FAK and PYK2 interact with SAP90/PSD-95-Associated Protein-3

Focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (PYK2) are two related non-receptor tyrosine kinases highly expressed in brain. Although they are both involved in synaptic plasticity, little is known about their specific neuronal partners. Using a yeast two-hybrid screen and GST pull-down assays we show that SAPAP3 (SAP90/PSD-95-Associated Protein-3) interacts with FAK (residues 676-840) and PYK2. The three proteins partly co-distribute in the same sucrose gradient fractions as the post-synaptic density protein PSD-95 and Src. Our results suggest that SAPAP3 is an anchoring protein for FAK and PYK2 in post-synaptic densities and may contribute to the synaptic function of these tyrosine kinases.     

Focal adhesion kinase regulates neuronal growth, synaptic plasticity and hippocampus-dependent spatial learning and memory

The focal adhesion kinase (FAK) is a non-receptor tyrosine kinase abundantly expressed in the mammalian brain and highly enriched in neuronal growth cones. Inhibitory and facilitatory activities of FAK on neuronal growth have been reported and its role in neuritic outgrowth remains controversial. Unlike other tyrosine kinases, such as the neurotrophin receptors regulating neuronal growth and plasticity, the relevance of FAK for learning and memory in vivo has not been clearly defined yet. A comprehensive study aimed at determining the role of FAK in neuronal growth, neurotransmitter release and synaptic plasticity in hippocampal neurons and in hippocampus-dependent learning and memory was therefore undertaken using the mouse model. Gain- and loss-of-function experiments indicated that FAK is a critical regulator of hippocampal cell morphology. FAK mediated neurotrophin-induced neuritic outgrowth and FAK inhibition affected both miniature excitatory postsynaptic potentials and activity-dependent hippocampal long-term potentiation prompting us to explore the possible role of FAK in spatial learning and memory in vivo. Our data indicate that FAK has a growth-promoting effect, is importantly involved in the regulation of the synaptic function and mediates in vivo hippocampus-dependent spatial learning and memory.     

Csk enhances insulin-stimulated dephosphorylation of focal adhesion proteins.

Insulin has pleiotropic effects on the regulation of cell physiology through binding to its receptor. The wide variety of tyrosine phosphorylation motifs of insulin receptor substrate 1 (IRS-1), a substrate for the activated insulin receptor tyrosine kinase, may account for the multiple functions of insulin. Recent studies have shown that activation of the insulin receptor leads to the regulation of focal adhesion proteins, such as a dephosphorylation of focal adhesion kinase (pp125FAK). We show here that C-terminal Src kinase (Csk), which phosphorylates C-terminal tyrosine residues of Src family protein tyrosine kinases and suppresses their kinase activities, is involved in this insulin-stimulated dephosphorylation of focal adhesion proteins. We demonstrated that the overexpression of Csk enhanced and prolonged the insulin-induced dephosphorylation of pp125FAK. Another focal adhesion protein, paxillin, was also dephosphorylated upon insulin stimulation, and a kinase-negative mutant of Csk was able to inhibit the insulin-induced dephosphorylation of pp125FAK and paxillin. Although we have shown that the Csk Src homology 2 domain can bind to several tyrosine-phosphorylated proteins, including pp125FAK and paxillin, a majority of protein which bound to Csk was IRS-1 when cells were stimulated by insulin. Our data also indicated that tyrosine phosphorylation levels of IRS-1 appear to be paralleled by the dephosphorylation of the focal adhesion proteins. We therefore propose that the kinase activity of Csk, through the insulin-induced complex formation of Csk with IRS-1, is involved in insulin's regulation of the phosphorylation levels of the focal adhesion proteins, possibly through inactivation of the kinase activity of c-Src family kinases.     

Integrin adhesions

Cell migration requires the coordination of adhesion site assembly and turnover. Canonical models for nascent adhesion formation postulate that integrin binding to extracellular matrix (ECM) proteins results in the rapid recruitment of cytoskeletal proteins such as talin and paxillin to integrin cytoplasmic domains. It is thought that integrin-talin clusters recruit and activate tyrosine kinases such as focal adhesion kinase (FAK). However, the molecular connections of this linkage remain unresolved. Our recent findings support an alternative model whereby FAK recruits talin to new sites of β1 integrin-mediated adhesion in mouse embryonic fibroblasts and human ovarian carcinoma cells. This is dependent on a direct binding interaction between FAK and talin and occurs independently of direct talin binding to β1 integrin. Herein, we discuss differences between nascent and mature adhesions, interactions between FAK, talin and paxillin, possible mechanisms of FAK activation and how this FAK-talin complex may function to promote cell motility through increased adhesion turnover.     

