p21-activated kinase 1 (Pak1) regulates cell motility in mammalian fibroblasts

PAK1-mediated activation of ERK1/2 regulates lamellipodial dynamics

PAK1 is a member of the p21-activated kinase (PAK) family of serine/threonine kinases that are activated by the Rho GTPases Rac and Cdc42, and are implicated in regulating morphological polarity, cell migration and adhesion. Here we investigate the function of PAK1 in cell motility using macrophages derived from PAK1-null mice. We show that CSF1, a macrophage chemoattractant, transiently stimulates PAK1 and MAPK activation, and that MAPK activation is reduced in PAK1-/- macrophages. PAK1 regulates the dynamics of lamellipodium extension as cells spread in response to adhesion but is not essential for macrophage migration or chemotaxis towards CSF1. Following adhesion, PAK1-/- macrophages spread more rapidly and have more lamellipodia than wild-type cells; however, these lamellipodia were less stable than those in wild-type macrophages. ERK1/2 activity was reduced in PAK1-/- macrophages during adhesion, and inhibition of ERK1/2 activation in wild-type macrophages was sufficient to increase the spread area and mimic the lamellipodial dynamics of PAK1-/- macrophages. Together, these data indicate that PAK1 signals via ERK1/2 to regulate lamellipodial stability.

Scaffolding function of PAK in the PDK1-Akt pathway

Many extracellular signals stimulate phosphatidylinositol-3-kinase, which in turn activates the Rac1 GTPase, the protein kinase Akt and the Akt Thr 308 upstream kinase PDK1. Active Rac1 stimulates a number of events, including substrate phosphorylation by a subgroup of the PAK family of kinases. The combined effects of Rac1, PDK1 and Akt are crucial for cell migration, growth, survival, metabolism and tumorigenesis. Here we show that Rac1 stimulates a second, kinase-independent function of PAK1. The PAK1 kinase domain serves as a scaffold to facilitate Akt stimulation by PDK1 and to aid recruitment of Akt to the membrane. PAK differentially activates subpopulations of Akt. These findings reveal scaffolding functions of PAK that regulate the efficiency, localization and specificity of the PDK1-Akt pathway.

Rac1 promotes intestinal epithelial restitution by increasing Ca2+ influx through interaction with phospholipase C-(gamma)1 after wounding

Intestinal mucosal restitution occurs as a consequence of epithelial cell migration and reseals superficial wounds after injury. This rapid reepithelialization is mediated in part by a phospholipase C-gamma1 (PLC-gamma1)-induced Ca(2+) signaling, but the exact mechanism underlying such signaling and its regulation remains elusive. The small GTP-binding protein Rac1 functions as a pivotal regulator of several signaling networks and plays an important role in regulating cell motility. The current study tests the hypothesis that Rac1 modulates intestinal epithelial cell migration after wounding by altering PLC-gamma1-induced Ca(2+) signaling. Inhibition of Rac1 activity by treatment with its inhibitor NSC-23766 or Rac1 silencing with small interfering RNA decreased store depletion-induced Ca(2+) influx and suppressed cell migration during restitution, whereas ectopic overexpression of Rac1 increased Ca(2+) influx and promoted cell migration. Rac1 physically interacted with PLC-gamma1 and formed Rac1/PLC-gamma1 complex in intestinal epithelial cells. PLC-gamma1 silencing in cells overexpressing Rac1 prevented stimulation of store depletion-induced Ca(2+) influx and cell migration after wounding. Polyamine depletion inhibited expression of both Rac1 and PLC-gamma1, decreased Rac1/PLC-gamma1 complex levels, reduced Ca(2+) influx, and repressed cell migration. Overexpression of Rac1 alone failed to rescue Ca(2+) influx after store depletion and cell migration in polyamine-deficient cells, because it did not alter PLC-gamma1 levels. These results indicate that Rac1 promotes intestinal epithelial cell migration after wounding by increasing Ca(2+) influx as a result of its interaction with PLC-gamma1.

Small GTPases in mechanosensitive regulation of endothelial barrier

Alterations in vascular permeability are defining feature of diverse processes including atherosclerosis, inflammation, ischemia/reperfusion injury, and ventilator-induced lung injury. Clinical observations and experimental studies support an essential role of mechanical forces in pathophysiologic regulation of lung barrier. Accumulating data demonstrate that decreased levels of blood flow and increased cyclic stretch of lung tissues associated with lung mechanical ventilation at high tidal volumes increase vascular permeability, activate inflammatory cytokine production, alveolar flooding, leukocyte infiltration, and hypoxemia, and increase morbidity and mortality. Potential synergism between pathologic mechanical stimulation and inflammatory molecules resulting in vascular leak and lung injury becomes increasingly recognized. This review will discuss a role of Rho family of small GTPases in the mechanochemical regulation of pulmonary endothelial permeability associated with ventilator induced lung injury.

Synthetic morphology: prospects for engineered, self-constructing anatomies

This paper outlines prospects for applying the emerging techniques of synthetic biology to the field of anatomy, with the aim of programming cells to organize themselves into specific, novel arrangements, structures and tissues. There are two main reasons why developing this hybrid discipline--synthetic morphology--would be useful. The first is that having a way to engineer self-constructing assemblies of cells would provide a powerful means of tissue engineering for clinical use in surgery and regenerative medicine. The second is that construction of simple novel systems according to theories of morphogenesis gained from study of real embryos will provide a means of testing those theories rigorously, something that is very difficult to do by manipulation of complex embryos. This paper sets out the engineering requirements for synthetic morphology, which include the development of a library of sensor modules, regulatory modules and effector modules that can be connected functionally within cells. A substantial number of sensor and regulatory modules already exist and this paper argues that some potential effector modules have already been identified. The necessary library may therefore be within reach. The paper ends by suggesting a set of challenges, ranging from simple to complex, the achievement of which would provide valuable proofs of concept.

Pak1 and Pak2 mediate tumor cell invasion through distinct signaling mechanisms

Pak kinases are thought to play critical roles in cell migration and invasion. Here, we analyze the roles of Pak1 and Pak2 in breast carcinoma cell invasion using the transient transfection of small interfering RNA. We find that although both Pak1 and Pak2 contribute to breast carcinoma invasion stimulated by heregulin, these roles are mediated by distinct signaling mechanisms. Thus, whereas the depletion of Pak1 interferes with the heregulin-mediated dephosphorylation of cofilin, the depletion of Pak2 does not. The depletion of Pak1 also has a stronger inhibitory effect on lamellipodial protrusion than does the depletion of Pak2. Interestingly, Pak1 and Pak2 play opposite roles in regulating the phosphorylation of the myosin light chain (MLC). Whereas the depletion of Pak1 decreases phospho-MLC levels in heregulin-stimulated cells, the depletion of Pak2 enhances MLC phosphorylation. Consistent with their opposite effects on MLC phosphorylation, Pak1 and Pak2 differentially modulate focal adhesions. Pak2-depleted cells display an increase in focal adhesion size, whereas in Pak1-depleted cells, focal adhesions fail to mature. We also found that the depletion of Pak2, but not Pak1, enhances RhoA activity and that the inhibition of RhoA signaling in Pak2-depleted cells decreases MLC phosphorylation and restores cell invasion. In summary, this work presents the first comprehensive analysis of functional differences between the Pak1 and Pak2 isoforms.

