Lamellipodium extension and cadherin adhesion: two cell responses to cadherin activation relying on distinct signalling pathways

The challenges of abundance: epithelial junctions and small GTPase signalling

Small GTPases of the Ras superfamily play critical roles in epithelial biogenesis. Many key morphogenetic functions occur when small GTPases act at epithelial junctions, where they mediate an increasingly complex interplay between cell-cell adhesion molecules and fundamental cellular processes, such as cytoskeletal activity, polarity and trafficking. Important recent advances in this field include the role of additional members of the Ras superfamily in cell-cell contact stability and the capacity for polarity determinants to regulate small GTPase signalling. Interestingly, small GTPases may participate in the cross-talk between different adhesive receptors: in tissues classical cadherins can selectively regulate other junctions through cell signalling rather than through a global influence on cell-cell cohesion.

Integrin-dependent actomyosin contraction regulates epithelial cell scattering

The scattering of Madin-Darby canine kidney cells in vitro mimics key aspects of epithelial-mesenchymal transitions during development, carcinoma cell invasion, and metastasis. Scattering is induced by hepatocyte growth factor (HGF) and is thought to involve disruption of cadherin-dependent cell-cell junctions. Scattering is enhanced on collagen and fibronectin, as compared with laminin1, suggesting possible cross talk between integrins and cell-cell junctions. We show that HGF does not trigger any detectable decrease in E-cadherin function, but increases integrin-mediated adhesion. Time-lapse imaging suggests that tension on cell-cell junctions may disrupt cell-cell adhesion. Varying the density and type of extracellular matrix proteins shows that scattering correlates with stronger integrin adhesion and increased phosphorylation of the myosin regulatory light chain. To directly test the role of integrin-dependent traction forces, substrate compliance was varied. Rigid substrates that produce high traction forces promoted scattering, in comparison to more compliant substrates. We conclude that integrin-dependent actomyosin traction force mediates the disruption of cell-cell adhesion during epithelial cell scattering.

Regulation of N-cadherin dynamics at neuronal contacts by ligand binding and cytoskeletal coupling

N-cadherin plays a key role in axonal outgrowth and synaptogenesis, but how neurons initiate and remodel N-cadherin-based adhesions remains unclear. We addressed this issue with a semiartificial system consisting of N-cadherin coated microspheres adhering to cultured neurons transfected for N-cadherin-GFP. Using optical tweezers, we show that growth cones are particularly reactive to N-cadherin coated microspheres, which they capture in a few seconds and drag rearward. Such strong coupling requires an intact connection between N-cadherin receptors and catenins. As they move to the basis of growth cones, microspheres slow down while gradually accumulating N-cadherin-GFP, demonstrating a clear delay between bead coupling to the actin flow and receptor recruitment. Using FRAP and photoactivation, N-cadherin receptors at bead-to-cell contacts were found to continuously recycle, consistently with a model of ligand-receptor reaction not limited by membrane diffusion. The use of N-cadherin-GFP receptors truncated or mutated in specific cytoplasmic regions show that N-cadherin turnover is exquisitely regulated by catenin partners. Turnover rates are considerably lower than those obtained previously in single molecule studies, demonstrating an active regulation of cadherin bond kinetics in intact cells. Finally, spontaneous neuronal contacts enriched in N-cadherin exhibited similar turnover rates, suggesting that such dynamics of N-cadherin may represent an intrinsic mechanism underlying the plasticity of neuronal adhesions.

Phosphorylation of N-cadherin-associated cortactin by Fer kinase regulates N-cadherin mobility and intercellular adhesion strength

Cortactin regulates the strength of nascent N-cadherin-mediated intercellular adhesions through a tyrosine phosphorylation-dependent mechanism. Currently, the functional significance of cortactin phosphorylation and the kinases responsible for the regulation of adhesion strength are not defined. We show that the nonreceptor tyrosine kinase Fer phosphorylates cadherin-associated cortactin and that this process is involved in mediating intercellular adhesion strength. In wild-type fibroblasts N-cadherin ligation-induced transient phosphorylation of Fer, indicating that junction formation activates Fer kinase. Tyrosine phosphorylation of cortactin after N-cadherin ligation was strongly reduced in fibroblasts expressing only catalytically inactive Fer (D743R), compared with wild-type cells. In wild-type cells, N-cadherin-coated bead pull-off assays induced fourfold greater endogenous N-cadherin association than in D743R cells. Fluorescence recovery after photobleaching showed that GFP-N-cadherin mobility at nascent contacts was 50% faster in wild-type than D743R cells. In shear wash-off assays, nascent intercellular adhesion strength was twofold higher in wild-type than D743R cells. Cortactin recruitment to adhesions was independent of Fer kinase activity, but was impacted by N-cadherin ligation-provoked Rac activation. We conclude that N-cadherin ligation induces Rac-dependent cortactin recruitment and Fer-dependent cortactin phosphorylation, which in turn promotes enhanced mobilization and interaction of surface expressed N-cadherin in contacting cells.

