Hilary E. Beggs

NCAM140 INTERACTS WITH THE FOCAL ADHESION KINASE P125FAK AND THE SRC-RELATED TYROSINE KINASE P59FYN

Axonal growth cones respond to adhesion molecules and extracellular matrix components by rapid morphological changes and growth rate modification. Neurite outgrowth mediated by the neural cell adhesion molecule (NCAM) requires the src family tyrosine kinase p59fyn in nerve growth cones, but the molecular basis for this interaction has not been defined. The NCAM140 isoform, which is found in migrating growth cones, selectively co-immunoprecipitated with p59fyn from nonionic detergent (Brij 96) extracts of early postnatal mouse cerebellum and transfected rat B35 neuroblastoma and COS-7 cells. p59fyn did not associate significantly with the NCAM180 isoform, which is found at sites of stable neural cell contacts, or with the glycophosphatidylinositol-linked NCAM120 isoform. pp60c-src, a tyrosine kinase that promotes neurite growth on the neuronal cell adhesion molecule L1, did not interact with any NCAM isoform. Whereas p59fyn was constitutively associated with NCAM140, the focal adhesion kinase p125fak, a nonreceptor tyrosine kinase known to mediate integrin-dependent signaling, became recruited to the NCAM140-p59fyn complex when cells were reacted with antibodies against the extracellular region of NCAM. Treatment of cells with a soluble NCAM fusion protein or with NCAM antibodies caused a rapid and transient increase in tyrosine phosphorylation of p125fak and p59fyn. These results suggest that NCAM140 binding interactions at the cell surface induce the assembly of a molecular complex of NCAM140, p125fak, and p59fyn and activate the catalytic function of these tyrosine kinases, initiating a signaling cascade that may modulate growth cone migration.

Netrin requires focal adhesion kinase and Src family kinases for axon outgrowth and attraction

Although netrins are an important family of neuronal guidance proteins, intracellular mechanisms that mediate netrin function are not well understood. Here we show that netrin-1 induces tyrosine phosphorylation of proteins including focal adhesion kinase (FAK) and the Src family kinase Fyn. Blockers of Src family kinases inhibited FAK phosphorylation and axon outgrowth and attraction by netrin. Dominant-negative FAK and Fyn mutants inhibited the attractive turning response to netrin. Axon outgrowth and attraction induced by netrin-1 were significantly reduced in neurons lacking the FAK gene. Our results show the biochemical and functional links between netrin, a prototypical neuronal guidance cue, and FAK, a central player in intracellular signaling that is crucial for cell migration.

Focal adhesion kinase modulates tension signaling to control actin and focal adhesion dynamics

In response to αβ1 integrin signaling, transducers such as focal adhesion kinase (FAK) become activated, relaying to specific machineries and triggering distinct cellular responses. By conditionally ablating Fak in skin epidermis and culturing Fak-null keratinocytes, we show that FAK is dispensable for epidermal adhesion and basement membrane assembly, both of which require αβ1 integrins. FAK is also dispensible for proliferation/survival in enriched medium. In contrast, FAK functions downstream of αβ1 integrin in regulating cytoskeletal dynamics and orchestrating polarized keratinocyte migration out of epidermal explants. Fak-null keratinocytes display an aberrant actin cytoskeleton, which is tightly associated with robust, peripheral focal adhesions and microtubules. We find that without FAK, Src, p190RhoGAP, and PKL–PIX–PAK, localization and/or activation at focal adhesions are impaired, leading to elevated Rho activity, phosphorylation of myosin light chain kinase, and enhanced tensile stress fibers. We show that, together, these FAK-dependent activities are critical to control the turnover of focal adhesions, which is perturbed in the absence of FAK.

