Interaction between PAK and nck: a template for Nck targets and role of PAK autophosphorylation

NCK1 Modulates Neuronal Actin Dynamics and Promotes Dendritic Spine, Synapse, and Memory Formation

Memory formation and maintenance is a dynamic process involving the modulation of the actin cytoskeleton at synapses. Understanding the signaling pathways that contribute to actin modulation is important for our understanding of synapse formation and function, as well as learning and memory. Here, we focused on the importance of the actin regulator, noncatalytic region of tyrosine kinase adaptor protein 1 (NCK1), in hippocampal dependent behaviors and development. We report that male mice lacking NCK1 have impairments in both short-term and working memory, as well as spatial learning. Additionally, we report sex differences in memory impairment showing that female mice deficient in NCK1 fail at reversal learning in a spatial learning task. We find that NCK1 is expressed in postmitotic neurons but is dispensable for neuronal proliferation and migration in the developing hippocampus. Morphologically, NCK1 is not necessary for overall neuronal dendrite development. However, neurons lacking NCK1 have lower dendritic spine and synapse densities in vitro and in vivo EM analysis reveal increased postsynaptic density (PSD) thickness in the hippocampal CA1 region of NCK1-deficient mice. Mechanistically, we find the turnover of actin-filaments in dendritic spines is accelerated in neurons that lack NCK1. Together, these findings suggest that NCK1 contributes to hippocampal-dependent memory by stabilizing actin dynamics and dendritic spine formation.SIGNIFICANCE STATEMENT Understanding the molecular signaling pathways that contribute to memory formation, maintenance, and elimination will lead to a better understanding of the genetic influences on cognition and cognitive disorders and will direct future therapeutics. Here, we report that the noncatalytic region of tyrosine kinase adaptor protein 1 (NCK1) adaptor protein modulates actin-filament turnover in hippocampal dendritic spines. Mice lacking NCK1 show sex-dependent deficits in hippocampal memory formation tasks, have altered postsynaptic densities, and reduced synaptic density. Together, our work implicates NCK1 in the regulation of actin cytoskeleton dynamics and normal synapse development which is essential for memory formation.

The molecular basis of p21-activated kinase-associated neurodevelopmental disorders: From genotype to phenotype

Although the identification of numerous genes involved in neurodevelopmental disorders (NDDs) has reshaped our understanding of their etiology, there are still major obstacles in the way of developing therapeutic solutions for intellectual disability (ID) and other NDDs. These include extensive clinical and genetic heterogeneity, rarity of recurrent pathogenic variants, and comorbidity with other psychiatric traits. Moreover, a large intragenic mutational landscape is at play in some NDDs, leading to a broad range of clinical symptoms. Such diversity of symptoms is due to the different effects DNA variations have on protein functions and their impacts on downstream biological processes. The type of functional alterations, such as loss or gain of function, and interference with signaling pathways, has yet to be correlated with clinical symptoms for most genes. This review aims at discussing our current understanding of how the molecular changes of group I p21-activated kinases (PAK1, 2 and 3), which are essential actors of brain development and function; contribute to a broad clinical spectrum of NDDs. Identifying differences in PAK structure, regulation and spatio-temporal expression may help understanding the specific functions of each group I PAK. Deciphering how each variation type affects these parameters will help uncover the mechanisms underlying mutation pathogenicity. This is a prerequisite for the development of personalized therapeutic approaches.

Minor Kinases with Major Roles in Cytokinesis Regulation

Cytokinesis, the conclusive act of cell division, allows cytoplasmic organelles and chromosomes to be faithfully partitioned between two daughter cells. In animal organisms, its accurate regulation is a fundamental task for normal development and for preventing aneuploidy. Cytokinesis failures produce genetically unstable tetraploid cells and ultimately result in chromosome instability, a hallmark of cancer cells. In animal cells, the assembly and constriction of an actomyosin ring drive cleavage furrow ingression, resulting in the formation of a cytoplasmic intercellular bridge, which is severed during abscission, the final event of cytokinesis. Kinase-mediated phosphorylation is a crucial process to orchestrate the spatio-temporal regulation of the different stages of cytokinesis. Several kinases have been described in the literature, such as cyclin-dependent kinase, polo-like kinase 1, and Aurora B, regulating both furrow ingression and/or abscission. However, others exist, with well-established roles in cell-cycle progression but whose specific role in cytokinesis has been poorly investigated, leading to considering these kinases as "minor" actors in this process. Yet, they deserve additional attention, as they might disclose unexpected routes of cell division regulation. Here, we summarize the role of multifunctional kinases in cytokinesis with a special focus on those with a still scarcely defined function during cell cleavage. Moreover, we discuss their implication in cancer.

Group I PAKs in myelin formation and repair of the central nervous system: what, when, and how

p21-activated kinases (PAKs) are a family of cell division control protein 42/ras-related C3 botulinum toxin substrate 1 (Cdc42/Rac1)-activated serine/threonine kinases. Group I PAKs (PAK1-3) have distinct activation mechanisms from group II PAKs (PAK4-6) and are the focus of this review. In transformed cancer cells, PAKs regulate a variety of cellular processes and molecular pathways which are also important for myelin formation and repair in the central nervous system (CNS). De novo mutations in group I PAKs are frequently seen in children with neurodevelopmental defects and white matter anomalies. Group I PAKs regulate virtually every aspect of neuronal development and function. Yet their functions in CNS myelination and remyelination remain incompletely defined. Herein, we highlight the current understanding of PAKs in regulating cellular and molecular pathways and discuss the status of PAK-regulated pathways in oligodendrocyte development. We point out outstanding questions and future directions in the research field of group I PAKs and oligodendrocyte development.

Nck adaptors at a glance

The non-catalytic region of tyrosine kinase (Nck) family of adaptors, consisting of Nck1 and Nck2, contributes to selectivity and specificity in the flow of cellular information by recruiting components of signaling networks. Known to play key roles in cytoskeletal remodeling, Nck adaptors modulate host cell-pathogen interactions, immune cell receptor activation, cell adhesion and motility, and intercellular junctions in kidney podocytes. Genetic inactivation of both members of the Nck family results in embryonic lethality; however, viability of mice lacking either one of these adaptors suggests partial functional redundancy. In this Cell Science at a Glance and the accompanying poster, we highlight the molecular organization and functions of the Nck family, focusing on key interactions and pathways, regulation of cellular processes, development, homeostasis and pathogenesis, as well as emerging and non-redundant functions of Nck1 compared to those of Nck2. This article thus aims to provide a timely perspective on the biology of Nck adaptors and their potential as therapeutic targets.

The Role of p21-Activated Kinases in Cancer and Beyond: Where Are We Heading?

The p21-activated kinases (PAKs), downstream effectors of Ras-related Rho GTPase Cdc42 and Rac, are serine/threonine kinases. Biologically, PAKs participate in various cellular processes, including growth, apoptosis, mitosis, immune response, motility, inflammation, and gene expression, making PAKs the nexus of several pathogenic and oncogenic signaling pathways. PAKs were proved to play critical roles in human diseases, including cancer, infectious diseases, neurological disorders, diabetes, pancreatic acinar diseases, and cardiac disorders. In this review, we systematically discuss the structure, function, alteration, and molecular mechanisms of PAKs that are involved in the pathogenic and oncogenic effects, as well as PAK inhibitors, which may be developed and deployed in cancer therapy, anti-viral infection, and other diseases. Furthermore, we highlight the critical questions of PAKs in future research, which provide an opportunity to offer input and guidance on new directions for PAKs in pathogenic, oncogenic, and drug discovery research.

