Cytoskeletal interactions of Rap1b in platelets

Talin‑1 interaction network in cellular mechanotransduction (Review)

The mechanical signals within the extracellular matrix (ECM) regulate cell growth, proliferation and differentiation, and integrins function as the hub between the ECM and cellular actin. Focal adhesions (FAs) are multi‑protein, integrin‑containing complexes, acting as tension‑sensing anchoring points that bond cells to the extracellular microenvironment. Talin‑1 serves as the central protein of FAs that participates in the activation of integrins and connects them with the actin cytoskeleton. As a cytoplasmic protein, Talin‑1 consists of a globular head domain and a long rod comprised of a series of α‑helical bundles. The unique structure of the Talin‑1 rod domain permits folding and unfolding in response to the mechanical stress, revealing various binding sites. Thus, conformation changes of the Talin‑1 rod domain enable the cell to convert mechanical signals into chemical through multiple signaling pathways. The present review discusses the binding partners of Talin‑1, their interactions, effects on the cellular processes, and their possible roles in diseases.

Structure of Rap1b bound to talin reveals a pathway for triggering integrin activation

Activation of transmembrane receptor integrin by talin is essential for inducing cell adhesion. However, the pathway that recruits talin to the membrane, which critically controls talin's action, remains elusive. Membrane-anchored mammalian small GTPase Rap1 is known to bind talin-F0 domain but the binding was shown to be weak and thus hardly studied. Here we show structurally that talin-F0 binds to human Rap1b like canonical Rap1 effectors despite little sequence homology, and disruption of the binding strongly impairs integrin activation, cell adhesion, and cell spreading. Furthermore, while being weak in conventional binary binding conditions, the Rap1b/talin interaction becomes strong upon attachment of activated Rap1b to vesicular membranes that mimic the agonist-induced microenvironment. These data identify a crucial Rap1-mediated membrane-targeting mechanism for talin to activate integrin. They further broadly caution the analyses of weak protein-protein interactions that may be pivotal for function but neglected in the absence of specific cellular microenvironments.

miR-181 subunits enhance the chemosensitivity of temozolomide by Rap1B-mediated cytoskeleton remodeling in glioblastoma cells

Glioblastoma multiforme (GBM) is the most malignant and frequent brain tumor, with an aggressive growth pattern and poor prognosis despite best treatment modalities. Although chemotherapy with temozolomide (TMZ) may restrain tumor growth for some months, TMZ resistance is also common and accounts for many treatment failures. Research into microRNA's role in GBM has shown that microRNAs play a key regulatory role in the GBM, making it a potential therapeutic target. In this study, we demonstrated that the lower expression of miR-181a/b/c/d subunits contributes to astrocytoma tumorigenesis, and their overexpression could inhibit the invasive proliferation of glioblastoma cells by targeting Rap1B-mediated cytoskeleton remodeling and related molecular (Cdc42, RhoA and N-cadherin) changes, suggesting that miR-181 was a critical regulator and might be an important target for glioblastoma treatment. TMZ as a standard chemotherapeutic agent for GBM inhibited the Rap1B expression and actin cytoskeleton remodeling to exert its cell killing by upregulating miR-181a/b/c/d subunits; conversely, each miR-181a/b/c/d subunit enhanced the chemosensitivity of TMZ in glioblastoma cells.

p40phox: The last NADPH oxidase subunit

The phagocytic NADPH-oxidase is a multiprotein system activated during the inflammatory response to produce superoxide anion (O2-), which is the substrate for formation of additional reactive oxygen species (ROS). The importance of this system for innate immunity is established by chronic granulomatous disease (CGD), a primary immunodeficiency caused by defects in the NADPH oxidase. In this review, we present and discuss recent knowledge about p40phox, the last NADPH oxidase component to be identified. Furthermore, its interaction with cellular pathways outside of the NADPH oxidase is discussed. Described in this review is evidence that p40phox participates in NADPH oxidase dynamics within cells, what is known about its role in the oxidase, the possibility that p40phox participates in non-NADPH oxidase processes in phagocytic and non-phagocytic cells and whether p40phox could mediate a similar function in other NADPH oxidases. An improved understanding of p40phox should provide new insights about NADPH oxidase, the physiology of phagocytic cells and the innate immune system.

