Activation of Pak by membrane localization mediated by an SH3 domain from the adaptor protein Nck

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.

A chemorepellent inhibits local Ras activation to inhibit pseudopod formation to bias cell movement away from the chemorepellent

The ability of cells to sense chemical gradients is essential during development, morphogenesis, and immune responses. Although much is known about chemoattraction, chemorepulsion remains poorly understood. Proliferating Dictyostelium cells secrete a chemorepellent protein called AprA. AprA prevents pseudopod formation at the region of the cell closest to the source of AprA, causing the random movement of cells to be biased away from the AprA. Activation of Ras proteins in a localized sector of a cell cortex helps to induce pseudopod formation, and Ras proteins are needed for AprA chemorepulsion. Here we show that AprA locally inhibits Ras cortical activation through the G protein–coupled receptor GrlH, the G protein subunits Gβ and Gα8, Ras protein RasG, protein kinase B, the p21-activated kinase PakD, and the extracellular signal–regulated kinase Erk1. Diffusion calculations and experiments indicate that in a colony of cells, high extracellular concentrations of AprA in the center can globally inhibit Ras activation, while a gradient of AprA that naturally forms at the edge of the colony allows cells to activate Ras at sectors of the cell other than the sector of the cell closest to the center of the colony, effectively inducing both repulsion from the colony and cell differentiation. Together, these results suggest that a pathway that inhibits local Ras activation can mediate chemorepulsion.

Regulation of Rac1 Activation in Choroidal Endothelial Cells: Insights into Mechanisms in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is one of the leading causes of blindness worldwide. Vision loss from the neovascular form is associated with the invasion of choroidal endothelial cells into the neural retina to form vision-threatening macular neovascularization (MNV). Anti-angiogenic agents are the current standard of care but are effective in only ~50% of AMD cases. The molecular mechanisms involved in invasive MNV point to the importance of regulating signaling pathways that lead to pathologic biologic outcomes. In studies testing the effects of AMD-related stresses, activation of the Rho GTPase, Rac1, was found to be important for the choroidal endothelial cell invasion into the neural retina. However, current approaches to prevent Rac1 activation are inefficient and less effective. We summarize active Rac1-mediated mechanisms that regulate choroidal endothelial cell migration. Specifically, we discuss our work regarding the role of a multidomain protein, IQ motif containing GTPase activating protein 1 (IQGAP1), in sustaining pathologic Rac1 activation and a mechanism by which active Rap1, a Ras-like GTPase, may prevent active Rac1-mediated choroidal endothelial cell migration.

Pak1 kinase controls cell shape through ribonucleoprotein granules

Fission yeast cells maintain a rod shape due to conserved signaling pathways that organize the cytoskeleton for polarized growth. We discovered a mechanism linking the conserved protein kinase Pak1 with cell shape through the RNA-binding protein Sts5. Pak1 (also called Shk1 and Orb2) prevents Sts5 association with P bodies by directly phosphorylating its intrinsically disordered region (IDR). Pak1 and the cell polarity kinase Orb6 both phosphorylate the Sts5 IDR but at distinct residues. Mutations preventing phosphorylation in the Sts5 IDR cause increased P body formation and defects in cell shape and polarity. Unexpectedly, when cells encounter glucose starvation, PKA signaling triggers Pak1 recruitment to stress granules with Sts5. Through retargeting experiments, we reveal that Pak1 localizes to stress granules to promote rapid dissolution of Sts5 upon glucose addition. Our work reveals a new role for Pak1 in regulating cell shape through ribonucleoprotein granules during normal and stressed growth conditions.

Sinner or Saint?: Nck Adaptor Proteins in Vascular Biology

The Nck family of modular adaptor proteins, including Nck1 and Nck2, link phosphotyrosine signaling to changes in cytoskeletal dynamics and gene expression that critically modulate cellular phenotype. The Nck SH2 domain interacts with phosphotyrosine at dynamic signaling hubs, such as activated growth factor receptors and sites of cell adhesion. The Nck SH3 domains interact with signaling effectors containing proline-rich regions that mediate their activation by upstream kinases. In vascular biology, Nck1 and Nck2 play redundant roles in vascular development and postnatal angiogenesis. However, recent studies suggest that Nck1 and Nck2 differentially regulate cell phenotype in the adult vasculature. Domain-specific interactions likely mediate these isoform-selective effects, and these isolated domains may serve as therapeutic targets to limit specific protein-protein interactions. In this review, we highlight the function of the Nck adaptor proteins, the known differences in domain-selective interactions, and discuss the role of individual Nck isoforms in vascular remodeling and function.

p21-Activated Kinases in Thyroid Cancer

The family of p21-activated kinases (PAKs) are oncogenic proteins that regulate critical cellular functions. PAKs play central signaling roles in the integrin/CDC42/Rho, ERK/MAPK, PI3K/AKT, NF-κB, and Wnt/β-catenin pathways, functioning both as kinases and scaffolds to regulate cell motility, mitosis and proliferation, cytoskeletal rearrangement, and other cellular activities. PAKs have been implicated in both the development and progression of a wide range of cancers, including breast cancer, pancreatic melanoma, thyroid cancer, and others. Here we will discuss the current knowledge on the structure and biological functions of both group I and group II PAKs, as well as the roles that PAKs play in oncogenesis and progression, with a focus on thyroid cancer and emerging data regarding BRAF/PAK signaling.

Nck1, But Not Nck2, Mediates Disturbed Flow-Induced p21-Activated Kinase Activation and Endothelial Permeability

Background

Alteration in hemodynamic shear stress at atheroprone sites promotes endothelial paracellular pore formation and permeability. The molecular mechanism remains unknown.

Methods and Results

We show that Nck (noncatalytic region of tyrosine kinase) deletion significantly ameliorates disturbed flow‐induced permeability, and selective isoform depletion suggests distinct signaling mechanisms. Only Nck1 deletion significantly reduces disturbed flow‐induced paracellular pore formation and permeability, whereas Nck2 depletion has no significant effects. Additionally, Nck1 re‐expression, but not Nck2, restores disturbed flow‐induced permeability in Nck1/2 knockout cells, confirming the noncompensating roles. In vivo, using the partial carotid ligation model of disturbed flow, Nck1 knockout prevented the increase in vascular permeability, as assessed by Evans blue and fluorescein isothiocyanate dextran extravasations and leakage of plasma fibrinogen into the vessel wall. Domain swap experiments mixing SH2 (phosphotyrosine binding) and SH3 (proline‐rich binding) domains between Nck1 and Nck2 showed a dispensable role for SH2 domains but a critical role for the Nck1 SH3 domains in rescuing disturbed flow‐induced endothelial permeability. Consistent with this, both Nck1 and Nck2 bind to platelet endothelial adhesion molecule‐1 (SH2 dependent) in response to shear stress, but only Nck1 ablation interferes with shear stress–induced PAK2 (p21‐activated kinase) membrane translocation and activation. A single point mutation into individual Nck1 SH3 domains suggests a role for the first domain of Nck1 in PAK recruitment to platelet endothelial cell adhesion molecule‐1 and activation in response to shear stress.

