PAK5, a new brain-specific kinase, promotes neurite outgrowth in N1E-115 cells

Phylogenetic and structural analysis of the Drosophila melanogaster p21-activated kinase DmPAK3

P21-activated kinases (PAKs) are a family of serine/threonine kinases whose diverse cellular functions in cytoskeletal reorganisation, cell motility, transformation and cell death are regulated both by the binding of the small RhoGTPases RAC and CDC42 and by RhoGTPase independent mechanisms. The genome of Drosophila melanogaster encodes three different PAK proteins: DmPAK1, DmMBT (DmPAK2) and DmPAK3. Although structurally related, DmPAK and DmMBT control different developmental processes and also differ in the regulation of their kinase activity through binding of RAC or CDC42. Here, we report the characterisation of DmPAK3. The phylogenetic analysis provides evidence that DmPAK3 and the related proteins from Drosophila pseudoobscura, Anopheles gambiae and Apis mellifera make up a distinct subgroup within the PAK protein family, which might be confined to insects. The structural differences of this PAK subgroup are also reflected by the selective binding of DmPAK3 to RAC-like RhoGTPases. Our biochemical analysis supports a model in which DmPAK3 can form homodimers where the N-terminal regulatory domain of one DmPAK3 protein can bind to and thereby inhibit the catalytic domain of the other DmPAK3 protein. Binding of activated RAC to the regulatory domain or mutation of the RAC-binding site in DmPAK3 relieves the inhibitory effect on kinase activity. Furthermore, our data indicate a function of DmPAK3 in reorganisation of the actin cytoskeleton.

The genetics of Pak

p21-activated kinases (Paks) are a highly conserved family of enzymes that bind to and are activated by small GTPases of the Cdc42 and Rac families. With the notable exception of plants, nearly all eukaryotes encode one or more Pak genes, indicating an ancient origin and important function for this family of enzymes. Genetic approaches in many different experimental systems, ranging from yeast to mice, are beginning to decipher the different functions of Paks. Although some of these functions are unique to a given organism, certain common themes have emerged, such as the activation of mitogen-activated protein kinase (MAPK) cascades and the regulation of cytoskeletal structure through effects on the actin and tubulin cytoskeletons.

PAK and other Rho-associated kinases--effectors with surprisingly diverse mechanisms of regulation

The Rho GTPases are a family of molecular switches that are critical regulators of signal transduction pathways in eukaryotic cells. They are known principally for their role in regulating the cytoskeleton, and do so by recruiting a variety of downstream effector proteins. Kinases form an important class of Rho effector, and part of the biological complexity brought about by switching on a single GTPase results from downstream phosphorylation cascades. Here we focus on our current understanding of the way in which different Rho-associated serine/threonine kinases, denoted PAK (p21-activated kinase), MLK (mixed-lineage kinase), ROK (Rho-kinase), MRCK (myotonin-related Cdc42-binding kinase), CRIK (citron kinase) and PKN (protein kinase novel), interact with and are regulated by their partner GTPases. All of these kinases have in common an ability to dimerize, and in most cases interact with a variety of other proteins that are important for their function. A diversity of known structures underpin the Rho GTPase-kinase interaction, but only in the case of PAK do we have a good molecular understanding of kinase regulation. The ability of Rho GTPases to co-ordinate spatial and temporal phosphorylation events explains in part their prominent role in eukaryotic cell biology.

The role of the Rho GTPases in neuronal development

Our brain serves as a center for cognitive function and neurons within the brain relay and store information about our surroundings and experiences. Modulation of this complex neuronal circuitry allows us to process that information and respond appropriately. Proper development of neurons is therefore vital to the mental health of an individual, and perturbations in their signaling or morphology are likely to result in cognitive impairment. The development of a neuron requires a series of steps that begins with migration from its birth place and initiation of process outgrowth, and ultimately leads to differentiation and the formation of connections that allow it to communicate with appropriate targets. Over the past several years, it has become clear that the Rho family of GTPases and related molecules play an important role in various aspects of neuronal development, including neurite outgrowth and differentiation, axon pathfinding, and dendritic spine formation and maintenance. Given the importance of these molecules in these processes, it is therefore not surprising that mutations in genes encoding a number of regulators and effectors of the Rho GTPases have been associated with human neurological diseases. This review will focus on the role of the Rho GTPases and their associated signaling molecules throughout neuronal development and discuss how perturbations in Rho GTPase signaling may lead to cognitive disorders.

PAK1 negatively regulates the activity of the Rho exchange factor NET1

Rho family small G-protein activity is controlled by guanine nucleotide exchange factors that stimulate the release of GDP, thus allowing GTP binding. Once activated, Rho proteins control cell signaling through interactions with downstream effector proteins, leading to changes in cytoskeletal organization and gene expression. The ability of Rho family members to modulate the activity of other Rho proteins is also intrinsic to these processes. In this work we show that the Rac/Cdc42hs-regulated protein kinase PAK1 down-regulates the activity of the RhoA-specific guanine nucleotide exchange factor NET1. Specifically, PAK1 phosphorylates NET1 on three sites in vitro: serines 152, 153, and 538. Replacement of serines 152 and 153 with glutamate residues down-regulates the activity of NET1 as an exchange factor in vitro and its ability to stimulate actin stress fiber formation in cells. Using a phospho-specific antibody that recognizes NET1 phosphorylated on serine 152, we show that PAK1 phosphorylates NET1 on this site in cells and that Rac1 stimulates serine 152 phosphorylation in a PAK1-dependent manner. Furthermore, coexpression of constitutively active PAK1 inhibits the ability of NET1 to stimulate actin polymerization only when serines 152 and 153 are present. These data provide a novel mechanism for the control of RhoA activity by Rac1 through the PAK-dependent phosphorylation of NET1 to reduce its activity as a guanine nucleotide exchange factor.