Integrin adhesions: Who’s on first? What's on second?

Cell migration requires the coordination of adhesion site assembly and turnover. Canonical models for nascent adhesion formation postulate that integrin binding to extracellular matrix (ECM) proteins results in the rapid recruitment of cytoskeletal proteins such as talin and paxillin to integrin cytoplasmic domains. It is thought that integrin-talin clusters recruit and activate tyrosine kinases such as focal adhesion kinase (FAK). However, the molecular connections of this linkage remain unresolved. Our recent findings support an alternative model whereby FAK recruits talin to new sites of β1 integrin-mediated adhesion in mouse embryonic fibroblasts and human ovarian carcinoma cells. This is dependent on a direct binding interaction between FAK and talin and occurs independently of direct talin binding to β1 integrin. Herein, we discuss differences between nascent and mature adhesions, interactions between FAK, talin and paxillin, possible mechanisms of FAK activation and how this FAK-talin complex may function to promote cell motility through increased adhesion turnover.     

RACK1 promotes neurite outgrowth by scaffolding AGAP2 to FAK

RACK1 binds proteins in a constitutive or transient manner and supports signal transmission by engaging in diverse and distinct signalling pathways. The emerging theme is that RACK1 functions as a signalling switch, recruiting proteins to form distinct molecular complexes. In focal adhesions, RACK1 is required for the regulation of FAK activity and for integrating a wide array of cellular signalling events including the integration of growth factor and adhesion signalling pathways. FAK is required for cell adhesion and migration and has a well-established role in neurite outgrowth and in the developing nervous system. However, the mechanism by which FAK activity is regulated in neurons remains unknown. Using neuronal cell lines, we determined that differentiation of these cells promotes an interaction between the scaffolding protein RACK1 and FAK. Disruption of the RACK1/FAK interaction leads to decreased neurite outgrowth suggesting a role for the interaction in neurite extension. We hypothesised that RACK1 recruits proteins to FAK, to regulate FAK activity in neuronal cells. To address this, we immunoprecipitated RACK1 from rat hippocampus and searched for interacting proteins by mass spectrometry. We identified AGAP2 as a novel RACK1-interacting protein. Having confirmed the RACK1–AGAP2 interaction biochemically, we show RACK1–AGAP2 to localise together in the growth cone of differentiated cells, and confirm that these proteins are in complex with FAK. This complex is disrupted when RACK1 expression is suppressed using siRNA or when mutants of RACK1 that do not interact with FAK are expressed in cells. Similarly, suppression of AGAP2 using siRNA leads to increased phosphorylation of FAK and increased cell adhesion resulting in decreased neurite outgrowth. Our results suggest that RACK1 scaffolds AGAP2 to FAK to regulate FAK activity and cell adhesion during the differentiation process.     

Direct interaction of focal adhesion kinase with p190RhoGEF

Focal adhesion kinase (FAK) is a protein-tyrosine kinase that associates with multiple cell surface receptors and signaling proteins through which it can modulate the activity of several intracellular signaling pathways. FAK activity can influence the formation of distinct actin cytoskeletal structures such as lamellipodia and stress fibers in part through effects on small Rho GTPases, although the molecular interconnections of these events are not well defined. Here, we report that FAK interacts with p190RhoGEF, a RhoA-specific GDP/GTP exchange factor, in neuronal cells and in brain tissue extracts by co-immunoprecipitation and co-localization analyses. Using a two-hybrid assay and deletion mutagenesis, the binding site of the FAK C-terminal focal adhesion targeting (FAT) domain was identified within the C-terminal coiled-coil domain of p190RhoGEF. Binding was independent of a LD-like binding motif within p190RhoGEF, yet FAK association was disrupted by a mutation (Leu-1034 to Ser) that weakens the helical bundle structure of the FAK FAT domain. Neuro-2a cell binding to laminin increased endogenous FAK and p190RhoGEF tyrosine phosphorylation, and co-transfection of a dominant-negative inhibitor of FAK activity, termed FRNK, inhibited lamininstimulated p190RhoGEF tyrosine phosphorylation and p21 RhoA GTP binding. Overexpression of FAK in Neuro-2a cells increased both endogenous p190RhoGEF tyrosine phosphorylation and RhoA activity, whereas these events were inhibited by FRNK co-expression. Because insulin-like growth factor 1 treatment of Neuro-2a cells increased FAK tyrosine phosphorylation and enhanced p190RhoGEF-mediated activation of RhoA, our results support the conclusion that FAK association with p190RhoGEF functions as a signaling pathway downstream of integrins and growth factor receptors to stimulate Rho activity.     