Conclusions

This work provides the first evidence that Nck1 but not the highly similar Nck2 plays a distinct role in disturbed flow‐induced vascular permeability by selective p21‐activated kinase activation.

PLCE1 regulates the migration, proliferation, and differentiation of podocytes

PLCE1 encodes phospholipase C epsilon, and its mutations cause recessive nephrotic syndrome. However, the mechanisms by which PLCE1 mutations result in defects associated with glomerular function are not clear. To address this, we investigated the function of PLCE1 in podocytes called glomerular epithelial cells, where the pathogenesis of nephrotic syndrome converges. PLCE1 colocalized with Rho GTPases in glomeruli. Further, it interacted with Rho GTPases through the pleckstrin homology domain and Ras GTP-binding domains 1/2. Knockdown or knockout of PLCE1 in podocytes resulted in decreased levels of GTP-bound Rac1 and Cdc42, but not those of RhoA, and caused a reduction in cell migration. PLCE1 interacted with NCK2 but not with NCK1. Similar to the PLCE1 knockout, NCK2 knockout resulted in decreased podocyte migration. Knockout of PLCE1 reduced the EGF-induced activation of ERK and cell proliferation in podocytes, whereas knockout of NCK2 did not affect proliferation. Further, the knockout of PLCE1 also resulted in decreased expression of podocyte markers, including NEPH1, NPHS1, WT1, and SYNPO, upon differentiation, but the knockout of NCK2 did not affect the expression of these markers. Therefore, our findings demonstrate that PLCE1 regulates Rho GTPase activity and cell migration through interacting with NCK2 and that PLCE1 also plays a role in the proliferation and differentiation of podocytes, regardless of the presence of NCK2.

A genetic mutation associated with kidney disease impairs the maturation and migration of cells that filter waste products out of the blood. Tiny tendrils from kidney cells called podocytes establish a tight meshwork that keeps blood proteins in circulation while allowing unwanted contaminants to pass through. Mutations in the PLCE1 gene disrupt this filter, leading to a disorder called nephrotic syndrome Researchers led by Heon Yung Gee at Yonsei University College of Medicine, Seoul, South Korea, have uncovered mechanisms underlying this malfunction. Working with cultured podocytes, they showed that loss of PLCE1 impairs cell migration, potentially undermining their ability to form a meshwork. The researchers also found that the protein encoded by PLCE1 interacts with other molecules that promote cell division and maturation, revealing another mechanism by which mutations could contribute to loss of podocyte function.

Cooperative recruitment of Arl4A and Pak1 to the plasma membrane contributes to sustained Pak1 activation for cell migration

Cell migration requires the coordination of multiple signaling pathways involved in membrane dynamics and cytoskeletal rearrangement. The Arf-like small GTPase Arl4A has been shown to modulate actin cytoskeleton remodeling. However, evidence of the function of Arl4A in cell migration is insufficient. Here, we report that Arl4A acts with the serine/threonine protein kinase Pak1 to modulate cell migration through their cooperative recruitment to the plasma membrane. We first observed that Arl4A and its isoform Arl4D interact with Pak1 and Pak2 and showed that Arl4A recruits Pak1 and Pak2 to the plasma membrane. The fibronectin-induced Pak1 localization at the plasma membrane is reduced in Arl4A-depleted cells. Unexpectedly, we found that Pak1, but not Arl4A-binding-defective Pak1, can recruit a cytoplasmic myristoylation-deficient Arl4A-G2A mutant to the plasma membrane. Furthermore, we found that the Arl4A-Pak1 interaction, which is independent of Rac1 binding to Pak1, is required for Arl4A-induced cell migration. Thus, we infer that there is feedback regulation between Arl4A and Pak1, in which they mutually recruit each other to the plasma membrane for Pak1 activation, thereby modulating cell migration through direct interaction.

Mixed lineage kinase 3 promotes breast tumorigenesis via phosphorylation and activation of p21-activated kinase 1

Mixed lineage kinase 3 (MLK3), a MAP3K member has been envisioned as a viable drug target in cancer, yet its detailed function and signaling is not fully elucidated. We identified that MLK3 tightly associates with an oncogene, PAK1. Mammalian PAK1 being a Ste20 (MAP4K) member, we tested whether it is an upstream regulator of MLK3. In contrast to our hypothesis, MLK3 activated PAK1 kinase activity directly, as well as in the cells. Although, MLK3 can phosphorylate PAK1 on Ser133 and Ser204 sites, PAK1S133A mutant is constitutively active, whereas, PAK1S204A is not activated by MLK3. Stable overexpression of PAK1S204A in breast cancer cells, impedes migration, invasion, and NFĸB activity. In vivo breast cancer cell tumorigenesis is significantly reduced in tumors expressing PAK1S204A mutant. These results suggest that mammalian PAK1 does not act as a MAP4K and MLK3-induced direct activation of PAK1 plays a key role in breast cancer tumorigenesis.

PAK4 signaling in health and disease: defining the PAK4-CREB axis

p21-Activated kinase 4 (PAK4), a member of the PAK family, regulates a wide range of cellular functions, including cell adhesion, migration, proliferation, and survival. Dysregulation of its expression and activity thus contributes to the development of diverse pathological conditions. PAK4 plays a pivotal role in cancer progression by accelerating the epithelial–mesenchymal transition, invasion, and metastasis. Therefore, PAK4 is regarded as an attractive therapeutic target in diverse types of cancers, prompting the development of PAK4-specific inhibitors as anticancer drugs; however, these drugs have not yet been successful. PAK4 is essential for embryonic brain development and has a neuroprotective function. A long list of PAK4 effectors has been reported. Recently, the transcription factor CREB has emerged as a novel effector of PAK4. This finding has broad implications for the role of PAK4 in health and disease because CREB-mediated transcriptional reprogramming involves a wide range of genes. In this article, we review the PAK4 signaling pathways involved in prostate cancer, Parkinson’s disease, and melanogenesis, focusing in particular on the PAK4-CREB axis.

