Coactivation of Rac1 and Cdc42 at lamellipodia and membrane ruffles induced by epidermal growth factor

Phosphorylation of Coronin 1B by Protein Kinase C Regulates Interaction with Arp2/3 and Cell Motility

Coronins are a conserved family of WD repeat-containing, actin-binding proteins that regulate cell motility in a variety of model organisms. Our results show that Coronin 1B is a ubiquitously expressed member of the mammalian Coronin gene family that co-localizes with the Arp2/3 complex at the leading edge of fibroblasts, and co-immunoprecipitates with this complex. Pharmacological experiments show that the interaction between Coronin 1B and the Arp2/3 complex is regulated by protein kinase C (PKC) phosphorylation. Coronin 1B is phosphorylated by PKC both in vitro and in vivo. Using tryptic peptide mapping and mutagenesis, we have identified serine 2 (Ser-2) on Coronin 1B as the major residue phosphorylated by PKC in vivo. Rat2 fibroblasts expressing the Coronin 1B S2A mutant show enhanced ruffling in response to phorbol 12-myristate 13-acetate (PMA) and increased speed in single cell tracking assays. Cells expressing the Coronin 1B S2D mutant have attenuated PMA-induced ruffling and slower cell speed. Expression of the S2A mutant partially protects cells from the inhibitory effects of PMA on cell speed, whereas expression of the S2D mutant renders cells hypersensitive to its effects. These data demonstrate that Coronin 1B regulates leading edge dynamics and cell motility in fibroblasts, and that its ability to control motility and interactions with the Arp2/3 complex are regulated by PKC phosphorylation at Ser-2. Furthermore, Coronin 1B phosphorylation is responsible for a significant fraction of the effects of PMA on fibroblast motility.

Directional persistence of EGF-induced cell migration is associated with stabilization of lamellipodial protrusions

Migrating cells can sustain a relatively constant direction of lamellipodial protrusion and locomotion over timescales ranging from minutes to hours. However, individual waves of lamellipodial extension occur over much shorter characteristic times. Little understanding exists regarding how cells might integrate biophysical processes across these disparate timescales to control the directional persistence of locomotion. We address this issue by examining the effects of epidermal growth factor (EGF) stimulation on long-timescale directional persistence and short-timescale lamellipodial dynamics of EGF receptor-transfected Chinese hamster ovary cells migrating on fibronectin-coated substrata. Addition of EGF increased persistence, with the magnitude of increase correlating with fibronectin coating concentration. Kymographic analysis of EGF-stimulated lamellipodial dynamics revealed that the temporal stability of lamellipodial protrusions similarly increased with fibronectin concentration. A soluble RGD peptide competitor reduced both the persistence of long-timescale cell paths and the stability of short-timescale membrane protrusions, indicating that cell-substratum adhesion concomitantly influences lamellipodial dynamics and directional persistence. These results reveal the importance of adhesion strength in regulating the directional motility of cells and suggest that the short-timescale kinetics of adhesion complex formation may play a key role in modulating directional persistence over much longer timescales.

Localized RhoA activation as a requirement for the induction of membrane ruffling

We examined the spatio-temporal activity of RhoA in migrating cells and growth factor-stimulated cells by using probes based on the principle of fluorescence resonance energy transfer. In HeLa cells migrating at a low cell density, RhoA was activated both at the contractile tail and at the leading edge. However, RhoA was activated only at the leading edge in MDCK cells migrating as a monolayer sheet. In growth factor-stimulated Cos1 and NIH3T3 cells, the activity of RhoA was greatly decreased at the plasma membrane, but remained high at the membrane ruffles in nascent lamellipodia. These observations are in agreement with the proposed role played by RhoA in stress fiber formation, but they also implicated RhoA in the regulation of membrane ruffling, the induction of which is a typical phenotype of activated Rac. In agreement with this view, dominant negative RhoA was found to inhibit membrane ruffling induced by active Rac. Furthermore, we found that Cdc42 activity was also required for high RhoA activity in membrane ruffles. Finally, we found that mDia1, but not ROCK, was stably associated with membrane ruffles. In conclusion, these results suggested that RhoA cooperates with Rac1 and Cdc42 to induce membrane ruffles via the recruitment of mDia.

Spatially distinct binding of Cdc42 to PAK1 and N-WASP in breast carcinoma cells

While a significant amount is known about the biochemical signaling pathways of the Rho family GTPase Cdc42, a better understanding of how these signaling networks are coordinated in cells is required. In particular, the predominant subcellular sites where GTP-bound Cdc42 binds to its effectors, such as p21-activated kinase 1 (PAK1) and N-WASP, a homolog of the Wiskott-Aldritch syndrome protein, are still undetermined. Recent fluorescence resonance energy transfer (FRET) imaging experiments using activity biosensors show inconsistencies between the site of local activity of PAK1 or N-WASP and the formation of specific membrane protrusion structures in the cell periphery. The data presented here demonstrate the localization of interactions by using multiphoton time-domain fluorescence lifetime imaging microscopy (FLIM). Our data here establish that activated Cdc42 interacts with PAK1 in a nucleotide-dependent manner in the cell periphery, leading to Thr-423 phosphorylation of PAK1, particularly along the lengths of cell protrusion structures. In contrast, the majority of GFP-N-WASP undergoing FRET with Cy3-Cdc42 is localized within a transferrin receptor- and Rab11-positive endosomal compartment in breast carcinoma cells. These data reveal for the first time distinct spatial association patterns between Cdc42 and its key effector proteins controlling cytoskeletal remodeling.