A novel function for cadherin-11 in the regulation of motor axon elongation and fasciculation

We previously observed that cadherin-11, a type II cadherin, is expressed in growing motor and sensory axons in the mouse embryo. Here, we assessed its functional involvement in the regulation of axon elongation and fasciculation by evaluating the activity of a specific cadherin-11 homophilic ligand, cad11-Fc (cadherin-11 extracellular region fused to Fc fragment of IgG), on the length and organization of motor axons outgrowing from embryonic ventral spinal cord explants. Cad11-Fc substrate enhanced axon growth and prevented interactions occurring between growing axons, providing evidences for a role of cadherin-11 in the control of growth cone progression. Comparison of cadherin-11 with N-cadherin, a type I cadherin concomitantly expressed by motor axons, revealed similarities in their functional properties, including the ability to reorganize the actin cytoskeleton through interactions with catenins, but differences in their axon growth-promoting activity, arguing for subtle differences in their contributions to peripheral nerve elongation.

ABI2-deficient mice exhibit defective cell migration, aberrant dendritic spine morphogenesis, and deficits in learning and memory

The Abl-interactor (Abi) family of adaptor proteins has been linked to signaling pathways involving the Abl tyrosine kinases and the Rac GTPase. Abi proteins localize to sites of actin polymerization in protrusive membrane structures and regulate actin dynamics in vitro. Here we demonstrate that Abi2 modulates cell morphogenesis and migration in vivo. Homozygous deletion of murine abi2 produced abnormal phenotypes in the eye and brain, the tissues with the highest Abi2 expression. In the absence of Abi2, secondary lens fiber orientation and migration were defective in the eye, without detectable defects in proliferation, differentiation, or apoptosis. These phenotypes were consistent with the localization of Abi2 at adherens junctions in the developing lens and at nascent epithelial cell adherens junctions in vitro. Downregulation of Abi expression by RNA interference impaired adherens junction formation and correlated with downregulation of the Wave actin-nucleation promoting factor. Loss of Abi2 also resulted in cell migration defects in the neocortex and hippocampus, abnormal dendritic spine morphology and density, and severe deficits in short- and long-term memory. These findings support a role for Abi2 in the regulation of cytoskeletal dynamics at adherens junctions and dendritic spines, which is critical for intercellular connectivity, cell morphogenesis, and cognitive functions.

Cell adhesion receptors, tyrosine kinases and actin modulators: a complex three-way circuitry

The interaction of cells with surrounding matrix and neighbouring cells governs many aspects of cell behaviour. Aside from transmitting signals from the external environment, adhesion receptors also receive signals from the cell interior. Here we review the interrelationship between adhesion receptors, tyrosine kinases (both growth factor receptor and non-receptor) and modulators of the actin cytoskeletal network. Deregulation of many aspects of these signalling pathways in cancer highlights the need for a better understanding of the complexities involved.

Recruitment of phosphoinositide 3-kinase defines a positive contribution of tyrosine kinase signaling to E-cadherin function

Classical cadherin adhesion molecules can function as adhesion-activated cell-signaling receptors. One key target for cadherin signaling is the lipid kinase phosphoinositide (PI) 3-kinase, which is recruited to cell-cell contacts and activated by E-cadherin. In this study, we sought to identify upstream factors necessary for E-cadherin to activate PI 3-kinase signaling. We found that inhibition of tyrosine kinase signaling blocked recruitment of PI 3-kinase to E-cadherin contacts and abolished the ability of E-cadherin to activate PI 3-kinase signaling. Tyrosine kinase inhibitors further perturbed several parameters of cadherin function, including cell adhesion and the ability of cells to productively extend nascent cadherin-adhesive contacts. Notably, the functional effects of tyrosine kinase blockade were rescued by expression of a constitutively active form of PI 3-kinase that restores PI 3-kinase signaling. Finally, using dominant negative Src mutants and Src-null cells, we identified Src as one key upstream kinase in the E-cadherin/PI 3-kinase-signaling pathway. Taken together, our findings indicate that tyrosine kinase activity, notably Src signaling, can contribute positively to cadherin function by supporting E-cadherin signaling to PI 3-kinase.