Prohibitin-1 maintains the angiogenic capacity of endothelial cells by regulating mitochondrial function and senescence

Prohibitin 1 (PHB1) is a highly conserved protein that is mainly localized to the inner mitochondrial membrane and has been implicated in regulating mitochondrial function in yeast. Because mitochondria are emerging as an important regulator of vascular homeostasis, we examined PHB1 function in endothelial cells. PHB1 is highly expressed in the vascular system and knockdown of PHB1 in endothelial cells increases mitochondrial production of reactive oxygen species via inhibition of complex I, which results in cellular senescence. As a direct consequence, both Akt and Rac1 are hyperactivated, leading to cytoskeletal rearrangements and decreased endothelial cell motility, e.g., migration and tube formation. This is also reflected in an in vivo angiogenesis assay, where silencing of PHB1 blocks the formation of functional blood vessels. Collectively, our results provide evidence that PHB1 is important for mitochondrial function and prevents reactive oxygen species-induced senescence and thereby maintains the angiogenic capacity of endothelial cells.

ERK5 differentially regulates PDGF-induced proliferation and migration of hepatic stellate cells

BACKGROUND/AIMS

Hepatic stellate cells (HSC) are liver-specific pericytes implicated in liver tissue repair. Activation of signaling pathways in HSC modulates hepatic fibrogenesis, but no information is available on the possible role of ERK5, a member of the mitogen-activated protein kinase family, in this process. In this study, we investigated the role of ERK5 in the biologic responses triggered by platelet-derived growth factor (PDGF) in HSC.

METHODS

Human HSC were cultured on plastic and studied in their myofibroblast-like phenotype.

RESULTS

PDGF-BB rapidly induced ERK5 activation and translocation to the nucleus. EGF and PDGF-DD were also found to activate ERK5. Interfering with Src activation blocked PDGF-BB-dependent ERK5 phosphorylation. To establish the biological significance of ERK5 activation, HSC were transfected with non-targeting siRNA or siRNA targeting ERK5. ERK5 silencing inhibited PDGF-BB-induced cell proliferation, and expression and activation of c-Jun. In contrast, depletion of ERK5 was associated with significantly increased cell migration, both in the presence or absence of PDGF-BB. This effect was associated with a redistribution of focal contacts, and with decreased phosphorylation of FAK, paxillin, and PAK.

CONCLUSIONS

ERK5 modulates PDGF-dependent biologic activities in human HSC, generating positive signals for cell proliferation downregulating the ability of the cells to migrate.

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.

Stimulus-dependent regulation of the phagocyte NADPH oxidase by a VAV1, Rac1, and PAK1 signaling axis

The p21-activated kinase-1 (PAK1) is best known for its role in the regulation of cytoskeletal and transcriptional signaling pathways. We show here in the microglia cell line Ra2 that PAK1 regulates NADPH oxidase (NOX-2) activity in a stimulus-specific manner. Thus, conditional expression of PAK1 dominant-positive mutants enhanced, whereas dominant-negative mutants inhibited, NADPH oxidase-mediated superoxide generation following formyl-methionyl-leucylphenylalanine or phorbol 12-myristate 13-acetate stimulation. Both Rac1 and the GTP exchange factor VAV1 were required as upstream signaling proteins in the formyl-methionyl-leucyl-phenylalanine-induced activation of endogenous PAK1. In contrast, PAK1 mutants had no effect on superoxide generation downstream of FcgammaR signaling during phagocytosis of IgG-immune complexes. We further present evidence that the effect of PAK1 on the respiratory burst is mediated through phosphorylation of p47(Phox), and we show that expression of a p47(Phox) (S303D/S304D/S320D) mutant, which mimics phosphorylation by PAK1, induced basal superoxide generation in vivo. In contrast PAK1 substrates LIMK-1 or RhoGDI are not likely to contribute to the PAK1 effect on NADPH oxidase activation. Collectively, our findings define a VAV1-Rac1-PAK1 signaling axis in mononuclear phagocytes regulating superoxide production in a stimulus-dependent manner.

Quantifying myosin light chain phosphorylation in single adherent cells with automated fluorescence microscopy

BACKGROUND

In anchorage dependent cells, myosin generated contractile forces affect events closely associated with adhesion such as the formation of stress fibers and focal adhesions, and temporally distal events such as entry of the cell into S-phase. As occurs in many signaling pathways, a phosphorylation reaction (in this case, phosphorylation of myosin light chain) is directly responsible for cell response. Western blotting has been useful in measuring intracellular phosphorylation events, but cells are lysed in the process of sample preparation for western blotting, and spatial information such as morphology, localization of the phosphorylated species, and the distribution of individual cell responses across the population is lost. We report here a reliable automated microscopy method for quantitative measurement of myosin light chain phosphorylation in adherent cells. This method allows us to concurrently examine cell morphology, cell-cell contact, and myosin light chain diphosphorylation in vascular smooth muscle cells.

RESULTS

Paraformaldehyde fixation and Triton X-100 permeabilization preserved cell morphology and myosin light chain phosphorylation better than the alternative fixation/permeabilization methods tested. We utilized automated microscopy methods to acquire three color images, determine cell spread area, and quantify the intensity of staining within each cell with anti-phospho-MLC antibody. Our results indicate that A10 rat aortic smooth muscle cells exhibit a re producible non-Gaussian distribution of MLC phosphorylation across a population of unsynchronized genetically identical cells. Adding an inhibitor of Rho kinase, Y27632, or plating cells on a low density of fibronectin, reduced phospho-myosin light chain signal as expected. On the other hand, adding calyculin A, an activator of contractility, increased myosin light chain phosphorylation. The IC50 for myosin light chain phosphorylation using Y27632 was determined to be 2.1 +/- 0.6 micrometers. We observed a positive linear relationship between cell area and myosin light chain diphosphorylation, which is consistent with what has been reported in the literature using other methods.

CONCLUSION

Our results show that using proper specimen fixation techniques and background subtraction methods, imaging cytometry can be used to reliably measure relative myosin light chain phosphorylation in individual adherent cells. Importantly, the ability to make this measurement in adherent cells allows for simultaneous measurement of and correlation with other parameters of cellular topography such as morphology and cell-cell proximity. This assay has potential application in screening for drug development.

