Expression of activated CDC42 induces T cell apoptosis in thymus and peripheral lymph organs via different pathways

Role of Rho GTPases in inflammatory bowel disease

Rat sarcoma virus homolog (Rho) guanosine triphosphatases (GTPases) function as "molecular switch" in cellular signaling regulation processes and are associated with the pathogenesis of inflammatory bowel disease (IBD). This chronic intestinal tract inflammation primarily encompasses two diseases: Crohn's disease and ulcerative colitis. The pathogenesis of IBD is complex and considered to include four main factors and their interactions: genetics, intestinal microbiota, immune system, and environment. Recently, several novel pathogenic components have been identified. In addition, potential therapies for IBD targeting Rho GTPases have emerged and proven to be clinically effective. This review mainly focuses on Rho GTPases and their possible mechanisms in IBD pathogenesis. The therapeutic possibility of Rho GTPases is also discussed.

RhoA and Cdc42 in T cells: Are they targetable for T cell-mediated inflammatory diseases?

Many inflammatory diseases are not curable, necessitating a better understanding of their pathobiology that may help identify novel biological targets. RhoA and Cdc42 of Rho family small GTPases regulate a variety of cellular functions such as actin cytoskeletal organization, cell adhesion, migration, proliferation, and survival. Recent characterization of mouse models of conditional gene knockout of RhoA and Cdc42 has revealed their physiological and cell type-specific roles in a number of cell types. In T lymphocytes, which play an important role in the pathogenesis of most, if not all, of the inflammatory diseases, we and others have investigated the effects of T cell-specific knockout of RhoA and Cdc42 on T cell development in the thymus, peripheral T cell homeostasis, activation, and differentiation to effector and regulatory T cells, and on T cell-mediated allergic airway inflammation and colitis. Here we highlight the phenotypes resulting from RhoA and Cdc42 deletion in T cells and discuss whether pharmacological targeting of RhoA and Cdc42 is feasible in treating asthma that is driven by allergic airway inflammation and colitis.

Reciprocal Regulation of Glycolysis-Driven Th17 Pathogenicity and Regulatory T Cell Stability by Cdc42

A balance between Th17 cells and regulatory T cells (Tregs) is important for host immunity and immune tolerance. The underlying molecular mechanisms remain poorly understood. Here we have identified Cdc42 as a central regulator of Th17/Treg balance. Deletion of Cdc42 in T cells enhanced Th17 differentiation but diminished induced Treg differentiation and suppressive function. Treg-specific deletion of Cdc42 decreased natural Tregs but increased effector T cells including Th17 cells. Notably, Cdc42-deficient Th17 cells became pathogenic associated with enhanced glycolysis and Cdc42-deficient Tregs became unstable associated with weakened glycolytic signaling. Inhibition of glycolysis in Cdc42-deficient Th17 cells diminished their pathogenicity and restoration of glycolysis in Cdc42-deficient Tregs rescued their instability. Intriguingly, Cdc42 deficiency in T cells led to exacerbated wasting disease in mouse models of colitis and Treg-specific deletion of Cdc42 caused early, fatal lymphoproliferative diseases. In summary, we show that Cdc42 is a bona fide regulator of peripheral tolerance through suppression of Th17 aberrant differentiation/pathogenicity and promotion of Treg differentiation/stability/function involving metabolic signaling and thus Cdc42 pathway might be harnessed in autoimmune disease therapy.

CDC42 Gtpase Activation Affects Hela Cell DNA Repair and Proliferation Following UV Radiation-Induced Genotoxic Stress