RSK4 and PAK5 Are Novel Candidate Genes in Diabetic Rat Kidney and Brain

The orphan hepatic nuclear factor (HNF) HNF4alpha is of pivotal importance for liver development and hepatocellular differentiation and plays an essential role in a regulatory circuitry to control a wide range of metabolic processes. It also targets genes in other organs, including pancreas, kidney, intestine, and colon; promotes expression of an epithelial phenotype; triggers de novo formation of functional tight junctions; and contributes to epithelial cell polarity. In particular, HNF4alpha dysfunction leads to metabolic disorders, including diabetes. We used the chromatin immunoprecipitation (ChIP) cloning procedure and a bioinformatic approach to search for candidate genes associated with impaired liver, pancreas, and kidney function. We identified two novel targets regulated by HNF4alpha, which participate in the control, at least in part, in cell-cycle regulation and are members of the mitogen-activated kinase pathway. In multiple ChIP assays, ribosomal S6 kinase 4 (RSK4) and p21-activated kinase 5 (PAK5) were confirmed, and in vitro binding of HNF4alpha was evidenced by electrophoretic mobility shift assays (EMSA) using oligonucleotides, which harbor novel binding sites. We also used EMSA to probe for binding sites in promoters of HNF1alpha, apolipoprotein B, alpha1-antitrypsin, and angiotensinogen. We further studied RSK4 and PAK5 kinase expression in streptozotocin-induced diabetic rat kidney and brain and observed significant repression of HNF4alpha, RSK4, and PAK5 as determined by quantitative real-time reverse transcriptase-polymerase chain reaction. RSK4 and PAK5 may provide a molecular rationale for late-stage complications in disease, and further studies are warranted to explore these targets for the treatment of diabetic nephro- and neuropathy, frequently seen in patients with HNF4alpha dysfunction.

Gα13 Signals via p115RhoGEF Cascades Regulating JNK1 and Primitive Endoderm Formation

The heterotrimeric G-protein G(13) mediates the formation of primitive endoderm from mouse P19 embryonal carcinoma cells in response to retinoic acid, signaling to the level of activation of c-Jun N-terminal kinase. The signal linkage map from MEKK1/MEKK4 to MEK1/MKK4 to JNK is obligate in this G alpha(13)-mediated pathway, whereas that between G alpha(13) and MEKKs is not known. The overall pathway to primitive endoderm formation was shown to be inhibited by treatment with Clostridium botulinum C3 exotoxin, a specific inactivator of RhoA family members. Constitutively active G alpha(13) was found to activate RhoA as well as Cdc42 and Rac1 in these cells. Although constitutively active Cdc42, Rac1, and RhoA all can activate JNK1, only the RhoA mutant was able to promote formation of primitive endoderm, mimicking expression of the constitutively activated G alpha(13). Expression of the constitutively active mutant form of p115RhoGEF (guanine nucleotide exchange factor) was found to activate RhoA and JNK1 activities. Expression of the dominant negative p115RhoGEF was able to inhibit activation of both RhoA and JNK1 in response to either retinoic acid or the expression of a constitutively activated mutant of G alpha(13). Expression of the dominant negative mutants of RhoA as well as those of either Cdc42 or Rac1, but not Ras, attenuated G alpha(13)-stimulated as well as retinoic acid-stimulated activation of all three of these small molecular weight GTPases, suggesting complex interrelationships among the three GTPases in this pathway. The formation of primitive endoderm in response to retinoic acid also could be blocked by expression of dominant negative mutants of RhoA, Cdc42, or Rac1. Thus, the signal propagated from G alpha(13) to JNK requires activation of p115RhoGEF cascades, including p115RhoGEF itself, RhoA, Cdc42, and Rac1. In a concerted effort, RhoA in tandem with Cdc42 and Rac1 activates the MEKK1/4, MEK1/MKK4, and JNK cascade, thereby stimulating formation of primitive endoderm.