Rac1 and EGFR cooperate to activate Pak in response to nutrient stress

The interplay between nutrient scarcity and signal transduction circuits is an important aspect of tumorigenesis that regulates many aspects of cancer progression. Glutamine is a critical nutrient for cancer cells, as it contributes to biosynthetic reactions that sustain cancer proliferation and growth. In tumors, because nutrient utilization can often outpace supply, glutamine levels can become limiting and oncogene-mediated metabolic rewiring triggers signaling cascades that support nutrient stress survival. Recently, we identified that in pancreatic ductal adenocarcinoma (PDAC) cells, glutamine depletion can trigger p21-activated kinase (Pak) activation through EGFR signaling as a means to circumvent metabolic stress. Here, we elucidate that glutamine starvation, as well EGF stimulation, can enhance the presence of many different Pak phosphoforms, and that this activation only occurs in a subset of PDAC cells. Pak is a well-established effector of Rac1, and while Rac1 mutant variants can modulate the metabolic induction of Pak phosphorylation, Rac1 inhibition only partially attenuates Pak activation upon glutamine depletion. We decipher that in order to efficiently suppress metabolic activation of Pak, both EGFR and Rac1 signaling must be inhibited. These results provide a mechanistic understanding of how glutamine-regulated signal transduction can control Pak activation in PDAC cells.

The Disordered Cellular Multi-Tasker WIP and Its Protein-Protein Interactions: A Structural View

WASp-interacting protein (WIP), a regulator of actin cytoskeleton assembly and remodeling, is a cellular multi-tasker and a key member of a network of protein-protein interactions, with significant impact on health and disease. Here, we attempt to complement the well-established understanding of WIP function from cell biology studies, summarized in several reviews, with a structural description of WIP interactions, highlighting works that present a molecular view of WIP's protein-protein interactions. This provides a deeper understanding of the mechanisms by which WIP mediates its biological functions. The fully disordered WIP also serves as an intriguing example of how intrinsically disordered proteins (IDPs) exert their function. WIP consists of consecutive small functional domains and motifs that interact with a host of cellular partners, with a striking preponderance of proline-rich motif capable of interactions with several well-recognized binding partners; indeed, over 30% of the WIP primary structure are proline residues. We focus on the binding motifs and binding interfaces of three important WIP segments, the actin-binding N-terminal domain, the central domain that binds SH3 domains of various interaction partners, and the WASp-binding C-terminal domain. Beyond the obvious importance of a more fundamental understanding of the biology of this central cellular player, this approach carries an immediate and highly beneficial effect on drug-design efforts targeting WIP and its binding partners. These factors make the value of such structural studies, challenging as they are, readily apparent.

Critical residues involved in tau binding to fyn: implications for tau phosphorylation in Alzheimer's disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterised by neuropathological deposits of amyloid plaques and neurofibrillary tangles comprised of β-amyloid and tau protein, respectively. In AD, tau becomes abnormally phosphorylated and aggregates to form intracellular deposits. However, the mechanisms by which tau exerts neurotoxicity in disease remain unclear. Recent studies have suggested that the presence of tau at synapses may indicate a role in neuronal signalling, which could be disrupted in pathological conditions. The non-receptor-associated tyrosine kinase fyn is located at the dendrite in neurons, where it was recently shown to interact with tau to stabilise receptor complexes at the post-synaptic density. Fyn also co-localises with tau in a proportion of neurons containing tau tangles in AD and fyn is also a tau kinase. Hence, tau-fyn interactions could play a pathogenic role in AD. Here we report the identification of critical proline residues, Pro213, Pro216, and Pro219, located within the fifth and sixth Pro-X-X-Pro motifs in the proline-rich region of tau, that are important for its binding to fyn. These residues in tau are flanked by numerous phosphorylation sites and therefore we investigated the relationship between fyn and the degree of tau phosphorylation in human post-mortem brain tissue. We found no difference in the amount of fyn present in control and AD brain. Notably, however, there was a significant correlation between fyn and phosphorylated tau at specific phospho-epitopes in control, but not in AD brain. Our results suggest that the pathological mechanisms underlying AD, that result in increased tau phosphorylation, may disrupt the physiological relationship between tau phosphorylation and fyn.

Group II p21-activated kinases as therapeutic targets in gastrointestinal cancer

P21-activated kinases (PAKs) are central players in various oncogenic signaling pathways. The six PAK family members are classified into group I (PAK1-3) and group II (PAK4-6). Focus is currently shifting from group I PAKs to group II PAKs. Group II PAKs play important roles in many fundamental cellular processes, some of which have particular significance in the development and progression of cancer. Because of their important functions, group II PAKs have become popular potential drug target candidates. However, few group II PAKs inhibitors have been reported, and most do not exhibit satisfactory kinase selectivity and “drug-like” properties. Isoform- and kinase-selective PAK inhibitors remain to be developed. This review describes the biological activities of group II PAKs, the importance of group II PAKs in the development and progression of gastrointestinal cancer, and small-molecule inhibitors of group II PAKs for the treatment of cancer.

Integration of linear and dendritic actin nucleation in Nck-induced actin comets

The role of the Nck adaptor protein in balancing linear versus branched actin nucleation in comet tails is evaluated. Nck recruits both linear and branched nucleation-promoting factors, both of which are necessary for the formation of actin comets. The findings highlight a novel role for Nck in pathogen-like actin motility.

The Nck adaptor protein recruits cytosolic effectors such as N-WASP that induce localized actin polymerization. Experimental aggregation of Nck SH3 domains at the membrane induces actin comet tails—dynamic, elongated filamentous actin structures similar to those that drive the movement of microbial pathogens such as vaccinia virus. Here we show that experimental manipulation of the balance between unbranched/branched nucleation altered the morphology and dynamics of Nck-induced actin comets. Inhibition of linear, formin-based nucleation with the small-molecule inhibitor SMIFH2 or overexpression of the formin FH1 domain resulted in formation of predominantly circular-shaped actin structures with low mobility (actin blobs). These results indicate that formin-based linear actin polymerization is critical for the formation and maintenance of Nck-dependent actin comet tails. Consistent with this, aggregation of an exclusively branched nucleation-promoting factor (the VCA domain of N-WASP), with density and turnover similar to those of N-WASP in Nck comets, did not reconstitute dynamic, elongated actin comets. Furthermore, enhancement of branched Arp2/3-mediated nucleation by N-WASP overexpression caused loss of the typical actin comet tail shape induced by Nck aggregation. Thus the ratio of linear to dendritic nucleation activity may serve to distinguish the properties of actin structures induced by various viral and bacterial pathogens.

p21-Activated Kinases 1, 2 and 4 in Endometrial Cancers: Effects on Clinical Outcomes and Cell Proliferation

p21-activated kinases (Paks) are serine/threonine protein kinases involved in biological events linked to malignant tumor progression. In this study, expression of Pak1, p-Pak2 Ser²⁰, Pak4, pPak4 Ser⁴⁷⁴ in 21 normal endometrium, 16 hyperplastic endometrium without atypia, 17 atypical complex hyperplasia and 67 endometrial cancers was assessed by immunohistochemistry and correlated with clinicopathological parameters. We also accessed the proliferative role and downstream targets of Pak1 in endometrial cancer. Pak1 was expressed in cytoplasm whereas Pak4 and p-Pak4 were expressed in both cytoplasm and nucleus of endometrial tissues. In normal endometrium, significantly higher Pak1 (P = 0.028) and cytoplasmic p-Pak2 (P = 0.048) expression was detected in proliferative endometrium than secretory endometrium. Pak1, cytoplasmic and nuclear Pak4 and nuclear p-Pak4 was significantly overexpressed in endometrial cancer when compared to atrophic endometrium (all P<0.05). Moreover, type I endometrioid carcinomas showed significantly higher Pak1 expression than type II non-endometrioid carcinomas (P<0.001). On the other hand, Pak1, Pak4 and p-Pak4 expression negatively correlated with histological grade (all P<0.05) while p-Pak2 and cytoplasmic Pak4 expression inversely correlated with myometrial invasion (all P<0.05). Furthermore, patients with endometrial cancers with lower cytoplasmic Pak4 expression showed poorer survival (P = 0.026). Multivariate analysis showed cytoplasmic Pak4 is an independent prognostic factor. Functionally, knockdown of Pak1, but not Pak4, in endometrial cancer cell line led to reduced cell proliferation along with reduced cyclin D1, estrogen receptor (ERα) and progestogen receptor (PR) expression. Significant correlation between Pak1 and PR expression was also detected in clinical samples. Our findings suggest that Pak1 and cytoplasmic p-Pak2 may promote cell proliferation in normal endometrium during menstral cycle. Pak1, cytoplasmic and nuclear Pak4 and nuclear p-Pak4 are involved in the pathogenesis of endometrial cancer especially in postmenopausal women. Pak1 promote endometrial cancer cell proliferation, particular in type I endometrioid carcinoma. Cytoplasmic Pak4 can be potential prognostic marker in endometrial cancer.