Sphingosine 1-phosphate receptor regulation of N-cadherin mediates vascular stabilization

Vascular stabilization, a process by which nascent vessels are invested with mural cells, is important in angiogenesis. Here we describe the molecular basis of vascular stabilization regulated by sphingosine 1-phosphate (S1P), a platelet-derived lipid mediator. S1P1 receptor-dependent cell-surface trafficking and activation of the cell-cell adhesion molecule N-cadherin is essential for interactions between endothelial and mural cells. Endothelial cell S1P1/Gi/Rac pathway induces microtubule polymerization, resulting in trafficking of N-cadherin to polarized plasma membrane domains. S1P treatment modulated the phosphorylation of N-cadherin as well as p120-catenin and induced the formation of cadherin/catenin/actin complexes containing novel regulatory and trafficking factors. The net result of endothelial cell S1P1 receptor activation is the proper trafficking and strengthening of N-cadherin-dependent cell-cell adhesion with mural cells. Perturbation of N-cadherin expression with small interfering RNA profoundly attenuated vascular stabilization in vitro and in vivo. S1P-induced trafficking and activation of N-cadherin provides a novel mechanism for the stabilization of nascent blood vessels by mural cells and may be exploited to control angiogenesis and vascular diseases.

Arp2/3 Activity Is Necessary for Efficient Formation of E-cadherin Adhesive Contacts

Classical cadherin adhesion molecules are fundamental determinants of cell-cell recognition that function in cooperation with the actin cytoskeleton. Productive cadherin-based cell recognition is characterized by a distinct morphological process of contact zone extension, where limited initial points of adhesion are progressively expanded into broad zones of contact. We recently demonstrated that E-cadherin ligation recruits the Arp2/3 actin nucleator complex to the plasma membrane in regions where cell contacts are undergoing protrusion and extension. This suggested that Arp2/3 might generate the protrusive forces necessary for cell surfaces to extend upon one another during contact assembly. We tested this hypothesis in mammalian cells by exogenously expressing the CA region of N-WASP. This fragment, which potently inhibits Arp2/3-mediated actin assembly in vitro, also effectively reduced actin assembly at cadherin adhesive contacts. Blocking Arp2/3 activity by this strategy profoundly reduced the ability of cells to extend cadherin adhesive contacts but did not affect cell adhesiveness. These findings demonstrate that Arp2/3 activity is necessary for cells to efficiently extend and assemble cadherin-based adhesive contacts.

Regulation of intercellular adhesion strength in fibroblasts

The regulation of adherens junction formation in cells of mesenchymal lineage is of critical importance in tumorigenesis but is poorly characterized. As actin filaments are crucial components of adherens junction assembly, we studied the role of gelsolin, a calcium-dependent, actin severing protein, in the formation of N-cadherin-mediated intercellular adhesions. With a homotypic, donor-acceptor cell model and plates or beads coated with recombinant N-cadherin-Fc chimeric protein, we found that gelsolin spatially co-localizes to, and is transiently associated with, cadherin adhesion complexes. Fibroblasts from gelsolin-null mice exhibited marked reductions in kinetics and strengthening of N-cadherin-dependent junctions when compared with wild-type cells. Experiments with lanthanum chloride (250 microm) showed that adhesion strength was dependent on entry of calcium ions subsequent to N-cadherin ligation. Cadherin-associated gelsolin severing activity was required for localized actin assembly as determined by rhodamine actin monomer incorporation onto actin barbed ends at intercellular adhesion sites. Scanning electron microscopy showed that gelsolin was an important determinant of actin filament architecture of adherens junctions at nascent N-cadherin-mediated contacts. These data indicate that increased actin barbed end generation by the severing activity of gelsolin associated with N-cadherin regulates intercellular adhesion strength.

N-cadherin activation substitutes for the cell contact control in cell cycle arrest and myogenic differentiation: involvement of p120 and beta-catenin

N-cadherin is expressed throughout skeletal myogenesis and has been proposed to be involved in the differentiation program of myogenic precursors. Here, we further characterize the N-cadherin involvement and its mechanism of action at the onset of differentiation, through controlled N-cadherin activation by plating isolated C2 myoblasts on surfaces coated with a chimeric Ncad-Fc homophilic ligand (N-cadherin ectodomain fused to the immunoglobulin G Fc fragment). We show that N-cadherin activation substitutes for the cell density in myogenic differentiation by promoting myogenin and troponin T expression. In addition, N-cadherin adhesion participates to the associated cell cycle arrest through the nuclear accumulation of cyclin-dependent kinase inhibitors p21 and p27. Mouse primary myoblast cultures exhibited similar responses to N-cadherin as C2 cells. RNA interference knockdowns of the N-cadherin-associated cytoplasmic proteins p120 and beta-catenin produced opposite effects on the differentiation pathway. p120 silencing resulted in a decreased myogenic differentiation, associated with a reduction in cadherin-catenin content, which may explain its action on myogenic differentiation. beta-Catenin silencing led to a stimulatory effect on myogenin expression, without any effect on cell cycle. Our results demonstrate that N-cadherin adhesion may account for cell-cell contact-dependent cell cycle arrest and differentiation of myogenic cells, involving regulation through p120 and beta-catenins.