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.

Localized activation of p21-activated kinase controls neuronal polarity and morphology

In the developing forebrain, neuronal polarization is a stepwise and initially reversible process that underlies correct migration and axon specification. Many aspects of cytoskeletal changes that accompany polarization are currently molecularly undefined and thus poorly understood. Here we reveal that the p21-activated kinase (Pak1) is essential for the specification of an axon and dendrites. In hippocampal neurons, activation of Pak1 is spatially restricted to the immature axon despite its uniform presence in all neurites. Hyperactivation of Pak1 at the membrane of all neurites or loss of Pak1 expression disrupts both neuronal morphology and the distinction between an axon and dendrites. We reveal that Pak1 acts on polarity in a kinase-dependent manner, by affecting the F-actin and microtubule cytoskeleton at least in part through Rac1 and cofilin. Our data are the first to demonstrate the importance of localized Pak1 kinase activation for neuronal polarization and differentiation.

2-amino-N-{4-[5-(2-phenanthrenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]-phenyl} acetamide (OSU-03012), a celecoxib derivative, directly targets p21-activated kinase

p21-Activated kinases (PAKs) are regulators of cell motility and proliferation. PAK activity is regulated in part by phosphoinositide-dependent kinase 1 (PDK1). We hypothesized that reduced PAK activity was involved in the effects of 2-amino-N-{4-[5-(2-phenanthrenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]-phenyl} acetamide (OSU-03012), a previously characterized PDK1 inhibitor derived from celecoxib. In three human thyroid cancer cell lines, OSU-03012 inhibited cell proliferation with reduced AKT phosphorylation by PDK1. OSU-03012 unexpectedly inhibited PAK phosphorylation at lower concentrations than PDK1-dependent AKT phosphorylation in two of the three lines. In cell-free kinase assays, OSU-03012 was shown to inhibit PAK activity and compete with ATP binding. In addition, computer modeling predicted a docking site for OSU-03012 in the ATP binding motif of PAK1. Finally, overexpression of constitutively activated PAK1 partially rescued the ability of motile NPA thyroid cancer cells to migrate during OSU-03012 treatment, suggesting that inhibition of PAK may be involved in the cellular effects of OSU-03012 in these cells. In summary, OSU-03012 is a direct inhibitor of PAK, and inhibition of PAK, either directly or indirectly, may be involved in its biological effects in vitro.

Tumorhead distribution to cytoplasmic membrane of neural plate cells is positively regulated by Xenopus p21-activated kinase 1 (X-PAK1)

Tumorhead (TH) regulates neural plate cell proliferation during Xenopus early development, and gain or loss of function prevents neural differentiation. TH shuttles between the nuclear and cytoplasmic/cortical cell compartments in embryonic cells. In this study, we show that subcellular distribution of TH is important for its functions. Targeting TH to the cell cortex/membrane potentiates a TH gain of function phenotype and results in neural plate expansion and inhibition of neuronal differentiation. We have found that TH subcellular localization is regulated, and that its shuttling between the nucleus and the cell cortex/cytoplasm is controlled by the catalytic activity of p21-activated kinase, X-PAK1. The phenotypes of embryos that lack, or have excess, X-PAK1 activity mimic the phenotypes induced by loss or gain of TH functions, respectively. We provide evidence that X-PAK1 is an upstream regulator of TH and discuss potential functions of TH at the cell cortex/cytoplasmic membrane and in the nucleus.

The atypical Rho family GTPase Wrch-1 regulates focal adhesion formation and cell migration

Wrch-1 (Wnt-regulated Cdc42 homolog) is a new member of the Rho family that was identified as a gene transcriptionally upregulated by Wnt-1. Wrch-1 has no detectable GTPase activity and displays very high intrinsic guanine nucleotide exchange, implying that it is constitutively GTP-bound. The biological functions of Wrch-1 largely remain to be characterized. Here, we report that Wrch-1 prominently localizes to focal adhesions. Depletion of Wrch-1 by small interfering RNA increases focal adhesion formation, whereas Wrch-1 overexpression disassembles focal adhesions. Wrch-1 depletion inhibits myosin-light-chain phosphorylation, which in turn leads to an increase in the number of focal adhesions and inhibits cell migration in response to wound healing. Depletion of Wrch-1 also inhibits Akt and JNK activation. Although pharmacological inhibitors of Akt and JNK inhibit cell migration, they do not affect focal adhesions. Thus, our data suggest that Wrch-1 regulates cell migration by multiple mechanisms: on the one hand Wrch-1 controls focal adhesions by regulating myosin light chain and on the other hand Wrch-1 stimulates the activation of Akt and JNK.

c-Abl-binding protein interacts with p21-activated kinase 2 (PAK-2) to regulate PDGF-induced membrane ruffles

p21-Activated kinases (PAKs) are serine/threonine kinases involved in multiple cellular functions including cytoskeleton regulation, proliferation and apoptosis. We performed a screen for proteins interacting with PAK-2, a ubiquitously expressed kinase involved in apoptotic signaling. Among the PAK-2 interacting proteins were different members of the Abl-binding protein family. Abl-binding proteins bound to a proline-rich region of PAK-2 located in the regulatory N terminus. Moreover, active PAK-2 phosphorylated Abl-binding proteins in vitro. Interestingly, we show that PAK-2 also interacted with c-Abl but via a different domain than with the Abl-binding proteins. PAK-2 and Abi-1 co-localized in the cytoplasm and to membrane dorsal ruffles induced by PDGF treatment. Expression of mutant PAK-2 deficient in binding to Abl-binding proteins or silencing of PAK-2 expression prevented the formation of membrane dorsal ruffles in response to PDGF. Our findings define a new class of PAK-interacting proteins, which play an important role in actin cytoskeletal reorganization.

A Rac-cGMP signaling pathway

The small GTPase Rac and the second messenger cGMP (guanosine 3',5'-cyclic monophosphate) are critical regulators of diverse cell functions. When activated by extracellular signals via membrane signaling receptors, Rac executes its functions through engaging downstream effectors such as p21-activated kinase (PAK), a serine/threonine protein kinase. However, the molecular mechanism by which membrane signaling receptors regulate cGMP levels is not known. Here we have uncovered a signaling pathway linking Rac to the increase of cellular cGMP. We show that Rac uses PAK to directly activate transmembrane guanylyl cyclases (GCs), leading to increased cellular cGMP levels. This Rac/PAK/GC/cGMP pathway is involved in platelet-derived growth factor-induced fibroblast cell migration and lamellipodium formation. Our findings connect two important regulators of cellular physiological functions and provide a general mechanism for diverse receptors to modulate physiological responses through elevating cellular cGMP levels.