Phosphorylation of tyrosine 285 of PAK1 facilitates βPIX/GIT1 binding and adhesion turnover

The p21-activated serine-threonine kinase (PAK1) regulates cell motility and adhesion. We have previously shown that the prolactin (PRL)-activated tyrosine kinase JAK2 phosphorylates PAK1 in vivo and in vitro and identified tyrosines 153, 201, and 285 in PAK1 as sites of JAK2 tyrosyl phosphorylation. Here, we further investigate the role of the tyrosyl phosphorylated PAK1 (pTyr-PAK1) in regulation of cell adhesion. We use human breast cancer T47D cell lines that stably overexpress PAK1 wild type or PAK1 Y3F mutant in which these 3 JAK2 phosphorylation sites were mutated to phenylalanine. We demonstrate that PRL/JAK2-dependent phosphorylation of these tyrosines promotes a motile phenotype in the cells upon adhesion, participates in regulation of cell adhesion on collagen IV, and is required for maximal PAK1 kinase activity. Down-regulation of PAK1 abolishes the effect of PAK1 on cell adhesion. We show that the tyrosyl phosphorylation of PAK1 promotes PAK1 binding to β-PAK1-interacting guanine-nucleotide exchange factor (βPIX) and G protein-coupled receptor kinase-interacting target 1 (GIT1), phosphorylation of paxillin on Ser273, and formation and distribution of adhesion complexes. Using phosphospecific antibodies (Abs) directed to single phosphorylated tyrosines on PAK1, we identified Tyr285 as a site of PRL-dependent phosphorylation of PAK1 by JAK2. Furthermore, using PAK1 Y285F mutant, we provide evidence for a role of pTyr285 in cell adhesion, enhanced βPIX/GIT1 binding, and adhesion turnover. Our immunohistochemistry analysis demonstrates that pTyr285- PAK1 may modulate PAK1 signaling during tumor progression.

PAK family kinases: Physiological roles and regulation

The p21-activated kinases (PAKs) are a family of Ser/Thr protein kinases that are represented by six genes in humans (PAK 1-6), and are found in all eukaryotes sequenced to date. Genetic and knockdown experiments in frogs, fish and mice indicate group I PAKs are widely expressed, required for multiple tissue development, and particularly important for immune and nervous system function in the adult. The group II PAKs (human PAKs 4-6) are more enigmatic, but their restriction to metazoans and presence at cell-cell junctions suggests these kinases emerged to regulate junctional signaling. Studies of protozoa and fungal PAKs show that they regulate cell shape and polarity through phosphorylation of multiple cytoskeletal proteins, including microtubule binding proteins, myosins and septins. This chapter discusses what we know about the regulation of PAKs and their physiological role in different model organisms, based primarily on gene knockout studies.

Reduced expression of p21-activated protein kinase 1 correlates with poor histological differentiation in pancreatic cancer

BACKGROUND

P21-activated protein kinase 1 (PAK1), a main downstream effector of small Rho GTPases, is overexpressed in many malignancies. PAK1 overexpression is associated with poor prognosis in some tumor types, including breast cancer, gastric cancer, and colorectal cancer. However, the expression and clinical relevance of PAK1 expression in human pancreatic cancer remains unknown.

METHODS

The present study investigated the clinical and prognostic significance of PAK1 expression in pancreatic carcinoma. We examined and scored the expression of PAK1 by immunohistochemistry in 72 primary pancreatic carcinoma samples and 20 liver metastatic samples. The relationships between PAK1 and clinicopathological parameters and prognosis in primary and metastatic pancreatic cancer were analyzed.

RESULTS

Among the total 92 cases, primary pancreatic cancer samples had a significantly higher rate (38/72, 52.8%) of high PAK1 expression than liver metastatic samples (5/20, 25.0%) (P=0.028). Among the 72 primary pancreatic cancer patients, high PAK1 expression was associated with younger age (P=0.038) and moderately or well differentiated tumor (P=0.007). Moreover, a positive relationship was found between high PAK1 expression and overall survival (OS) (P<0.005). Patients with high PAK1 expression had a better OS than those with low PAK1 expression. Univariate and multivariate analysis by Cox regression including PAK1 and other prognostic pathological markers demonstrated high PAK1 immunostaining as a prognostic factor for survival in pancreatic cancer patients (P<0.005).

CONCLUSIONS

We report for the first time that PAK1 is a novel prognostic marker for pathologically confirmed human pancreatic cancer. Reduced expression of PAK1 correlates with poor histological differentiation in pancreatic cancer.

Role of p-21-activated kinases in cancer progression

The p-21-activated kinases (PAKs) are downstream effectors of Rho GTPases Rac and Cdc42. The PAK family consists of six members which are segregated into two subgroups (Group I and Group II) based on sequence homology. Group I PAKs (PAK1-3) are the most extensively studied but there is increasing interest in the functionality of Group II PAKs (PAK4-6). The PAK family proteins are thought to play an important role in many different cellular processes, some of which have particular significance in the context of cancer progression. This review explores established and more recent data, linking the PAK family kinases to cancer progression including expression profiles, evasion of apoptosis, promotion of cell survival, and regulation of cell invasion. Finally, we discuss attempts to therapeutically target the PAK family and outline the major obstacles that still need to be overcome.

Molecular mechanisms of SH2- and PTB-domain-containing proteins in receptor tyrosine kinase signaling

Intracellular signaling is mediated by reversible posttranslational modifications (PTMs) that include phosphorylation, ubiquitination, and acetylation, among others. In response to extracellular stimuli such as growth factors, receptor tyrosine kinases (RTKs) typically dimerize and initiate signaling through phosphorylation of their cytoplasmic tails and downstream scaffolds. Signaling effectors are recruited to these phosphotyrosine (pTyr) sites primarily through Src homology 2 (SH2) domains and pTyr-binding (PTB) domains. This review describes how these conserved domains specifically recognize pTyr residues and play a major role in mediating precise downstream signaling events.

