A domain containing the Cdc42/Rac interactive binding (CRIB) region of p65PAK inhibits transcriptional activation and cell transformation mediated by the Ras-Rac pathway

R-Ras subfamily proteins elicit distinct physiologic effects and phosphoproteome alterations in neurofibromin-null MPNST cells

Background

Loss of the Ras GTPase-activating protein neurofibromin promotes nervous system tumor pathogenesis in patients with neurofibromatosis type 1 (NF1). Neurofibromin loss potentially hyperactivates classic Ras (H-Ras, N-Ras, K-Ras), M-Ras, and R-Ras (R-Ras, R-Ras2/TC21) subfamily proteins. We have shown that classic Ras proteins promote proliferation and survival, but not migration, in malignant peripheral nerve sheath tumor (MPNST) cells. However, it is unclear whether R-Ras, R-Ras2 and M-Ras are expressed and hyperactivated in MPNSTs and, if so, whether they contribute to MPNST pathogenesis. We assessed the expression and activation of these proteins in MPNST cells and inhibited them to determine the effect this had on proliferation, migration, invasion, survival and the phosphoproteome.

Methods

NF1-associated (ST88-14, 90-8, NMS2, NMS-PC, S462, T265-2c) and sporadic (STS-26T, YST-1) MPNST lines were used. Cells were transfected with doxycycline-inducible vectors expressing either a pan-inhibitor of the R-Ras subfamily [dominant negative (DN) R-Ras] or enhanced green fluorescent protein (eGFP). Methodologies used included immunoblotting, immunocytochemistry, PCR, Transwell migration, ³H-thymidine incorporation, calcein cleavage assays and shRNA knockdowns. Proteins in cells with or without DN R-Ras expression were differentially labeled with SILAC and mass spectrometry was used to identify phosphoproteins and determine their relative quantities in the presence and absence of DN R-Ras. Validation of R-Ras and R-Ras2 action and R-Ras regulated networks was performed using genetic and/or pharmacologic approaches.

Results

R-Ras2 was uniformly expressed in MPNST cells, with R-Ras present in a major subset. Both proteins were activated in neurofibromin-null MPNST cells. Consistent with classical Ras inhibition, DN R-Ras and R-Ras2 knockdown inhibited proliferation. However, DN R-Ras inhibition impaired migration and invasion but not survival. Mass spectrometry-based phosphoproteomics identified thirteen protein networks distinctly regulated by DN R-Ras, including multiple networks regulating cellular movement and morphology. ROCK1 was a prominent mediator in these networks. DN R-Ras expression and RRAS and RRAS2 knockdown inhibited migration and ROCK1 phosphorylation; ROCK1 inhibition similarly impaired migration and invasion, altered cellular morphology and triggered the accumulation of large intracellular vesicles.

Conclusions

R-Ras proteins function distinctly from classic Ras proteins by regulating distinct signaling pathways that promote MPNST tumorigenesis by mediating migration and invasion.

Plain English Summary

Mutations of the NF1 gene potentially results in the activation of multiple Ras proteins, which are key regulators of many biologic effects. The protein encoded by the NF1 gene, neurofibromin, acts as an inhibitor of both classic Ras and R-Ras proteins; loss of neurofibromin could cause these Ras proteins to become persistently active, leading to the development of cancer. We have previously shown that three related Ras proteins (the classic Ras proteins) are highly activated in malignant peripheral nerve sheath tumor (MPNST) cells with neurofibromin loss and that they drive cancer cell proliferation and survival by activating multiple cellular signaling pathways. Here, we examined the expression, activation and action of R-Ras proteins in MPNST cells that have lost neurofibromin. Both R-Ras and R-Ras2 are expressed in MPNST cells and activated. Inhibition of R-Ras action inhibited proliferation, migration and invasion but not survival. We examined the activation of cytoplasmic signaling pathways in the presence and absence of R-Ras signaling and found that R-Ras proteins regulated 13 signaling pathways distinct from those regulated by classic Ras proteins. Closer study of an R-Ras regulated pathway containing the signaling protein ROCK1 showed that inhibition of either R-Ras, R-Ras2 or ROCK1 similarly impaired cellular migration and invasion and altered cellular morphology. Inhibition of R-Ras/R-Ras2 and ROCK1 signaling also triggered the accumulation of abnormal intracellular vesicles, indicating that these signaling molecules regulate the movement of proteins and other molecules in the cellular interior.