Identification and characterization of PS-GAP as a novel regulator of caspase-activated PAK-2

p21-activated protein kinase (PAK)-2 is a member of the PAK family of serine/threonine kinases. PAKs are activated by the p21 G-proteins Rac and Cdc42 in response to a variety of extracellular signals and act in pathways controlling cell growth, shape, motility, survival, and death. PAK-2 is unique among the PAK family members because it is also activated through proteolytic cleavage by caspase-3 or similar proteases to generate the constitutively active PAK-2p34 fragment. Activation of full-length PAK-2 by Rac or Cdc42 stimulates cell survival and protects cells from cell death, whereas caspase-activated PAK-2p34 induces a cell death response. Caspase-activated PAK-2p34 is rapidly degraded by the 26 S proteasome, but full-length PAK-2 is not. Stabilization of PAK-2p34 by preventing its polyubiquitination and degradation results in a dramatic stimulation of cell death. Although many proteins have been shown to interact with and regulate full-length PAK-2, little is known about the regulation of caspase-activated PAK-2p34. Here, we identify PS-GAP as a regulator of caspase-activated PAK-2p34. PS-GAP is a GTPase-activating protein for Cdc42 and RhoA that was originally identified by its interaction with the tyrosine kinase PYK-2. PS-GAP interacts specifically with caspase-activated PAK-2p34, but not active or inactive full-length PAK-2, through a region between the GAP and SH3 domains. The interaction with PS-GAP inhibits the protein kinase activity of PAK-2p34 and changes the localization of PAK-2p34 from the nucleus to the perinuclear region. Furthermore, PS-GAP decreases the stimulation of cell death induced by stabilization of PAK-2p34.

GEFT, a Rho family guanine nucleotide exchange factor, regulates neurite outgrowth and dendritic spine formation

The Rho family of small GTPases controls a wide range of cellular processes in eukaryotic cells, such as normal cell growth, proliferation, differentiation, gene regulation, actin cytoskeletal organization, cell fate determination, and neurite outgrowth. The activation of Rho-GTPases requires the exchange of GDP for GTP, a process catalyzed by the Dbl family of guanine nucleotide exchange factors. We demonstrate that a newly identified guanine nucleotide exchange factor, GEFT, is widely expressed in the brain and highly concentrated in the hippocampus, and the Purkinje and granular cells of the cerebellum. Exogenous expression of GEFT promotes dendrite outgrowth in hippocampal neurons, resulting in spines with larger size as compared with control spines. In neuroblastoma cells, GEFT promotes the active GTP-bound state of Rac1, Cdc42, and RhoA and increases neurite outgrowth primarily via Rac1. Furthermore, we demonstrated that PAK1 and PAK5, both downstream effectors of Rac1/Cdc42, are necessary for GEFT-induced neurite outgrowth. AP-1 and NF-kappaB, two transcriptional factors involved in neurite outgrowth and survival, were up-regulated in GEFT-expressing cells. Together, our data suggest that GEFT enhances dendritic spine formation and neurite outgrowth in primary neurons and neuroblastoma cells, respectively, through the activation of Rac/Cdc42-PAK signaling pathways.

The integrin-binding protein Nischarin regulates cell migration by inhibiting PAK

Nischarin, a novel intracellular protein, was originally identified as a binding partner for the alpha5beta1 integrin. Here we show that Nischarin also interacts with members of the PAK family of kinases. The amino terminus of Nischarin preferentially binds to the carboxy-terminal domain of PAK1 when the kinase is in its activated conformation. Nischarin binding to PAK1 is enhanced by active Rac, with the three proteins forming a complex, while expression of the alpha5beta1 integrin also increases the Nischarin/PAK1 association. Interaction with Nischarin strongly inhibits the ability of PAK1 to phosphorylate substrates. This effect on PAK kinase activity closely parallels Nischarin's ability to inhibit cell migration. Conversely, reduction of endogenous levels of Nischarin by RNA interference promotes cell migration. In addition, PAK1 and Nischarin colocalize in membrane ruffles, structures known to be involved in cell motility. Thus, Nischarin may regulate cell migration by forming inhibitory complexes with PAK family kinases.

Activated Ras induces a proangiogenic phenotype in primary endothelial cells

Angiogenic factors alter endothelial cell phenotype to promote the formation of new blood vessels, a process critical for a number of normal and pathological conditions. Ras is required for the induction of the angiogenic phenotype in response to vascular endothelial growth factor (VEGF). However, VEGF generates many signals, several of which are not dependent upon Ras activation. Our current study investigates the sufficiency of Ras activation for driving angiogenic responses. An activated Ras(V12) mutant induces prominent membrane ruffling, branching morphogenesis on three-dimensional collagen, DNA synthesis, and cell migration in primary endothelial cells. An upregulation of PI3K/AKT, Erk, and Jnk signaling pathways accompany these phenotypic changes. The inhibition of Erk blocked cell proliferation, but only partially attenuated migration. Blocking PI3K had no effect on DNA synthesis, but caused a modest reduction in cell migration. Lastly, Jnk played a significant role in both the proliferation and migration response. These effects of Ras(V12) are not the result of increased autocrine secretion of VEGF. These data suggest that the acquisition of activating Ras mutations can lead to a proangiogenic conversion in the phenotype of primary endothelial cells. Furthermore, these data raise the possibility that chronic Ras activation in endothelial cells may be sufficient to promote angiogenesis and the development of vascular anomalies.