Multiple cytoskeletal pathways and PI3K signaling mediate CDC-42-induced neuronal protrusion in C. elegans

Rho GTPases are key regulators of cellular protrusion and are involved in many developmental events including axon guidance during nervous system development. Rho GTPase pathways display functional redundancy in developmental events, including axon guidance. Therefore, their roles can often be masked when using simple loss-of-function genetic approaches. As a complement to loss-of-function genetics, we constructed a constitutively activated CDC-42(G12V) expressed in C. elegans neurons. CDC-42(G12V) drove the formation of ectopic lamellipodial and filopodial protrusions in the PDE neurons, which resembled protrusions normally found on migrating growth cones of axons. We then used a candidate gene approach to identify molecules that mediate CDC-42(G12V)-induced ectopic protrusions by determining if loss of function of the genes could suppress CDC-42(G12V). Using this approach, we identified 3 cytoskeletal pathways previously implicated in axon guidance, the Arp2/3 complex, UNC-115/abLIM, and UNC-43/Ena. We also identified the Nck-interacting kinase MIG-15/NIK and p21-activated kinases (PAKs), also implicated in axon guidance. Finally, PI3K signaling was required, specifically the Rictor/mTORC2 branch but not the mTORC1 branch that has been implicated in other aspects of PI3K signaling including stress and aging. Our results indicate that multiple pathways can mediate CDC-42-induced neuronal protrusions that might be relevant to growth cone protrusions during axon pathfinding. Each of these pathways involves Rac GTPases, which might serve to integrate the pathways and coordinate the multiple CDC-42 pathways. These pathways might be relevant to developmental events such as axon pathfinding as well as disease states such as metastatic melanoma.

p21-Activated kinase (PAK) regulates cytoskeletal reorganization and directional migration in human neutrophils

Neutrophils serve as a first line of defense in innate immunity owing in part to their ability to rapidly migrate towards chemotactic factors derived from invading pathogens. As a migratory function, neutrophil chemotaxis is regulated by the Rho family of small GTPases. However, the mechanisms by which Rho GTPases orchestrate cytoskeletal dynamics in migrating neutrophils remain ill-defined. In this study, we characterized the role of p21-activated kinase (PAK) downstream of Rho GTPases in cytoskeletal remodeling and chemotactic processes of human neutrophils. We found that PAK activation occurred upon stimulation of neutrophils with f-Met-Leu-Phe (fMLP), and PAK accumulated at the actin-rich leading edge of stimulated neutrophils, suggesting a role for PAK in Rac-dependent actin remodeling. Treatment with the pharmacological PAK inhibitor, PF3758309, abrogated the integrity of RhoA-mediated actomyosin contractility and surface adhesion. Moreover, inhibition of PAK activity impaired neutrophil morphological polarization and directional migration under a gradient of fMLP, and was associated with dysregulated Ca(2+) signaling. These results suggest that PAK serves as an important effector of Rho-family GTPases in neutrophil cytoskeletal reorganization, and plays a key role in driving efficient directional migration of human neutrophils.

WIP provides an essential link between Nck and N-WASP during Arp2/3-dependent actin polymerization

Nck links phosphotyrosine-based signaling to Arp2/3-dependent actin polymerization during many different cellular processes as well as actin-based motility of enteropathogenic Escherichia coli (EPEC), vaccinia, and other vertebrate poxviruses by interacting with N-WASP/WASP. Nck also binds WASP-interacting protein (WIP), which inhibits the ability of N-WASP to activate the Arp2/3 complex until it receives an appropriate signaling input. Using mouse embryonic fibroblasts (MEFs) lacking Nck, WIP, or N-WASP, we have investigated whether an interaction of Nck with both WIP and N-WASP is required for their recruitment to vaccinia during Arp2/3-dependent actin assembly. We find that WIP or its homolog WIRE is required for N-WASP recruitment and actin-based motility of the virus. WIP contains two Nck-binding sites and is recruited to the virus, bound to N-WASP, by interacting with the second SH3 domain of Nck. N-WASP also contains two Nck-binding sites, but its recruitment is dependent on its interaction with WIP rather than Nck. The first and third SH3 domains of Nck are not required to recruit the WIP:N-WASP complex but are essential to stimulate actin assembly. We have established that WIP acts as an essential link between Nck and N-WASP. Our observations provide important insights into the hierarchy and connections in one of the major cellular signaling networks stimulating Arp2/3 complex-dependent actin polymerization.

The Rac GTPase effector p21-activated kinase is essential for hematopoietic stem/progenitor cell migration and engraftment

The p21-activated kinases (Paks) are serine/threonine kinases that are major effectors of the Rho guanosine 5'\x{2011}triphosphatase, Rac, and Cdc42. Rac and Cdc42 are known regulators of hematopoietic stem and progenitor cell (HSPC) function, however, a direct role for Paks in HSPCs has yet to be elucidated. Lin(-)Sca1(+)c-kit(+) (LSK) cells from wild-type mice were transduced with retrovirus expressing Pak inhibitory domain (PID), a well-characterized inhibitor of Pak activation. Defects in marrow homing and in vitro cell migration, assembly of the actin cytoskeleton, proliferation, and survival were associated with engraftment failure of PID-LSK. The PID-LSK demonstrated decreased phosphorylation of extracellular signal-regulated kinase (ERK), whereas constitutive activation of ERK in these cells led to rescue of hematopoietic progenitor cell proliferation in vitro and partial rescue of Pak-deficient HSPC homing and engraftment in vivo. Using conditional knock-out mice, we demonstrate that among group A Paks, Pak2(-/-) HSPC show reduced homing to the bone marrow and altered cell shape similar to PID-LSK cells in vitro and are completely defective in HSPC engraftment. These data demonstrate that Pak proteins are key components of multiple engraftment-associated HSPC functions and play a direct role in activation of ERK in HSPCs, and that Pak2 is specifically essential for HSPC engraftment.

Altered nitric oxide production mediates matrix-specific PAK2 and NF-κB activation by flow

PAK2 mediates shear stress–induced NF-κB activation. Basement membrane proteins limit the proinflammatory response to shear by blocking the interaction of PAK2 with the adaptor protein Nck. This uncoupling response requires protein kinase A–dependent nitric oxide production and subsequent PAK2 phosphorylation on Ser-20 in the Nck-binding domain.

Shear stress generated by distinct blood flow patterns modulates endothelial cell phenotype to spatially restrict atherosclerotic plaque development. Signaling through p21-activated kinase (PAK) mediates several of the deleterious effects of shear stress, including enhanced NF-κB activation and proinflammatory gene expression. Whereas shear stress activates PAK in endothelial cells on a fibronectin matrix, basement membrane proteins limit shear-induced PAK activation and inflammation through a protein kinase A–dependent pathway; however, the mechanisms underlying this regulation were unknown. We show that basement membrane proteins limit membrane recruitment of PAK2, the dominant isoform in endothelial cells, by blocking its interaction with the adaptor protein Nck. This uncoupling response requires protein kinase A–dependent nitric oxide production and subsequent PAK2 phosphorylation on Ser-20 in the Nck-binding domain. Of importance, shear stress does not stimulate nitric oxide production in endothelial cells on fibronectin, resulting in enhanced PAK activation, NF-κB phosphorylation, ICAM-1 expression, and monocyte adhesion. These data demonstrate that differential flow–induced nitric oxide production regulates matrix-specific PAK signaling and describe a novel mechanism of nitric oxide–dependent NF-κB inhibition.

LAT-independent Erk activation via Bam32-PLC-γ1-Pak1 complexes: GTPase-independent Pak1 activation

In T cells, the adaptor Bam32 is coupled to Erk activation downstream of the TCR by an unknown mechanism. We characterized in Jurkat cells and primary T lymphocytes a pathway dependent on Bam32-PLC-γ1-Pak1 complexes, in which Pak1 kinase activates Raf-1 and Mek-1, both upstream of Erk. In the Bam32-PLC-γ1-Pak1 complex, catalytically inactive PLC-γ1 is used as a scaffold linking Bam32 to Pak1. PLC-γ1(C-SH2) directly binds S141 of Bam32, preventing LAT-mediated activation of Ras by PLC-γ1. The Bam32-PLC-γ1 interaction enhances the binding of the SH3 domain of the phospholipase with Pak1. The PLC-γ1(SH3)-Pak1 interaction activates Pak1 independently of the small GTPases Rac1/Cdc42, previously described as being the only activators of Pak1 in T cells. Direct binding of the SH3 domain of PLC-γ1 to Pak1 dissociates inactive Pak1 homodimers, a mechanism required for Pak1 activation. We have thus uncovered a LAT/Ras-independent, Bam32-nucleated pathway that activates Erk signaling in T cells.