F-actin-dependent Translocation of the Rap1 GDP/GTP Exchange Factor RasGRP2

RasGRPs constitute a new group of diacylglycerol-dependent GDP/GTP exchange factors that activate Ras subfamily GTPases. Despite a common structure, Ras-GRPs diverge in their GTPase specificity, subcellular distribution, and downstream biological effects. The more divergent family member is RasGRP2, a Rap1-specific exchange factor with low affinity toward diacylglycerol. The regulation of RasGRP2 during signal transduction has remained elusive up to now. In this report, we show that the subcellular localization of Ras-GRP2 is highly dependent on actin dynamics. Thus, the induction of F-actin by cytoskeletal regulators such as Vav, Vav2, Dbl, and Rac1 leads to the shift of RasGRP2 from the cytosol to membrane ruffles and its co-localization with F-actin. Treatment of cells with cytoskeletal disrupting drugs abolishes this effect, leading to an abnormal localization of RasGRP2 in cytoplasmic clusters of actin. The use of Rac1 effector mutants indicates that the RasGRP2 translocation is linked exclusively to actin polymerization and is independent of other pathways such as p21-activated kinase JNK, or superoxide production. Biochemical experiments demonstrate that the translocation of RasGRP2 to membrane ruffles is mediated by the direct association of this protein with F-actin, a property contained within its 150 first amino acids. Finally, we show that the RasGRP2/F-actin interaction promotes the regionalized activation of Rap1 in juxtamembrane areas of the cell. These results reveal a novel function of the actin cytoskeleton in mediating the spatial activation of Ras subfamily GTPases through the selective recruitment of GDP/GTP exchange factors.

Use of large-scale expression cloning screens in the Xenopus laevis tadpole to identify gene function

We have conducted an expression cloning screen of approximately 50, 000 cDNAs from a tadpole stage Xenopus laevis cDNA library to functionally identify genes affecting a wide range of cellular and developmental processes. Fifty-seven cDNAs were isolated for their ability to alter gross tadpole morphology or the expression patterns of tissue-specific markers. Thirty-seven of the cDNAs have not been previously described for Xenopus, and 15 of these show little or no similarity to sequences in the NCBI database. The screen and the identified genes are presented in this paper to demonstrate the power, ease, speed, and flexibility of expression cloning in the X. laevis embryo. Future screens such as this one can be done on a larger scale and will complement the sequence-based screens and genome-sequencing projects which are producing a large body of novel genes without ascribed functions.

Involvement of the adapter protein CRKL in integrin-mediated adhesion

CRKL, an SH2-SH3-SH3 adapter protein, is one of the major tyrosine phosphoproteins detected in primary leukemic neutrophils from patients with CML. CRKL binds directly to BCR/ABL through its N-terminal SH3 domain, suggesting it may be involved in BCR/ABL signal transduction. However, the biological function of CRKL in either normal or leukemic cells is still largely unknown. In this study, we have examined the effects of overexpressing full length or deletion mutants of CRKL in hematopoietic cell lines. Full length, SH2- and SH3(N)-domain deletion mutants of CRKL were transfected into an interleukin-3-dependent hematopoietic cell line, Ba/F3, and 3-5 individual sublines which stably overexpressed each transgene were obtained [Ba/F-CRKL, Ba/F-CRKL deltaSH2, and Ba/F-CRKL deltaSH3(N)]. The growth properties of these transfected cells in the presence or absence of IL-3 were not different from mock transfected or untransfected Ba/F3 cells. However, Ba/F3 cells overexpressing full length CRKL, but not deletion mutants of CRKL, were found to have an increase in their ability to bind to fibronectin-coated surfaces. Further, expression of full length, but not deltaSH2- or deltaSH3-CRKL deletion mutants, was found to alter cell morphology on fibronectin-coated plates, an effect which was further enhanced by certain kinds of stress stimuli, such as ionizing radiation. Similar results were obtained when CRKL was transiently overexpressed in Ba/F3 cells, and were also obtained in a second IL-3 dependent hematopoietic cell line, 32Dcl3. Adhesion to fibronectin was blocked by anti-beta1 integrin monoclonal antibody, but overexpression of CRKL did not affect surface expression of beta1 integrins, nor did it spontaneously induce expression of the beta1 integrin 'activation' epitope recognized by the 9EG7 monoclonal antibody. These data suggest a role for CRKL in signaling pathways which regulate adhesion to fibronectin.