Membrane/cytoskeleton communication

Accumulations of particular lipids in ordered arrays in the membrane (termed microdomains or lipid rafts) can attract proteins with specific targeting domains. Both the lipid and protein components of rafts communicate with the cytoskeleton directly thereby regulating cellular responses. Recent evidence implicating phosphoinositide 1,5 bisphosphate (PIP2) in cytoskeletal regulation shows that agonist sensitive regulation of PIP2 homoeostasis occurs specifically rafts, which appear to provide a major structural substrate for its function. The crucial role of PIP2 in generating cytoskeletal responses is chiefly achieved by regulating proteins that control actin dynamics directly. Many of these regulatory proteins are also specifically enriched in rafts either directly (by insertion into the lipid bilayer via acetylation motifs), or indirectly via interactions with other raft components. The notion that rafts form membrane platforms or modules that mediate signaling responses has been most extensively demonstrated in the immune synapse (IS) of T cells, a complex assemblage of rafts that integrates signaling cascades originating from the simultaneous activation of a wide variety of receptors. The IS is essential for both the amplification and maintenance of T-cell activation, and its assembly at the antigen presenting site depends on the interactions between rafts and the actin cytoskeleton that regulates coalescence of smaller raft components into the larger IS complex. Likewise the neuron, which represents the most highly polarized cell in the body, utilizes the regulation of actin dynamics in response to a plethora of extracellular signals to control axon pathfinding thereby sculpting nervous system cytoarchitecture with utmost precision. It is now becoming clear, that as in the T-cell, lipid rafts in the growing axon can assemble into highly specific, yet malleable and dynamic, signaling modules that regulate actin dynamics in a fashion that is also PIP2-dependent and that utilizes both familiar and novel regulatory mechanisms. It seems clear that raft mediated cytoskeletal regulation represents a highly conserved mechanism to integrate cellular responses to diverse signals.

Biology of the p21-activated kinases

The p21-activated kinases (PAKs) 1-3 are serine/threonine protein kinases whose activity is stimulated by the binding of active Rac and Cdc42 GTPases. Our understanding of the regulation and biology of these important signaling proteins has increased tremendously since their discovery in the mid-1990s. PAKs 1-3 are activated by a variety of GTPase-dependent and -independent mechanisms. This complexity reflects the contributions of PAK function in many cellular signaling pathways and the need to carefully control PAK action in a highly localized manner. PAKs serve as important regulators of cytoskeletal dynamics and cell motility, transcription through MAP kinase cascades, death and survival signaling, and cell-cycle progression. Consequently, PAKs have also been implicated in a number of pathological conditions and in cell transformation. We propose here a key role for PAK action in coordinating the dynamics of the actin and microtubule cytoskeletons during directional motility of cells, as well as in other functions requiring cytoskeletal polarization.

Death receptor-induced activation of initiator caspase 8 is antagonized by serine/threonine kinase PAK4

Normal cell growth requires a precisely controlled balance between cell death and survival. This involves activation of different types of intracellular signaling cascades within the cell. While some types of signaling proteins regulate apoptosis, or programmed cell death, other proteins within the cell can promote survival. The serine/threonine kinase PAK4 can protect cells from apoptosis in response to several different types of stimuli. As is the case for other members of the p21-activated kinase (PAK) family, one way that PAK4 may promote cell survival is by phosphorylating and thereby inhibiting the proapoptotic protein Bad. This leads in turn to the inhibition of effector caspases such as caspase 3. Here we show that in response to cytokines which activate death domain-containing receptors, such as the tumor necrosis factor and Fas receptors, PAK4 can inhibit the death signal by a different mechanism. Under these conditions, PAK4 inhibits apoptosis early in the caspase cascade, antagonizing the activation of initiator caspase 8. This inhibition, which does not require PAK4's kinase activity, may involve inhibition of caspase 8 recruitment to the death domain receptors. This role in regulating initiator caspases is an entirely novel role for the PAK proteins and suggests a new mechanism by which these proteins promote cell survival.

The mechanism of p21-activated kinase 2 autoactivation

The p21-activated kinases (PAKs) play an important role in diverse cellular processes. PAK2 is activated by autophosphorylation upon binding of small G proteins such as Cdc42 and Rac in the GTP-bound state. However, the mechanism of PAK2 autophosphorylation in vitro is unclear. In the present study, the kinetic theory of the substrate reaction during modification of enzyme activity has been applied to a study of the autoactivation of PAK2. On the basis of the kinetic equation of the substrate reaction during the autophosphorylation of PAK2, the activation rate constants for the free enzyme and enzyme-substrate complex have been determined. The results indicate that 1) in the presence of Cdc42, PAK2 autophosphorylation is a bipartite mechanism, with the regulatory domain autophosphorylated at multiple residues, whereas activation coincides with autophosphorylation of the catalytic domain at Thr-402; 2) the autophosphorylation reactions in regulatory domain are either a nonlimiting step or not required for activation of enzyme; 3) the autophosphorylation at site Thr-402 on the catalytic domain occurs by an intermolecular mechanism and is required for phosphorylation of exogenous substrates examined; 4) binding of the exogenous protein/peptide substrates at the active site of PAK2 has little or no effect on the autoactivation of PAK2, suggesting that multiple regions of PAK2 are involved in the enzyme-substrate recognition. The present method also provides a novel approach for studying autophosphorylation reactions. Since the experimental conditions used resemble more closely the in vivo situation where the substrate is constantly being turned over while the enzyme is being modified, this new method would be particularly useful when the regulatory mechanisms of the reversible phosphorylation reaction toward certain enzymes are being assessed.