Cell division control protein 42 (CDC42) homolog is a small Rho GTPase enzyme that participates in such processes as cell cycle progression, migration, polarity, adhesion, and transcription. Recent studies suggest that CDC42 is a potent tumor suppressor in different tissues and is related to aging processes. Although DNA damage is crucial in aging, a potential role for CDC42 in genotoxic stress remains to be explored. Migration, survival/proliferation and DNA damage/repair experiments were performed to demonstrate CDC42 involvement in the recovery of HeLa cells exposed to ultraviolet radiation-induced stress. Sub-lines of HeLa cells ectopically expressing the constitutively active CDC42-V12 mutant were generated to examine whether different CDC42-GTP backgrounds might reflect different sensitivities to UV radiation. Our results show that CDC42 constitutive activation does not interfere with HeLa cell migration after UV radiation. However, the minor DNA damage exhibited by the CDC42-V12 mutant exposed to UV radiation most likely results in cell cycle arrest at the G2/M checkpoint and reduced proliferation and survival. HeLa cells and Mock clones, which express endogenous wild-type CDC42 and show normal activity, are more resistant to UV radiation. None of these effects are altered by pharmacological CDC42 inhibition. Finally, the phosphorylation status of the DNA damage response proteins γ-H2AX and p-Chk1 was found to be delayed and attenuated, respectively, in CDC42-V12 clones. In conclusion, the sensitivity of HeLa cells to ultraviolet radiation increases with CDC42 over-activation due to inadequate DNA repair signaling, culminating in G2/M cell accumulation, which is translated into reduced cellular proliferation and survival.

Promotion of immune and glycaemic functions in streptozotocin-induced diabetic rats treated with un-denatured camel milk whey proteins

T cell mediated autoimmune diabetes is characterized by immune cell infiltration of pancreatic islets and destruction of insulin-producing β-cells. This study was designed to assess the effect of whey proteins (WP) on the responsiveness of lymphocytes in rats after four months of Streptozotocin (STZ)-induced Type 1 diabetes (T1D). A diabetic group was supplemented with WP daily for five weeks at a dose of 100 mg/kg. Ribonucleic acid (RNA) was extracted from stimulated lymphocytes in order to analyse gene expressions using real time PCR and RT-PCR. PCR results were confirmed with ELISA. The proliferation capacity of lymphocytes and their homing to the spleen were studied. Antigen-activated lymphocytes showed that diabetes impaired the mRNA expression of the protein kinase B (Akt1), Cdc42, and the co-stimulatory molecule, CD28, which are important for cell survival, actin polymerization and T cell activation, respectively. Accordingly, proliferation of lymphocytes was found to be suppressed in diabetic rats, both in vivo and in vitro. WP was found to restore Akt1, Cdc42 and CD28 mRNA expression during diabetes to normal levels. WP, therefore, served to activate the proliferation of B lymphocytes in diabetic rats both in vivo and in vitro. Although WP was found to up-regulate mRNA expression of both interleukin (IL)-2 and interferon gamma (IFN-γ), it suppressed the proliferation activity of almost all T cell subsets. This was confirmed by WP normalizing the structure and function of ß cells. Meanwhile, WP was found to down regulate the mRNA expression of Tumor necrosis factor-alpha (TNF-α) and its programmed cell death-receptor (Fas). Taken together, the results of this study provide evidence for the potential impact of WP in the treatment of immune impairment in T1D, suggesting that it serves to reverse autoimmunity by suppressing autoreactive T cells and down regulating TNF-α and Fas, resulting in improved pancreatic ß cell structure and function.

Rho-GTPases as key regulators of T lymphocyte biology

Rho-GTPases belong to the Ras superfamily and are crucial signal transducing proteins downstream of many receptors. In general, the Rho-GTPases function as molecular switches, cycling between inactive (GDP-bound) and active (GTP-bound) states. The activated GTP bound Rho-GTPases interact with a broad spectrum of effectors to regulate a plethora of biological pathways including cytoskeletal dynamics, motility, cytokinesis, cell growth, apoptosis, transcriptional activity and nuclear signaling. Recently, gene targeting in mice allowed the selective inactivation of different Rho-GTPases and has advanced our understanding of the physiological role of these proteins, particularly in the immune system. Particularly, these proteins are key signaling molecules in T lymphocytes, which are generated in the thymus and are major players in the immune system. The scope of this review is to discuss recent data obtained in Rho-GTPases deficient mice by focusing on the role-played by Rho-GTPases in T-lymphocyte development, migration, activation and differentiation.