An enzyme that regulates an important controller of gene expression may offer a therapeutic target for cancer and other diseases. cAMP response element-binding protein (CREB) interacts with various other proteins to switch a myriad of target genes on and off in different cells. A review by Eung-Gook Kim, Eun-Young Shin and colleagues at Chungbuk National University, Cheongju, South Korea, explores the interplay between CREB and an enzyme called p21-activated kinase 4 (PAK4) in human health and disease. PAK4, for example, has been shown to promote CREB’s gene-activating function in prostate cancer, and PAK4 overexpression is a feature of numerous other tumor types. Disruptions in PAK4-mediated regulation of CREB activity have also been observed in neurons affected by Parkinson’s disease. The authors see strong clinical promise in further exploring the biology of the PAK4-CREB pathway.

p27Kip1 promotes invadopodia turnover and invasion through the regulation of the PAK1/Cortactin pathway

p27^Kip1 (p27) is a cyclin-CDK inhibitor and negative regulator of cell proliferation. p27 also controls other cellular processes including migration and cytoplasmic p27 can act as an oncogene. Furthermore, cytoplasmic p27 promotes invasion and metastasis, in part by promoting epithelial to mesenchymal transition. Herein, we find that p27 promotes cell invasion by binding to and regulating the activity of Cortactin, a critical regulator of invadopodia formation. p27 localizes to invadopodia and limits their number and activity. p27 promotes the interaction of Cortactin with PAK1. In turn, PAK1 promotes invadopodia turnover by phosphorylating Cortactin, and expression of Cortactin mutants for PAK-targeted sites abolishes p27’s effect on invadopodia dynamics. Thus, in absence of p27, cells exhibit increased invadopodia stability due to impaired PAK1-Cortactin interaction, but their invasive capacity is reduced compared to wild-type cells. Overall, we find that p27 directly promotes cell invasion by facilitating invadopodia turnover via the Rac1/PAK1/Cortactin pathway.

DOI:http://dx.doi.org/10.7554/eLife.22207.001

When animals develop from embryos to adults, or try to heal wounds later in life, their cells have to move. Moving means that the cells must invade into their surroundings, a dense network of proteins called the extracellular matrix. The cell first attaches to the extracellular matrix; degrades it; and then moves into the newly opened space. Cells have developed specialized structures called invadosomes to enable all these steps. Invadosomes are never static, they first assemble where cells interact with extracellular matrix, they then release proteins that loosen the matrix, and finally disassemble again to allow cells to move. Invadosomes in cancer cells often become overactive, and can allow the tumor cells to spread throughout the body.

A lot of different proteins are involved in controlling how and when cells move. p27 is a well-known protein usually found in a cell’s nucleus along with the cell’s DNA. Inside the nucleus, p27 suppresses tumor growth by stopping cells from dividing. However, often in cancer cells p27 moves outside of the cell’s nucleus where it contributes to cell movement via an unknown mechanism.

To answer how p27 controls cell invasion, Jeannot et al. used a biochemical technique to uncover which proteins p27 binds to when it is outside of the nucleus. One of its interaction partners was called Cortactin. This protein is known to be an important building block of invadosomes, and is involved in both the assembly and disassembly of these structures. In further experiments, Jeannot studied mouse cells with or without p27 and human cancer cells that can be grown in the laboratory. The experiments revealed that p27 promotes an enzyme called PAK1 to also bind to Cortactin. PAK1 then modified Cortactin, causing whole invadosomes to disassemble, which in turn allowed cells to de-attach from the matrix and move forward. In contrast, cells lacking p27 had more stable invadosomes, attached more strongly to the matrix and were better at degrading it, but could not invade as well as cells with p27.

Overall these experiments showed a new way that p27 promotes cell invasion. The next steps will include finding out exactly how the modification of Cortactin causes the invadosomes to disassemble. Furthermore, it will be important to study whether forcing p27 back into the nucleus can reduce the spread of cancer cells in the body.

DOI:http://dx.doi.org/10.7554/eLife.22207.002

Dock8 interacts with Nck1 in mediating Schwann cell precursor migration

During embryonic development of the peripheral nervous system (PNS), Schwann cell precursors migrate along neuronal axons to their final destinations, where they will myelinate the axons after birth. While the intercellular signals controlling Schwann cell precursor migration are well studied, the intracellular signals controlling Schwann cell precursor migration remain elusive. Here, using a rat primary cell culture system, we show that Dock8, an atypical Dock180-related guanine-nucleotide exchange factor (GEF) for small GTPases of the Rho family, specifically interacts with Nck1, an adaptor protein composed only of Src homology (SH) domains, to promote Schwann cell precursor migration induced by platelet-derived growth factor (PDGF). Knockdown of Dock8 or Nck1 with its respective siRNA markedly decreases PDGF-induced cell migration, as well as Rho GTPase activation, in precursors. Dock8, through its unique N-terminal proline-rich motif, interacts with the SH3 domain of Nck1, but not with other adaptor proteins composed only of SH domains, e.g. Grb2 and CrkII, and not with the adaptor protein Elmo1. Reintroduction of the proline-rich motif mutant of Dock8 in Dock8 siRNA-transfected Schwann cell precursors fails to restore their migratory abilities, whereas that of wild-type Dock8 does restore these abilities. These results suggest that Nck1 interaction with Dock8 mediates PDGF-induced Schwann cell precursor migration, demonstrating not only that Nck1 and Dock8 are previously unanticipated intracellular signaling molecules involved in the regulation of Schwann cell precursor migration but also that Dock8 is among the genetically-conservative common interaction subset of Dock family proteins consisting only of SH domain adaptor proteins.

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Dock8, a Rho family GEF, regulates Schwann cell precursor migration.

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Nck1 adaptor protein regulates Schwann cell precursor migration.

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Dock8 uniquely interacts with Nck1.

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The interaction of Dock8 with Nck1 contributes to migration.

p21-activated kinase 2 regulates HSPC cytoskeleton, migration, and homing via CDC42 activation and interaction with β-Pix

Cytoskeletal remodeling of hematopoietic stem and progenitor cells (HSPCs) is essential for homing to the bone marrow (BM). The Ras-related C3 botulinum toxin substrate (Rac)/cell division control protein 42 homolog (CDC42) effector p21-activated kinase (Pak2) has been implicated in HSPC homing and engraftment. However, the molecular pathways mediating Pak2 functions in HSPCs are unknown. Here, we demonstrate that both Pak2 kinase activity and its interaction with the PAK-interacting exchange factor-β (β-Pix) are required to reconstitute defective ITALIC! Pak2 (ITALIC! Δ/Δ)HSPC homing to the BM. Pak2 serine/threonine kinase activity is required for stromal-derived factor-1 (SDF1α) chemokine-induced HSPC directional migration, whereas Pak2 interaction with β-Pix is required to regulate the velocity of HSPC migration and precise F-actin assembly. Lack of SDF1α-induced filopodia and associated abnormal cell protrusions seen in ITALIC! Pak2 (ITALIC! Δ/Δ)HSPCs were rescued by wild-type (WT) Pak2 but not by a Pak2-kinase dead mutant (KD). Expression of a β-Pix interaction-defective mutant of Pak2 rescued filopodia formation but led to abnormal F-actin bundles. Although CDC42 has previously been considered an upstream regulator of Pak2, we found a paradoxical decrease in baseline activation of CDC42 in ITALIC! Pak2 (ITALIC! Δ/Δ)HSPCs, which was rescued by expression of Pak2-WT but not by Pak2-KD; defective homing of ITALIC! Pak2-deleted HSPCs was rescued by constitutive active CDC42. These data demonstrate that both Pak2 kinase activity and its interaction with β-Pix are essential for HSPC filopodia formation, cytoskeletal integrity, and homing via activation of CDC42. Taken together, we provide mechanistic insights into the role of Pak2 in HSPC migration and homing.