Endothelial FAK is essential for vascular network stability, cell survival, and lamellipodial formation

Morphogenesis of a vascular network requires dynamic vessel growth and regression. To investigate the cellular mechanism underlying this process, we deleted focal adhesion kinase (FAK), a key signaling mediator, in endothelial cells (ECs) using Tie2-Cre mice. Targeted FAK depletion occurred efficiently early in development, where mutants exhibited a distinctive and irregular vasculature, resulting in hemorrhage and lethality between embryonic day (e) 10.5 and 11.5. Capillaries and intercapillary spaces in yolk sacs were dilated before any other detectable abnormalities at e9.5, and explants demonstrate that the defects resulted from the loss of FAK and not from organ failure. Time-lapse microscopy monitoring EC behavior during vascular formation in explants revealed no apparent decrease in proliferation or migration but revealed increases in cell retraction and death leading to reduced vessel growth and increased vessel regression. Consistent with this phenotype, ECs derived from mutant embryos exhibited aberrant lamellipodial extensions, altered actin cytoskeleton, and nonpolarized cell movement. This study reveals that FAK is crucial for vascular morphogenesis and the regulation of EC survival and morphology.

Ras- and PI3K-dependent breast tumorigenesis in mice and humans requires focal adhesion kinase signaling

Cancer cells require sustained oncogenic signaling in order to maintain their malignant properties. It is, however, unclear whether they possess other dependencies that can be exploited therapeutically. We report here that in a large fraction of human breast cancers, the gene encoding focal adhesion kinase (FAK), a core component of integrin signaling, was amplified and FAK mRNA was overexpressed. A mammary gland-specific deletion of Fak in mice did not seem to affect normal mammary epithelial cells, and these mice were protected from tumors initiated by the polyoma middle T oncoprotein (PyMT), which activates Ras and PI3K. FAK-deficient PyMT-transformed cells displayed both growth arrest and apoptosis, as well as diminished invasive and metastatic capacity. Upon silencing of Fak, mouse mammary tumor cells transformed by activated Ras became senescent and lost their invasive ability. Further, Neu-transformed cells also underwent growth arrest and apoptosis if integrin beta4-dependent signaling was simultaneously inactivated. Human breast cancer cells carrying oncogenic mutations that activate Ras or PI3K signaling displayed similar responses upon silencing of FAK. Mechanistic studies indicated that FAK sustains tumorigenesis by promoting Src-mediated phosphorylation of p130Cas. These results suggest that FAK supports Ras- and PI3K-dependent mammary tumor initiation, maintenance, and progression to metastasis by orchestrating multiple core cellular functions, including proliferation, survival, and avoidance of senescence.

Control of axonal branching and synapse formation by focal adhesion kinase

The formation of neuronal networks in the central nervous system (CNS) requires precise control of axonal branch development and stabilization. Here we show that cell-specific ablation of the murine gene Ptk2 (more commonly known as fak), encoding focal adhesion kinase (FAK), increases the number of axonal terminals and synapses formed by neurons in vivo. Consistent with this, fak mutant neurons also form greater numbers of axonal branches in culture because they have increased branch formation and reduced branch retraction. Expression of wild-type FAK, but not that of several FAK variants that prevent interactions with regulators of Rho family GTPases including the p190 Rho guanine nuclear exchange factor (p190RhoGEF), rescues the axonal arborization phenotype observed in fak mutant neurons. In addition, expression of a mutant p190RhoGEF that cannot associate with FAK results in a phenotype very similar to that of neurons lacking FAK. Thus, FAK functions as a negative regulator of axonal branching and synapse formation, and it seems to exert its actions, in part, through Rho family GTPases.

Focal adhesion kinase is required for the spatial organization of the leading edge in migrating cells

The process of cell migration is initiated by protrusion at the leading edge of the cell, the formation of peripheral adhesions, the exertion of force on these adhesions, and finally the release of the adhesions at the rear of the cell. Focal adhesion kinase (FAK) is intimately involved in the regulation of this process, although the precise mechanism(s) whereby FAK regulates cell migration is unclear. We have used two approaches to reduce FAK expression in fibroblasts. Treatment of cells with FAK-specific siRNAs substantially reduced FAK expression and inhibited the spreading of fibroblasts in serum-free conditions, but did not affect the rate of spreading in the presence of serum. In contrast with the wild-type cells, the FAK siRNA-treated cells exhibited multiple extensions during cell spreading. The extensions appeared to be inappropriately formed lamellipodia as evidenced by the localization of cortactin to lamellipodial structures and the inhibition of such structures by expression of dominant-negative Rac. The wild-type phenotype was restored by reexpressing wild-type FAK in the knockdown cells, but not by expression of FAK containing a point mutation at the autophosphorylation site (FAK Y397F). In wound-healing assays, FAK knockdown cells failed to form broad lamellipodia, instead forming multiple leading edges. Similar results were obtained using primary mouse embryo fibroblasts from FAK-flox mice in which Cre-mediated excision was used to ablate the expression of FAK. These data are consistent with a role for FAK in regulating the formation of a leading edge during cell migration by coordinating integrin signaling to direct the correct spatial activation of membrane protrusion.

