Rac Isoforms Research Articles

Rho family GTPases, Rac and Cdc42, control the localization of neonatal dentate granule cells during brain development

The hippocampus is generally considered as a brain center for learning and memory. We have recently established an electroporation-mediated gene transfer method to investigate the development of neonatal dentate granule cells in vivo. Using this new technique, we introduced knockdown vectors against Rac1 small GTPase into precursors for dentate granule cells at postnatal day 0. After 21 days, Rac1-deficient cells were frequently mispositioned between the granule cell layer (GCL) and hilus. About 60% of these mislocalized cells expressed a dentate granule cell marker, Prox1. Both the dendritic spine density and the ratio of mature spine were reduced when Rac1 was silenced. Notably, the deficient cells have immature thin processes during migrating in the early neonatal period. Knockdown of another Rac isoform, Rac3, also resulted in mislocalization of neonatally born dentate granule cells. In addition, knockdown of Cdc42, another Rho family protein, also caused mislocalization of the cell, although the effects were moderate compared to Rac1 and 3. Despite the ectopic localization, Rac3- or Cdc42-disrupted mispositioned cells expressed Prox1. These results indicate that Rho signaling pathways differentially regulate the proper localization and differentiation of dentate granule cells.

RPM-1 regulates axon termination by affecting growth cone collapse and microtubule stability.

Axon termination is essential for efficient and accurate nervous system construction. At present, relatively little is known about how growth cone collapse occurs prior to axon termination in vivo Using the mechanosensory neurons of C. elegans, we found collapse prior to axon termination is protracted, with the growth cone transitioning from a dynamic to a static state. Growth cone collapse prior to termination is facilitated by the signaling hub RPM-1. Given the prominence of the cytoskeleton in growth cone collapse, we assessed the relationship between RPM-1 and regulators of actin dynamics and microtubule stability. Our results reveal several important findings about how axon termination is orchestrated: (1) RPM-1 functions in parallel to RHO-1 and CRMP/UNC-33, but is suppressed by the Rac isoform MIG-2; (2) RPM-1 opposes the function of microtubule stabilizers, including tubulin acetyltransferases; and (3) genetic epistasis suggests the microtubule-stabilizing protein Tau/PTL-1 potentially inhibits RPM-1. These findings provide insight into how growth cone collapse is regulated during axon termination in vivo, and suggest that RPM-1 signaling destabilizes microtubules to facilitate growth cone collapse and axon termination.

Rac2 Functions in Both Neutrophils and Macrophages To Mediate Motility and Host Defense in Larval Zebrafish.

Leukocyte motility is required for host defense responses. Rac-family Rho GTPases are implicated in leukocyte function; however, the distinct roles of different Rac isoforms in host defense in vivo have remained unclear. In this study, we generated Rac2-deficient zebrafish using transcription activator-like effector nucleases to directly compare the role of Rac2 in vivo in neutrophils and macrophages in motility and the response to infection. This zebrafish larval model is highly amenable to live imaging of leukocyte behavior, and we report that in rac2-/- larvae both neutrophils and macrophages are defective in basic motility, leading to impaired responses to localized wounds or infections. rac2-/- larvae are highly susceptible to infection with Pseudomonas aeruginosa, which can be almost fully rescued by ectopic expression of either Rac2 or Rac1 specifically in neutrophils, indicating that these isoforms have partially overlapping functions in vivo. Rescue of Rac2 expression specifically in macrophages also confers resistance to Pseudomonas infection, highlighting an important role for Rac2 in this leukocyte population as well. Surprisingly, in contrast to neutrophils expressing a Rac2 dominant inhibitory human disease mutation, rac2-/- neutrophils do not have altered polarity or mobilization from hematopoietic tissue, suggesting that a different Rac isoform, such as Rac1, also contributes to these phenotypes in vivo.

Suppressive action of miRNAs to ARP2/3 complex reduces cell migration and proliferation via RAC isoforms in Hirschsprung disease.