PTEN regulates colorectal epithelial apoptosis through Cdc42 signalling

Background:

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) regulation of the Rho-like GTPase Cdc42 has a central role in epithelial polarised growth, but effects of this molecular network on apoptosis remain unclear.

Methods:

To investigate the role of Cdc42 in PTEN-dependent cell death, we used flow cytometry, in vitro pull-down assays, poly(ADP ribose) polymerase (PARP) cleavage and other immunoblots in isogenic PTEN-expressing and -deficient colorectal cells (HCT116PTEN^+/+, HCT116PTEN^−/−, Caco2 and Caco2 ShPTEN cells) after transfection or treatment strategies.

Results:

The PTEN knockout or suppression by short hairpin RNA or small interfering RNA (siRNA) inhibited Cdc42 activity, PARP cleavage and/or apoptosis in flow cytometry assays. Transfection of cells with wild-type or constitutively active Cdc42 enhanced PARP cleavage, whereas siRNA silencing of Cdc42 inhibited PARP cleavage and/or apoptosis. Pharmacological upregulation of PTEN by sodium butyrate (NaBt) treatment enhanced Cdc42 activity, PARP cleavage and apoptosis, whereas Cdc42 siRNA suppressed NaBt-induced PARP cleavage. Cdc42-dependent signals can suppress glycogen synthase kinase-β (GSK3β) activity. Pharmacological inhibition of GSK3β by lithium chloride treatment mimicked effects of Cdc42 in promotion of PARP cleavage and/or apoptosis.

Conclusion:

Phosphatase and tensin homologue deleted on chromosome 10 may influence apoptosis in colorectal epithelium through Cdc42 signalling, thus providing a regulatory framework for both polarised growth and programmed cell death.

Distinct roles of Cdc42 in thymopoiesis and effector and memory T cell differentiation

Cdc42 of the Rho GTPase family has been implicated in cell actin organization, proliferation, survival, and migration but its physiological role is likely cell-type specific. By a T cell-specific deletion of Cdc42 in mouse, we have recently shown that Cdc42 maintains naïve T cell homeostasis through promoting cell survival and suppressing T cell activation. Here we have further investigated the involvement of Cdc42 in multiple stages of T cell differentiation. We found that in Cdc42(-/-) thymus, positive selection of CD4(+)CD8(+) double-positive thymocytes was defective, CD4(+) and CD8(+) single-positive thymocytes were impaired in migration and showed an increase in cell apoptosis triggered by anti-CD3/-CD28 antibodies, and thymocytes were hyporesponsive to anti-CD3/-CD28-induced cell proliferation and hyperresponsive to anti-CD3/-CD28-stimulated MAP kinase activation. At the periphery, Cdc42-deficient naive T cells displayed an impaired actin polymerization and TCR clustering during the formation of mature immunological synapse, and showed an enhanced differentiation to Th1 and CD8(+) effector and memory cells in vitro and in vivo. Finally, Cdc42(-/-) mice exhibited exacerbated liver damage in an induced autoimmune disease model. Collectively, these data establish that Cdc42 is critically involved in thymopoiesis and plays a restrictive role in effector and memory T cell differentiation and autoimmunity.

Signaling role of Cdc42 in regulating mammalian physiology

Cdc42 is a member of the Rho GTPase family of intracellular molecular switches regulating multiple signaling pathways involved in actomyosin organization and cell proliferation. Knowledge of its signaling function in mammalian cells came mostly from studies using the dominant-negative or constitutively active mutant overexpression approach in the past 2 decades. Such an approach imposes a number of experimental limitations related to specificity, dosage, and/or clonal variability. Recent studies by conditional gene targeting of cdc42 in mice have revealed its tissue- and cell type-specific role and provide definitive information of the physiological signaling functions of Cdc42 in vivo.

Rho GTPase Cdc42 is essential for human T-cell development

BACKGROUND

Rho GTPases are involved in the regulation of many cell functions, including some related to the actin cytoskeleton. Different Rho GTPases have been shown to be important for T-cell development in mice. However, their role in human T-cell development has not yet been explored.