Gene expression changes in long term expanded human neural progenitor cells passaged by chopping lead to loss of neurogenic potential in vivo

Numerous cell culture protocols have been described for the proliferation of multipotent human neural progenitor cells (HNPCs). The mitogen combinations used to expand HNPCs vary, and it is not clear to what extent this may affect the subsequent differentiation of these cells. In this study human foetal cortical tissue was cultured in the presence of either EGF, or FGF-2, or a combination of both using a unique chopping method in which cell to cell contact is maintained. The differentiation potential of neurospheres following mitogen withdrawal was assessed at early (8 weeks) and late (20 weeks) times of expansion, both in vitro and in vivo. In addition, changes in gene expression with time were analysed by microarray experiments. Results show that the presence of FGF-2 was highly predictive of neuronal differentiation after short term culture both in vitro and in vivo. In addition, time in culture had a significant effect on transplant size and neural constituents suggesting that cells have a limited life span and restricted lineage potential. Array analysis confirms that following extensive time in culture cells are entering growth arrest with fundamental expression changes in genes associated with cell cycle regulation, apoptosis and immune functions.

Early herpes simplex virus type 1 infection is dependent on regulated Rac1/Cdc42 signalling in epithelial MDCKII cells

The aim of this study was to understand how molecular determinants of epithelial cells influence initial infection by herpes simplex virus type 1 (HSV-1). Upon infection of the epithelial MDCKII cell line, enhanced association of virus particles with cells forming actin protrusions was observed, suggesting a putative role of actin dynamics in HSV-1 infection. Thus, the impact of the small Rho-like GTPases Rac1, Cdc42 and RhoA acting as key regulators of actin dynamics was addressed. Endogenous Rac1 and Cdc42 were temporarily activated at 15 and 30 min after HSV-1 infection. When constitutively active Cdc42 or Rac1 mutants were expressed transiently, a significant decrease in infectivity was observed, whereas expression of RhoA mutants had no influence. Furthermore, dominant-negative Cdc42 led to decreased infectivity, whereas dominant-negative Rac1 had no effect. So far, the study of potential effectors indicated that Rac1/Cdc42 mutants inhibited infectivity independently of p21-activated kinase (Pak1). The inhibitory effect of Rac1/Cdc42 mutant expression on HSV-1 infection was characterized further and it was found that binding, internalization and transport of HSV-1 were not affected by expression of Rac1/Cdc42 mutants. Thus, these results provide the first evidence for a role of Rac1/Cdc42 signalling during early HSV-1 infection and suggest a mechanism relying on virus-induced regulation of Rac1/Cdc42 activities.

Update on HIV-associated nephropathy

PURPOSE OF REVIEW

HIV-associated nephropathy is characterized by a constellation of pathologic findings including a collapsing glomerulopathy, tubular dilatation, and interstitial infiltration with leukocytes. This review summarizes some of the recent advances in our understanding of the gene products and signaling pathways that contribute to the pathogenesis of HIV-associated nephropathy.

RECENT FINDINGS

Podocytes infected with HIV-associated nephropathy exhibit podocyte proliferation and de-differentiation. Restriction of HIV-1 transgene expression to the podocyte in a murine model supports the belief that podocyte infection is pivotal to the development of the disease. Recent studies have provided compelling in-vitro and in-vivo evidence that expression of the HIV-1 accessory gene nef is critical in altering the phenotype of mature podocytes and causing injury to these cells. An in-vitro study suggests that nef's effects in the podocyte appear to be mediated through Src kinase-dependent activation of the signal transducer and activator of transcription 3 and mitogen-activated protein kinase 1,2 signaling pathways.

SUMMARY

Recent evidence demonstrates that the viral protein nef plays a critical role in the development of HIV-associated nephropathy and provides a foundation for developing new therapeutic strategies for patients afflicted with this disease.

Increased p21-activated kinase-1 expression is associated with invasive potential in uveal melanoma

Prognosis in patients with uveal melanoma is poor as approximately half of all tumors metastasize and currently there are no effective treatments for disseminated disease. Differences in invasiveness between uveal melanomas could therefore be of major significance regarding clinical outcome. To identify genes associated with invasive potential, we have used microarray expression profiling combined with phenotypic characterization of uveal melanoma and melanocyte cell lines to define a gene signature associated with cellular invasion. A panel of 14 uveal cell cultures was assessed using three assays of invasiveness: matrigel invasion chamber system, scratch wound closure and cell motility. We identified a set of 853 differentially expressed transcripts (Wilcoxon-Mann-Whitney test, P<0.01) that discriminated between samples with high or low invasive capacity based on a composite phenotype that takes into account behavior across all three assays. Of particular interest, expression of two members of the p21-activated kinase (PAK) family, PAK1 and PAK7, was elevated in the more invasive cultures. PAK1 has previously been shown to play a central role in regulating cell motility and invasiveness in other cell types, and increased expression has been observed in breast and colorectal carcinomas. Using small interfering RNA-mediated PAK1 knockdown, we showed up to a five-fold decrease in invasion through matrigel, indicating that elevated levels of PAK1 are associated with invasive potential in uveal melanoma. These data implicate PAK1 as a potential new target for therapy of these tumors.

The kinase-inhibitory domain of p21-activated kinase 1 (PAK1) inhibits cell cycle progression independent of PAK1 kinase activity

p21-activated kinase 1 (PAK1) is a mediator of downstream signaling from the small GTPases Rac and Cdc42. In its inactive state, PAK1 forms a homodimer where two kinases inhibit each other in trans. The kinase inhibitory domain (KID) of one molecule of PAK1 binds to the kinase domain of its counterpart and keeps it inactive. Therefore, the isolated KID of PAK1 has been widely used to specifically inhibit and study PAK function. Here, we show that the isolated KID induced a cell cycle arrest with accumulation of cells in the G1 phase of the cell cycle with an inhibition of cyclin D1 and D2 expression. This cell cycle arrest required the intact KID and was also induced by a mutated KID unable to block PAK1 kinase activity. Furthermore, the KID-induced cell cycle arrest could not be rescued by the expression of a constitutively active PAK1-T423E mutant, concluding that this arrest occurs independently of PAK1 kinase activity. Our results suggest that PAK1 through its KID inhibits cyclin D expression and thereby enforces a cell cycle arrest. Our results also call for serious precaution in the use of KID to study PAK function.