Hirschsprung disease (HSCR) is a congenital disorder caused by the defective function of the embryonic enteric neural crest. The impaired migration of embryonic enteric neural crest plays an important role in the pathogenesis of this disease. Recent studies showed that the ARP2/3 complex and RAC isoforms had effects on actin cytoskeleton remodelling, which contributes to migration. Moreover, some regulatory relationships were identified between ARP2/3 complex and RAC isoforms. Although microRNAs (miRNAs) have been known to modulate target gene expression on the post‐transcriptional level, little is known about the regulation among miRNAs, ARP2/3 complex and RAC isoforms. Here, we report that down‐regulation of ARP2 and ARP3, two main subunits of ARP2/3 complex, suppressed migration and proliferation in 293T and SH‐SY5Y cell lines via the inhibition of RAC1 and RAC2. Meanwhile, as the target genes, ARP2 and ARP3 are reduced by increased miR‐24‐1* and let‐7a*, respectively, in 70 HSCR samples as compared with 74 normal controls. Co‐immunoprecipitation showed that aberrant reduction in ARP2 and ARP3 could weaken the function of ARP2/3 complex. Our study demonstrates that the miR‐24‐1*/let‐7a*‐ARP2/3 complex‐RAC isoforms pathway may represent a novel pathogenic mechanism for HSCR.

The Role of Rac and Pak in Neutrophil Histone Hypercitrullination and Neutrophil Extracellular Traps Formation

Under specific activating conditions, polymorphonuclear neutrophils (PMNs) release neutrophil extracellular traps (NETs) composed of decondensed chromatin lined with microbicidal protein such as neutrophil elastase and myeloperoxidase. NETs contribute to innate immunity but can also foster autoimmune diseases and thrombus formation. NET formation (NETosis) requires reactive oxygen species (ROS) production by NADPH oxidase and histone hypercitrullination by peptidylarginine deiminase 4 (PAD4), allowing for chromatin decondensation. Rac GTPases are expressed in three isoforms: Rac1 is ubiquitously expressed and plays a role in PMN migration and oxidase function; Rac2 is hematopoietic-specific and the major isoform in PMNs and Rac3 is mostly neuronal. Rac1 and Rac2 regulate the cytoskeleton in PMNs, controlling actin polymerization, cell shape, adhesion and migration and are essential components of the NADPH oxidase complex. The present study aimed to explore the role of the Rac pathway on NETosis in PMNs, including the upstream guanosine exchange factor (GEF) activator, Vav, and a downstream effector of Rac, p21 activated kinase, Pak. We developed a flow cytometry-based quantification of H3 hypercitrullination (H3Cit). In response to phorbol myristate ester (PMA) stimulation, H3Cit is increased to 136% of basal in WT cells, compared with 103% in Rac2-/- (P<0.01) (Table). H3Cit levels observed by flow were confirmed in a NET formation assay. Rac2-/- PMNs formed significantly fewer NETs both spontaneously and after PMA stimulation (WT unstimulated 2.79%, Rac2-/- unstimulated 0.72%, WT+PMA 10.84%, Rac2-/-+PMA 1.39%, P< 0.05 for all pair comparisons). Furthermore, Rac2-/- mice demonstrated a trend towards reduced frequency of provoked thrombosis in an in vivo vena cava stenosis model (WT 78% and Rac2-/- 56% of mice with thrombus). Deletion of floxed Rac1 sequences in a Rac2-/- background in vivoallows generation and purification of PMNs lacking both Rac isoforms. Rac1Δ/Δ,Rac2-/- PMNs, which are defective in actin polymerization, had reduced basal H3Cit and a nearly complete lack of PMA-induced increase in H3Cit (136% vs 69%, WT vs Rac1Δ/Δ,Rac2-/-, P<0.01) (Table). Null knockouts of the GEFs Vav1 (hematopoietic-specific), Vav2, Vav3 or both Vav1 and 3 did not impair H3Cit response to PMA (Table). We next studied downstream effectors of Rac. Group A Paks include Pak1, 2 and 3 isoforms. Pretreatment of wild-type PMNs with either PF3758309 or IPA-3, two group A Pak inhibitors with distinct mechanisms of action, led to reduced H3Cit after PMA stimulation (induction reduced from 36% to 11% for both PF3758309 and IPA-3 treated (Table). To validate this in a genetic model, we studied Pak2Δ/Δ PMNs, since we have recently demonstrated the dependence of hematopoietic stem cell migration on Pak2. Pak2Δ/Δ demonstrated a reduced basal level of H3Cit and a significantly reduced PMA-induced increase in H3Cit (136% vs 94%, WT vs Pak2Δ/Δ, P<0.05, Table). In summary, we describe a flow-based assay that quantitates the early processes of NET formation and validated that this assay reliably predicts agonist-induced NET formation in a genetic model. The results establish that both Rac1 and Rac2, and the downstream effector Pak2, regulate histone H3 hypercitrullination and NET formation in PMNs, while suggesting that Vav does not activate the Rac pathway in PMA-induced NET formation. These data further delineate the role of the Rac pathway in NETosis, linking cytoskeleton and oxidase functions. Furthermore, these data indicate Pak could represent a therapeutic target for a wide array of pathological processes related to NETosis such as thrombosis and numerous autoimmune diseases.TableIntensity of H3Cit staining as determined by flow cytometry-based assay.Basal H3Cit levelPMA-induced H3CitPMA-induced changeWT PMN100±2%136±5%36%Rac2-/-82±9% ns103±15.3%**21%Rac1Δ/Δ, Rac2-/-64±10%***69±10%**5%Vav1-/-86±9% ns145±15% ns59%Vav2-/-91±4% ns134±18% ns43%Vav3-/-153±30%*171±28% ns18%Vav1,3-/-125±20% ns144±24% ns19%WT+PF 5nM76±17% ns87±8%*11%WT+ IPA 5µM100±4% ns111±10% ns11%Pak2Δ/Δ75±7%*94±11%*19%Results are expressed as mean±SEM % of the untreated WT control of each experiment. Results are from ≥3 independent experiments. * P<0.05, **P<0.01. P<0.001, ns non-significant, by two-tailed t-test. DisclosuresNo relevant conflicts of interest to declare.