DESIGN AND METHODS

We examined the expression and activation of Rho GTPases along different stages of T-cell development in the human thymus. Early stage human thymocytes were transduced with constitutively active and dominant negative mutants of different Rho GTPases to explore their role in human T-cell development, as analyzed in fetal thymus organ cultures. The use of these mutants as well as Rho GTPase-specific inhibitors allowed us to explore the role of GTPases in thymocyte migration.

RESULTS

We found that the expression of several Rho GTPases is differently regulated during successive stages of T-cell development in man, suggesting a specific role in human thymopoiesis. In chimeric fetal thymus organ culture, T-cell development was not or only mildly affected by expression of dominant negative Rac1 and Rac2, but was severely impaired in the presence of dominant negative Cdc42, associated with enhanced apoptosis and reduced proliferation. Kinetic analysis revealed that Cdc42 is necessary in human T-cell development both before and after expression of the pre-T-cell receptor. Using inhibitors and retrovirally transferred mutants of the aforementioned Rho GTPases, we showed that only Rac1 is necessary for migration of different thymocyte subsets, including the early CD34(+) fraction, towards stromal cell-derived factor-1 alpha. Constitutively active mutants of Rac1, Rac2 and Cdc42 all impaired migration towards stromal cell-derived factor-1 alpha and T-cell development to different degrees.

CONCLUSIONS

This is the first report on Rho GTPases in human T-cell development, showing the essential role of Cdc42. Our data suggest that enhanced apoptotic death and reduced proliferation rather than disturbed migration explains the decreased thymopoiesis induced by dominant negative Cdc42.

Rho GTPases in hematopoiesis and hemopathies

Rho family GTPases are intracellular signaling proteins regulating multiple pathways involved in cell actomyosin organization, adhesion, and proliferation. Our knowledge of their cellular functions comes mostly from previous biochemical studies that used mutant overexpression approaches in various clonal cell lines. Recent progress in understanding Rho GTPase functions in blood cell development and regulation by gene targeting of individual Rho GTPases in mice has allowed a genetic understanding of their physiologic roles in hematopoietic progenitors and mature lineages. In particular, mouse gene-targeting studies have provided convincing evidence that individual members of the Rho GTPase family are essential regulators of cell type-specific functions and stimuli-specific pathways in regulating hematopoietic stem cell interaction with bone marrow niche, erythropoiesis, and red blood cell actin dynamics, phagocyte migration and killing, and T- and B-cell maturation. In addition, deregulation of Rho GTPase family members has been associated with multiple human hematologic diseases such as neutrophil dysfunction, leukemia, and Fanconi anemia, raising the possibility that Rho GTPases and downstream signaling pathways are of therapeutic value. In this review we discuss recent genetic studies of Rho GTPases in hematopoiesis and several blood lineages and the implications of Rho GTPase signaling in hematologic malignancies, immune pathology. and anemia.

Rho GTPase function in tumorigenesis

Malignant tumor cells display uncontrolled proliferation, loss of epithelial cell polarity, altered interactions with neighboring cells and the surrounding extracellular matrix, and enhanced migratory properties. Proteins of the Rho GTPase family regulate all these processes in cell culture and, for that reason, Rho GTPases, their regulators, and their effectors have been suggested to control tumor formation and progression in humans. However, while the tumor-relevant functions of Rho GTPases are very well documented in vitro, we are only now beginning to assess their contribution to cancer in human patients and in animal models. This review will give a very brief overview of Rho GTPase function in general and then focus on in vivo evidence for a role of Rho GTPases in malignant tumors, both in human patients and in genetically modified mice.