Mechanism of polarized protrusion formation on neuronal precursors migrating in the developing chicken cerebellum

Directed cell migration results from the polarization of the cellular motile apparatus by integration of extracellular signals, which are presented in a three-dimensional, spatiotemporal manner in living organisms. To investigate the mechanism underlying the highly polarized and directional nature of migration in vivo, we have developed an imaging system for observing rhombic lip cell migration in the developing chicken cerebellum. First, we show that Cdc42 is the central regulator of the overall polarity, morphology and protrusion formation in these cells. However, perturbation of canonical polarity effectors of Cdc42, e.g. the Par6-Par3-aPKC complex, does not disrupt the cell asymmetry, whereas it affects orientation of the tip of the leading process. In contrast to Cdc42, Rac is required for the generation of protrusions but not the overall polarity. Function interference of class IA phosphoinositide 3-kinase abrogates both directional extension and maintenance of the long leading process, whereas PTEN modulates the size of the protrusion. Actomyosin contractility is important for coordinated spreading of the tip of the leading process in situ. Finally, ErbB4 functions in the generation of protrusions on the rhombic lip cells. These results suggest that polarized protrusion formation on neuronal precursors may occur by a more divergent and complex mechanism than that seen in studies of other cell types growing on planar substrates.

Prostate-specific membrane antigen regulates angiogenesis by modulating integrin signal transduction

The transmembrane peptidase prostate-specific membrane antigen (PSMA) is universally upregulated in the vasculature of solid tumors, but its functional role in tumor angiogenesis has not been investigated. Here we show that angiogenesis is severely impaired in PSMA-null animals and that this angiogenic defect occurs at the level of endothelial cell invasion through the extracellular matrix barrier. Because proteolytic degradation of the extracellular matrix is a critical component of endothelial invasion in angiogenesis, it is logical to assume that PSMA participates in matrix degradation. However, we demonstrate a novel and more complex role for PSMA in angiogenesis, where it is a principal component of a regulatory loop that is tightly modulating laminin-specific integrin signaling and GTPase-dependent, p21-activated kinase 1 (PAK-1) activity. We show that PSMA inhibition, knockdown, or deficiency decreases endothelial cell invasion in vitro via integrin and PAK, thus abrogating angiogenesis. Interestingly, the neutralization of beta(1) or the inactivation of PAK increases PSMA activity, suggesting that they negatively regulate PSMA. This negative regulation is mediated by the cytoskeleton as the disruption of interactions between the PSMA cytoplasmic tail and the anchor protein filamin A decreases PSMA activity, integrin function, and PAK activation. Finally, the inhibition of PAK activation enhances the PSMA/filamin A interaction and, thus, boosts PSMA activity. These data imply that PSMA participates in an autoregulatory loop, wherein active PSMA facilitates integrin signaling and PAK activation, leading to both productive invasion and downregulation of integrin beta(1) signaling via reduced PSMA activity. Therefore, we have identified a novel role for PSMA as a true molecular interface, integrating both extracellular and intracellular signals during angiogenesis.

A Pak- and Pix-dependent branch of the SDF-1alpha signalling pathway mediates T cell chemotaxis across restrictive barriers

Pak (p21-activated kinase) serine/threonine kinases have been shown to mediate directional sensing of chemokine gradients. We hypothesized that Pak may also mediate chemokine-induced shape changes, to facilitate leucocyte chemotaxis through restrictive barriers, such as the extracellular matrix. A potent inhibitor, Pak(i), was characterized and used to probe the role of Pak-family kinases in SDF-1alpha (stromal-cell derived factor-1alpha/CXCL12)-induced chemotaxis in a T cell model. Pak(i) potently inhibited SDF-1alpha-induced Pak activation by a bivalent mechanism, as indicated by its complete inactivation upon point mutation of two binding sites, but partial inactivation upon mutation of either site alone. Importantly, Pak(i) was not toxic to cells over the time frame of our experiments, since it did not substantially affect cell surface expression of CXCR4 (CXC chemokine receptor 4) or integrins, cell cycle progression, or a number of ligand-induced responses. Pak(i) produced dose-dependent inhibition of SDF-1alpha-induced migration through rigid filters bearing small pores; but unexpectedly, did not substantially affect the magnitude or kinetics of chemotaxis through filters bearing larger pores. SDF-1alpha-induced Pak activation was partly dependent on PIX (Pak-interactive exchange factor); correspondingly, an allele of beta-PIX that cannot bind Pak inhibited SDF-1alpha-induced chemotaxis through small, but not large pores. By contrast, other key players in chemotaxis: G(i), PI3K (phosphoinositide 3-kinase), and the Rho-family G-proteins, Rac and Cdc42 (cell division cycle 42), were required for SDF-1alpha-induced migration regardless of the barrier pore-size. These studies have revealed a distinct branch of the SDF-1alpha signalling pathway, in which the Rac/Cdc42 effector, Pak, and its partner, PIX, specifically regulate the cellular events required for chemokine-induced migration through restrictive barriers.

Activation of myosin in HeLa cells causes redistribution of focal adhesions and F-actin from cell center to cell periphery

Activation of actomyosin II by phosphorylation of its regulatory light chain is one of the main factors involved in the regulation of cytoskeletal dynamics. Phosphorylation of myosin regulatory light chain may be mediated directly and indirectly by several kinases including myosin light chain kinase (MLCK) and kinases activated by small GTP-binding proteins. Most of the myosin kinases, including PAK, can also interact with other proteins through binding sites located outside of their catalytic domains. In an attempt to study the effects due only to phosphorylation of myosin light chain, we expressed the constitutively active catalytic domain of ameba PAK in HeLa cells. The catalytic domain phosphorylates myosin light chain in vitro with high specific activity but has none of the sequences that target mammalian PAK to other proteins and membranes. Expression of the catalytic domain caused disassembly of focal adhesions and stress fibers in the cell center and accumulation of focal adhesions and F-actin at the cell periphery. There was a twofold increase in the phosphorylation level of endogenous myosin light chain and changes in cell shape consistent with enhanced cell contractility. The phenotype was independent of MLCK, ROCK, MEK, Rac, and Rho activities but was abolished by blebbistatin, a specific inhibitor of myosin II activity. Our data are consistent with myosin being directly phosphorylated by the expressed catalytic domain of ameba PAK with the induced phenotype resulting from cell retraction driven by contraction of peripheral actomyosin. The phenotype induced by expression of the catalytic domain is reminiscent of that caused by expression of active mammalian PAK, suggesting that myosin phosphorylation may play an important role in PAK-induced cytoskeletal changes. The catalytic domain of ameba PAK may be a useful tool for studying the effects of myosin light chain phosphorylation in other cells.