Structural Determinants of RGS-RhoGEF Signaling Critical to Entamoeba histolytica Pathogenesis

G protein signaling pathways, as key components of physiologic responsiveness and timing, are frequent targets for pharmacologic intervention. Here, we identify an effector for heterotrimeric G proteinα subunit (EhGα1) signaling from Entamoeba histolytica, the causative agent of amoebic colitis. EhGα1 interacts with this effector and guanosine triphosphatase-accelerating protein, EhRGS-RhoGEF, in a nucleotide state-selective fashion. Coexpression of EhRGS-RhoGEF with constitutively active EhGα1 and EhRacC leads to Rac-dependent spreading in Drosophila S2 cells. EhRGS-RhoGEF overexpression in E.histolytica trophozoites leads to reduced migration toward serum and lower cysteine protease activity, as well as reduced attachment to, and killingof, host cells. A 2.3Å crystal structure of thefull-length EhRGS-RhoGEF reveals a putative inhibitory helix engaging the Dbl homology domain Rho-binding surface and the pleckstrin homology domain. Mutational analysis of the EhGα1/EhRGS-RhoGEF interface confirms a canonical "regulator of G protein signaling" domain rather than a RhoGEF-RGS ("rgRGS") domain, suggesting a convergent evolution toward heterotrimeric and small G protein cross-talk.