Rho GTPase-mediated pathways in mature CD4+ T cells

Effective immune responses require the appropriate activation and differentiation of peripheral CD4(+) T cells. These processes need to be followed by the timely elimination of the responding T cells in order to restore T cell homeostasis. Defects in the appropriate regulation of T cell activation, expansion, and survival underlie the pathogenesis of many autoimmune disorders including SLE. The molecular machinery employed by T cells to properly control these processes and prevent the onset of autoimmunity has not been fully elucidated. Rho GTPases (which include the Rac, Cdc42, and Rho subfamilies) are molecular switches that control a wide range of cellular processes. Their fundamental role in biology is due to their ability to regulate both cytoskeletal dynamics and a large number of signal transduction pathways. Activation of Rho GTPases is now recognized as a key event in the coordination of immune responses and, particularly, in the activation of T cells. In this review, we will first provide an overview of the role of Rho GTPase-mediated pathways in mature CD4(+) T cells and then we will discuss recent studies, which suggest that deregulation of these pathways may play a role in the pathogenesis of SLE.

Rac GTPase isoforms Rac1 and Rac2 play a redundant and crucial role in T-cell development

Rac GTPases have been implicated in the regulation of diverse functions in various blood cell lineages, but their role in T-cell development is not well understood. We have carried out conditional gene targeting to achieve hematopoietic stem cell (HSC)- or T-cell lineage-specific deletion of Rac1 or Rac1/Rac2 by crossbreeding the Mx-Cre or Lck-Cre transgenic mice with Rac1(loxp/loxp) or Rac1(loxp/loxp);Rac2(-/-) mice. We found that (1) HSC deletion of both Rac1 and Rac2 inhibited production of common lymphoid progenitors (CLPs) in bone marrow and suppressed T-cell development in thymus and peripheral organs, whereas deletion of Rac1 moderately affected CLP production and T-cell development. (2) T cell-specific deletion of Rac1 did not affect T-cell development, whereas deletion of both Rac1 and Rac2 reduced immature CD4(+)CD8(+) and mature CD4(+) populations in thymus as well as CD4(+) and CD8(+) populations in spleen. (3) The developmental defects of Rac1/Rac2 knockout T cells were associated with proliferation, survival, adhesion, and migration defects. (4) Rac1/Rac2 deletion suppressed T-cell receptor-mediated proliferation, IL-2 production, and Akt activation in thymocytes. Thus, Rac1 and Rac2 have unique roles in CLP production and share a redundant but essential role in later stages of T-cell development by regulating survival and proliferation signals.

The RhoA transcriptional program in pre-T cells

The GTPase RhoA is essential for the development of pre-T cells in the thymus. To investigate the mechanisms used by RhoA to control thymocyte development we have used Affymetrix gene profiling to identify RhoA regulated genes in T cell progenitors. The data show that RhoA plays a specific and essential role in pre-T cells because it is required for the expression of transcription factors of the Egr-1 and AP-1 families that have critical functions in thymocyte development. Loss of RhoA function in T cell progenitors causes a developmental block that pheno-copies the consequence of losing pre-TCR expression in Recombinase gene 2 (Rag2) null mice. Transcriptional profiling reveals both common and unique gene targets for RhoA and the pre-TCR indicating that RhoA participates in the pre-TCR induced transcriptional program but also mediates pre-TCR independent gene transcription.

Rac1-mediated Bcl-2 induction is critical in antigen-induced CD4 single-positive differentiation of a CD4+CD8+ immature thymocyte line

Rac1, one of the Rho family small guanosine triphosphatases, has been shown to work as a "molecular switch" in various signal transduction pathways. To assess the function of Rac1 in the differentiation process of CD4 single-positive (CD4-SP) T cells from CD4CD8 double-positive (DP) cells, we used a DP cell line DPK, which can differentiate into CD4-SP cells upon TCR stimulation in vitro. DPK expressing dominant-negative (dn)Rac1 underwent massive apoptosis upon TCR stimulation and resulted in defective differentiation of CD4-SP cells. Conversely, overexpression of dnRac2 did not affect differentiation. TCR-dependent actin polymerization was inhibited, whereas early ERK activation was unaltered in dnRac1-expressing DPK. We found that TCR-dependent induction of Bcl-2 was suppressed greatly in dnRac1-expressing DPK, and this suppression was independent of actin rearrangement. Furthermore, introduction of exogenous Bcl-2 inhibited TCR-dependent induction of apoptosis and restored CD4-SP generation in dnRac1-expressing DPK without restoring TCR-induced actin polymerization. Collectively, these data indicate that Rac1 is critical in differentiation of CD4-SP from the DP cell line by preventing TCR-induced apoptosis via Bcl-2 up-regulation.