Regulation of synaptojanin 2 5'-phosphatase activity by Src

Synaptojanin 2 (SYNJ2) is a phosphatidylinositol (PI) phosphatase that controls two distinct functions, clathrin-mediated endocytosis and tumor cell invadopodia formation and invasion. Here, we identify a number of novel SYNJ2 binding partners, several of which have previously been shown to be necessary for invadopodia formation or clathrin-mediated endocytosis. We focus on Src family kinases. We found that Src phosphorylates SYNJ2 on Tyr⁴⁹⁰, thereby stimulating SYNJ2 5′-phosphatase activity in vitro. We also provide evidence that Src-mediated phosphorylation of SYNJ2 contributes to invadopodia formation.

Pak1 regulates the orientation of apical polarization and lumen formation by distinct pathways

The development of the basic architecture of branching tubules enclosing a central lumen that characterizes most epithelial organs crucially depends on the apico-basolateral polarization of epithelial cells. Signals from the extracellular matrix control the orientation of the apical surface, so that it faces the lumen interior, opposite to cell-matrix adhesion sites. This orientation of the apical surface is thought to be intrinsically linked to the formation of single lumens. We previously demonstrated in three-dimensional cyst cultures of Madin-Darby canine kidney (MDCK) cells that signaling by β1 integrins regulates the orientation of the apical surface, via a mechanism that depends on the activity of the small GTPase Rac1. Here, we investigated whether the Rac1 effector Pak1 is a downstream effector in this pathway. Expression of constitutive active Pak1 phenocopies the effect of β1 integrin inhibition in that it misorients the apical surface and induces a multilumen phenotype. The misorientation of apical surfaces depends on the interaction of active Pak1 with PIX proteins and is linked to defects in basement membrane assembly. In contrast, the multilumen phenotype was independent of PIX and the basement membrane. Therefore, Pak1 likely regulates apical polarization and lumen formation by two distinct pathways.

Breast cancer anti-estrogen resistance 3 (BCAR3) protein augments binding of the c-Src SH3 domain to Crk-associated substrate (p130cas)

The focal adhesion adapter protein p130(cas) regulates adhesion and growth factor-related signaling, in part through Src-mediated tyrosine phosphorylation of p130(cas). AND-34/BCAR3, one of three NSP family members, binds the p130(cas) carboxyl terminus, adjacent to a bipartite p130(cas) Src-binding domain (SBD) and induces anti-estrogen resistance in breast cancer cell lines as well as phosphorylation of p130(cas). Only a subset of the signaling properties of BCAR3, specifically augmented motility, are dependent upon formation of the BCAR3-p130(cas) complex. Using GST pull-down and immunoprecipitation studies, we show that among NSP family members, only BCAR3 augments the ability of p130(cas) to bind the Src SH3 domain through an RPLPSPP motif in the p130(cas) SBD. Although our prior work identified phosphorylation of the serine within the p130(cas) RPLPSPP motif, mutation of this residue to alanine or glutamic acid did not alter BCAR3-induced Src SH3 domain binding to p130(cas). The ability of BCAR3 to augment Src SH3 binding requires formation of a BCAR3-p130(cas) complex because mutations that reduce association between these two proteins block augmentation of Src SH3 domain binding. Similarly, in MCF-7 cells, BCAR3-induced tyrosine phosphorylation of the p130(cas) substrate domain, previously shown to be Src-dependent, was reduced by an R743A mutation that blocks BCAR3 association with p130(cas). Immunofluorescence studies demonstrate that BCAR3 expression alters the intracellular location of both p130(cas) and Src and that all three proteins co-localize. Our work suggests that BCAR3 expression may regulate Src signaling in a BCAR3-p130(cas) complex-dependent fashion by altering the ability of the Src SH3 domain to bind the p130(cas) SBD.

MNT inhibits the migration of human hepatocellular carcinoma SMMC7721 cells

Max binding protein (MNT) is a member of the Myc/Max/Mad network that plays a role in cell proliferation, differentiation and apoptosis. We previously observed that MNT was differentially expressed in hepatocellular carcinoma (HCC) and interacted with Nck1 by 2-DE. Nck family adaptor proteins function to couple tyrosine phosphorylation signals, regulate actin cytoskeletal reorganization and lead to cell motility. In order to investigate the regulatory role of MNT in HCC migration, we used transient transfection with a MNT expressing vector to overexpress MNT protein in SMMC7721 cells, and MNT siRNA to knockdown MNT expression. Rho Family Small GTPase activation assay, Western blots and transwell assay were used to determine the migration potential of cells. We found that knockdown of MNT expression might promote SMMC7721 cell migration, while the overexpressed MNT could significantly inhibit cell migration. It further emphasized the role of MNT in inhibition of cell migration that might be a promising target for HCC chemotherapy.

p21-Activated kinase 3 (PAK3) protein regulates synaptic transmission through its interaction with the Nck2/Grb4 protein adaptor

Mutations in the p21-activated kinase 3 gene (pak3) are responsible for nonsyndromic forms of mental retardation. Expression of mutated PAK3 proteins in hippocampal neurons induces abnormal dendritic spine morphology and long term potentiation anomalies, whereas pak3 gene invalidation leads to cognitive impairments. How PAK3 regulates synaptic plasticity is still largely unknown. To better understand how PAK3 affects neuronal synaptic plasticity, we focused on its interaction with the Nck adaptors that play a crucial role in PAK signaling. We report here that PAK3 interacts preferentially with Nck2/Grb4 in brain extracts and in transfected cells. This interaction is independent of PAK3 kinase activity. Selective uncoupling of the Nck2 interactions in acute cortical slices using an interfering peptide leads to a rapid increase in evoked transmission to pyramidal neurons. The P12A mutation in the PAK3 protein strongly decreases the interaction with Nck2 but only slightly with Nck1. In transfected hippocampal cultures, expression of the P12A-mutated protein has no effect on spine morphogenesis or synaptic density. The PAK3-P12A mutant does not affect synaptic transmission, whereas the expression of the wild-type PAK3 protein decreases the amplitude of spontaneous miniature excitatory currents. Altogether, these data show that PAK3 down-regulates synaptic transmission through its interaction with Nck2.

Use of a decoy peptide to purify p21 activated kinase-1 in cardiac muscle and identification of ceramide-related activation

The p²¹ activated kinase-1 (Pak1) is a serine-threonine protein kinase directly activated by Cdc42 and Rac1. In cardiac myocytes, Pak1 activation leads to dephosphorylation of cTnI and C-protein through upregulation of phosphatase-2A (PP2A). Pak1 activity is directly correlated with its autophosphorylation, which occurs upon binding to the small GTPases and to some small organic molecules as well. In this report, we describe a novel method for rapid purification of endogenous Pak1 from bovine ventricle muscle. The method is simple and easy to carry out. The purified Pak1 demonstrated autophosphorylation in vitro that was enhanced by D-erythro-sphingosine-1, N-acetyl-D-erythro-sphingosine (C₂-ceramide), and N-hexanoyl-D-erythro-sphingosine (C₆-ceramide). Dihydro-L-threo-sphingosine (saphingol) also had some effect on Pak1 autophosphorylation. The method we developed provides a useful tool to study Pak1 activity and regulation in the heart. Moreover, our results indicate a potential role of the sphingolipids as unique signaling molecules inducing a direct activation of Pak1 that may modulate different cardiac functions.