Rac2 expression and its role in neutrophil functions of zebrafish (Danio rerio)

The neutrophil contributes significantly to the immune response. In particular, their phagocytosis and pathogen-killing functions are vital for defense from invading pathogens. Rac2, a Rho small GTPase, is involved in many key neutrophil functions. Loss of Rac2 activity results in severe bacterial infections and neutrophil function deficits in humans and mice. While the genes rac1, 2, and 3 have been identified in the zebrafish genome, their expression has not been well-characterized. We describe rac1, 2, and 3 expression over the first three days of development, as well as the presence and localization of Rac2 protein in adult zebrafish neutrophils. The mRNA for each Rac isoform was detected in zebrafish embryos as early as 12 h post fertilization. Immunocytochemistry and confocal microscopy of adult zebrafish neutrophils confirmed diffuse Rac2 protein within the cytoplasm. Only rac2 was found in sorted neutrophil samples. Armed with knowledge of its presence and exclusive expression, the role of Rac2 in key antimicrobial zebrafish neutrophil responses was examined by small molecule inhibition of Rac during respiratory burst, NET release, and phagocytosis assays. Inhibition of Rac2 during these assays produced a dose-dependent decrease in each function, as was expected due to previous work in mammals. The expression pattern and role of Rac2 in zebrafish neutrophil function allows for comparative studies of innate immune responses in this animal model.

A Role for Rac3 GTPase in the Regulation of Autophagy

The process of autophagy is situated at the intersection of multiple cell signaling pathways, including cell metabolism, growth, and death, and hence is subject to multiple forms of regulation. We previously reported that inhibition of isoprenylcysteine carboxylmethyltransferase (Icmt), which catalyzes the final step in the post-translational prenylation of so-called CAAX proteins, results in the induction of autophagy which enhances cell death in some cancer cells. In this study, using siRNA-mediated knockdown of a group of small GTPases that are predicted Icmt substrates, we identify Rac3 GTPase as a negative regulator of the process of autophagy. Knockdown of Rac3, but not the closely related isoforms Rac1 and Rac2, results in induction of autophagy. Ectopic expression of Rac3, significantly rescues cells from autophagy and cell death induced by Icmt inhibition, strengthening the notion of an isoform-specific autophagy regulatory function of Rac3. This role of Rac3 was observed in multiple cell lines with varying Rac subtype expression profiles, suggesting its broad involvement in the process. The identification of this less-studied Rac member as a novel regulator provides new insight into autophagy and opens opportunities in identifying additional regulatory inputs of the process.

Involvement of Rac GTPase activation in phosphatidylcholine hydroperoxide-induced THP-1 cell adhesion to ICAM-1

Increasing evidence indicates that phospholipid oxidation plays important roles in atherosclerosis. Here, we investigated the involvement of Rho-family GTPases inphosphatidylcholine hydroperoxide (PCOOH)-induced THP-1 cell adhesion to ICAM-1. Isoprenoid depletion by fluvastatin and geranylgeranyltransferase inhibition by GGTI-286 suppressed PCOOH-induced cell adhesion to ICAM-1 and F-actin-rich membrane protrusion formation. Pull-down assays demonstrated the activation of Rac1 and Rac2 in PCOOH-treated cells. Pan-Rho-family GTPase inhibitor Clostridium difficile toxin B, Rac-specific inhibitor NSC23776, and RNA interference of the Rac isoforms suppressed the cell adhesion. These findings indicate the involvement of Rac GTPase activation in PCOOH-induced cell adhesion to ICAM-1 via actin reorganization.