Cdc42GAP regulates c-Jun N-terminal kinase (JNK)-mediated apoptosis and cell number during mammalian perinatal growth

Rho family GTPase Cdc42 is known to regulate polarity and growth in lower eukaryotes, but its physiologic function in mammals has yet to be determined. Here we have disrupted cdc42gap, a ubiquitously expressed negative regulator of Cdc42, in mice. Cdc42GAP(-/-) embryonic fibroblasts and various organs displayed significantly elevated Cdc42 activity. The embryonic and neonatal homozygous mice were reduced in size by approximately 25-40% and suffered severe growth retardation. Major organs from Cdc42GAP(-/-) mice were proportionally smaller because of decreased cell number. Basal apoptosis was increased in Cdc42GAP(-/-) cells and tissues, and this was attributed to altered c-Jun N-terminal kinase apoptotic signals. These results reveal a role of Cdc42GAP in mammalian perinatal growth and implicate the c-Jun N-terminal kinase-mediated apoptosis machinery as a Cdc42 effector pathway in vivo.

The Immunological Synapse and Rho GTPases

Rho GTPases are molecular switches controlling a broad range of cellular processes including lymphocyte activation. Not surprisingly, Rho GTPases are now recognized as pivotal regulators of antigen-specific T cell activation by APCs and immunological synapse formation. This review summarizes recent advances in our understanding of how Rho GTPase-dependent pathways control T lymphocyte motility, polarization and activation.

The RhoA- and CDC42-specific exchange factor Dbs promotes expansion of immature thymocytes and deletion of double-positive and single-positive thymocytes

Specific members of the Rho family of GTPases exert unique influences on thymocyte proliferation, differentiation and deletion. Dbs is a guanine nucleotide exchange factor which is expressed throughout thymocyte development and is able to activate the Rho family GTPases CDC42, RhoA and RhoG. Transgenic mice expressing an activated form of Dbs had increased numbers of double-negative thymocytes. The Dbs transgene promoted expansion of double-negative thymocytes in the absence of pre-TCR, but had no effect on pre-TCR-dependent differentiation of double-negative thymocytes into double-positive thymocytes. Transgenic double-positive thymocytes were proliferative in vivo, but were also susceptible to apoptosis in vivo and in vitro. The transgenic single-positive thymocytes had attenuated proliferative responses following TCR ligation, and were depleted rather than expanded during culture in the presence of anti-CD3. When expressing a positively selectable TCR, transgenic double-positive thymocytes were increased in number and activated, but the output of single-positive thymocytes was reduced. Transgenic double-positive thymocytes were acutely sensitive to deletion by TCR ligation in vivo. These results indicate that activation of Dbs has the potential to promote proliferation throughout thymocyte development, but also sensitizes double-positive and single-positive thymocytes to deletion.

Immunological function in mice lacking the Rac-related GTPase RhoG

RhoG is a low-molecular-weight GTPase highly expressed in lymphocytes that activates gene transcription and promotes cytoskeletal reorganization in vitro. To study the in vivo function of RhoG, we generated mice homozygous for a targeted disruption of the RhoG gene. Despite the absence of RhoG, the development of B and T lymphocytes was unaffected. However, there was an increase in the level of serum immunoglobulin G1 (IgG1) and IgG2b as well as a mild increase of the humoral immune response to thymus-dependent antigens. In addition, B- and T-cell proliferation in response to antigen receptor cross-linking was slightly increased. Although RhoG deficiency produces a mild phenotype, our experiments suggest that RhoG may contribute to the negative regulation of immune responses. The lack of a strong phenotype could indicate a functional redundancy of RhoG with other Rac proteins in lymphocytes.