Nck Binds to the T Cell Antigen Receptor Using Its SH3.1 and SH2 Domains in a Cooperative Manner, Promoting TCR Functioning

Ligand binding to the TCR causes a conformational change at the CD3 subunits to expose the CD3ε cytoplasmic proline-rich sequence (PRS). It was suggested that the PRS is important for TCR signaling and T cell activation. It has been shown that the purified, recombinant SH3.1 domain of the adaptor molecule noncatalytic region of tyrosine kinase (Nck) can bind to the exposed PRS of CD3ε, but the molecular mechanism of how full-length Nck binds to the TCR in cells has not been investigated so far. Using the in situ proximity ligation assay and copurifications, we show that the binding of Nck to the TCR requires partial phosphorylation of CD3ε, as it is based on two cooperating interactions. First, the SH3.1(Nck) domain has to bind to the nonphosphorylated and exposed PRS, that is, the first ITAM tyrosine has to be in the unphosphorylated state. Second, the SH2(Nck) domain has to bind to the second ITAM tyrosine in the phosphorylated state. Likewise, mutations of the SH3.1 and SH2 domains in Nck1 resulted in the loss of Nck1 binding to the TCR. Furthermore, expression of an SH3.1-mutated Nck impaired TCR signaling and T cell activation. Our data suggest that the exact pattern of CD3ε phosphorylation is critical for TCR functioning.

Specification of Dendritogenesis Site in Drosophila aCC Motoneuron by Membrane Enrichment of Pak1 through Dscam1

Precise positioning of dendritic branches is a critical step in the establishment of neuronal circuitry. However, there is limited knowledge on how environmental cues translate into dendrite initiation or branching at a specific position. Here, through a combination of mutation, RNAi, and imaging experiments, we found that a Dscam-Dock-Pak1 hierarchical interaction defines the stereotypical dendrite growth site in the Drosophila aCC motoneuron. This interaction localizes the Cdc42 effector Pak1 to the plasma membrane at the dendrite initiation site before the activation of Cdc42. Ectopic expression of membrane-anchored Pak1 overrides this spatial specification of dendritogenesis, confirming its function in guiding Cdc42 signaling. We further discovered that Dscam1 localization in aCC occurs through an inter-neuronal contact that involves Dscam1 in the partner MP1 neuron. These findings elucidate a mechanism by which Dscam1 controls neuronal morphogenesis through spatial regulation of Cdc42 signaling and, subsequently, cytoskeletal remodeling.

The Role of the Pleckstrin Homology Domain-containing Protein CKIP-1 in Activation of p21-activated Kinase 1 (PAK1)

Upon growth factor stimulation, PAK1 is recruited to the plasma membrane and activated by a mechanism that requires its phosphorylation at Ser-223 by the protein kinase CK2. However, the upstream signaling molecules that regulate this phosphorylation event are not clearly defined. Here, we demonstrate a major role of the CK2α-interacting protein CKIP-1 in activation of PAK1. CK2α, CKIP-1, and PAK1 are translocated to membrane ruffles in response to the epidermal growth factor (EGF), where CKIP-1 mediates the interaction between CK2α and PAK1 in a PI3K-dependent manner. Consistently, PAK1 mediates phosphorylation and modulation of the activity of p41-Arc, one of its plasma membrane substrate, in a fashion that requires PI3K and CKIP-1. Moreover, CKIP-1 knockdown or PI3K inhibition suppresses PAK1-mediated cell migration and invasion, demonstrating the physiological significance of the PI3K-CKIP-1-CK2-PAK1 signaling pathway. Taken together, these findings identify a novel mechanism for the activation of PAK1 at the plasma membrane, which is critical for cell migration and invasion.

NCK is critical for the development of deleted in colorectal cancer (DCC) sensitive spinal circuits

As our understanding of motor circuit function increases, our need to understand how circuits form to ensure proper function becomes increasingly important. Recently, deleted in colorectal cancer (DCC) has been shown to be important in the development of spinal circuits necessary for gait. Importantly, humans with mutation in DCC show mirror movement disorders pointing to the significance of DCC in the development of spinal circuits for coordinated movement. Although DCC binds a number of ligands, the intracellular signaling cascade leading to the aberrant spinal circuits remains unknown. Here, we show that the non-catalytic region of tyrosine kinase adaptor (NCK) proteins 1 and 2 are distributed in the developing spinal cord. Using dissociated dorsal spinal neuron cultures we show that NCK proteins are necessary for the outgrowth and growth cone architecture of DCC(+ve) dorsal spinal neurons. Consistent with a role for NCK in DCC signaling, we show that loss of NCK proteins leads to a reduction in the thickness of TAG1(+ve) commissural bundles in the floor plate and loss of DCC mRNA in vivo. We suggest that DCC signaling functions through NCK1 and NCK2 and that both proteins are necessary for the establishment of normal spinal circuits necessary for gait. Reduction in NCK proteins in the developing CNS leads to a reduction in TAG1(+ve) commissural tract thickness, a reduction in growth cone complexity of DCC(+ve) spinal interneurons, and a reduction in DCC mRNA. These are consistent with an in vivo role for NCK in the development of critical DCC spinal circuits, and may be important for the normal development of spinal circuits critical for walking.

Recruitment of the adaptor protein Nck to PECAM-1 couples oxidative stress to canonical NF-κB signaling and inflammation

Oxidative stress stimulates nuclear factor κB (NF-κB) activation and NF-κB-dependent proinflammatory gene expression in endothelial cells during several pathological conditions, including ischemia/reperfusion injury. We found that the Nck family of adaptor proteins linked tyrosine kinase signaling to oxidative stress-induced activation of NF-κB through the classic IκB kinase-dependent pathway. Depletion of Nck prevented oxidative stress induced by exogenous hydrogen peroxide or hypoxia/reoxygenation injury from activating NF-κB in endothelial cells, increasing the abundance of the proinflammatory molecules ICAM-1 (intracellular adhesion molecule-1) and VCAM-1 (vascular cell adhesion molecule-1) and recruiting leukocytes. Nck depletion also attenuated endothelial cell expression of genes encoding proinflammatory factors but not those encoding antioxidants. Nck promoted oxidative stress-induced activation of NF-κB by coupling the tyrosine phosphorylation of PECAM-1 (platelet endothelial cell adhesion molecule-1) to the activation of p21-activated kinase, which mediates oxidative stress-induced NF-κB signaling. Consistent with this mechanism, treatment of mice subjected to ischemia/reperfusion injury in the cremaster muscle with a Nck inhibitory peptide blocked leukocyte adhesion and emigration and the accompanying vascular leak. Together, these data identify Nck as an important mediator of oxidative stress-induced inflammation and a potential therapeutic target for ischemia/reperfusion injury.