PAK1 hyperactivation is sufficient for mammary gland tumor formation

Emerging data suggest that p21-activated kinase 1 (Pak1), a downstream signaling molecule of the small GTPases, growth factors, and lipid signaling, is upregulated or hyperactivated in human breast cancer. Until now, however, no direct causative role had been found for Pak1 in mammary tumor formation. We therefore sought to identify the role that Pak1 plays in mammary gland tumorigenesis. Our results showed that in a transgenic mouse model, overexpression of catalytically active Pak1 leads to the development of malignant mammary tumors and to a variety of other breast lesions, including focal solid nodules, ductal hyperplasia, and mini-intraductal neoplasm and adenoma. We also found that Pak1 hyperactivation increases the stimulation of downstream proliferative signaling effectors MEK1/2 and p38-MAPK in mammary tumor epithelial cells. Moreover, in our study, we detected expression of estrogen receptor-alpha expression and progesterone receptor expression during early stages of the lesions, but their expression was lost during the cells' transition to malignant invasive tumors. Finally, we found that consistent with a role in breast tumor progression, Pak1 expression and its nuclear accumulation was increased progressively during the transition from ductal hyperplasia to ductal carcinoma in situ to adenocarcinoma in widely used multistep polyoma-middle T-antigen transgenic mice. Together, these findings provide the first direct evidence that Pak1 deregulation may be sufficient for the formation of mammary gland tumors.

p21-activated kinases in cancer

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

Association between Pak1 expression and subcellular localization and tamoxifen resistance in breast cancer patients

BACKGROUND

p21-activated kinase 1 (Pak1) phosphorylates many proteins in both normal and transformed cells. Its ability to phosphorylate and thereby activate the estrogen receptor alpha (ERalpha) potentially limits the effectiveness of antiestrogen treatment in breast cancer. Here we studied associations between Pak1 expression and subcellular localization in tumor cells and tamoxifen resistance.

METHODS

Pak1 protein expression was evaluated in 403 primary breast tumors from premenopausal patients who had been randomly assigned to 2 years of adjuvant tamoxifen or no treatment. Tamoxifen response was evaluated by comparing recurrence-free survival in relation to Pak1 and ERalpha expression in untreated versus tamoxifen-treated patients. Tamoxifen responsiveness of human MCF-7 breast cancer cells that inducibly expressed constitutively active Pak1 or that transiently overexpressed wild-type Pak1 (Wt-Pak1) or Pak1 that lacked functional nuclear localization signals (Pak1DeltaNLS) was evaluated by analyzing cyclin D1 promoter activation and protein levels as markers for ERalpha activation. The response to tamoxifen in relation to Pak1 expression was analyzed in naturally tamoxifen-resistant Ishikawa human endometrial cancer cells. All statistical tests were two-sided.

RESULTS

Among patients who had ERalpha-positive tumors with low Pak1 expression, those treated with tamoxifen had better recurrence-free survival than those who received no treatment (hazard ratio [HR] = 0.502, 95% confidence interval [CI] = 0.331 to 0.762; P = .001) whereas there was no difference in recurrence-free survival between treatment groups for patients whose tumors had high cytoplasmic (HR = 0.893, 95% CI = 0.420 to 1.901; P = .769) or any nuclear Pak1 expression (HR = 0.955, 95% CI = 0.405 to 2.250; P = .916). In MCF-7 cells, overexpression of Wt-Pak1, but not of Pak1DeltaNLS, compromised tamoxifen response by stimulating cyclin D1 expression. Treatment of Ishikawa cells with tamoxifen led to an increase in the amount of nuclear Pak1 and Pak1 kinase activity, suggesting that tamoxifen, to some extent, regulates Pak1 expression.

CONCLUSIONS

Our data support a role for Pak1, particular Pak1 localized to the nucleus, in ERalpha signaling and in tamoxifen resistance.

Genetic deletion of Rac1 GTPase reveals its critical role in actin stress fiber formation and focal adhesion complex assembly

Rac1 is an intracellular signal transducer regulating a variety of cell functions. Previous studies by overexpression of dominant-negative or constitutively active mutants of Rac1 in clonal cell lines have established that Rac1 plays a key role in actin lamellipodia induction, cell-matrix adhesion, and cell anoikis. In the present studies, we have examined the cellular behaviors of Rac1 gene-targeted primary mouse embryonic fibroblasts (MEFs) after Cre recombinase-mediated deletion of Rac1 gene. Rac1-null MEFs became contracted and elongated in morphology and were defective in lamellipodia formation, cell spreading, cell-fibronectin adhesion, and focal contact formation in response to platelet-derived growth factor or serum. Unexpectedly, deletion of Rac1 also abolished actin stress fibers in the cells without detectable alteration of endogenous RhoA activity. Although the expression and/or activation status of focal adhesion complex components such as Src, FAK, and vinculin were not affected by Rac1 deletion, the number and size of adhesion plaques were significantly reduced, and the molecular complex between Src, FAK, and vinculin was dissembled in Rac1-null cells. Overexpression of an active RhoA mutant or ROK failed to rescue the stress fiber and adhesion plaque defects of the Rac1-null cells. Although Rac1 deletion caused a significant reduction in phospho-PAK1, -AKT, and -ERK under serum stimulation, reconstitution of active PAK1, but not AKT or MEK1, was able to rescue the actin cytoskeleton and adhesion phenotypes of the Rac1-deficient cells. Furthermore, Rac1 deletion led to a marked increase in spontaneous apoptosis that could be rescued by active PAK1, AKT, or MEK1 expression. Our results obtained from gene-targeted primary MEFs indicate that Rac1 is essential not only for lamellipodia induction but also for the RhoA-regulated actin stress fiber and focal adhesion complex formation and that Rac1 is involved in cell survival regulation through anoikis-dependent as well as -independent mechanisms.

Re-expression of the tumor suppressor NF2/merlin inhibits invasiveness in mesothelioma cells and negatively regulates FAK

The neurofibromatosis type 2 NF2 gene product, merlin, is a tumor suppressor frequently inactivated in malignant mesothelioma (MM). To investigate a possible correlation between merlin inactivation and MM invasiveness, we restored merlin expression in NF2-deficient MM cells. Re-expression of merlin markedly inhibited cell motility, spreading and invasiveness, properties connected with the malignant phenotype of MM cells. To test directly whether merlin inactivation promotes invasion in a nonmalignant system, we used small interfering RNA to silence Nf2 in mouse embryonic fibroblasts (MEFs) and found that downregulation of merlin resulted in enhanced cell spreading and invasion. To delineate signaling events connected with this phenotype, we investigated the effect of merlin expression on focal adhesion kinase (FAK), a key component of cellular pathways affecting migration and invasion. Expression of merlin attenuated FAK phosphorylation at the critical phosphorylation site Tyr397 and disrupted the interaction of FAK with its binding partners Src and p85, the regulatory subunit of phosphatidylinositol-3-kinase. In addition, NF2-null MM cells stably overexpressing FAK showed increased invasiveness, which decreased significantly when merlin expression was restored. Collectively, these findings suggest that merlin inactivation is a critical step in MM pathogenesis and is related, at least in part, with upregulation of FAK activity.