Wiskott-Aldrich syndrome protein and the cytoskeletal dynamics of dendritic cells

The regulated migration and spatial localization of dendritic cells in response to environmental signals are critical events during the initiation of physiological immune responses and maintenance of tolerance. Cells deficient in the Wiskott-Aldrich syndrome protein (WASP) have been used to demonstrate the importance of the dynamic remodelling of the actin-based cytoskeleton during the selective adhesion and migration of these cells. Unlike most cell types, macrophages, dendritic cells, and osteoclasts utilize a specialized adhesive array termed the podosome in order to migrate. Podosomes are composed of many of the same structural and regulatory proteins as seen in the more commonly found focal adhesion, but are unique in their requirement for WASP. Without WASP, podosomes cannot form and the affected cells are obliged to use focal adhesions for their migratory activities. Once activated by a series of upstream regulatory proteins, WASP acts as a scaffold for the binding of the potent actin nucleating protein complex known as Arp2/3. This article reviews the available evidence that suggests that failures in the regulation of the actin cytoskeleton may contribute significantly to the immunopathology of the Wiskott-Aldrich syndrome.

RalA activation at nascent lamellipodia of epidermal growth factor-stimulated Cos7 cells and migrating Madin-Darby canine kidney cells

RalA, a member of the Ras-family GTPases, regulates various cellular functions such as filopodia formation, endocytosis, and exocytosis. On epidermal growth factor (EGF) stimulation, activated Ras recruits guanine nucleotide exchange factors (GEFs) for RalA, followed by RalA activation. By using fluorescence resonance energy transfer-based probes for RalA activity, we found that the EGF-induced RalA activation in Cos7 cells was restricted at the EGF-induced nascent lamellipodia, whereas under a similar condition both Ras activation and Ras-dependent translocation of Ral GEFs occurred more diffusely at the plasma membrane. This EGF-induced RalA activation was not observed when lamellipodial protrusion was suppressed by a dominant negative mutant of Rac1, a GTPase-activating protein for Cdc42, inhibitors of phosphatidylinositol 3-kinase, or inhibitors of actin polymerization. On the other hand, EGF-induced lamellipodial protrusion was inhibited by microinjection of the RalA-binding domains of RalBP1 and Sec5. Furthermore, we found that RalA activity was high at the lamellipodia of migrating Madin-Darby canine kidney cells and that the migration of Madin-Darby canine kidney cells was perturbed by the microinjection of RalBP1-RalA-binding domain. Thus, RalA activation is required for the induction of lamellipodia, and conversely, lamellipodial protrusion seems to be required for the RalA activation, suggesting the presence of a positive feedback loop between RalA activation and lamellipodial protrusion. Our observation also demonstrates that the spatial regulation of RalA is conducted by a mechanism distinct from the temporal regulation conducted by Ras-dependent plasma membrane recruitment of Ral guanine nucleotide exchange factors.

Visualization of Molecular Activities Inside Living Cells with Fluorescent Labels

The major task of modern cell biology is to identify the function and relation of the many different gene products, discovered by genomics and proteomics approaches, in the context of the living cell. To achieve this goal, an increasing toolbox of custom-designed biosensors based on fluorescent labels is available to study the molecular activities of the cellular machinery. An overview of the current status of the young field of molecular-cellular physiology is presented that includes the application of fluorescent labels in the design of biosensors and the major detection schemes used to extract their sensing information. In particular, the use of the photophysical phenomenon of Förster resonance energy transfer (FRET) as a powerful indicator of cellular biochemical events is discussed. In addition, we will point out the challenges and directions of the field and project the short-term future for the application of fluorescence-based biosensors in biology.

Integrin ?2?1 modulates EGF stimulation of Rho GTPase-dependent morphological changes in adherent human rhabdomyosarcoma RD cells

The ability of cells to undergo shape changes is essential for diverse cellular functions including cell growth, differentiation, and movement. The present study examines how an integration of the function of alpha2beta1 integrin with that of the receptor for epidermal growth factor (EGFR) modulates EGF-stimulated morphological changes in human rhabdomyosarcoma RD transfectant cells. Upon EGF stimulation, RD transfectant cells that lacked alpha2beta1 integrin expression (RDpF) underwent contraction; in contrast, expression of alpha2beta1 on RD cells (RDX2C2) resulted in transient cell spreading. Integrin alpha2 cytoplasmic domain played a critical role in the observed alpha2beta1-mediated conversion from a cell rounding to a cell spreading phenotype. Thus, the expression of an alpha2 cytoplasmic domain deletion variant (X2C0) or a chimeric alpha2beta1 containing the cytoplasmic domain of alpha4 (X2C4) or alpha5 (X2C5), instead of alpha2, failed to mediate spreading upon EGF stimulation. Using dominant negative (DN) mutants of RhoGTPases, results revealed that RhoA activation was required for both EGF-stimulated responses of cell rounding and spreading, Cdc42 functioned in the re-spreading of cells after undergoing EGF-stimulated contraction, and Rac1 was required in alpha2beta1-mediated RD cell spreading. Therefore, alpha2beta1 integrin function can switch the Rho GTPase-dependent cell shape changes in RD cells from an EGF-stimulated cell contraction to a spreading morphology. Together, results show that integrin alpha2 cytoplasmic domain plays an indispensable role in the ability of integrin alpha2beta1 to modulate EGF stimulation of Rho-GTPase-dependent morphological changes in RD cells.