PAK4 kinase is essential for embryonic viability and for proper neuronal development

The serine/threonine kinase PAK4 is a target for the Rho GTPase Cdc42 and has been shown to regulate cell morphology and cytoskeletal organization in mammalian cells. To examine the physiological and developmental functions of PAK4, we have disrupted the PAK4 gene in mice. The absence of PAK4 led to lethality by embryonic day 11.5, a result most likely due to a defect in the fetal heart. Striking abnormalities were also evident in the nervous systems of PAK4-deficient embryos. These embryos had dramatic defects in neuronal development and axonal outgrowth. In particular, spinal cord motor neurons and interneurons failed to differentiate and migrate to their proper positions. This is probably related to the role for PAK4 in the regulation of cytoskeletal organization and cell and/or extracellular matrix adhesion. PAK4-null embryos also had defects in proper folding of the caudal portion of the neural tube, suggesting an important role for PAK4 in neural tube development.

Targeted disruption of the gene for the PAK5 kinase in mice

PAK5 is a member of the group B family of PAK serine/threonine kinases and is an effector for the Rho GTPase Cdc42. PAK5 is highly expressed in the brain and is expressed at lower levels in several other tissues. In cell lines, PAK5 has been shown to play a role in filopodia formation and neurite outgrowth. To examine the biological function of PAK5, we deleted the PAK5 gene in mice. The phenotypes of the PAK5-null mice are completely different from those of mice null for PAK4, another member of the group B PAK family. Unlike PAK4-null mice, which are embryonic lethal, PAK5-null mice develop normally and are fertile. The nervous system appears normal in the absence of PAK5, as do other tissues in which PAK5 is normally expressed. Our results suggest functional redundancy between PAK5 and other Rho GTPase targets.

Identification of an autoinhibitory domain of p21-activated protein kinase 5

The p21-activated protein kinases (Paks) are serine/threonine protein kinases activated by binding to Rho family small GTPases, Rac and Cdc42. Recently, Pak family members have been subdivided into two groups, I and II. Group II Paks, including Pak4, Pak5, and Pak6, does not contain the highly conserved autoinhibitory domain that is found in the group I Paks members, i.e. Pak1, Pak2, and Pak3. In the present study, we have purified the glutathione S-transferase fusion form of Pak5 and shown for the first time that Pak5 autophosphorylation can be activated by GTP bound form of Cdc42. Mutation of histidine residues 19 and 22 to leucine on the p21-binding domain of Pak5 completely abolished the binding of Cdc42 and the Cdc42-mediated autophosphorylation. On the other hand, mutation of tyrosine 40 to cysteine of Cdc42 did not knockout the binding of Pak5. Analysis of C-terminal deletion mutants has identified an autoinhibitory fragment of Pak5 that is absent from other group II Pak family members. Taken together, these results suggest that Pak5, like Pak1, contains an autoinhibitory domain and its activity is regulated by Cdc42.

p21-Activated kinase 5 (Pak5) localizes to mitochondria and inhibits apoptosis by phosphorylating BAD

Pak5 is the most recently identified and least understood member of the p21-activated kinase (Pak) family. This kinase is known to promote neurite outgrowth in vitro, but its localization, substrates, and effects on cell survival have not been reported. We show here that Pak5 has unique properties that distinguish it from all other members of the Pak family. First, Pak5, unlike Pak1, cannot complement an STE20 mutation in Saccharomyces cerevisiae. Second, Pak5 binds to the GTPases Cdc42 and Rac, but these GTPases do not regulate Pak5 kinase activity, which is constitutive and stronger than any other Pak. Third, Pak5 prevents apoptosis induced by camptothecin and C2-ceramide by phosphorylating BAD on Ser-112 in a protein kinase A-independent manner and prevents the localization of BAD to mitochondria, thereby inhibiting the apoptotic cascade that leads to apoptosis. Finally, we show that Pak5 itself is constitutively localized to mitochondria, and that this localization is independent of kinase activity or Cdc42 binding. These features make Pak5 unique among the Pak family and suggest that it plays an important role in apoptosis through BAD phosphorylation.

Inhibition of contraction and myosin light chain phosphorylation in guinea-pig smooth muscle by p21-activated kinase 1

The p21-activated protein kinases (PAKs) have been implicated in cytoskeletal rearrangements and modulation of non-muscle contractility. Little, however, is known about the role of the PAK family members in smooth muscle contraction. Therefore, we investigated the effect of the predominant isoform in vascular smooth muscle cells, PAK1, on contraction and phosphorylation of the regulatory light chains of myosin (r-MLC) in Triton-skinned guinea-pig smooth muscle. We also investigated which of the three putative substrates at the contractile apparatus - MLCK, caldesmon or r-MLC - is phosphorylated by PAK1 in smooth muscle tissue. Incubation of Triton-skinned carotid artery and taenia coli from guinea-pig with an active mutant of PAK1 in relaxing solution for 30-60 min resulted in inhibition of submaximal force by about 50 %. The mechanism of inhibition of force was studied in the Triton-skinned taenia coli. In this preparation, inhibition of force was associated with a respective inhibition of r-MLC phosphorylation. In the presence of the myosin phosphatase inhibitor, microcystin-LR (10 microM), the rate of contraction and r-MLC phosphorylation elicited at pCa 6.79 were both decreased. Because under these conditions the rate of r-MLC phosphorylation is solely dependent on MLCK activity, this result suggests that the inhibitory effect of PAK1 on steady-state force and r-MLC phosphorylation is due to inhibition of MLCK. In line with this, we found that MLCK was significantly phosphorylated by PAK1 while there was very little 32P incorporation into caldesmon. PAK1 phosphorylated isolated r-MLC but not those in the skinned fibres or in purified smooth muscle myosin II. In conclusion, these results suggest that PAK1 attenuates contraction of skinned smooth muscle by phosphorylating and inhibiting MLCK.