UnPAKing RUNX3 functions-Both sides of the coin

Post translational modifications of RUNX3 have been shown to play an important role in directing RUNX3 functions. In this review we highlight the phosphorylation dependent functions of RUNX3 as regulated by PAK1 and its implications on tumorigenesis.

Molecular pathways: targeting RAC-p21-activated serine-threonine kinase signaling in RAS-driven cancers

Cancers driven by oncogenic Ras proteins encompass some of the most deadly human cancer types, and there is a pressing need to develop therapies for these diseases. Although recent studies suggest that mutant Ras proteins may yet be druggable, the most promising and advanced efforts involve inhibitors of Ras effector signaling. Most efforts to target Ras signaling have been aimed at the ERK mitogen-activated protein kinase and the phosphoinositide 3-kinase signaling networks. However, to date, no inhibitors of these Ras effector pathways have been effective against RAS-mutant cancers. This ineffectiveness is due, in part, to the involvement of additional effectors in Ras-dependent cancer growth, such as the Rac small GTPase and the p21-activated serine-threonine kinases (PAK). PAK proteins are involved in many survival, cell motility, and proliferative pathways in the cell and may present a viable new target in Ras-driven cancers. In this review, we address the role and therapeutic potential of Rac and group I PAK proteins in driving mutant Ras cancers.

The role of p21-activated kinases in hepatocellular carcinoma metastasis

The p21-activated kinases (PAKs) are downstream effectors of the Rho family small GTPases as well as a wide variety of mitogenic factors and have been implicated in cancer formation, development and metastasis. PAKs phosphorylate a wide spectrum of substrates to mediate extracellular signals and regulate cytoskeletal remodeling, cell motility and survival. In this review, we aim to summarize the findings regarding the oncogenic role and the underlying mechanisms of PAKs signaling in various cancers, and in particular highlight the prime importance of PAKs in hepatocellular carcinoma (HCC) progression and metastasis. Recent studies exploring the potential therapeutic application of PAK inhibitors will also be discussed.

Up-regulation of p21-activated kinase 1 by in vitro treatment with interleukin 1-beta and its increased expression in ovarian endometriotic cysts

We evaluated whether the proinflammatory cytokines can regulate p21-activated kinase (Pak1) expression in endometrial cells as well as whether its expression is increased in endometriotic cysts. We found that interleukin-1β up-regulates Pak1 expression in endometrial stromal cells (ESC) and that the immunoreactivity of Pak1 is increased in the endometriotic cysts.

Pak protein kinases and their role in cancer

Some of the characteristics of cancer cells are high rates of cell proliferation, cell survival, and the ability to invade surrounding tissue. The cytoskeleton has an essential role in these processes. Dynamic changes in the cytoskeleton are necessary for cell motility and cancer cells are dependent on motility for invasion and metastasis. The signaling pathways behind the reshaping and migrating properties of the cytoskeleton in cancer cells involve a group of Ras-related small GTPases and their effectors, including the p21-activated kinases (Paks). Paks are a family of serine/threonine protein kinases comprised of six isoforms (Pak 1-6), all of which are direct targets of the small GTPases Rac and Cdc42. Besides their role in cytoskeletal dynamics, Paks have recently been shown to regulate various other cellular activities, including cell survival, mitosis, and transcription. Paks are overexpressed and/or hyperactivated in several human tumors and their role in cell transformation makes them attractive therapeutic targets. Pak-targeted therapeutics may efficiently inhibit certain types of tumors and efforts to identify selective Pak-inhibitors are underway.

Overexpression of protease-activated receptors-1,-2, and-4 (PAR-1, -2, and -4) in prostate cancer

BACKGROUND

Although protease-activated receptors (PARs) have been described to play a role in different malignancies, their expression and biological activity in prostate cancer are mostly unknown.

METHODS

PAR expression in radical prostatectomy specimens was investigated by immunohistochemistry (IHC, 40 patients) and RT-PCR. Their role in LNCaP prostate cancer cell migration and Rac1/Cdc42 signaling was assessed with Boyden chamber analysis and Western blot, respectively.

RESULTS

PAR mRNA expression was higher in cancer, and protein expression was increased in PAR-1 (45%), PAR-2 (42%), and PAR-4 (68%), compared to normal glands. Increased PAR-1 (periglandular stroma) was associated with higher rates of biochemical recurrence (median follow-up, 5 years; P = 0.006). LNCaP migration was enhanced twofold and Rac1/Cdc42 signaling was activated by stimulation of PAR-1 and PAR-2.