Increased SHP-1 protein expression by high glucose levels reduces nephrin phosphorylation in podocytes

Nephrin, a critical podocyte membrane component that is reduced in diabetic nephropathy, has been shown to activate phosphotyrosine signaling pathways in human podocytes. Nephrin signaling is important to reduce cell death induced by apoptotic stimuli. We have shown previously that high glucose level exposure and diabetes increased the expression of SHP-1, causing podocyte apoptosis. SHP-1 possesses two Src homology 2 domains that serve as docking elements to dephosphorylate tyrosine residues of target proteins. However, it remains unknown whether SHP-1 interacts with nephrin and whether its elevated expression affects the nephrin phosphorylation state in diabetes. Here we show that human podocytes exposed to high glucose levels exhibited elevated expression of SHP-1, which was associated with nephrin. Coexpression of nephrin-CD16 and SHP-1 reduced nephrin tyrosine phosphorylation in transfected human embryonic kidney 293 cells. A single tyrosine-to-phenylalanine mutation revealed that rat nephrin Tyr(1127) and Tyr(1152) are required to allow SHP-1 interaction with nephrin. Overexpression of dominant negative SHP-1 in human podocytes prevented high glucose-induced reduction of nephrin phosphorylation. In vivo, immunoblot analysis demonstrated that nephrin expression and phosphorylation were decreased in glomeruli of type 1 diabetic Akita mice (Ins2(+/C96Y)) compared with control littermate mice (Ins2(+/+)), and this was associated with elevated SHP-1 and cleaved caspase-3 expression. Furthermore, immunofluorescence analysis indicated increased colocalization of SHP-1 with nephrin in diabetic mice compared with control littermates. In conclusion, our results demonstrate that high glucose exposure increases SHP-1 interaction with nephrin, causing decreased nephrin phosphorylation, which may, in turn, contribute to diabetic nephropathy.

Adaptive responses to dasatinib-treated lung squamous cell cancer cells harboring DDR2 mutations

DDR2 mutations occur in approximately 4% of lung squamous cell cancer (SCC) where the tyrosine kinase inhibitor dasatinib has emerged as a new therapeutic option. We found that ERK and AKT phosphorylation was weakly inhibited by dasatinib in DDR2-mutant lung SCC cells, suggesting that dasatinib inhibits survival signals distinct from other oncogenic receptor tyrosine kinases (RTK) and/or compensatory signals exist that dampen dasatinib activity. To gain better insight into dasatinib's action in these cells, we assessed altered global tyrosine phosphorylation (pY) after dasatinib exposure using a mass spectrometry-based quantitative phosphoproteomics approach. Overlaying protein-protein interaction relationships upon this dasatinib-regulated pY network revealed decreased phosphorylation of Src family kinases and their targets. Conversely, dasatinib enhanced tyrosine phosphorylation in a panel of RTK and their signaling adaptor complexes, including EGFR, MET/GAB1, and IGF1R/IRS2, implicating a RTK-driven adaptive response associated with dasatinib. To address the significance of this observation, these results were further integrated with results from a small-molecule chemical library screen. We found that dasatinib combined with MET and insulin-like growth factor receptor (IGF1R) inhibitors had a synergistic effect, and ligand stimulation of EGFR and MET rescued DDR2-mutant lung SCC cells from dasatinib-induced loss of cell viability. Importantly, we observed high levels of tyrosine-phosphorylated EGFR and MET in a panel of human lung SCC tissues harboring DDR2 mutations. Our results highlight potential RTK-driven adaptive-resistant mechanisms upon DDR2 targeting, and they suggest new, rationale cotargeting strategies for DDR2-mutant lung SCC.

Untangling the complexity of PAK1 dynamics: The future challenge

PAK1 kinase is a crucial regulator of a variety of cellular processes, such as motility, cell division, gene transcription and apoptosis. Its deregulation is involved in several pathologies, including cancer, viral infection and neurodegenerative diseases. Due to this strong implication in human health, the complex network of signaling pathways centered on PAK1 is a subject of intensive investigations. This review summarizes the present knowledge on the multiple PAK1 intracellular localizations and on its shuttling between different compartments. The dynamics of PAK1 localization and activation are finely tuned by the cell and it is this tight control that underlies the capacity of PAK1 to participate in the regulation of many fundamental cell functions. Recently, PAK1 biosensors have been developed to visualize PAK1 activation in live cells. These new imaging tools should be of great help to better understand PAK1 biology and to conceive strategies for efficient and specific PAK1 inhibitors.

Proteasomal degradation of Nck1 but not Nck2 regulates RhoA activation and actin dynamics

The ubiquitously expressed adapter proteins Nck1/2 interact with a multitude of effector molecules to regulate diverse cellular functions including cytoskeletal dynamics. Here we show that Nck1, but not Nck2, is a substrate of c-Cbl-mediated ubiquitination. We uncover lysine 178 in Nck1 as the evolutionarily conserved ubiquitin acceptor site. We previously reported that synaptopodin, a proline-rich actin-binding protein, induces stress fibres by blocking the Smurf1-mediated ubiquitination of RhoA. We now find that synaptopodin competes with c-Cbl for binding to Nck1, which prevents the ubiquitination of Nck1 by c-Cbl. Gene silencing of c-Cbl restores Nck1 protein abundance and stress fibres in synaptopodin knockdown cells. Similarly, expression of c-Cbl-resistant Nck1(K178R) or Nck2 containing the SH3 domain 2 of Nck1 restores stress fibres in synaptopodin-depleted podocytes through activation of RhoA signalling. These findings reveal proteasomal regulation as a key factor in the distinct and non-redundant effects of Nck on RhoA-mediated actin dynamics.

EphA4-dependent Brachyury expression is required for dorsal mesoderm involution in the Xenopus gastrula

Xenopus provides a well-studied model of vertebrate gastrulation, but a central feature, the movement of the mesoderm to the interior of the embryo, has received little attention. Here, we analyze mesoderm involution at the Xenopus dorsal blastopore lip. We show that a phase of rapid involution - peak involution - is intimately linked to an early stage of convergent extension, which involves differential cell migration in the prechordal mesoderm and a new movement of the chordamesoderm, radial convergence. The latter process depends on Xenopus Brachyury, the expression of which at the time of peak involution is controlled by signaling through the ephrin receptor, EphA4, its ligand ephrinB2 and its downstream effector p21-activated kinase. Our findings support a conserved role for Brachyury in blastopore morphogenesis.