Intracellular Location and Cell Context-Dependent Function of Protein Kinase D

Protein kinase D (PKD) is an antigen receptor-activated serine kinase localized at either the plasma membrane or the cytosol of lymphocytes. To probe PKD function at these different locations, transgenesis was used to target active PKD either to the membrane or cytosol of pre-T cells. In recombinase gene null pre-T cells, membrane and cytosolic active PKD both induced differentiation reminiscent of beta selection: downregulation of CD25 and upregulation of CD2 and CD5. Active PKDs also induced pre-T cell proliferation, although this response was not universal to all thymocyte subsets. There were two striking differences between the actions of the differentially localized PKDs. Membrane but not cytosolic PKD could induce expression of CD8 and CD4 in recombinase null mice; cytosolic but not membrane PKD suppressed Vbeta to DJbeta rearrangements of the TCRbeta chain locus in wild-type T cells. PKD function is thus determined by its intracellular location and cell context.

Guanine nucleotide exchange factor, Tiam1, directly binds to c-Myc and interferes with c-Myc-mediated apoptosis in rat-1 fibroblasts

The transcription factor c-Myc is important for the control of cell growth, cell cycle progression, neoplasia, and apoptotic cell death. Recently, c-Myc-binding proteins, which bind either to the N-terminal domain or the C-terminal domain of c-Myc, have been proposed as the key molecules to realize the mechanisms of these multiple c-Myc functions. We report in the present study on another protein, Tiam1, which is a specific guanine nucleotide exchange factor of Rac1 and which binds to c-Myc and modulates several of its biological functions. We were able to detect the direct binding and in vivo association between c-Myc and Tiam1. The necessary role in this interaction of the Myc box II of c-Myc was revealed in the cell extracts. The additional discovery of the intranuclear localization of Tiam1 in Rat1 cells and in neuronal cells of the mouse brain suggests this interaction may occur in the nucleus. Overexpression of Tiam1 repressed the luciferase activity of c-Myc and also inhibited the c-Myc apoptotic activity through this protein-protein interaction. Taken together, we concluded that Tiam1 is another c-Myc regulator, working in the nuclei to control c-Myc-related apoptosis.

RhoG regulates gene expression and the actin cytoskeleton in lymphocytes

RhoG, a member of the Rho family of GTPases, has been implicated as a regulator of the actin cytoskeleton. In this study, we show a novel function for the small GTPase RhoG on the regulation of the interferon-gamma promoter and nuclear factor of activated T cells (NFAT) gene transcription in lymphocytes. Optimal function of RhoG for the expression of these genes requires a calcium signal, normally provided by the antigen receptor. In addition, RhoG potentiation of NFAT requires the indirect activity of Rac and Cdc42; however, pathways distinct from those activated by Rac and Cdc42 mediate RhoG activation of NFAT-dependent transcription. Using effector domain mutants of RhoG we found that its ability to potentiate NFAT-dependent transcription correlates with its capacity to increase actin polymerization, supporting the suggestion that NFAT-dependent transcription is an actin-dependent process. RhoG also promotes T-cell spreading on fibronectin, a property that is independent of its ability to enhance NFAT-dependent transcription. Hence, these results implicate RhoG in leukocyte trafficking and the control of gene expression induced in response to antigen encounter.

Understanding Rho/Rac biology in T-cells using animal models

Experiments with cell lines have unveiled the implication of the Rho/Rac family of GTPases in cytoskeletal organization, mitogenesis, and cell migration. However, there have not been adequate animal models to investigate the role of these proteins in more physiological settings. This scenario has changed recently in the case of the T-cell lineage after the generation of animal models for Rho/Rac family members, their regulators, and effectors. These studies have revealed the implication of these GTPases on multiple regulatory layers of T-cells, including the coordination of cytoskeletal change, activation of kinase cascades, stimulation of calcium fluxes, and the induction of gene expression. These pathways affect the transition of different T-cell maturation stages, the positive/negative selection of thymocytes, T-cell responses to antigens, and the homeostasis of peripheral T-lymphocytes. Moreover, these animals have revealed interesting cross-talks between Rho/Rac pathways and other signal transduction routes that participate in lymphocyte responses.