CONCLUSIONS

PARs are overexpressed in prostate cancer and may serve as potential predictors of recurrence. The data suggest potential role of PARs in autocrine and paracrine mechanisms of prostate cancer.

The alliance for cellular signaling plasmid collection: a flexible resource for protein localization studies and signaling pathway analysis

Cellular responses to inputs that vary both temporally and spatially are determined by complex relationships between the components of cell signaling networks. Analysis of these relationships requires access to a wide range of experimental reagents and techniques, including the ability to express the protein components of the model cells in a variety of contexts. As part of the Alliance for Cellular Signaling, we developed a robust method for cloning large numbers of signaling ORFs into Gateway entry vectors, and we created a wide range of compatible expression platforms for proteomics applications. To date, we have generated over 3000 plasmids that are available to the scientific community via the American Type Culture Collection. We have established a website at www.signaling-gateway.org/data/plasmid/ that allows users to browse, search, and blast Alliance for Cellular Signaling plasmids. The collection primarily contains murine signaling ORFs with an emphasis on kinases and G protein signaling genes. Here we describe the cloning, databasing, and application of this proteomics resource for large scale subcellular localization screens in mammalian cell lines.

The activation of TC10, a Rho small GTPase, contributes to v-Rel-mediated transformation

v-Rel is the oncogenic member of the Rel/NF-kappaB family of transcription factors and transforms hematopoietic cells and fibroblasts. Differential display was employed to identify target genes that exhibit altered expression in v-Rel transformed cells. One of the cDNAs identified encodes the chicken ortholog of TC10, a member of the Rho small GTPase family. The expression of TC10 was increased in v-Rel-transformed chicken embryonic fibroblasts (CEFs) 3 to 6-fold relative to control cells at both the RNA and protein levels. An elevated level of active, GTP-bound TC10 was also detected in v-Rel-transformed cells relative to control cells. Expression of a dominant-negative TC10 mutant (TC10T32N) decreased the colony formation potential of v-Rel-transformed cells. Furthermore, overexpression of wild-type TC10 or a gain-of-function mutant (TC10Q76L) greatly enhanced the ability of v-Rel transformed CEFs to form colonies in soft agar. In addition to enhance the transformation potential of v-Rel, the overexpression of wild-type TC10 or the gain-of-function mutant alone enhanced the saturation density of CEFs and was sufficient for their anchorage-independent growth in vitro. These results indicate that elevated TC10 activity contributes to v-Rel-mediated transformation of CEFs and demonstrate for the first time that a Rho factor alone is capable of inducing the in vitro transformation of primary cells.

p21-activated kinases in cancer

The pivotal role of kinases in signal transduction and cellular regulation has lent them considerable appeal as pharmacological targets across a broad spectrum of cancers. p21-activated kinases (Paks) are serine/threonine kinases that function as downstream nodes for various oncogenic signalling pathways. Paks are well-known regulators of cytoskeletal remodelling and cell motility, but have recently also been shown to promote cell proliferation, regulate apoptosis and accelerate mitotic abnormalities, which results in tumour formation and cell invasiveness. Alterations in Pak expression have been detected in human tumours, which makes them an attractive new therapeutic target.

Activation of p21-activated kinase 6 by MAP kinase kinase 6 and p38 MAP kinase

The p21-activated kinases (PAKs) contain an N-terminal Cdc42/Rac interactive binding domain, which in the group 1 PAKs (PAK1, 2, and 3) regulates the activity of an adjacent conserved autoinhibitory domain. In contrast, the group 2 PAKs (PAK4, 5, and 6) lack this autoinhibitory domain and are not activated by Cdc42/Rac binding, and the mechanisms that regulate their kinase activity have been unclear. This study found that basal PAK6 kinase activity was repressed by a p38 mitogen-activated protein (MAP) kinase antagonist and could be strongly stimulated by constitutively active MAP kinase kinase 6 (MKK6), an upstream activator of p38 MAP kinases. Mutation of a consensus p38 MAP kinase target site at serine 165 decreased PAK6 kinase activity. Moreover, PAK6 was directly activated by MKK6, and mutation of tyrosine 566 in a consensus MKK6 site (threonine-proline-tyrosine, TPY) in the activation loop of the PAK6 kinase domain prevented activation by MKK6. PAK6 activation by MKK6 was also blocked by mutation of an autophosphorylated serine (serine 560) in the PAK6 activation loop, indicating that phosphorylation of this site is necessary for MKK6-mediated activation. PAK4 and PAK5 were similarly activated by MKK6, consistent with a conserved TPY motif in their activation domains. The activation of PAK6 by both p38 MAP kinase and MKK6 suggests that PAK6 plays a role in the cellular response to stress-related signals.