PAK1-Nck regulates cyclin D1 promoter activity in response to prolactin

Prolactin (PRL) is critical for alveolar proliferation and differentiation in normal mammary development and is also implicated in breast cancer. PRL influences cell proliferation and growth by altering the expression of cyclin D1. Cyclin D1 expression is directly regulated by PRL through the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5-mediated transcriptional activation of the cyclin D1 promoter. A p21-activated serine-threonine kinase (PAK)1 has also been implicated in the regulation of cyclin D1 gene expression. We have previously demonstrated that JAK2 directly phosphorylates PAK1 and extend these data here to demonstrate that PAK1 activates the cyclin D1 promoter in response to PRL. We show that mutation of PAK1 Tyr 153, 201, and 285 (sites of JAK2 phosphorylation; PAK1 Y3F) decreases both PAK1 nuclear translocation in response to PRL and PRL-induced cyclin D1 promoter activity by 55%. Mutation of the PAK1 nuclear localization signals decreases PRL-induced cyclin D1 promoter activity by 46%. A PAK1 Y3F mutant lacking functional nuclear localization signals decreases PRL-induced cyclin D1 activity by 68%, suggesting that there is another PAK1-dependent mechanism to activate the cyclin D1 promoter. We have found that adapter protein Nck sequesters PAK1 in the cytoplasm and that coexpression of both PAK1 and Nck inhibits the amplifying effect of PRL-induced PAK1 on cyclin D1 promoter activity (95% inhibition). This inhibition is partially abolished by disruption of PAK1-Nck binding. We propose two PAK1-dependent mechanisms to activate cyclin D1 promoter activity in response to PRL: via nuclear translocation of tyrosyl-phosphorylated PAK1 and via formation of a Nck-PAK1 complex that sequesters PAK1 in the cytoplasm.

The adapter protein Nck: role of individual SH3 and SH2 binding modules for protein interactions in T lymphocytes

Nck is a ubiquitously expressed, primarily cytosolic adapter protein consisting of one SH2 domain and three SH3 domains. It links receptor and nonreceptor tyrosine kinases to actin cytoskeleton reorganizing proteins. In T lymphocytes, Nck is a crucial component of signaling pathways for T cell activation and effector function. It recruits actin remodeling proteins to T cell receptor (TCR)-associated activation clusters and thereby initiates changes in cell polarity and morphology. Moreover, Nck is crucial for the TCR-induced mobilization of secretory vesicles to the cytotoxic immunological synapse. To identify the interactome of Nck in human T cells, we performed a systematic screen for interaction partners in untreated or pervanadate-treated cells. We used GST fusion proteins containing full length Nck, the combined SH3 domains or the individual SH3 and SH2 domains to precipitate putative Nck interactors from cellular lysates. Protein bands were excised from gels, processed by tryptic in-gel digestion and analyzed by mass spectrometry. Using this approach, we confirmed previously established interactions (e.g., with Slp76, CD3 epsilon, WASP, and WIPF1) and identified several novel putative Nck-binding proteins. We subsequently verified the SH2 domain binding to the actin-binding protein HIP55 and to FYB/ADAP, and the SH3-mediated binding to the nuclear proteins SFPQ/NONO. Using laser scanning microscopy, we provide new evidence for a nuclear localization of Nck in human T cells. Our data highlight the fundamental role of Nck in the TCR-to-cytoskeleton crosstalk and point to yet unknown nuclear functions of Nck also in T lymphocytes.

p21-Activated kinase-1 promotes aggressive phenotype, cell proliferation, and invasion in gestational trophoblastic disease

Gestational trophoblastic disease (GTD) includes hydatidiform mole (HM), which can develop persistent gestational trophoblastic neoplasia requiring chemotherapy; choriocarcinoma, which is a frankly malignant tumor; placental site trophoblastic tumor; and epithelioid trophoblastic tumor. p21-Activated kinases (PAKs) promote malignant tumor progression. Therefore, this study investigated PAK1, PAK2, and p-PAK2 Ser(20) in the pathogenesis of GTD. By real-time PCR, PAK1 mRNA was significantly higher in HMs, particularly metastatic HMs (P = 0.046) and HMs that developed persistent disease (P = 0.011), when compared with normal placentas. By immunohistochemistry, significantly increased cytoplasmic PAK1 immunoreactivity in cytotrophoblasts was also detected in HMs (P = 0.042) and choriocarcinomas (P = 0.003). In addition, HMs that developed persistent disease displayed higher PAK1 immunoreactivity than those that regressed (P = 0.016), and elevated PAK1 immunoreactivity was observed in placental site trophoblastic tumors. Indeed, there was significant positive correlation between PAK1 expression and the proliferative indices Ki-67 (P = 0.016) and MCM7 (P = 0.026). Moreover, higher PAK1 mRNA and protein expression was confirmed in the choriocarcinoma cell-lines JEG-3 and JAR; however, PAK2 mRNA and p-PAK2 immunoreactivity showed a similar expression pattern in normal first trimester placentas and GTD. Knockdown of PAK1 in JEG-3 and JAR reduced cell proliferation, migration, and invasion ability, up-regulated p16, and down-regulated vascular endothelial growth factor and MT1-MMP expression. This is the first report revealing the involvement of PAK1 in the pathogenesis and clinical progress of GTD.

alphaPix interacts with Helicobacter pylori CagA to induce IL-8 expression in gastric epithelial cells

OBJECTIVE

Helicobacter pylori CagA, translocated into gastric epithelial cells, induces IL-8 expression through the signalling pathways, including extracellular signal-regulated kinase (ERK) and nuclear factor-kappaB (NF-kappaB). We previously demonstrated that CagA interacts with host alphaPix. The present study was purposed to determine the role of the interaction of alphaPix with CagA on the signalling pathways for IL-8 expression in H. pylori-infected gastric epithelial cells.

MATERIAL AND METHODS

H. pylori HP99 strain (CagA+, VacA+) was infected to gastric epithelial AGS cells transfected with non-targeting (NT) or alphaPix- targeting siRNA. Activation of signalling molecules including p21-activated kinase (PAK), ERK and NF-kappaB, and expression of IL-8 in the cells were assessed.

RESULTS

H. pylori CagA was delivered into AGS cells and then interacted with alphaPix at 4 h following H. pylori infection. PAK1, ERK and NF-kappaB were activated in the cells containing NT and alphaPix siRNA at 1-2 h following H. pylori infection. However, after 4 h, the time when CagA was delivered into the cells, the activations of PAK1, ERK and NF-kappaB were inhibited by down-regulation of alphaPix using siRNA but not by NT siRNA. The results indicate that alphaPix is required for H. pylori-mediated signalling of PAK1, ERK and NF-kappaB. Additionally, alphaPix siRNA suppressed IL-8 induction after translocation of CagA into the cells, indicating that interaction of CagA with alphaPix is critical for CagA-mediating signalling for IL-8 expression.

CONCLUSIONS

The interaction of alphaPix with CagA activates PAK1, ERK and NF-kappaB, which induces IL-8 expression in H. pylori-infected gastric epithelial cells.

The emerging importance of group II PAKs

The Rho-family GTPases Rho Rac and Cdc42 regulate many intracellular processes through their interaction with downstream effector proteins. The PAKs (p21-activated kinases) are a family of effector proteins for Rac and Cdc42. PAKs are important regulators of actin cytoskeletal dynamics, neurite outgrowth, cell survival, hormone signalling and gene transcription. There are six mammalian PAKs that can be divided into two groups: group I PAKs (PAK1-3) and group II PAKs (PAK4-6). Although the two PAK groups are architecturally similar, there are differences in their mode of regulation, suggesting that their cellular functions are likely to be different. Whereas much is known about group I PAKs, less is known about the more recently discovered PAK4, PAK5 and PAK6. This review will focus on the latest structural and functional results relating to the group II PAKs and discuss the emerging importance of group II PAKs in disease progression.