The C-terminal kinase domain of the p34cdc2-related PITSLRE protein kinase (p110C) associates with p21-activated kinase 1 and inhibits its activity during anoikis

The PITSLRE protein kinases are parts of the large family of p34cdc2-related kinases. During apoptosis induced by some stimuli, specific PITSLRE isoforms are cleaved by caspase to produce a protein that contains the C-terminal kinase domain of the PITSLRE proteins (p110C). The p110C induces apoptosis when it is ectopically expressed in Chinese hamster ovary cells. In our study, similar induction of this p110C was observed during anoikis in NIH3T3 cells. To investigate the molecular mechanism of apoptosis mediated by p110C, we used the yeast two-hybrid system to screen a human fetal liver cDNA library and identified p21-activated kinase 1 (PAK1) as an interacting partner of p110C. The association of p110C with PAK1 was further confirmed by in vitro binding assay, in vivo coimmunoprecipitation, and confocal microscope analysis. The interaction of p110C with PAK1 occurred within the residues 210-332 of PAK1. Neither association between p58PITSLRE or p110PITSLRE and PAK1 nor association between p110C and PAK2 or PAK3 was observed. Anoikis was increased and PAK1 activity was inhibited when NIH3T3 cells were transfected with p110C. Furthermore, the binding of p110C with PAK1 and inhibition of PAK1 activity were also observed during anoikis. Taken together, these data suggested that PAK1 might participate in the apoptotic pathway mediated by p110C.

[Rho proteins: their function in neurons]

Neurons possess a polarized morphology. In general, each neuron has several dendrites but only one axon. Such morphology is the basis for directionalized rapid signaling, information flowing from the short dendrites to the long axon. The mechanisms involved in the establishment of the neuronal polarity remain largely unknown. However, recently, members of Rho family proteins have been implicated in the regulation of neuronal morphology especially development of neuronal polarity, axon outgrowth and guidance, dendritic tree elaboration and synapse formation. Moreover, the Rho GTPases have been reported to be directly or indirectly involved in some neurological conditions such as X-linked mental retardation as well as Alzheimer's and Parkinson's diseases. These findings demonstrate the importance of Rho GTPases in the development, maintenance and function of the nervous system.

Mbt, a Drosophila PAK protein, combines with Cdc42 to regulate photoreceptor cell morphogenesis

The Drosophila gene mushroom bodies tiny (mbt) encodes a putative p21-activated kinase (PAK), a family of proteins that has been implicated in a multitude of cellular processes including regulation of the cytoskeleton, cell polarisation, control of MAPK signalling cascades and apoptosis. The mutant phenotype of mbt is characterised by fewer neurones in the brain and the eye, indicating a role of the protein in cell proliferation, differentiation or survival. We show that mutations in mbt interfere with photoreceptor cell morphogenesis. Mbt specifically localises at adherens junctions of the developing photoreceptor cells. A structure-function analysis of the Mbt protein in vitro and in vivo revealed that the Mbt kinase domain and the GTPase binding domain, which specifically interacts with GTP-loaded Cdc42, are important for Mbt function. Besides regulation of kinase activity, another important function of Cdc42 is to recruit Mbt to adherens junctions. We propose a role for Mbt as a downstream effector of Cdc42 in photoreceptor cell morphogenesis.

The GDP exchange factor AND-34 is expressed in B cells, associates with HEF1, and activates Cdc42

AND-34, a novel GDP exchange factor, is expressed constitutively at significant levels in murine splenic B cells, but not in murine splenic T cells or thymocytes. In B cell lines, anti-IgM treatment up-regulates AND-34 transcript levels. B cell AND-34 associates with both the docking molecules p130Cas and HEF1. AND-34 binds by its GDP exchange factor domain to the C terminus of HEF1, a region of HEF1 previously implicated in apoptotic, adhesion, and cell cycle-regulated signaling. Overexpression of AND-34 in murine B cell lines activates the Rho family GTPase Cdc42, but not Rac, Rho, RalA, or Rap1. Consistent with this, a subpopulation of AND-34 overexpressing B cells have long filamentous actin-containing cellular extensions. AND-34 overexpression augments both autophosphorylation and kinase activity of the Cdc42/Rac-responsive serine/threonine kinase PAK1. As previously reported for lymphoid cells transfected with constitutively active Cdc42, AND-34 overexpression inhibits SDF-1alpha-induced B cell polarization. These studies suggest that p130Cas and HEF1-associated AND-34 may regulate B cell adhesion and motility through a Cdc42-mediated signaling pathway.