Vinexin beta regulates the anchorage dependence of ERK2 activation stimulated by epidermal growth factor

ERK is activated by soluble growth factors in adherent cells. However, activation of ERK is barely detectable and not sufficient for cell proliferation in non-adherent cells. Here, we show that exogenous expression of vinexin beta, a novel focal adhesion protein, allows anchorage-independent ERK2 activation stimulated by epidermal growth factor. In contrast, expression of vinexin beta had no effect on ERK2 activation in adherent cells, suggesting that vinexin beta regulates the anchorage dependence of ERK2 activation. Analyses using deletion mutants demonstrated that a linker region between the second and third SH3 domains of vinexin beta, but not the SH3 domains, is required for this function of vinexin beta. To evaluate the pathway regulating the anchorage dependence of ERK2 activation, we used a dominant-negative mutant of p21-activated kinase (PAK) and a specific inhibitor (H89) of cAMP-dependent protein kinase (PKA) because PAK and PKA are known to regulate the anchorage dependence of ERK2 activation. The dominant-negative mutant of PAK suppressed the anchorage-independent ERK2 activation induced by expression of vinexin beta. The dominant-negative mutant of vinexin beta inhibited the anchorage-independent ERK2 activation induced by the PKA inhibitor. Together, these observations indicate that vinexin beta plays a key role in regulating the anchorage dependence of ERK2 activation through PKA-PAK signaling.

Requirement for PAK4 in the anchorage-independent growth of human cancer cell lines

p21-activated protein kinase (PAK) serine/threonine kinases are important effectors of Rho family GTPases and have been implicated in the regulation of cell morphology and motility, as well as in cell transformation. To further investigate the possible involvement of PAK kinases in tumorigenesis, we analyzed the expression of several family members in tumor cell lines. Here we demonstrate that PAK4 is frequently overexpressed in human tumor cell lines of various tissue origins. We also have identified serine (Ser-474) as the likely autophosphorylation site in the kinase domain of PAK4 in vivo. Mutation of this serine to glutamic acid (S474E) results in constitutive activation of the kinase. Phosphospecific antibodies directed against serine 474 detect activated PAK4 on the Golgi membrane when PAK4 is co-expressed with activated Cdc42. Furthermore, expression of the active PAK4 (S474E) mutant has transforming potential, leading to anchorage-independent growth of NIH3T3 cells. A kinase-inactive PAK4 (K350A,K351A), on the other hand, efficiently blocks transformation by activated Ras and inhibits anchorage-independent growth of HCT116 colon cancer cells. Taken together, our data strongly implicate PAK4 in oncogenic transformation and suggest that PAK4 activity is required for Ras-driven, anchorage-independent growth.

MST4, a new Ste20-related kinase that mediates cell growth and transformation via modulating ERK pathway

In this study, we report the cloning and characterization of a novel human Ste20-related kinase that we designated MST4. The 416 amino acid full-length MST4 contains an amino-terminal kinase domain, which is highly homologous to MST3 and SOK, and a unique carboxy-terminal domain. Northern blot analysis indicated that MST4 is highly expressed in placenta, thymus, and peripheral blood leukocytes. Wild-type but not kinase-dead MST4 can phosphorylate myelin basic protein in an in vitro kinase assay. MST4 specifically activates ERK but not JNK or p38 MAPK in transient transfected cells or in stable cell lines. Overexpression of dominant negative MEK1 or treatment with PD98059 abolishes MST4-induced ERK activity, whereas dominant-negative Ras or c-Raf-1 mutants failed to do so, indicating MST4 activates MEK1/ERK via a Ras/Raf-1 independent pathway. HeLa and Phoenix cell lines overexpressing wild-type, but not kinase-dead, MST4 exhibit increased growth rate and form aggressive soft-agar colonies. These phenotypes can be inhibited by PD98059. These results provide the first evidence that MST4 is biologically active in the activation of MEK/ERK pathway and in mediating cell growth and transformation.