Platelet sarco/endoplasmic reticulum Ca2+ATPase isoform 3b and Rap 1b: interrelation and regulation in physiopathology

Platelet Ca2+ signalling involves intracellular Ca2+ pools, whose content is controlled by sarco/endoplasmic reticulum Ca2+ATPases (SERCAs). Among these, a key role is played by the inositol trisphosphate-sensitive Ca2+ pool, associated with the SERCA 3b isoform. We have investigated the control of this Ca2+ pool through the cAMP-dependent phosphorylation of the GTP-binding protein, Rap (Ras-proximate) 1b. We first looked for this Ca2+ pool target of regulation by studying the expression of the different SERCA and Rap 1 proteins in human platelets and various cell lines, by Western blotting and reverse transcription-PCR. Since co-expression of Rap 1b and SERCA 3b was obtained, we looked for their protein-protein interaction as a function of the cAMP-dependent phosphorylation of Rap 1b. Co-immunoprecipitations of SERCA 3b and Rap 1b proteins were found in the absence of phosphorylation, induced by the catalytic subunit of the cAMP-dependent protein kinase (csPKA). In contrast, upon pre-treatment of platelet membranes with csPKA, the SERCA 3b dissociated from the Rap 1b protein, in agreement with a role of its phosphorylated state in their interaction. Finally, we looked for adaptation of this complex in a platelet pathological model of hypertension. We investigated the expression of both proteins, as well as the cAMP-dependent phosphorylation of Rap 1b and SERCA 3b activity in platelets from control normotensive Wistar-Kyoto rats and from spontaneously hypertensive rats (SHRs). A decrease in SERCA 3b activity was associated with a decrease in Rap 1b endogenous phosphorylation in SHR platelets, consistent with a functional role in the regulation of the SERCA 3b-associated Ca2+ pool.

Role of small G proteins in yeast cell polarization and wall biosynthesis

In the vegetative (mitotic) cycle and during sexual conjugation, yeast cells display polarized growth, giving rise to a bud or to a mating projection, respectively. In both cases one can distinguish three steps in these processes: choice of a growth site, organization of the growth site, and actual growth and morphogenesis. In all three steps, small GTP-binding proteins (G proteins) and their regulators play essential signaling functions. For the choice of a bud site, Bud1, a small G protein, Bud2, a negative regulator of Bud1, and Bud5, an activator, are all required. If any of them is defective, the cell loses its ability to select a proper bud position and buds randomly. In the organization of the bud site or of the site in which a mating projection appears, Cdc42, its activator Cdc24, and its negative regulators play a fundamental role. In the absence of Cdc42 or Cdc24, the actin cytoskeleton does not become organized and budding does not take place. Finally, another small G protein, Rho1, is required for activity of beta (1-->3)glucan synthase, the enzyme that catalyzes the synthesis of the major structural component of the yeast cell wall. In all of the above processes, G proteins can work as molecular switches because of their ability to shift between an active GTP-bound state and an inactive GDP-bound state.

Relationship between Rap1 protein phosphorylation and regulation of Ca2+ transport in platelets: a new approach

Although the interrelationship between the two messengers Ca2+ and cyclic AMP in platelet function is well documented, its mechanism of action still remains to be established. We investigated here the question of the regulation of platelet Ca(2+)-ATPases by cyclic AMP through the phosphorylation of the Rap1 protein using a pathological model. We first found experimental conditions where Ca(2+)-transport by platelet membrane vesicles appeared to be dependent on the phosphorylation of the Rap1 protein. Then, we studied platelets of patients with congestive heart failure for their expression of the potential 97 kDa Ca(2+)-ATPase target of regulation through the Rap1 protein as well as the phosphorylation of the Rap1 protein using the catalytic subunit of the cyclic AMP-dependent protein kinase (C. Sub.). In the first patients studied, we found no significant modification in the expression of the 97 kDa Ca(2+)-ATPase by Western blotting using the PL/IM 430 monoclonal antibody which specifically recognized this isoform. In contrast, the Rap1 protein was differentially phosphorylated when using 15 micrograms/ml of the C. Sub. These results allowed us to use these pathological platelets to study the relationship between the expression of Rap1 protein and the regulation of Ca2+ transport by selecting a patient with severe heart failure. We could show a decrease in the expression as well as in the phosphorylation of Rap1 protein and demonstrate a lower effect of C. Sub. on Ca2+ transport. Finally, by studying a further series of patients, we could confirm that the decrease in Rap1 protein expression in heart failure, whatever its extent, was variable, and could strictly correlate the expression of Rap1 protein with the stimulatory effect of C. Sub. on Ca2+ transport. Besides the evidence for regulation of the expression of the Rap1 protein in platelets from patients with heart failure, these findings constitute a new approach in favour of the regulation of platelet Ca2+ transport through the phosphorylation of the Rap1 protein.