XIAP reverses various functional activities of FRNK in endothelial cells

In endothelial cells, focal adhesion kinase (FAK) regulates cell proliferation, migration, adhesion, and shear-stimulated activation of MAPK. We recently found that FAK is recruited into focal adhesion (FA) sites through interactions with XIAP (X-chromosome linked inhibitor of apoptosis protein) and activated by Src kinase in response to shear stress. In this study, we examined which domain(s) of FAK is(are) important for various vascular functions such as FA recruiting, XIAP-binding and shear stress-stimulated ERK activation. Through a series of experiments, we determined that the FRNK domain is recruited into FA sites and promotes endothelial cell adhesion. Interestingly, XIAP knockdown was shown to reduce FA recruitment of FRNK and the cell adhesive effect of FRNK. In addition, we found that XIAP interacts with FRNK, suggesting cross-talk between XIAP and FRNK. We also demonstrated that FRNK inhibits endothelial cell migration and shear-stimulated ERK activation. These inhibitory effects of FRNK were reversed by XIAP knockdown. Taken together, we can conclude that XIAP plays a key role in vascular functions of FRNK or FRNK domain-mediated vascular functions of FAK.     

Focal adhesion kinase: protein interactions and cellular functions

Integrin-mediated cell adhesion to extracellular matrix (ECM) plays important roles in a variety of biological processes. Recent studies suggested that integrins mediate signal transduction across the plasma membrane via activating several intracellular signaling pathways. Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that has been shown to be a major mediator of integrin signal transduction pathways. Upon activation by integrins, FAK undergoes autophosphorylation as well as associations with several other intracellular signaling molecules. These interactions in the signaling pathways have been shown to regulation a variety of cellular functions such as cell spreading, migration, cell proliferation, apoptosis and cell survival. Recent progress in the understanding of FAK interactions with other proteins in the regulation of these cellular functions will be discussed in this review.     

FAK/mTOR信号通在肿瘤细胞中的调控作用

粘附斑激酶（focal adhesion kinase,FAK）是一种胞质非受体酪氨酸激酶,是整合素信号通路里一个重要的调节因子,在肿瘤细胞中高表达,与细胞迁移、粘附和侵袭有关。mTOR (mammalian target of rapamycin)是非典型性的Ser/Thr激酶,属于PIKK（phosphatidylinositol kinase related kinase）超家族,介导营养信号调控细胞生长、分化及代谢等生理过程。近年的研究发现FAK通过三条途径与mTOR相关联,组成FAK/mTOR信号通路,在肿瘤细胞的增殖、迁移及肿瘤微环境中发挥着重要的调控作用。本文综述了FAK、mTOR和FAK/mTOR信号通路的特点及对肿瘤细胞调控作用的研究概况,为肿瘤治疗提供参考。     

Identification of Novel Focal Adhesion Kinase Substrates: Role for FAK in NFκB Signaling

Focal adhesion kinase (FAK) is a major signaling molecule which functions downstream of integrins or in conjunction with mitogenic signaling pathways. FAK is overexpressed and/or activated in many types of human tumors, in which it promotes cell adhesion, survival, migration and invasion. In addition to FAK''s ability to regulate signaling through its scaffolding activities, FAK encodes an intrinsic kinase activity. Although some FAK substrates have been identified, a more comprehensive analysis of substrates is lacking. In this study, we use a protein microarray to screen the human proteome for FAK substrates. We confirm that several of the proteins identified are bona fide in vitro FAK substrates, including several factors which are known to regulate the NFκB pathway. Finally, we identify a role for FAK''s kinase activity in both canonical and non-canonical NFκB signaling. Our screen therefore represents the first high throughput screen for FAK substrates and provides the basis for future in-depth analysis of the role of FAK''s kinase activity in the processes of tumorigenesis.     