P21-activated kinase 1: convergence point in PDGF- and LPA-stimulated collagen matrix contraction by human fibroblasts

Fibroblast three-dimensional collagen matrix culture provides a tissue-like model that can be used to analyze cell form and function. The physiological agonists platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA) both stimulate human fibroblasts to contract floating collagen matrices. In this study, we show that the PDGF and LPA signaling pathways required for matrix contraction converge on p21-activated kinase 1 (PAK1) and its downstream effector cofilin1 and that contraction depends on cellular ruffling activity, rather than on the protrusion and retraction of cellular dendritic extensions. We also show that, depending on the agonist, different Rho effectors cooperate with PAK1 to regulate matrix contraction, Rho kinase in the case of PDGF and mDia1 in the case of LPA. These findings establish a unified framework for understanding the cell signaling pathways involved in fibroblast contraction of floating collagen matrices.

A GIT1/PIX/Rac/PAK signaling module regulates spine morphogenesis and synapse formation through MLC

Three of seven recently identified genes mutated in nonsyndromic mental retardation are involved in Rho family signaling. Two of the gene products, alpha-p-21-activated kinase (PAK) interacting exchange factor (alphaPIX) and PAK3, form a complex with the synaptic adaptor protein G-protein-coupled receptor kinase-interacting protein 1 (GIT1). Using an RNA interference approach, we show that GIT1 is critical for spine and synapse formation. We also show that Rac is locally activated in dendritic spines using fluorescence resonance energy transfer. This local activation of Rac is regulated by PIX, a Rac guanine nucleotide exchange factor. PAK1 and PAK3 serve as downstream effectors of Rac in regulating spine and synapse formation. Active PAK promotes the formation of spines and dendritic protrusions, which correlates with an increase in the number of excitatory synapses. These effects are dependent on the kinase activity of PAK, and PAK functions through phosphorylating myosin II regulatory light chain (MLC). Activated MLC causes an increase in dendritic spine and synapse formation, whereas inhibiting myosin ATPase activity results in decreased spine and synapse formation. Finally, both activated PAK and activated MLC can rescue the defects of GIT1 knockdown, suggesting that PAK and MLC are downstream of GIT1 in regulating spine and synapse formation. Our results point to a signaling complex, consisting of GIT1, PIX, Rac, and PAK, that plays an essential role in the regulation of dendritic spine and synapse formation and provides a potential mechanism by which alphaPIX and PAK3 mutations affect cognitive functions in mental retardation.

The GIT-associated kinase PAK targets to the centrosome and regulates Aurora-A

Previously, we showed PAK-PIX-GIT targets and regulates focal adhesions; here, we uncover a different function for the complex at the centrosome. Active PAK1 is particularly evident in mitosis and phosphorylates the centrosomal adaptor GIT1 on serine 517. Interestingly, direct centrosome targeting activates the kinase via a process not requiring Rho GTPases; excision of the centrosome prevents this activation. Once activated, PAK1 dissociates from PIX/GIT but can bind to and phosphorylate the important centrosomal kinase Aurora-A. PAK1 promotes phosphorylation of Aurora-A on Thr288 and Ser342, which are key sites for kinase activation in mitosis. In vivo PAK activation causes an accumulation of activated Aurora-A; conversely, when betaPIX is depleted or PAK is inhibited, there is a delay in centrosome maturation. These observations may underlie reported effects of active PAK on cells, including histone H3 phosphorylation, alterations in centrosome number, and progression through mitosis.

Requirement of reversible caldesmon phosphorylation at P21-activated kinase-responsive sites for lamellipodia extensions during cell migration

Caldesmon is believed to be one of the key regulators for actin dynamics and thereby cell polarity, membrane extension, and cell motility. We have shown previously that stress fiber formation and cell movement are severely impaired in the cells expressing human fibroblast caldesmon fragment defective in Ca2+/CaM binding sites. Both Ser458 and Ser489, adjacent to the Ca2+/CaM-binding sites, are phosphorylated by p21-activated kinase (PAK) in vitro. Here we report that Ser458 is phosphorylated in response to cell movement. We substituted Ser458 and Ser489 on C-terminal caldesmon (CaD39) with alanine or glutamic acid to mimic under-phosphorylated (CaD39-PAKA) or constitutively phosphorylated (CaD39-PAKE) caldesmon. In vitro, CaD39-PAKE, but not CaD39-PAKA, fails to inhibit myosin ATPase activity and exhibits reduced binding to Ca2+/CaM. When stably expressed in Chinese Hamster Ovary cells, both CaD39-PAKA and CaD39-PAKE incorporate into stress fibers and localize to the leading edge of the migrating cell. Expression of CaD39-PAKE, but not CaD39-PAKA, fails to protect stress fibers from cytochalasin depolymerization. However, both mutations inhibit cell polarization and lead to defects in membrane extension and cell migration. We conclude that phosphorylation of caldesmon by PAK is a dynamic process required to regulate actin dynamics and membrane protrusions in wound-induced cell migration.

Activation of p21-activated kinase 2 and its association with Nef are conserved in murine cells but are not sufficient to induce an AIDS-like disease in CD4C/HIV transgenic mice

A well conserved feature of human immunodeficiency virus, type 1 (HIV-1) and simian immunodeficiency virus (SIV) Nef is the interaction with and activation of the human p21-activated kinase 2 (PAK2). The conservation of this interaction in other species and its significance for Nef pathogenesis in vivo are poorly documented. In the present study, we measured these parameters in Nef-expressing thymocytes, macrophages, and dendritic cells of a transgenic (Tg) mouse model of AIDS (CD4C/HIV). We found that Nef binds to and activates PAK2, but not PAK1 and -3, in these three cell subsets. Nef associates with only a small fraction of PAK2. The Nef-PAK2 complex also comprises beta-PIX-COOL. The impact of the Nef-PAK2 association on disease development was also analyzed in Tg mice expressing 10 different Nef mutant alleles. CD4C/HIV Tg mice expressing Nef alleles defective in Nef-PAK2 association (P69A, P72A/P75A, R105A/R106A, Delta56-66, or G2A (myristoylation site)) failed to develop disease of the non-lymphoid organs (kidneys and lungs). Among these, only Tg mice expressing Nef(P69A) and Nef(G2A) showed some depletion of CD4(+) T cells, although a down-regulation of the CD4 surface protein was documented in all these Tg lines, except those expressing Nef(Delta56-66). Among other Tg mice expressing Nef mutants having conserved the Nef-PAK2 association (RD35AA, D174K, P147A/P150A, Delta8-17, and Delta25-65), only Tg mice expressing Nef(Delta8-17) develop kidney and lung diseases, but all showed partial CD4(+) T cell depletion despite some being defective for CD4 down-regulation (RD35AA and D174K). Therefore, Nef can activate murine PAK2 and associate with a small fraction of it, as in human cells. Such activation and binding of PAK2 is clearly not sufficient but may be required to induce a multiorgan AIDS-like disease in Tg mice.