Intact Rac Signaling Is Important for Leukemia Cell Survival

AbstractAbstract 2885Rho GTPases, Ras-related small G proteins, regulate multiple cell processes in hematopoietic cells. There is growing evidence that acute myeloid leukemia (AML) blasts and particularly MLL-rearranged AML blasts, rely on Rac activity (Mulloy JC et al, Blood, 2010). However, little is known about the role of these GTPases in acute lymphoblastic leukemia (ALL) and particularly precursor B cell ALL. To investigate the role of Rac and potential compensation by other GTPases in ALL, we first assessed the protein expression and activation of Rac in a number of B-ALL cell lines (SEM; RS4,11; REH; Nalm 6; Raji), compared with a T-ALL cell line (Jurkat) and several AML cell lines (ML2; MV4,11). Of these cell lines SEM; RS4,11; ML2 and MV4,11 are characterized by MLL-fusion genes. Jurkat and MLL-rearranged AML cell lines show higher expression of Rac proteins compared to B cell leukemia lines (Table 1). Overall, B-ALL cell lines exhibit highly variable levels of Rac expression and activity with no obvious correlation to the presence of MLL-fusion proteins. We then investigated proliferation and apoptosis in cell lines treated with the small molecule inhibitor NSC23766 (NSC), which blocks interaction of a subset of guanine exchange factors (GEFs) with Rac and thus inhibits its activation. Treatment with NSC led to ∼2-fold increase in cells arrested at G0/G1 and induced apoptosis in a dose-dependent fashion at NSC concentrations previously demonstrated to be non-toxic in normal hematopoietic cells (Muller LUW et al., Leukemia, 2008) (Table 2). The lymphoid cell lines Jurkat, Raji and SEM appeared less responsive to NSC with no increased apoptosis at 40μM NSC. There was no correlation between NSC response and baseline expression or activation status of Rac. However, cell lines resistant to NSC exhibited a paradoxical and transient early increase in Rac activation, suggesting the existence of compensatory activation mechanisms. To determine if the relative resistance observed in some cell lines was related to dependence on GEFs not targeted by NSC and to validate that the inhibitory effect of NSC was specifically due to Rac inhibition in sensitive cells, shRNAs were utilized to knock-down different members of the Rac subfamily. Effective shRNA-mediated knockdown was validated by western blot. Knockdown of Rac1 or Rac2 consistently induced apoptosis compared to non-targeting vector controls in NSC sensitive cell lines ML2 and Nalm6, with ML2 cells appearing slightly more sensitive to knock-down of Rac2 (Table 3). Knock-down of either Rac1 or Rac2 had little effect upon Jurkat cells which are resistant to NSC treatment. These data suggest that Jurkat cells are not dependent upon Rac signaling for survival; however we cannot discount the possibility that some compensation may occur between Rac1 and Rac2. These experiments demonstrate the importance of intact Rac signaling pathways for the survival of the majority of leukemia cell lines tested and demonstrate that dependence on Rac signaling is not restricted to leukemias characterized by MLL-rearrangements. Our observations also suggest that activation of different Rac isoforms may influence sensitivity towards pharmacological Rac inhibition. Disclosures:No relevant conflicts of interest to declare.

Rac1 regulates neuronal polarization through the WAVE complex.

Neuronal migration and axon growth, key events during neuronal development, require distinct changes in the cytoskeleton. Although many molecular regulators of polarity have been identified and characterized, relatively little is known about their physiological role in this process. To study the physiological function of Rac1 in neuronal development, we have generated a conditional knock-out mouse, in which Rac1 is ablated in the whole brain. Rac1-deficient cerebellar granule neurons, which do not express other Rac isoforms, showed impaired neuronal migration and axon formation both in vivo and in vitro. In addition, Rac1 ablation disrupts lamellipodia formation in growth cones. The analysis of Rac1 effectors revealed the absence of the Wiskott-Aldrich syndrome protein (WASP) family verprolin-homologous protein (WAVE) complex from the plasma membrane of knock-out growth cones. Loss of WAVE function inhibited axon growth, whereas overexpression of a membrane-tethered WAVE mutant partially rescued axon growth in Rac1-knock-out neurons. In addition, pharmacological inhibition of the WAVE complex effector Arp2/3 also reduced axon growth. We propose that Rac1 recruits the WAVE complex to the plasma membrane to enable actin remodeling necessary for axon growth.

Activation of the A3 adenosine receptor inhibits fMLP-induced Rac activation in mouse bone marrow neutrophils

