

10.1083/jcb.200403043 ◽

2004 ◽

Vol 167 (6) ◽

pp. 1183-1194 ◽

Cited By ~ 271

Author(s):

Yeh-Shiu Chu ◽

William A. Thomas ◽

Olivier Eder ◽

Frederic Pincet ◽

Eric Perez ◽

...

Keyword(s):

Cell Adhesion ◽

Actin Cytoskeleton ◽

Actin Polymerization ◽

Small Gtpases ◽

Dominant Negative ◽

Dominant Negative Form ◽

Cytoskeleton Remodeling ◽

Dependent Cell ◽

Separation Force ◽

E Cadherin

We have used a modified, dual pipette assay to quantify the strength of cadherin-dependent cell–cell adhesion. The force required to separate E-cadherin–expressing paired cells in suspension was measured as an index of intercellular adhesion. Separation force depended on the homophilic interaction of functional cadherins at the cell surface, increasing with the duration of contact and with cadherin levels. Severing the link between cadherin and the actin cytoskeleton or disrupting actin polymerization did not affect initiation of cadherin-mediated adhesion, but prevented it from developing and becoming stronger over time. Rac and Cdc42, the Rho-like small GTPases, were activated when E-cadherin–expressing cells formed aggregates in suspension. Overproduction of the dominant negative form of Rac or Cdc42 permitted initial E-cadherin–based adhesion but affected its later development; the dominant active forms prevented cell adhesion outright. Our findings highlight the crucial roles played by Rac, Cdc42, and actin cytoskeleton dynamics in the development and regulation of strong cell adhesion, defined in terms of mechanical forces.

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Translocation of cortactin to the cell periphery is mediated by the small GTPase Rac1

Journal of Cell Science ◽

10.1242/jcs.111.16.2433 ◽

1998 ◽

Vol 111 (16) ◽

pp. 2433-2443 ◽

Cited By ~ 9

Author(s):

S.A. Weed ◽

Y. Du ◽

J.T. Parsons

Keyword(s):

Actin Polymerization ◽

Intracellular Localization ◽

Adhesion Formation ◽

Small Gtpases ◽

Small Gtpase ◽

Dominant Negative ◽

Actin Binding ◽

Cell Cortex ◽

Cell Periphery ◽

Membrane Ruffling

Small GTPases of the Rho family regulate signaling pathways that control actin cytoskeletal structures. In Swiss 3T3 cells, RhoA activation leads to stress fiber and focal adhesion formation, Rac1 to lamellipoda and membrane ruffles, and Cdc42 to microspikes and filopodia. Several downstream molecules mediating these effects have been recently identified. In this report we provide evidence that the intracellular localization of the actin binding protein cortactin, a Src kinase substrate, is regulated by the activation of Rac1. Cortactin redistributes from the cytoplasm into membrane ruffles as a result of growth factor-induced Rac1 activation, and this translocation is blocked by expression of dominant negative Rac1N17. Expression of constitutively active Rac1L61 evoked the translocation of cortactin from cytoplasmic pools into peripheral membrane ruffles. Expression of mutant forms of the serine/threonine kinase PAK1, a downstream effector of Rac1 and Cdc42 recently demonstrated to trigger cortical actin polymerization and membrane ruffling, also led to the translocation of cortactin to the cell cortex, although this was effectively blocked by coexpression of Rac1N17. Collectively these data provide evidence for cortactin as a putative target of Rac1-induced signal transduction events involved in membrane ruffling and lamellipodia formation.

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