Phosphorylation of p85 beta PIX, a Rac/Cdc42-specific guanine nucleotide exchange factor, via the Ras/ERK/PAK2 pathway is required for basic fibroblast growth factor-induced neurite outgrowth

Guanine nucleotide exchange factors (GEFs) have been implicated in growth factor-induced neuronal differentiation through the activation of small GTPases. Although phosphorylation of these GEFs is considered an activation mechanism, little is known about the upstream of PAK-interacting exchange factor (PIX), a member of the Dbl family of GEFs. We report here that phosphorylation of p85 betaPIX/Cool/p85SPR is mediated via the Ras/ERK/PAK2 pathway. To understand the role of p85 betaPIX in basic fibroblast growth factor (bFGF)-induced neurite outgrowth, we established PC12 cell lines that overexpress the fibroblast growth factor receptor-1 in a tetracycline-inducible manner. Treatment with bFGF induces the phosphorylation of p85 betaPIX, as determined by metabolic labeling and mobility shift upon gel electrophoresis. Interestingly, phosphorylation of p85 betaPIX is inhibited by PD98059, a specific MEK inhibitor, suggesting the involvement of the ERK cascade. PAK2, a major PAK isoform in PC12 cells as well as a binding partner of p85 betaPIX, also functions upstream of p85 betaPIX phosphorylation. Surprisingly, PAK2 directly binds to ERK, and its activation is dependent on ERK. p85 betaPIX specifically localizes to the lamellipodia at neuronal growth cones in response to bFGF. A mutant form of p85 betaPIX (S525A/T526A), in which the major phosphorylation sites are replaced by alanine, shows significant defect in targeting. Moreover, expression of the mutant p85 betaPIX efficiently blocks PC12 cell neurite outgrowth. Our study defines a novel signaling pathway for bFGF-induced neurite outgrowth that involves activation of the PAK2-p85 betaPIX complex via the ERK cascade and subsequent translocation of this complex.

Signaling mechanisms that regulate actin-based motility processes in the nervous system

Actin-based motility is critical for nervous system development. Both the migration of neurons and the extension of neurites require organized actin polymerization to push the cell membrane forward. Numerous extracellular stimulants of motility and axon guidance cues regulate actin-based motility through the rho GTPases (rho, rac, and cdc42). The rho GTPases reorganize the actin cytoskeleton, leading to stress fiber, filopodium, or lamellipodium formation. The activity of the rho GTPases is regulated by a variety of proteins that either stimulate GTP uptake (activation) or hydrolysis (inactivation). These proteins potentially link extracellular signals to the activation state of rho GTPases. Effectors downstream of the rho GTPases that directly influence actin polymerization have been identified and are involved in neurite development. The Arp2/3 complex nucleates the formation of new actin branches that extend the membrane forward. Ena/VASP proteins can cause the formation of longer actin filaments, characteristic of growth cone actin morphology, by preventing the capping of barbed ends. Actin-depolymerizing factor (ADF)/cofilin depolymerizes and severs actin branches in older parts of the actin meshwork, freeing monomers to be re-incorporated into actively growing filaments. The signaling mechanisms by which extracellular cues that guide axons to their targets lead to direct effects on actin filament dynamics are becoming better understood.

Emerging functions of p21-activated kinases in human cancer cells

The p21 activated kinases (Paks), an evolutionarily conserved family of serine/threonine kinases, are important for a variety of cellular functions including cell morphogenesis, motility, survival, mitosis, and angiogenesis. Paks are widely expressed in numerous tissues and are activated by growth factors and extracellular signals through GTPase-dependent and -independent mechanisms. Overexpression of Paks in epithelial cancer cells has been shown to increase migration potential, increase anchorage independent growth, and cause abnormalities in mitosis. Dysregulation of Paks has been reported in several human tumors and neurodegenerative diseases. A growing list of novel Pak interacting proteins has opened up exciting avenues of investigation by which to understand the functions of Paks in tumorigenesis. In this review, we will summarize the current knowledge of the Paks family with respect to emerging cellular functions and possible contributions to cancer.

Protein kinase MARK/PAR-1 is required for neurite outgrowth and establishment of neuronal polarity

Protein kinases of the microtubule affinity-regulating kinase (MARK) family were originally discovered because of their ability to phosphorylate certain sites in tau protein (KXGS motifs in the repeat domain). This type of phosphorylation is enhanced in abnormal tau from Alzheimer brain tissue and causes the detachment of tau from microtubules. MARK-related kinases (PAR-1 and KIN1) occur in various organisms and are involved in establishing and maintaining cell polarity. Herein, we report the ability of MARK2 to affect the differentiation and outgrowth of cell processes from neuroblastoma and other cell models. MARK2 phosphorylates tau protein at the KXGS motifs; this results in the detachment of tau from microtubules and their destabilization. The formation of neurites in N2a cells is blocked if MARK2 is inactivated, either by transfecting a dominant negative mutant, or by MARK2 inhibitors such as hymenialdisine. Alternatively, neurites are blocked if the target KXGS motifs on tau are rendered nonphosphorylatable by point mutations. The results suggest that MARK2 contributes to the plasticity of microtubules needed for neuronal polarity and the growth of neurites.