Adenosine is released from injured or hypoxic tissues where it exerts numerous anti-inflammatory effects including suppression of neutrophil functions. Although most previous work has implicated the A2AAR, we have recently shown that selective activation of the abundantly expressed A3AR inhibits neutrophil superoxide production and chemotaxis providing a potential mechanistic explanation for the efficacy of A3AR agonists in experimental animal models of inflammation. In this study, we hypothesized that the A3AR suppresses neutrophil functions by inhibiting the monomeric GTPase Rac, a central regulator of chemokine-directed neutrophil migration and superoxide production. We found that pre-treating neutrophils with the highly selective A3AR agonist CP-532,903 reduced fMLP-induced Rac activation using an ELISA-based assay that detects all three Rac isoforms. CP-532,903 also inhibited fMLP-induced F-actin formation, a downstream effector function of Rac relevant to neutrophil migration, but not activation of ERK1/2 or p38. Pre-treating neutrophils with CP-532,903 did not stimulate cAMP production or alter fMLP-induced calcium transients, implicating that A3AR stimulation does not inhibit Rac activation or neutrophil activities by suppressing Ca2+ signaling, elevating the intracellular concentration of cAMP, or by cross-desensitizing fMLP receptors. Our results suggest that activation of the A3AR signals to suppress neutrophil functions by interfering with the monomeric GTPase Rac, thus contributing to the ant-inflammatory actions of adenosine.

Sequestering of Rac by the Yersinia Effector YopO Blocks Fcγ Receptor-mediated Phagocytosis

Pathogenic Yersinia species neutralize innate immune mechanisms by injecting type three secretion effectors into immune cells, altering cell signaling. Our study elucidates how one of these effectors, YopO, blocks phagocytosis. We demonstrate using different phagocytic models that YopO specifically blocks Rac-dependent Fcgamma receptor internalization pathway but not complement receptor 3-dependent uptake, which is controlled by Rho activity. We show that YopO prevents Rac activation but does not affect Rac accumulation at the phagocytic cup. In addition, we show that plasma membrane localization and the guanine-nucleotide dissociation inhibitor (GDI)-like domain of YopO cooperate for maximal anti-phagocytosis. Although YopO has the same affinity for Rac1, Rac2, and RhoA in vitro, it selectively interacts with Rac isoforms in cells. This is due to the differential localization of the Rho family G proteins in resting cells; Rac isoforms partially exist as a GDI-free pool at the membrane of resting cells, whereas RhoA is trapped in the cytosol by RhoGDIalpha. We propose that YopO exploits this basic difference in localization and availability to selectively inhibit Rac-dependent phagocytosis.

Human Neutrophils Coordinate Chemotaxis by Differential Activation of Rac1 and Rac2

Rac1 and Rac2, members of the small Rho GTPase family, play essential roles in coordinating directional migration and superoxide production during neutrophil responses to chemoattractants. Although earlier studies in Rac1 and Rac2 knockout mice have demonstrated unique roles for each Rac isoform in chemotaxis and NADPH oxidase activation, it is still unclear how human neutrophils use Rac1 and Rac2 to achieve their immunological responses to foreign agent stimulation. In the current study, we used TAT dominant-negative Rac1-T17N and Rac2-T17N fusion proteins to acutely alter the activity of Rac1 and Rac2 individually in human neutrophils. We demonstrate distinct activation kinetics and different roles for Rac1 and Rac2 in response to low vs high concentrations of fMLP. These observations were verified using neutrophils from mice in which Rac1 or Rac2 was genetically absent. Based on these results, we propose a model to explain how human neutrophils kill invading microbes while limiting oxidative damage to the adjacent surrounding healthy tissue through the differential activation of Rac1 and Rac2 in response to different concentrations of chemoattractant.

Superoxide from NADPH oxidase upregulates type 5 phosphodiesterase in human vascular smooth muscle cells: inhibition with iloprost and NONOate

To determine whether there is an association between vascular NADPH oxidase (NOX), superoxide, the small GTPase Rac(1) and PDE type 5 (PDE5) in human vascular smooth muscle cell (hVSMCs). hVSMCs were incubated with xanthine-xanthine oxidase (X-XO; a superoxide generating system) or the thromboxane A(2) analogue, U46619 (+/-superoxide dismutase (SOD) or apocynin) for 16 h. The expression of PDE5 and NOX-1 was assessed using Western blotting and superoxide measured. The role of Rac(1) in superoxide generation was assessed by overexpressing either the dominant-negative or constitutively active Rac isoforms. The effects of iloprost, DETA-NONOate and the Rho-kinase inhibitor, Y27632, on PDE5 and NOX-1 expression were also studied. Following 16 h incubation, U46619 and X-XO promoted the expression of PDE5 and NOX-1, an effect blocked by SOD or apocynin when co-incubated over the same time course. X-XO and U46619 both promoted the formation of superoxide. Overexpression of dominant-negative Rac(1) or addition of iloprost, DETA-NONOate or Y27632 completely blocked both superoxide release and PDE5 protein expression and activity. These data demonstrate that superoxide derived from NOX upregulates the expression of PDE5 in human VSMCs. As PDE5 hydrolyses cyclic GMP, this effect may blunt the vasculoprotective actions of NO.