Simultaneous tyrosine and serine phosphorylation of STAT3 transcription factor is involved in Rho A GTPase oncogenic transformation

Stats (signal transducers and activators of transcription) are latent cytoplasmic transcription factors that on a specific stimulus migrate to the nucleus and exert their transcriptional activity. Here we report a novel signaling pathway whereby RhoA can efficiently modulate Stat3 transcriptional activity by inducing its simultaneous tyrosine and serine phosphorylation. Tyrosine phosphorylation is exerted via a member of the Src family of kinases (SrcFK) and JAK2, whereas the JNK pathway mediates serine phosphorylation. Furthermore, cooperation of both tyrosine as well as serine phosphorylation is necessary for full activation of Stat3. Induction of Stat3 activity depends on the effector domain of RhoA and correlates with induction of both Src Kinase-related and JNK activities. Activation of Stat3 has biological implications. Coexpression of an oncogenic version of RhoA along with the wild-type, nontransforming Stat3 gene, significantly enhances its oncogenic activity on human HEK cells, suggesting that Stat3 is an essential component of RhoA-mediated transformation. In keeping with this, dominant negative Stat3 mutants or inhibition of its tyrosine or serine phosphorylation completely abrogate RhoA oncogenic potential. Taken together, these results indicate that Stat3 is an important player in RhoA-mediated oncogenic transformation, which requires simultaneous phosphorylation at both tyrosine and serine residues by specific signaling events triggered by RhoA effectors.

Searching new targets for anticancer drug design: the families of Ras and Rho GTPases and their effectors

The Ras superfamily of low-molecular-weight GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell growth and development, apoptosis, lipid metabolism, cytoarchitecture, membrane trafficking, and transcriptional regulation. More than 100 genes of this superfamily grouped in six subfamilies have been described so far, pointing to the complexities and specificities of their cellular functions. Dysregulation of members of at least two of these families (the Ras and the Rho families) is involved in the events that lead to the uncontrolled proliferation and invasiveness of human tumors. In recent years, the cloning and characterization of downstream effectors for Ras and Rho proteins have given crucial clues to the specific pathways that lead to aberrant cellular growth and ultimately to tumorigenesis. A direct link between the functions of some of these effectors with the appearance of transformed cells and their ability to proliferate and invade surrounding tissues has been made. Accordingly, drugs that specifically alter their functions display antineoplasic properties, and some of these drugs are already under clinical trials. In this review, we survey the progress made in understanding the underlying molecular connections between carcinogenesis and the specific cellular functions elicited by some of these effectors. We also discuss new drugs with antineoplastic or antimetastatic activity that are targeted to specific effectors for Ras or Rho proteins.

Rho signals to cell growth and apoptosis

Ras and Rho GTPases are among the best studied signaling molecules in molecular biology. Essential cellular processes, such as cell growth, lipid metabolism, cytoarchitecture, membrane trafficking, transcriptional regulation, apoptosis, and response to genotoxic agents, are directly modulated by different members of this superfamily of proteins. Not until recently have we begun to understand the physiological implications of Ras and Rho GTPases, linking them to processes such as embryonic development, tissue remodeling, tumorigenesis and metastasis. In this sense, uncontrolled activation, due to overexpression of different members of the Rho family in a variety of tissues, leads to uncontrolled proliferation and invasiveness of human tumors. In this review, an attempt to briefly integrate recent findings in transcriptional regulation by Rho GTPases in the context of carcinogenesis and metastasis as well as apoptosis is made.

SPECs, small binding proteins for Cdc42

The Rho GTPase, Cdc42, regulates a wide variety of cellular activities including actin polymerization, focal complex assembly, and kinase signaling. We have identified a new family of very small Cdc42-binding proteins, designated SPECs (for Small Protein Effector of Cdc42), that modulates these regulatory activities. The two human members, SPEC1 and SPEC2, encode proteins of 79 and 84 amino acids, respectively. Both contain a conserved N-terminal region and a centrally located CRIB (Cdc42/Rac Interactive Binding) domain. Using a yeast two-hybrid system, we found that both SPECs interact strongly with Cdc42, weakly with Rac1, and not at all with RhoA. Transfection analysis revealed that SPEC1 inhibited Cdc42-induced c-Jun N-terminal kinase (JNK) activation in COS1 cells in a manner that required an intact CRIB domain. Immunofluorescence experiments in NIH-3T3 fibroblasts demonstrated that both SPEC1 and SPEC2 showed a cortical localization and induced the formation of cell surface membrane blebs, which was not dependent on Cdc42 activity. Cotransfection experiments demonstrated that SPEC1 altered Cdc42-induced cell shape changes both in COS1 cells and in NIH-3T3 fibroblasts and that this alteration required an intact CRIB domain. These results suggest that SPECs act as novel scaffold molecules to coordinate and/or mediate Cdc42 signaling activities.