The role of p21-activated kinases in hepatocellular carcinoma metastasis

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

Coordinated activation of the Rac-GAP β2-chimaerin by an atypical proline-rich domain and diacylglycerol

Chimaerins, a family of GTPase activating proteins (GAPs) for the small G-protein Rac, have been implicated in development, neuritogenesis, and cancer. These Rac-GAPs are regulated by the lipid second messenger diacylglycerol (DAG) generated by tyrosine-kinases such as the epidermal growth factor receptor (EGFR). Here we identify an atypical Pro-rich motif in chimaerins that binds to the adaptor protein Nck1. Unlike most Nck1 partners, chimaerins bind to the third SH3 domain of Nck1. This association is mediated by electrostatic interactions of basic residues within the Pro-rich motif with acidic clusters in the SH3 domain. EGF promotes the binding of β2-chimaerin to Nck1 in the cell periphery in a DAG-dependent manner. Moreover, β2-chimaerin translocation to the plasma membrane and its peripheral association with Rac1 requires Nck1. Our studies underscore a coordinated mechanism for β2-chimaerin activation that involves lipid interactions via the C1 domain and protein-protein interactions via the N-terminal Pro-rich region.

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.

Phosphatase of regenerating liver 3 (PRL3) provokes a tyrosine phosphoproteome to drive prometastatic signal transduction

Phosphatase of regenerating liver 3 (PRL3) is suspected to be a causative factor toward cellular metastasis when in excess. To date, the molecular basis for PRL3 function remains an enigma, making efforts at distilling a concerted mechanism for PRL3-mediated metastatic dissemination very difficult. We previously discovered that PRL3 expressing cells exhibit a pronounced increase in protein tyrosine phosphorylation. Here we take an unbiased mass spectrometry-based approach toward identifying the phosphoproteins exhibiting enhanced levels of tyrosine phosphorylation with a goal to define the "PRL3-mediated signaling network." Phosphoproteomic data support intracellular activation of an extensive signaling network normally governed by extracellular ligand-activated transmembrane growth factor, cytokine, and integrin receptors in the PRL3 cells. Additionally, data implicate the Src tyrosine kinase as the major intracellular kinase responsible for "hijacking" this network and provide strong evidence that aberrant Src activation is a major consequence of PRL3 overexpression. Importantly, the data support a PDGF(α/β)-, Eph (A2/B3/B4)-, and Integrin (β1/β5)-receptor array as being the predominant network coordinator in the PRL3 cells, corroborating a PRL3-induced mesenchymal-state. Within this network, we find that tyrosine phosphorylation is increased on a multitude of signaling effectors responsible for Rho-family GTPase, PI3K-Akt, STAT, and ERK activation, linking observations made by the field as a whole under Src as a primary signal transducer. Our phosphoproteomic data paint the most comprehensive picture to date of how PRL3 drives prometastatic molecular events through Src activation.

Evidence for a novel mechanism of the PAK1 interaction with the Rho-GTPases Cdc42 and Rac

P21-activated kinase 1 (PAK1) is activated by binding to GTP-bound Rho GTPases Cdc42 and Rac via its CRIB domain. Here, we provide evidence that S79 in the CRIB domain of PAK1 is not directly involved in this binding but is crucial for PAK1 activation. S79A mutation reduces the binding affinity of PAK1 for the GTPases and inhibits autophosphorylation and kinase activity of PAK1. Thus, this mutation abrogates the ability of PAK1 to induce changes in cell morphology and motility and to promote malignant transformation of prostate epithelial cells. We also show that growth of the prostate cancer cell line PC3 is inhibited by the treatment of a PAK1-inhibiting peptide comprising 19 amino acids centered on S79, but not by the PAK1 peptide containing the S79A mutation, and that this growth inhibition is correlated with reduced autophosphorylation activity of PAK1. Together, these findings demonstrate a significant role of S79 in PAK1 activation and provide evidence for a novel mechanism of the CRIB-mediated interaction of PAK1 with Cdc42 and Rac.

Protein kinase CK2 phosphorylates and activates p21-activated kinase 1

Activation of the p21-activated kinase 1 (PAK1) is achieved through a conformational change that converts an inactive PAK1 dimer to an active monomer. In this paper, we show that this change is necessary but not sufficient to activate PAK1 and that it is, rather, required for CK2-dependent PAK1(S223) phosphorylation that converts a monomeric PAK1 into a catalytically active form. This phosphorylation appears to be essential for autophosphorylation at specific residues and overall activity of PAK1. A phosphomimetic mutation (S223E) bypasses the requirement for GTPases in PAK1 activation, whereas the constitutive activity of the PAK1 mutant (PAK1(H83,86L)), postulated to mimic GTPase-induced structural changes, is abolished by inhibition of S223 phosphorylation. Thus, S223 is likely accessible to CK2 upon conformational changes of PAK1 induced by GTPase-dependent and GTPase-independent stimuli, suggesting that S223 phosphorylation may play a key role in the final step of the PAK1 activation process. The physiological significance of this phosphorylation is reinforced by the observations that CK2 is responsible for epidermal growth factor-induced PAK1 activation and that inhibition of S223 phosphorylation abrogates PAK1-mediated malignant transformation of prostate epithelial cells. Taken together, these findings identify CK2 as an upstream activating kinase of PAK1, providing a novel mechanism for PAK1 activation.

Integration of signaling and cytoskeletal remodeling by Nck in directional cell migration

Planar and apical-basal cellular polarization of epithelia and endothelia are crucial during morphogenesis. The establishment of these distinct polarity states and their transitions are regulated by signaling networks that include polarity complexes, Rho GTPases, and phosphoinositides. The spatiotemporal coordination of signaling by these molecules modulates cytoskeletal remodeling and vesicle trafficking to specify membrane domains, a prerequisite for the organization of tissues and organs. Here we present an overview of how activation of the WASp/Arp2/3 pathway of actin remodeling by Nck coordinates directional cell migration and speculate on its role as a signaling integrator in the coordination of cellular processes involved in endothelial cell polarity and vascular lumen formation.