Mammary Epithelial-Specific Disruption of Focal Adhesion Kinase Retards Tumor Formation and Metastasis in a Transgenic Mouse Model of Human Breast Cancer

Focal adhesion kinase (FAK) is a central regulator of the focal adhesion, influencing cell proliferation, survival, and migration. Despite evidence demonstrating FAK overexpression in human cancer, its role in tumor initiation and progression is not well understood. Using Cre/LoxP technology to specifically knockout FAK in the mammary epithelium, we showed that FAK is not required for tumor initiation but is required for tumor progression. The mechanistic underpinnings of these results suggested that FAK regulates clinically relevant gene signatures and multiple signaling complexes associated with tumor progression and metastasis, such as Src, ERK, and p130Cas. Furthermore, a systems-level analysis identified FAK as a major regulator of the tumor transcriptome, influencing genes associated with adhesion and growth factor signaling pathways, and their cross talk. Additionally, FAK was shown to down-regulate the expression of clinically relevant proliferation- and metastasis-associated gene signatures, as well as an enriched group of genes associated with the G(2) and G(2)/M phases of the cell cycle. Computational analysis of transcription factor-binding sites within ontology-enriched or clustered gene sets suggested that the differentially expressed proliferation- and metastasis-associated genes in FAK-null cells were regulated through a common set of transcription factors, including p53. Therefore, FAK acts as a primary node in the activated signaling network in transformed motile cells and is a prime candidate for novel therapeutic interventions to treat aggressive human breast cancers.

Regulation of lamellipodial persistence, adhesion turnover, and motility in macrophages by focal adhesion kinase.

Macrophages are a key component of the innate immune system. In this study, we investigate how focal adhesion kinase (FAK) and the related kinase Pyk2 integrate adhesion signaling and growth factor receptor signaling to regulate diverse macrophage functions. Primary bone marrow macrophages isolated from mice in which FAK is conditionally deleted from cells of the myeloid lineage exhibited elevated protrusive activity, altered adhesion dynamics, impaired chemotaxis, elevated basal Rac1 activity, and a marked inability to form stable lamellipodia necessary for directional locomotion. The contribution of FAK to macrophage function in vitro was substantiated in vivo by the finding that recruitment of monocytes to sites of inflammation was impaired in the absence of FAK. Decreased Pyk2 expression in primary macrophages also resulted in a diminution of invasive capacity. However, the combined loss of FAK and Pyk2 had no greater effect than the loss of either molecule alone, indicating that both kinases function within the same pathway to promote invasion.

Myocyte-Restricted Focal Adhesion Kinase Deletion Attenuates Pressure Overload–Induced Hypertrophy

Focal adhesion kinase (FAK) is a ubiquitously expressed cytoplasmic tyrosine kinase strongly activated by integrins and neurohumoral factors. Previous studies have shown that cardiac FAK activity is enhanced by hypertrophic stimuli before the onset of overt hypertrophy. Herein, we report that conditional deletion of FAK from the myocardium of adult mice did not affect basal cardiac performance, myocyte viability, or myofibrillar architecture. However, deletion of FAK abolished the increase in left ventricular posterior wall thickness, myocyte cross-sectional area, and hypertrophy-associated atrial natriuretic factor induction following pressure overload. Myocyte-restricted deletion of FAK attenuated the initial wave of extracellular signal-regulated kinase activation and cFos expression induced by adrenergic agonists and biomechanical stress. In addition, we found that persistent challenge of mice with myocyte-restricted FAK inactivation leads to enhanced cardiac fibrosis and cardiac dysfunction in comparison to challenged genetic controls. These studies show that loss of FAK impairs normal compensatory hypertrophic remodeling without a concomitant increase in apoptosis in response to cardiac pressure overload and highlight the possibility that FAK activation may be a common requirement for the initiation of this compensatory response.