PAKing up to the endothelium

Angiogenesis recapitulates the growth of blood vessels that progressively expand and remodel into a highly organized and stereotyped vascular network. During adulthood, endothelial cells that formed the vascular wall retain their plasticity and can be engaged in neo-vascularization in response to physiological stimuli, such as hypoxia, wound healing and tissue repair, ovarian cycle and pregnancy. In addition, numerous human diseases and pathological conditions are characterized by an excessive, uncontrolled and aberrant angiogenesis. The signalling pathways involving the small Rho GTPase, Rac and its downstream effector the p21-activated serine/threonine kinase (PAK) had recently emerged as pleiotropic modulators in these processes. Indeed, Rac and PAK were found to modulate endothelial cell biology, such as sprouting, migration, polarity, proliferation, lumen formation, and maturation. Elucidating the Rac/PAK molecular circuitry will provide essential information for the development of new therapeutic agents designed to normalize the blood vasculature in human diseases.

Regulation of process retraction and cell migration by EphA3 is mediated by the adaptor protein Nck1

The Eph family of tyrosine kinase receptors and their ligands, the ephrins, participates in the regulation of a wide variety of biological functions under normal and pathological conditions. During embryonic development, interactions between the ligands and receptors define tissue boundaries, guide migrating axons, and regulate angiogenesis, as well as bone morphogenesis. These molecules have also been shown to modify neural activity in the adult nervous system and influence tumor progression. However, the molecular mechanisms underlying these diverse functions are not completely understood. In this study, we conducted a yeast two-hybrid screen to identify molecules that physically interact with Eph receptors using the cytoplasmic domain of EphA3 as "bait". This study identified Nck1 as a strong binding partner of EphA3 as assayed using both GST fusion protein pull down and co-immunoprecipitation techniques. The interaction is mediated through binding of the Nck1 SH2 domain to the phosphotyrosine residue at position 602 (Y602) of the EphA3 receptor. The removal of the SH2 domain or the mutation of the Y602 residue abolishes the interaction. We further demonstrated that EphA3 activation inhibits cell migration and process outgrowth, and these inhibiting effects are partially alleviated by dominant-negative Nck1 mutants that lack functional SH2 or SH3 domains, but not by the wild-type Nck1 gene. These results suggest that Nck1 interacts with EphA3 to regulate cell migration and process retraction.

Relationship of p21-activated kinase (PAK) and filopodia to persistence and oncogenic transformation

Previously, we found that oncogenically transformed cells had fewer filopodia and more large, p21-activated kinase (PAK)-dependent features than normal cells. These large protrusions (LPs) were increased in cells expressing RhoA(N19) with Cdc42-associated kinase (ACK). Here, we determine how GTPase-mediated mechanisms of focal contact (FC) regulation affect these protrusions. Constructs encoding various proteins were introduced into cells which were then studied by microscopy and computerized image processing and analysis. Constructs that prevented PAK recruitment by PAK-interacting exchange factor (PIX) or restricted PAK residence time on FCs decreased both protrusions. Thus, filopodia were also PAK-dependent. A comparison of FC distribution in cells expressing PAK in the presence or absence of PAK kinase inhibitor domain (KID) suggested that PAK enlarged FCs without affecting the prevalence of either protrusion. KID or Nck expression increased LPs but not filopodia. Nck failed to synergize with KID or ACK and RhoA(N19) in enhancing LPs. Nck and KID synergistically enhanced filopodia, possibly because Nck recruited PAK to FCs while KID prevented their dissociation by PAK-mediated autophosphorylation. Coexpression of Nck, ACK, and RhoA(N19) abrogated filopodia and replicated the transformed phenotype. Since Nck recruitment of PAK is implicated in persistence of directional movement, we studied the PAK-Nck interface. Filopodia were eliminated by the Nck PAK-binding domain and LPs by the PAK Nck-binding domain. The results suggested that filopodia formation has more stringent requirements than LP formation, and Nck and PAK are used differently in the protrusions. Loss of filopodia in transformed cells may reflect defective regulation of GTPase mechanisms.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Regulation of cardiac excitation and contraction by p21 activated kinase-1

Cardiac excitation and contraction are regulated by a variety of signaling molecules. Central to the regulatory scheme are protein kinases and phosphatases that carry out reversible phosphorylation of different effectors. The process of beta-adrenergic stimulation mediated by cAMP dependent protein kinase (PKA) forms a well-known pathway considered as the most significant control mechanism in excitation and contraction as well as many other regulatory mechanisms in cardiac function. However, although dephosphorylation pathways are critical to these regulatory processes, signaling to phosphatases is relatively poorly understood. Emerging evidence indicates that regulation of phosphatases, which dampen the effect of beta-adrenergic stimulation, is also important. We review here functional studies of p21 activated kinase-1 (Pak1) and its potential role as an upstream signal for protein phosphatase PP2A in the heart. Pak1 is a serine/threonine protein kinase directly activated by the small GTPases Cdc42 and Rac1. Pak1 is highly expressed in different regions of the heart and modulates the activities of ion channels, sarcomeric proteins, and other phosphoproteins through up-regulation of PP2A activity. Coordination of Pak1 and PP2A activities is not only potentially involved in regulation of normal cardiac function, but is likely to be important in patho-physiological conditions.

PAK signalling in neuronal physiology

Group I p21-activated kinases are a family of key effectors of Rac1 and Cdc42 and they regulate many aspects of cellular function, such as cytoskeleton dynamics, cell movement and cell migration, cell proliferation and differentiation, and gene expression. The three genes PAK1/2/3 are expressed in brain and recent evidence indicates their crucial roles in neuronal cell fate, in axonal guidance and neuronal polarisation, and in neuronal migration. Moreover they are implicated in neurodegenerative diseases and play an important role in synaptic plasticity, with PAK3 being specifically involved in mental retardation. The main goal of this review is to describe the molecular mechanisms that govern the different functions of group I PAK in neuronal signalling and to discuss the specific functions of each isoform.

Nck-1 interacts with PKR and modulates its activation by dsRNA

Activation of the double-stranded RNA (dsRNA)-activated protein kinase PKR results in inhibition of general translation through phosphorylation of the eukaryotic initiation factor 2 alpha-subunit on serine 51 (eIF2alphaSer51). Previously, we have reported that the adaptor protein Nck-1 modulates eIF2alphaSer51 phosphorylation by a subset of eIF2alpha kinases, including PKR. Herein, we demonstrate that Nck-1 prevents efficient activation of PKR by dsRNA, revealing that Nck-1 acts at the level of PKR. In agreement, Nck-1 impairs p38MAPK activation and attenuates cell death induced by dsRNA, in addition to diminish eIF2alphaSer51 phosphorylation. Our data show that the inhibitory effect of Nck-1 on PKR is reversible, as it could be overcome by increasing levels of dsRNA. Interestingly, we found that Nck-1 interacts with the inactive form of PKR, independently of its Src homology domains. Furthermore, we uncovered that Nck-1 is substrate of PKR in vitro. All together, our data provide the first evidence identifying Nck-1 as a novel endogenous regulator of PKR and support the notion that Nck-1-PKR interaction could be a way to limit PKR activation.