Nck enables directional cell migration through the coordination of polarized membrane protrusion with adhesion dynamics

Directional migration requires the coordination of cytoskeletal changes essential for cell polarization and adhesion turnover. Extracellular signals that alter tyrosine phosphorylation drive directional migration by inducing reorganization of the actin cytoskeleton. It is recognized that Nck is an important link between tyrosine phosphorylation and actin dynamics, however, the role of Nck in cytoskeletal remodeling during directional migration and the underlying molecular mechanisms remain largely undetermined. In this study, a combination of molecular genetics and quantitative live cell microscopy was used to show that Nck is essential in the establishment of front-back polarity and directional migration of endothelial cells. Time-lapse differential interference contrast and total internal reflection fluorescence microscopy showed that Nck couples the formation of polarized membrane protrusions with their stabilization through the assembly and maturation of cell-substratum adhesions. Measurements by atomic force microscopy showed that Nck also modulates integrin α5β1-fibronectin adhesion force and cell stiffness. Fluorescence resonance energy transfer imaging revealed that Nck depletion results in delocalized and increased activity of Cdc42 and Rac. In contrast, the activity of RhoA and myosin II phosphorylation were reduced by Nck knockdown. Thus, this study identifies Nck as a key coordinator of cytoskeletal changes that enable cell polarization and directional migration which are critical processes in development and disease.

p130Cas: a key signalling node in health and disease

p130Cas/breast cancer anti-oestrogen resistance 1 (BCAR1) is a member of the Cas (Crk-associated substrate) family of adaptor proteins, which have emerged as key signalling nodes capable of interactions with multiple proteins, with important regulatory roles in normal and pathological cell function. The Cas family of proteins is characterised by the presence of multiple conserved motifs for protein-protein interactions, and by extensive tyrosine and serine phosphorylations. Recent studies show that p130Cas contributes to migration, cell cycle control and apoptosis. p130Cas is essential during early embryogenesis, with a critical role in cardiovascular development. Furthermore, p130Cas has been reported to be involved in the development and progression of several human cancers. p130Cas is able to perform roles in multiple processes due to its capacity to regulate a diverse array of signalling pathways, transducing signals from growth factor receptor tyrosine kinases, non-receptor tyrosine kinases, and integrins. In this review we summarise the current understanding of the structure, function, and regulation of p130Cas, and discuss the importance of p130Cas in both physiological and pathophysiological settings, with a focus on the cardiovascular system and cancer.

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.

EphB2 receptor forward signaling controls cortical growth cone collapse via Nck and Pak

EphB receptors and their ephrinB ligands transduce bidirectional signals that mediate contact-dependent axon guidance primarily by promoting growth cone repulsion. However, how EphB receptor-mediated forward signaling induces axonal repulsion remains poorly understood. Here, we identify Nck and Pak proteins as essential forward signaling components of EphB2-dependent growth cone collapse in cortical neurons. We show that kinase-active EphB2 binds to Pak and promotes growth cone repulsion via Pak kinase activity, Pak-Nck binding, RhoA signaling and endocytosis. However, Pak's function in this context appears to be independent of Rac/Cdc42-GTP, consistent with the absence of Rac-GTP production after ephrinB treatment of cortical neurons. Taken together, our findings suggest that ephrinB-activated EphB2 receptors recruit a novel Nck/Pak signaling complex to mediate repulsive cortical growth cone guidance, which may be relevant for EphB forward signaling-dependent axon guidance in vivo.

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.

A Rac1/PAK1 cascade controls β-catenin activation in colon cancer cells

P21-activated kinase 1 (PAK1) is associated with colon cancer progression and metastasis, whereas the molecular mechanism remains elusive. Here, we show that downregulation of PAK1 in colon cancer cells reduces total β-catenin level, as well as cell proliferation. Mechanistically, PAK1 directly phosphorylates β-catenin proteins at Ser675 site and this leads to more stable and transcriptional active β-catenin. Corroborating these results, PAK1 is required for full Wnt signaling, and superactivation of β-catenin is achieved by simultaneous knockdown of adenomatous polyposis coli protein and activation of PAK1. Moreover, we show that Rac1 functions upstream of PAK1 in colon cancer cells and contributes to β-catenin phosphorylation and accumulation. We conclude that a Rac1/PAK1 cascade controls β-catenin S675 phosphorylation and full activation in colon cancer cells. Supporting this conclusion, overexpression of PAK1 is observed in 70% of colon cancer samples and is correlated with massive β-catenin accumulation.

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.

Dexamethasone increases the phosphorylation of nephrin in cultured podocytes

BACKGROUND

We reported that nephrin is phosphorylated at Y1204 and Y1228 under normal conditions and that the phosphorylation is decreased in puromycin nephrosis and in human minimal change nephrosis. These results indicate that the phosphorylation of nephrin is important for maintaining normal podocyte function. However, little is known about the regulation of nephrin phosphorylation. Here, we investigated whether glucocorticoid, a drug used to treat glomerular diseases with proteinuria, might affect the phosphorylation of nephrin.

METHODS

Human cultured podocytes transiently expressing human nephrin were treated with dexamethasone (Dex), and the phosphorylation of nephrin was determined by immunoblot with the anti-pY1228 antibody.

RESULTS

Dex treatment for 24 h increased the phosphorylation of nephrin; this increased phosphorylation was inhibited by the glucocorticoid receptor antagonist but not by the mineral corticoid receptor antagonist. A shorter incubation time (30 min) did not increase the phosphorylation, and actinomycin D and cycloheximide treatments abolished the increased phosphorylation. The activation of Src-family kinases was correlated with nephrin phosphorylation, both of which were abolished by small interfering RNA (siRNA) treatment for serum/glucocorticoid-induced kinase 1 (SGK1).

CONCLUSIONS

These results clarify a novel action of glucocorticoid on nephrin phosphorylation through SGK1. Glucocorticoid treatment for human glomerulonephritis may exert its function by regulating the phosphorylation of nephrin.

The C. elegans nck-1 gene encodes two isoforms and is required for neuronal guidance

The NCK adaptor proteins are composed entirely of SH3 and SH2 domains and serve as protein interaction bridges for several receptors during signal transduction events. Here we report the molecular and genetic analysis of the Caenorhabditis elegans nck-1 gene. C. elegans nck-1 encodes two isoforms: NCK-1A and a shorter isoform that lacks the first SH3 domain, NCK-1B. C. elegans nck-1 mutants exhibit defects in axon guidance and neuronal cell position, as well as defects in the excretory canal cell, gonad, and male mating. NCK-1 is broadly expressed in neurons and epithelial cells with NCK-1B being the most abundant isoform. NCK-1A and NCK-1B share a similar expression pattern in parts of the nervous system, but also have independent expression patterns in other tissues. Interestingly, NCK-1B is localized to the nuclei of many cells. Genetic rescue experiments show that NCK-1 functions cell autonomously and, in general, either NCK-1A or NCK-1B is sufficient to function in axon guidance. However, there appears to be specific roles for each isoform, for example NCK-1B is required for HSN cell migration while NCK-1A is required for efficient male mating. Genetic epistasis experiments show that NCK-1 functions redundantly with the LAR Receptor Tyrosine Phosphatase, PTP-3, and the Netrin receptor UNC-40.