Essential role of CIB1 in regulating PAK1 activation and cell migration

p21-activated kinases (PAKs) regulate many cellular processes, including cytoskeletal rearrangement and cell migration. In this study, we report a direct and specific interaction of PAK1 with a 22-kD Ca²⁺-binding protein, CIB1, which results in PAK1 activation both in vitro and in vivo. CIB1 binds to PAK1 within discrete regions surrounding the inhibitory switch domain in a calcium-dependent manner, providing a potential mechanism of CIB1-induced PAK1 activation. CIB1 overexpression significantly decreases cell migration on fibronectin as a result of a PAK1-and LIM kinase–dependent increase in cofilin phosphorylation. Conversely, the RNA interference–mediated depletion of CIB1 increases cell migration and reduces normal adhesion-induced PAK1 activation and cofilin phosphorylation. Together, these results demonstrate that endogenous CIB1 is required for regulated adhesion-induced PAK1 activation and preferentially induces a PAK1-dependent pathway that can negatively regulate cell migration. These results point to CIB1 as a key regulator of PAK1 activation and signaling.

Rho GTPases and the control of cell behaviour

Rho, Rac and Cdc42, three members of the Rho family of small GTPases, each control a signal transduction pathway linking membrane receptors to the assembly and disassembly of the actin cytoskeleton and of associated integrin adhesion complexes. Rho regulates stress fibre and focal adhesion assembly, Rac regulates the formation of lamellipodia protrusions and membrane ruffles, and Cdc42 triggers filopodial extensions at the cell periphery. These observations have led to the suggestion that wherever filamentous actin is used to drive a cellular process, Rho GTPases are likely to play an important regulatory role. Rho GTPases have also been reported to control other cellular activities, such as the JNK (c-Jun N-terminal kinase) and p38 MAPK (mitogen-activated protein kinase) cascades, an NADPH oxidase enzyme complex, the transcription factors NF-κB (nuclear factor κB) and SRF (serum-response factor), and progression through G1 of the cell cycle. Thus Rho, Rac and Cdc42 can regulate the actin cytoskeleton and gene transcription to promote co-ordinated changes in cell behaviour. We have been analysing the biochemical contributions of Rho GTPases in cell movement and have found that Rac controls cell protrusion, while Cdc42 controls cell polarity.

Regulation of phagocytosis by Rho GTPases

Phagocytosis is the mechanism of internalization used by specialized cells such as macrophages, dendritic cells, and neutrophils to internalize, degrade, and eventually present peptides derived from particulate antigens. The phagocytic process comprises several sequential and complex events initiated by the recognition ofligands on the surface of the particles by specific receptors on the surface of the phagocytic cells. Receptor clustering at the attachment site generates a phagocytic signal that in turn leads to local polymerization of actin filaments and to particle internalization. Depending on the particles and receptors involved, it appears that the structures and mechanisms associated with particle ingestion are diverse. However, work during the past few years has highlighted the importance of small GTP-binding proteins of the Rho family in various types of phagocytosis. As reviewed here, Rho family GTPases, their activators, and their downstream effectors control the local reorganization of the actin cytoskeleton beneath bound particles.

PAK1 induces podosome formation in A7r5 vascular smooth muscle cells in a PAK-interacting exchange factor-dependent manner

Remodeling of the vascular smooth muscle cytoskeleton is essential for cell motility involved in the development of diseases such as arteriosclerosis and restenosis. The p21-activated kinase (PAK), which is an effector of the Rho GTPases Rac and Cdc42, has been shown to be involved in cytoskeletal remodeling and cell motility. We show herein that expression of cytoskeletally active constructs of PAK1 is able to induce the formation of dynamic, podosome-like F-actin columns in the A7r5 vascular smooth muscle cell line. Most of these actin columns appear at the junctions between stress fibers and focal adhesions and contain several known podosomal protein markers, such as cortactin, Arp2/3, α-actinin, and vinculin. The kinase activity of PAK plays a role in the regulation of the turnover rates of these actin columns but is not essential for their formation. The ability of PAK to interact with the PAK-interacting exchange factor (PIX) but not with Rac or Cdc42, however, is required for the formation of the actin columns as well as for the translocation of PIX and G protein-coupled receptor kinase-interacting protein (GIT) to focal adhesions adjacent to the actin columns. These findings suggest that interaction between PAK and PIX, as well as the recruitment of PIX and GIT to focal adhesions, plays an important role in the formation of actin columns that resemble podosomes induced by phorbol ester in vascular smooth muscle cells.

Biochemistry and biomechanics of cell motility

Cell motility is an essential cellular process for a variety of biological events. The process of cell migration requires the integration and coordination of complex biochemical and biomechanical signals. The protrusion force at the leading edge of a cell is generated by the cytoskeleton, and this force generation is controlled by multiple signaling cascades. The formation of new adhesions at the front and the release of adhesions at the rear involve the outside-in and inside-out signaling mediated by integrins and other adhesion receptors. The traction force generated by the cell on the extracellular matrix (ECM) regulates cell-ECM adhesions, and the counter force exerted by ECM on the cell drives the migration. The polarity of cell migration can be amplified and maintained by the feedback loop between the cytoskeleton and cell-ECM adhesions. Cell migration in three-dimensional ECM has characteristics distinct from that on two-dimensional ECM. The migration of cells is initiated and modulated by external chemical and mechanical factors, such as chemoattractants and the mechanical forces acting on the cells and ECM, as well as the surface density, distribution, topography, and rigidity of the ECM.

Symmetry-breaking in mammalian cell cohort migration during tissue pattern formation: role of random-walk persistence

Coordinated, cohort cell migration plays an important role in the morphogenesis of tissue patterns in metazoa. However, individual cells intrinsically move in a random walk-like fashion when studied in vitro. Hence, in the absence of an external orchestrating influence or template, the emergence of cohort cell migration must involve a symmetry-breaking event. To study this process, we used a novel experimental system in which multiple capillary endothelial cells exhibit spontaneous and robust cohort migration in the absence of chemical gradients when cultured on micrometer-scale extracellular matrix islands fabricated using microcontact printing. A computational model suggested that directional persistence of random-walk and dynamic mechanical coupling of adjacent cells are the critical control parameters for this symmetry-breaking behavior that is induced in spatially-constrained cell ensembles. The model predicted our finding that fibroblasts, which exhibit a much shorter motility persistence time than endothelial cells, failed to undergo symmetry breaking or produce cohort migration on the matrix islands. These findings suggest that cells have intrinsic motility characteristics that are tuned to match their role in tissue patterning. Our results underscore the importance of studying cell motility in the context of cell populations, and the need to address emergent features in multicellular organisms that arise not only from cell-cell and cell-matrix interactions, but also from properties that are intrinsic to individual cells.