Signaling from G protein-coupled receptors to ERK5/Big MAPK 1 involves Galpha q and Galpha 12/13 families of heterotrimeric G proteins. Evidence for the existence of a novel Ras AND Rho-independent pathway

The regulation of gene expression by cell surface receptors often involves the stimulation of signaling pathways including one or more members of the MAPK superfamily of serine-threonine kinases. Upon their activation in the cytosol, MAPKs can translocate to the nucleus and affect the activity of a variety of transcription factors. Recently, it has been observed that a novel member of the MAPK superfamily, ERK5, can be potently activated by transforming G protein-coupled receptors (GPCRs) and that ERK5 participates in the regulation of c-jun expression through the activation of MEF2 transcription factors. How cell surface receptors, including GPCRs, stimulate ERK5 is still poorly understood. In this study, we have used transiently transfected COS-7 cells to begin delineating the biochemical route linking GPCRs to ERK5. We show that receptors that can couple to the G(q) and G(12/13) families of heterotrimeric G proteins, m1 and thrombin receptors, respectively, but not those coupled to G(i), such as m2 receptors, are able to regulate the activity of ERK5. To investigate which heterotrimeric G proteins signal to ERK5, we used a chimeric system by which Galpha(q)- and Galpha(13)-mediated signaling pathways can be conditionally activated upon ligand stimulation. Using this system, as well as the expression of activated forms of G protein subunits, we show that the Galpha(q) and Galpha(12/13) families of heterotrimeric G proteins, but not the Galpha(i), Galpha(s), and betagamma subunits, are able to regulate ERK5. Furthermore, we provide evidence that the stimulation of ERK5 by GPCRs involves a novel signaling pathway, which is distinct from those regulated by Ras and Rho GTPases.

Signaling to Rho GTPases

Rho GTPases regulate many important processes in all eukaryotic cells, including the organization of the actin cytoskeleton, gene transcription, cell cycle progression, and membrane trafficking. Their activity is regulated by signals originating from different classes of surface receptors including G-protein-coupled receptors, tyrosine kinase receptors, cytokine receptors, and adhesion receptors. Recent work has identified multiple mechanisms by which receptors can signal to Rho GTPases and this will be the major focus of this review. In addition, there is growing evidence for cross-talk within the Rho GTPase family as well as between the Rho and Ras GTPase families. These signaling networks are thought to provide the cooperative and coordinated interactions that are crucial for regulating complex biological processes such as cell migration.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

PAK3 mutation in nonsyndromic X-linked mental retardation

Nonsyndromic X-linked mental retardation (MRX) syndromes are clinically homogeneous but genetically heterogeneous disorders, whose genetic bases are largely unknown. Affected individuals in a multiplex pedigree with MRX (MRX30), previously mapped to Xq22, show a point mutation in the PAK3 (p21-activated kinase) gene, which encodes a serine-threonine kinase. PAK proteins are crucial effectors linking Rho GTPases to cytoskeletal reorganization and to nuclear signalling. The mutation produces premature termination, disrupting kinase function. MRI analysis showed no gross defects in brain development. Immunofluorescence analysis showed that PAK3 protein is highly expressed in postmitotic neurons of the developing and postnatal cerebral cortex and hippocampus. Signal transduction through Rho GTPases and PAK3 may be critical for human cognitive function.

Dual role of Ras and Rho proteins: at the cutting edge of life and death

Small GTP-binding proteins of the Ras superfamily are master controllers of the cell physiology. The range of processes in which these proteins are involved include cell cycle progression, cell division, regulation of cell morphology and motility and intracellular trafficking of molecules and organelles. The study of apoptosis, the physiological form of cell suicide, is progressively linking the functions of small G proteins to the control of the mechanisms that trigger the genetic programmes of cell death. To date, isoforms of the Ras and Rho groups have been related to both promotion and suppression of apoptosis. Further, signalling pathways driven by these proteins have been associated with the function and/or expression of molecules that regulate apoptotic responses. Thus, all available evidence points to a critical role for Ras and Rho proteins as major gatekeepers of the decision between cellular life and death.