Focal adhesion kinase (FAK), a multifunctional protein

Focal adhesion kinase (FAK) is a cytoplasmic protein tyrosine kinase localized to regions called focal adhesions. Many stimuli can induce tyrosine phosphorylation and activation of FAK, including integrins and growth factors. The major site of autophosphorylation, tyrosine 397, is a docking site for the SH2 domains of Src family proteins. The other sites of phosphorylation are phosphorylated by Src kinases. Phosphorylated FAK binds proteins of focal adhesion and can activate them directly or indirectly by phosphorylation. These activated proteins forming the FAK complex facilitate the generation of downstream signals necessary to regulate cell functions, like motility, survival and proliferation. Dysregulation of FAK could participate in the development of cancer. This review will focus upon the mechanisms by which FAK transmits biochemical signals and elicits biological effects.     

Identification of Tyr-397 as the primary site of tyrosine phosphorylation and pp60src association in the focal adhesion kinase, pp125FAK.

A number of cellular processes, such as proliferation, differentiation, and transformation, are regulated by cell-extracellular matrix interactions. Previous studies have identified a novel tyrosine kinase, the focal adhesion kinase p125FAK, as a component of cell adhesion plaques. p125FAK was identified as a 125-kDa tyrosine-phosphorylated protein in cells transformed by the v-src oncogene. p125FAK is an intracellular protein composed of three domains: a central domain with homology to protein tyrosine kinases, flanked by two noncatalytic domains of 400 amino acids which bear no significant homology to previously cloned proteins. p125FAK is believed to play an important regulatory role in cell adhesion because it localizes to cell adhesion plaques and because its phosphorylation on tyrosine residues is regulated by binding of cell surface integrins to the extracellular matrix. Recent studies have shown that Src, through its SH2 domain, stably associates with pp125FAK and that this association prevents dephosphorylation of pp125FAK in vitro by protein tyrosine phosphatases. In this report, we identify Tyr-397 as the primary in vivo and in vitro site of p125FAK tyrosine phosphorylation and association with Src. Substituting phenylalanine for tyrosine at position 397 significantly reduces p125FAK tyrosine phosphorylation and association with Src but does not abolish p125FAK kinase activity. In addition, p125FAK kinase is able to trans-phosphorylate Tyr-397 in vitro in a kinase-deficient p125FAK variant. Phosphorylation of Tyr-397 provides a site [Y(P)AEI] that fits the consensus sequence for the binding of Src.     

Regulation of focal adhesion kinase by a novel protein inhibitor FIP200

Focal adhesion kinase (FAK) is a major mediator of integrin signaling pathways. The mechanisms of regulation of FAK activity and its associated cellular functions are not very well understood. Here, we present data suggesting that a novel protein FIP200 functions as an inhibitor for FAK. We show the association of endogenous FIP200 with FAK, which is decreased upon integrin-mediated cell adhesion concomitant with FAK activation. In vitro- and in vivo-binding studies indicate that FIP200 interacts with FAK through multiple domains directly. FIP200 bound to the kinase domain of FAK inhibited its kinase activity in vitro and its autophosphorylation in vivo. Overexpression of FIP200 or its segments inhibited cell spreading, cell migration, and cell cycle progression, which correlated with their inhibition of FAK activity in vivo. The inhibition of these cellular functions by FIP200 could be rescued by coexpression of FAK. Last, we show that disruption of the functional interaction between endogenous FIP200 with FAK leads to increased FAK phosphorylation and partial restoration of cell cycle progression in cells plated on poly-L-lysine, providing further support for FIP200 as a negative regulator of FAK. Together, these results identify FIP200 as a novel protein inhibitor for FAK.     