PAK4 is activated via PI3K in HGF-stimulated epithelial cells

The p21-activated kinases (PAKs) are divided into two subgroups based on sequence homology. Group 1 PAKs (PAK1-3) are involved in cell migration, and are activated by pro-migratory stimuli and by Cdc42/Rac GTPases. In contrast,little is known about the regulation of the recently identified group II PAKs(PAK4-6). Here we report that PAK4 is activated by HGF, a migratory stimulus for epithelial cells. In unstimulated MDCK cells, activated PAK4 induces a decrease in stress fibres, and when cells are stimulated with HGF, it induces a loss of focal complexes and cell rounding. This response is dependent on PAK4 kinase activity but does not require Cdc42 interaction. Activated PAK4 localises to the cell periphery but not specifically in lamellipodia, and HGF induces localisation of wild-type PAK4 to the cell periphery. LY294002, a phosphoinositide 3-kinase (PI3K) inhibitor, inhibits HGF-induced PAK4 kinase activation, relocalisation, and cell rounding. However, the isolated C-terminal kinase domain of PAK4 can induce cell rounding in the presence of LY294002, suggesting that the N-terminal region acts as a negative regulator of PAK4 activity. These results indicate that HGF stimulates PAK4 through PI3K, and that PAK4 could contribute to HGF-induced changes in actin organisation and cell-substratum adhesion.

Nerve growth factor signals through TrkA, phosphatidylinositol 3-kinase, and Rac1 to inactivate RhoA during the initiation of neuronal differentiation of PC12 cells

In PC12 rat pheochromocytoma cells, nerve growth factor (NGF)-induced neuronal differentiation is blocked by constitutively active dominant mutants of RhoA but augmented by negative ones, suggesting a not yet elucidated inhibitory signaling link between NGF receptors and RhoA. Here we show that NGF treatment rapidly translocates RhoA from the plasma membrane to the cytosol and simultaneously decreases RhoA affinity to its target Rho-associated kinase (ROK), a key mediator of neurite outgrowth. This effect was transient, because after 2 days of NGF treatment, RhoA relocated from the cytosol to the plasma membrane, and its GTP loading returned to a level found in undifferentiated cells. Inhibition of RhoA is mediated by activation of the TrkA receptor, because NGF failed to induce RhoA translocation and inhibition of ROK binding in nnr5 cells that lack TrkA, whereas the inhibition was reconstituted in receptor add-back B5 cells. In MM17-26 cells, which due to expression of dominant negative Ras do not differentiate, NGF-stimulated transient RhoA inhibition was unaffected. The inhibitory pathway from TrkA to RhoA involves phosphatidylinositol-3-kinase (PI3K), because the inhibitors LY294002 or wortmannin prevented NGF-induced RhoA translocation and increased RhoA association with ROK. Furthermore, inhibition of PI3K significantly reduced NGF- mediated Rac1 activation, whereas dominant negative Rac1 abolished the inhibitory signaling to RhoA. Taken together, these data indicate that NGF-mediated activation of TrkA receptor stimulates PI3K, which in turn increases Rac1 activity to induce transient RhoA inactivation during the initial phase of neurite outgrowth.

Neuritogenesis: polarization of constitutive exocytosis by effectors of Rho-family GTPases?

The sprouting of neurites from a neuron represents a highly specialized form of cellular morphogenesis that must involve coordinated changes in two major cellular processes at two membrane locations: reorganization of the cytoskeleton and redirection of membrane traffic from the trans-Golgi network to the plasma membrane of the growth tip. How exactly are these two processes linked and how is spatial and temporal coordination achieved at the first instance of neurite sprouting? Recent advances may have already revealed some, if not most of the pieces in the puzzle. We discuss below, with some extrapolations, of what has recently come to light, and what more is needed to construct a coherent picture.

Cloning and characterization of PAK5, a novel member of mammalian p21-activated kinase-II subfamily that is predominantly expressed in brain

The p21-activated kinase (PAK) family of protein kinases has recently attracted considerable attention as an effector of Rho family of small G proteins and as an upstream regulator of MAPK signalling pathways during cellular events such as re-arrangement of the cytoskeleton and apoptosis. We have cloned a novel human PAK family kinase that has been designated as PAK5. PAK5 contains a CDC42/Rac1 interactive binding (CRIB) motif at the N-terminus and a Ste20-like kinase domain at the C-terminus. PAK5 is structurally most related to PAK4 and PAK6 to make up the PAK-II subfamily. We have shown that PAK5 preferentially binds to CDC42 in the presence of GTP and that CRIB motif is essential for this interaction. PAK5 is a functional protein kinase but unlike PAK-I family kinases (PAK1, 2, and 3), the kinase activity of PAK5 does not seem to require the binding of CDC42. Overexpression of PAK5 activates the JNK kinase pathway but not p38 or ERK pathways. PAK5 transcript is predominantly expressed in brain as revealed by Northern blot and in situ hybridization. The expression pattern of PAK5 is distinct from that of PAK4 and PAK6, suggesting a functional division among PAK-II subfamily kinases based on differential tissue distribution.