The Pak1 kinase: an important regulator of neuronal morphology and function in the developing forebrain

The mammalian central nervous system (CNS) represents a highly complex unit, the correct function of which relies on the appropriate differentiation and survival of its neurones. It is becoming apparent that the Rho family of small GTPases and their downstream targets have a major function in regulating CNS development. Among the effectors, the role of the Pak family of kinases, especially Pak1, is becoming increasingly evident. Although highest levels of Pak1 expression and activation are detected in the developing nervous system, much remains undiscovered concerning its function in neurones. This review summarises what is currently known regarding the biological and molecular role of Pak1 in the mammalian forebrain. It emphasises the importance of Pak1 in regulating neuronal polarity, morphology, migration and synaptic function. Consequently, there are also strong indications that Pak1 is required for normal cognitive function. Furthermore, loss of Pak1 has been associated with the progression of neurodegenerative disorders, particularly Alzheimer's disease, while up-regulation and de-regulation may be responsible for oncogenic transformation of support cells within the CNS, especially astrocyte progenitors. Together, these new and exciting findings encourage the future exploration into the function of Pak1 in the nervous system, thus, paving the way for novel strategies towards improved diagnosis and therapeutic treatment of diseases that affect the CNS.

PAK is regulated by PI3K, PIX, CDC42, and PP2Calpha and mediates focal adhesion turnover in the hyperosmotic stress-induced p38 pathway

Fractionation of brain extracts and functional biochemical assays identified PP2Calpha, a serine/threonine phosphatase, as the major biochemical activity inhibiting PAK1. PP2Calpha dephosphorylated PAK1 and p38, both of which were activated upon hyperosmotic shock with the same kinetics. In comparison to growth factors, hyperosmolality was a more potent activator of PAK1. Therefore we characterize the PAK signaling pathway in the hyperosmotic shock response. Endogenous PAKs were recruited to the p38 kinase complex in a phosphorylation-dependent manner. Overexpression of a PAK inhibitory peptide or dominant negative Cdc42 revealed that p38 activation was dependent on PAK and Cdc42 activities. PAK mutants deficient in binding to Cdc42 or PAK-interacting exchange factor were not activated. Using a panel of kinase inhibitors, we identified PI3K acting upstream of PAK, which correlated with PAK repression by pTEN overexpression. RNA interference knockdown of PAK expression reduced stress-induced p38 activation and conversely, PP2Calpha knockdown increased its activation. Hyperosmotic stress-induced PAK translocation away from focal adhesions to the perinuclear compartment and resulted in disassembly of focal adhesions, which are hallmarks of PAK activation. Inhibition of PAK by overexpression of PP2Calpha or the kinase inhibitory domain prevented sorbitol-induced focal adhesion dissolution. Inhibition of MAPK pathways showed that MEK-ERK signaling but not p38 is required for full PAK activation and focal adhesion turnover. We conclude that 1) PAK plays a required role in hyperosmotic signaling through the PI3K/pTEN/Cdc42/PP2Calpha/p38 pathway, and 2) PAK and PP2Calpha modulate the effects of this pathway on focal adhesion dynamics.

Roles of P21-activated kinases and associated proteins in epithelial wound healing

The primary function of epithelia is to provide a barrier between the extracellular environment and the interior of the body. Efficient epithelial repair mechanisms are therefore crucial for homeostasis. The epithelial wound-healing process involves highly regulated morphogenetic changes of epithelial cells that are driven by dynamic changes of the cytoskeleton. P21-activated kinases are serine/threonine kinases that have emerged as important regulators of the cytoskeleton. These kinases, which are activated downsteam of the Rho GTPases Rac and cd42, were initially mostly implicated in the regulation of cell migration. More recently, however, these kinases were shown to have many additional functions that are relevant to the regulation of epithelial wound healing. Here, we provide an overview of the morphogenetic changes of epithelial cells during wound healing and the many functions of p21-activated kinases in these processes.

Protein kinase A-dependent phosphorylation modulates beta1Pix guanine nucleotide exchange factor activity through 14-3-3beta binding

beta(1)Pix is a guanine nucleotide exchange factor (GEF) for the small GTPases Rac and Cdc42 which has been shown to mediate signaling pathways leading to cytoskeletal reorganization. In the present study, we show that the basal association between endogenous betaPix and endogenous 14-3-3beta was increased after forskolin stimulation and significantly inhibited by protein kinase A inhibitor. However, forskolin stimulation failed to increase the interaction between 14-3-3beta and a beta(1)Pix mutant that is insensitive to protein kinase A phosphorylation, beta(1)Pix(S516A, T526A). We present evidence indicating that forskolin-induced binding of 14-3-3beta to beta(1)Pix results in inhibition of Rac1 GTP loading in 293 cells and in vitro. Furthermore, we show that deletion of 10 amino acid residues within the leucine zipper domain is sufficient to block beta(1)Pix homodimerization and 14-3-3beta binding and modulates beta(1)Pix-GEF activity. These residues also play a crucial role in beta(1)Pix intracellular localization. These results indicate that 14-3-3beta negatively affects the GEF activity of dimeric beta(1)Pix only. Altogether, these results provide a mechanistic insight into the role of 14-3-3beta in modulating beta(1)Pix-GEF activity.

JAK2 Tyrosine Kinase Phosphorylates PAK1 and Regulates PAK1 Activity and Functions

The serine-threonine kinase PAK1 is activated by small GTPase-dependent and -independent mechanisms and promotes cell survival. However, the role of tyrosyl phosphorylation in the regulation of PAK1 function is poorly understood. In this study, we have shown that the prolactin-activated tyrosine kinase JAK2 phosphorylates PAK1 in vivo. Wild type, but not kinase-dead, JAK2 directly phosphorylates PAK1 in cells and in an in vitro kinase assay. PAK1 tyrosines 153, 201, and 285 were identified as sites of JAK2 tyrosyl phosphorylation by mass spectrometry and two-dimensional peptide mapping. Mutation of PAK1 tyrosines 153, 201, and 285 to phenylalanines individually or in combination implicated these PAK1 tyrosines in the regulation of PAK1 kinase activity. Tyrosyl phosphorylation by JAK2 significantly increases PAK1 kinase activity, whereas similar phosphorylation of the PAK1 Y153F,Y201F,Y285F mutant has no effect on PAK1 activity. Tyrosyl phosphorylation of wild type PAK1 decreases apoptosis induced by serum deprivation and staurosporine treatment and increases cell motility. In contrast, these parameters are unaltered in the PAK1 Y153F,Y201F,Y285F mutant. Our findings indicate that JAK2 phosphorylates PAK1 at these specific tyrosines and that this phosphorylation plays an important role in cell survival and motility.

Reciprocal regulation of SH3 and SH2 domain binding via tyrosine phosphorylation of a common site in CD3epsilon

Recruitment of cellular signaling proteins by the CD3 polypeptides of the TCR complex mediates T cell activation. We have screened a human Src homology 3 (SH3) domain phage display library for proteins that can bind to the proline-rich region of CD3epsilon. This screening identified Eps8L1 (epidermal growth factor receptor pathway substrate 8-like 1) together with the N-terminal SH3 domain of Nck1 and Nck2 as its preferred SH3 partners. Studies with recombinant proteins confirmed strong binding of CD3epsilon to Eps8L1 and Nck SH3 domains. CD3epsilon bound well also to Eps8 and Eps8L3, and modestly to Eps8L2, but not detectably to other SH3 domains tested. Interestingly, binding of Nck and Eps8L1 SH3 domains was mapped to a PxxDY motif that shared its tyrosine residue (Y166) with the ITAM of CD3epsilon. Phosphorylation of this residue abolished binding of Eps/Nck SH3 domains in peptide spot filter assays, as well as in cells cotransfected with a dominantly active Lck kinase. TCR ligation-induced binding and phosphorylation-dependent loss of binding were also demonstrated between Eps8L1 and endogenous CD3epsilon in Jurkat T cells. Thus, phosphorylation of Y166 serves as a molecular switch during T cell activation that determines the capacity of CD3epsilon to interact with either SH3 or SH2 domain-containing proteins.