Identifying the Relative Contributions of Rac1 and Rac2 to Osteoclastogenesis

Rac small GTPases may play an important regulatory role in osteoclastogenesis. Our in vitro and in vivo results show that both Rac1 and Rac2 are required for optimal osteoclast differentiation, but Rac1 is more critical. Rac1 is the key Rac isoform responsible for regulating ROS generation and the actin cytoskeleton during the multiple stages of osteoclast differentiation. Recent evidence suggests that the Rac small GTPases may play an important regulatory role in osteoclastogenesis. This finding is important because bisphosphonates may regulate their antiresorptive/antiosteoclast effects through the modification of Rho family of small GTPases. To elucidate the specific roles of the Rac1 and Rac2 isoforms during osteoclastogenesis, we used mice deficient in Rac1, Rac2, or both Rac1 and Rac2 in monocyte/osteoclast precursors. Macrophage-colony stimulating factor (M-CSF)- and RANKL-mediated osteoclastogenesis in vitro was studied by using bone marrow-derived mononucleated preosteoclast precursors (MOPs). The expression of osteoclast-specific markers was examined using quantitative real-time PCR and Western blot analysis. Free actin barbed ends in bone marrow MOPs after M-CSF stimulation was determined. The ability of MOPs to migrate toward M-CSF was assayed using Boyden chambers. Margin spreading on heparin sulfate-coated glass and RANKL-induced reactive oxygen species generation were also performed. Functional assays of in vitro-generated osteoclasts were ascertained using dentine sections from narwal tusks. Osteoclast levels in vivo were counted in TRACP and immunohistochemically stained distal tibial sections. In vivo microarchitexture of lumbar vertebrate was examined using microCT 3D imaging and analysis. We show here that, although both Rac isoforms are required for normal osteoclast differentiation, Rac1 deletion results in a more profound reduction in osteoclast formation in vitro because of its regulatory role in pre-osteoclast M-CSF-mediated chemotaxis and actin assembly and RANKL-mediated reactive oxygen species generation. This Rac1 cellular defect also manifests at the tissue level with increased trabecular bone volume and trabeculae number compared with wildtype and Rac2-null mice. This unique mouse model has shown for the first time that Rac1 and Rac2 play different and nonoverlapping roles during osteoclastogenesis and will be useful for identifying the key roles played by these two proteins during the multiple stages of osteoclast differentiation. Rac1 and Rac2 play different and nonoverlapping roles during osteoclastogenesis. This model showed that Rac1 is the key Rac isoform responsible for regulating ROS generation and the actin cytoskeleton during the multiple stages of osteoclast differentiation.

Genetic and pharmacologic evidence that Rac1 GTPase is involved in regulation of platelet secretion and aggregation.