Recognition of LD motifs by the focal adhesion targeting domains of focal adhesion kinase and proline‐rich tyrosine kinase 2‐beta: Insights from molecular dynamics simulations

The focal adhesion kinase (FAK) and the proline‐rich tyrosine kinase 2‐beta (PYK2) are implicated in cancer progression and metastasis and represent promising biomarkers and targets for cancer therapy. FAK and PYK2 are recruited to focal adhesions (FAs) via interactions between their FA targeting (FAT) domains and conserved segments (LD motifs) on the proteins Paxillin, Leupaxin, and Hic‐5. A promising new approach for the inhibition of FAK and PYK2 targets interactions of the FAK domains with proteins that promote localization at FAs. Advances toward this goal include the development of surface plasmon resonance, heteronuclear single quantum coherence nuclear magnetic resonance (HSQC‐NMR) and fluorescence polarization assays for the identification of fragments or compounds interfering with the FAK‐Paxillin interaction. We have recently validated this strategy, showing that Paxillin mimicking polypeptides with 2 to 3 LD motifs displace FAK from FAs and block kinase‐dependent and independent functions of FAK, including downstream integrin signaling and FA localization of the protein p130Cas. In the present work we study by all‐atom molecular dynamics simulations the recognition of peptides with the Paxillin and Leupaxin LD motifs by the FAK‐FAT and PYK2‐FAT domains. Our simulations and free‐energy analysis interpret experimental data on binding of Paxillin and Leupaxin LD motifs at FAK‐FAT and PYK2‐FAT binding sites, and assess the roles of consensus LD regions and flanking residues. Our results can assist in the design of effective inhibitory peptides of the FAK‐FAT: Paxillin and PYK2‐FAT:Leupaxin complexes and the construction of pharmacophore models for the discovery of potential small‐molecule inhibitors of the FAK‐FAT and PYK2‐FAT focal adhesion based functions.     

Dibutyryl cyclic AMP-induced process formation in astrocytes is associated with a decrease in tyrosine phosphorylation of focal adhesion kinase and paxillin.

Focal adhesion kinase (FAK or pp125FAK) is a cytosolic protein tyrosine kinase which plays an important role in integrin-mediated signal transduction. Adhesion of cells to the substratum correlates with an increase in tyrosine phosphorylation of FAK as well as an associated protein, paxillin. In this report we show that the tyrosine phosphorylation of FAK and paxillin are decreased during dibutyryl cyclic AMP-induced (dB-cAMP) process formation in astrocytes. When astrocytes in suspension are treated with dB-cAMP, no alteration in morphology or tyrosine phosphorylation is observed, suggesting that both phenomena are linked and adhesion dependent. Furthermore, genistein, a tyrosine kinase inhibitor, can induce process formation in such cells, underscoring the significance of protein tyrosine kinases in maintaining the morphology of adherent cells. Finally, endothelin-1, a vasopeptide which is known to inhibit process formation in astrocytes, inhibited the tyrosine dephosphorylation of proteins associated with dB-cAMP treatment. These results suggest that the formation of asymmetric processes in astrocytes results from a coordinated set of alterations in the actin cytoskeleton as well as the adhesion of the cell to the substratum. Modification of the properties of such molecules is required for process formation and the dynamic modulation of astrocytic morphology in vitro and in vivo.     

The C terminus of talin links integrins to cell cycle progression

Integrins are cell adhesion receptors that sense the extracellular matrix (ECM) environment. One of their functions is to regulate cell fate decisions, although the question of how integrins initiate intracellular signaling is not fully resolved. In this paper, we examine the role of talin, an adapter protein at cell-matrix attachment sites, in outside-in signaling. We used lentiviral small hairpin ribonucleic acid to deplete talin in mammary epithelial cells. These cells still attached to the ECM in an integrin-dependent manner and spread. They had a normal actin cytoskeleton, but vinculin, paxillin, focal adhesion kinase (FAK), and integrin-linked kinase were not recruited to adhesion sites. Talin-deficient cells showed proliferation defects, and reexpressing a tail portion of the talin rod, but not its head domain, restored integrin-mediated FAK phosphorylation, suppressed p21 expression, and rescued cell cycle. Thus, talin recruits and activates focal adhesion proteins required for proliferation via the C terminus of its rod domain. Our study reveals a new function for talin, which is to link integrin adhesions with cell cycle progression.     