Rac1 modulation of the apical domain is negatively regulated by β (Heavy)-spectrin

Epithelial polarity and morphogenesis require the careful coordination of signaling and cytoskeletal elements. In this paper, we describe multiple genetic interactions between the apical cytoskeletal protein β(H) and Rac1 signaling in Drosophila: activation of Rac1 signaling by expression of the exchange factor Trio, is strongly enhanced by reducing β(H) levels, and such reductions in β(H) levels alone are shown to cause an increase in GTP-Rac1 levels. In contrast, co-expression of a C-terminal fragment of β(H) (βH33) suppresses the Trio expression phenotype. In addition, sustained expression of βH33 alone in the eye induces a strong dominant phenotype that is similar to the expression of dominant negative Rac1(N17), and this phenotype is also suppressed by the co-expression of Trio or by knockdown of RacGAP50C. We further demonstrate that a loss-of-function allele in pak, a Rac1 effector and negative regulator of β(H)' dominantly suppresses larval lethality arising loss-of-function karst (β(H)) alleles. Furthermore, expression of constitutively active Pak(myr) in the larval salivary gland induces expansion of the apical membrane and destabilization of the apical polarity determinants Crumbs and aPKC. These effects resemble a Rac1 activation phenotype and are suppressed by βH33. Together, our data suggest that apical proteins including β(H) are negatively regulated by Rac1 activation, but that Rac1 signaling is also suppressed by β(H) through its C-terminal domain. Such a system would be bistable with either Rac1 or β(H) predominant. We suggest a model for apical domain maintenance wherein Rac1 down-regulation of β(H) (via Pak) is opposed by β(H)-mediated down-regulation of Rac1 signaling.

Phosphorylation status of nephrin in human membranous nephropathy

BACKGROUND

We recently reported that nephrin, a major slit-diaphragm protein, is phosphorylated at Y1204 and Y1228 in normal human glomeruli and that phosphorylation decreased significantly in minimal-change nephrosis. These results indicate that phosphorylation of nephrin is important for maintenance of normal podocyte morphology and function. On the other hand, phosphorylation of nephrin was reportedly increased in certain animal models of glomerular injury.

METHODS

We performed immunofluorescent and immunoelectron staining of phosphorylated nephrin in human kidney biopsy specimens of membranous nephropathy (MN) to investigate whether phosphorylation of nephrin was altered in human MN and whether it correlated with MN staging.

RESULTS

Although aberrant localization of phosphorylated nephrin was detected using immunoelectron microscopy in stage I MN, a decrease in the immunofluorescent intensity of phosphorylated nephrin was not observed in stage I, and only a slight decrease was seen in stages II, III, and IV compared with controls. No significant correlation between nephrin phosphorylation and proteinuria was observed.

CONCLUSION

Nephrin phosphorylation was not significantly decreased in the early stage of MN.

Role of p21-activated kinase in cell polarity and directional mesendoderm migration in the Xenopus gastrula

The p21 activated kinases (Paks) are prominently involved in the regulation of cell motility. Using a kinase-dead mutant of xPak1, we show that during Xenopus gastrulation, the kinase activity of Pak1 is required upstream of Cdc42 for the establishment of cell polarity in the migrating mesendoderm. Overactivation of Pak1 function by the expression of constitutively active xPak1 compromises the maintenance of cell polarity, by indirectly inhibiting RhoA function. Inhibition of cell polarization does not affect the migration of single mesendoderm cells. However, Pak1 inhibition interferes with the guidance of mesendoderm migration by directional cues residing in the extracellular matrix of the blastocoel roof, and with mesendoderm translocation in the embryo.

Pak protein kinases and their role in cancer

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

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.

Dissecting activation of the PAK1 kinase at protrusions in living cells

The p21-activated kinase (PAK) 1 kinase, an effector of the Cdc42 and Rac1 GTPases, regulates cell protrusions and motility by controlling actin and adhesion dynamics. Its deregulation has been linked to human cancer. We show here that activation of PAK1 is necessary for protrusive activity during cell spreading. To investigate PAK1 activation dynamics at live protrusions, we developed a conformational biosensor, based on fluorescence resonance energy transfer. This novel PAK1 biosensor allowed the spatiotemporal visualization of PAK1 activation during spreading of COS-7 cells and during motility of normal rat kidney cells. By using this imaging approach in COS-7 cells, the following new insights on PAK1 regulation were unveiled. First, PAK1 acquires an intermediate semi-open conformational state upon recruitment to the plasma membrane. This semi-open PAK1 species is selectively autophosphorylated on serines in the N-terminal regulatory region but not on the critical threonine 423 in the catalytic site. Second, this intermediate PAK1 state is hypersensitive to stimulation by Cdc42 and Rac1. Third, interaction with PIX proteins contributes to PAK1 stimulation at membrane protrusions, in a GTPase-independent way. Finally, trans-phosphorylation events occur between PAK1 molecules at the membrane possibly playing a relevant role for its activation. This study leads to a model for the complex and accurate regulation of PAK1 kinase in vivo at cell protrusions.

Sns and Kirre, the Drosophila orthologs of Nephrin and Neph1, direct adhesion, fusion and formation of a slit diaphragm-like structure in insect nephrocytes

The Immunoglobulin superfamily (IgSF) proteins Neph1 and Nephrin are co-expressed within podocytes in the kidney glomerulus, where they localize to the slit diaphragm (SD) and contribute to filtration between blood and urine. Herein, we demonstrate that their Drosophila orthologs Kirre (Duf) and Sns are co-expressed within binucleate garland cell nephrocytes (GCNs) that contribute to detoxification of the insect hemolymph by uptake of molecules through an SD-like nephrocyte diaphragm (ND) into labyrinthine channels that are active sites of endocytosis. The functions of Kirre and Sns in the embryonic musculature, to mediate adhesion and fusion between myoblasts to form multinucleate muscle fibers, have been conserved in the GCNs, where they contribute to adhesion of GCNs in the ;garland' and to their fusion into binucleate cells. Sns and Kirre proteins localize to the ND at the entry point into the labyrinthine channels and, like their vertebrate counterparts, are essential for its formation. Knockdown of Kirre or Sns drastically reduces the number of NDs at the cell surface. These defects are associated with a decrease in uptake of large proteins, suggesting that the ND distinguishes molecules of different sizes and controls access to the channels. Moreover, mutations in the Sns fibronectin-binding or immunoglobulin domains lead to morphologically abnormal NDs and to reduced passage of proteins into the labyrinthine channels for uptake by endocytosis, suggesting a crucial and direct role for Sns in ND formation and function. These data reveal significant similarities between the insect ND and the SD in mammalian podocytes at the level of structure and function.

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.