Rac1 GTPase, a member of the Ras-related Rho GTPase family, is the major Rac isoform present in platelets and has been shown to be involved in cell actin cytoskeleton reorganization and adhesion. Agonists that induce platelet secretion and aggregation also activate Rac1 GTPase, raising the possibility that Rac1 GTPase may be involved in regulation of platelet function. To rigorously define the role of Rac1 in platelet regulation. We have used a dual approach of gene targeting in mice and pharmacologic inhibition of Rac1 by NSC23766, a rationally designed specific small molecule inhibitor, to study the role of Rac1 in platelet function. Platelets from mice as well as human platelets treated with NSC23766 exhibited a significant decrease in: (i) active Rac1 species and phosphorylation of the Rac effector, p21-activated kinase; (ii) expression of P-selectin and secretion of adenosine triphosphate induced by thrombin or U46619; and (iii) aggregation induced by adenosine 5'-diphosphate, collagen, thrombin and U46619, a stable analog of thromboxane A(2). NSC23766 did not alter the cAMP or cGMP levels in platelets. Consistent with the requirement of Rac1 for normal platelet function, the bleeding times in Rac1(-/-) mice or mice given NSC23766 were significantly prolonged. Our data show that deficiency or inhibition of Rac1 GTPase blocks platelet secretion. The inhibition of secretion, at least in part, is responsible for diminished platelet aggregation and prolonged bleeding times observed in Rac1 knockout or Rac1 inhibitor-treated mice.

Formin the Way

Reorganization of the actin cytoskeleton requires nucleating factors, which initiate actin fiber formation. In this issue of Immunity, the functions of two classes of actin nucleators are examined in T lymphocytes, giving insight into the regulation of T cell polarity (Gomez et al., 2007).

Vav Proteins in Neutrophils Are Required for FcγR-Mediated Signaling to Rac GTPases and Nicotinamide Adenine Dinucleotide Phosphate Oxidase Component p40(phox)

Phagocytes generate reactive oxygen species, the regulation of which is important in eliminating ingested microbes while limiting tissue damage. Clustering of FcgammaRs results in the activation of Vav proteins, Rho/Rac guanine nucleotide exchange factors, and results in robust superoxide generation through the NADPH oxidase. In this study, studies in neutrophils isolated from mice deficient in Vav or Rac isoforms demonstrate a critical role for Vav3 in Rac2-dependent activation of the NADPH oxidase following FcgammaR clustering. However, studies in cytokine-primed cells revealed a strict requirement for Vav1 and Vav3 and Rac1 and Rac2 in the FcgammaR-mediated oxidative burst. In comparison, Vav was not essential for PMA or G protein-coupled receptor-mediated superoxide generation. The FcgammaR-mediated oxidative burst defect in Vav-deficient cells was linked to aberrant Rac activation as well as Rac- and actin-polymerization-independent, but PI3K-dependent, phosphorylation of the NADPH oxidase component p40(phox). In macrophages, Vav regulation of Rac GTPases was required specifically in FcgammaR-mediated activation of the oxidative burst, but not in phagocytosis. Thus, Vav proteins specifically couple FcgammaR signaling to NADPH oxidase function through a Rac-dependent as well as an unexpected Rac-independent signal that is proximal to NADPH oxidase activation and does not require actin polymerization.

Rac1 and Rac2 regulate macrophage morphology but are not essential for migration

Rac GTPases are believed to contribute to migration in leukocytes by transducing signals from cell surface receptors to the actin and microtubule cytoskeletons. Mammals have three closely related Rac isoforms, Rac1, Rac2 and Rac3, and it is widely assumed that cell migration requires the activity of these Rac GTPases. We have previously shown that Rac1-null mouse macrophages have altered cell shape and reduced membrane ruffling but normal migration speed. Here we investigate the behaviour of macrophages lacking Rac2 (Rac2(-/-)) or Rac1 and Rac2 (Rac1/2(-/-)). Rac2(-/-) macrophages have reduced F-actin levels and lack podosomes, which are integrin-based adhesion sites, and their migration speed is similar to or slightly slower than wild-type macrophages, depending on the substrate. Unexpectedly, Rac1/2(-/-) macrophages, which do not express Rac1, Rac2 or Rac3, migrate at a similar speed to wild-type macrophages on a variety of substrates and perform chemotaxis normally, although their morphology and mode of migration is altered. However, Rac1(-/-) and Rac1/2(-/-) but not Rac2(-/-) macrophages are impaired in their ability to invade through Matrigel. Together, these data show that Rac1 and Rac2 have distinct roles in regulating cell morphology, migration and invasion, but are not essential for macrophage migration or chemotaxis.