Focal adhesion kinase is negatively regulated by phosphorylation at tyrosine 407

Focal adhesion kinase (FAK) mediates signal transduction in response to multiple extracellular inputs via tyrosine phosphorylation at specific residues. Although several tyrosine phosphorylation events have been linked to FAK activation and downstream signal transduction, the function of FAK phosphorylation at Tyr(407) was previously unknown. Here, we show for the first time that phosphorylation of FAK Tyr(407) increases during serum starvation, contact inhibition, and cell cycle arrest, all conditions under which activating FAK Tyr(397) phosphorylation decreases. Transfection of NIH3T3 cells with a phosphorylation-mimicking FAK 407E mutant decreased autophosphorylation at Tyr(397) and inhibited both FAK kinase activity in vitro and FAK-mediated functions such as cell adhesion, spreading, proliferation, and migration. The opposite effects were observed in cells transfected with nonphosphorylatable mutant FAK 407F. Taken together, these data suggest the novel concept that FAK Tyr(407) phosphorylation negatively regulates the enzymatic and biological activities of FAK.     

Signaling through focal adhesion kinase

Integrin receptor binding to extracellular matrix proteins generates intracellular signals via enhanced tyrosine phosphorylation events that are important for cell growth, survival, and migration. This review will focus on the functions of the focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) and its role in linking integrin receptors to intracellular signaling pathways. FAK associates with several different signaling proteins such as Src-family PTKs, p130^Cas, Shc, Grb2, PI 3-kinase, and paxillin. This enables FAK to function within a network of integrin-stimulated signaling pathways leading to the activation of targets such as the ERK and JNK/mitogen-activated protein kinase pathways. Focus will be placed on the structural domains and sites of FAK tyrosine phosphorylation important for FAK-mediated signaling events and how these sites are conserved in the FAK-related PTK, Pyk2. We will review what is known about FAK activation by integrin receptor-mediated events and also non-integrin stimuli. In addition, we discuss the emergence of a consensus FAK substrate phosphorylation sequence. Emphasis will also be placed on the role of FAK in generating cell survival signals and the cleavage of FAK during caspase-mediated apoptosis. An in-depth discussion will be presented of integrin-stimulated signaling events occurring in the FAK knockout fibroblasts (FAK⁻) and how these cells exhibit deficits in cell migration. FAK re-expression in the FAK⁻ cells confirms the role of this PTK in the regulation of cell morphology and in promoting cell migration events. In addition, these results reinforce the potential role for FAK in promoting an invasive phenotype in human tumors.     

Dual role of Fyn in the regulation of FAK+6,7 by cannabinoids in hippocampus

In hippocampus endocannabinoids modulate synaptic function and plasticity and increase tyrosine phosphorylation of several proteins, including focal adhesion kinase (FAK). Autophosphorylation of FAK on Tyr-397 is generally a critical step for its activation, allowing the recruitment of Src family kinases, and phosphorylation of FAK and associated proteins. We have examined the mechanisms of the regulation of FAK by cannabinoids in rat and mouse hippocampal slices. Anandamide and 2-arachidonoylglycerol, two endocannabinoids, and Delta9-tetrahydrocannabinol, stimulated tyrosine phosphorylation of FAK+6,7, a neuronal splice isoform of FAK, on several residues including Tyr-397. Cannabinoids increased phosphorylation of p130-Cas, a protein associated with FAK, but had no effect on PYK2, a tyrosine kinase related to FAK and enriched in hippocampus. Pharmacological experiments and the use of knockout mice demonstrated that the effects of cannabinoids were mediated through CB1 receptors. These effects were sensitive to manipulation of cAMP-dependent protein kinase, suggesting that they were mediated by inhibition of a cAMP pathway. PP2, an Src family kinase inhibitor, prevented the effects of cannabinoids on p130-Cas and on FAK+6,7 tyrosines 577 and 925, but not 397, indicating that FAK autophosphorylation was upstream of Src family kinases in response to CB1-R stimulation. Endocannabinoids increased the association of Fyn, but not Src, with FAK+6,7. In hippocampal slices from Fyn -/- mice, the levels of p130-Cas were increased, and the effects of endocannabinoids on tyrosine phosphorylation, including of Tyr-397, were completely abolished. These results demonstrate the specific functional association of Fyn with FAK+6,7 in a pathway regulated by endocannabinoids, in which Fyn may play roles dependent and independent of its catalytic activity.