RPTPα controls epithelial adherens junctions, linking E-cadherin engagement to c-Src-mediated phosphorylation of cortactin

A novel noncanonical function for IRF6 in the recycling of E-cadherin

Interferon Regulatory Factor 6 (IRF6) is a transcription factor essential for keratinocyte cell-cell adhesions. Previously, we found that recycling of E-cadherin was defective in the absence of IRF6, yet total E-cadherin levels were not altered, suggesting a previously unknown, nontranscriptional function for IRF6. IRF6 protein contains a DNA binding domain (DBD) and a protein binding domain (PBD). The transcriptional function of IRF6 depends on its DBD and PBD, however, whether the PBD is necessary for the interaction with cytoplasmic proteins has yet to be demonstrated. Here, we show that an intact PBD is required for recruitment of cell-cell adhesion proteins at the plasma membrane, including the recycling of E-cadherin. Colocalizations and coimmunoprecipitations reveal that IRF6 forms a complex in recycling endosomes with Rab11, Myosin Vb, and E-cadherin, and that the PBD is required for this interaction. These data indicate that IRF6 is a novel effector of the endosomal recycling of E-cadherin and demonstrate a non-transcriptional function for IRF6 in regulating cell-cell adhesions.

Manipulating PTPRD function with ectodomain antibodies

Protein tyrosine phosphatases (PTPs) are critical regulators of signal transduction but have yet to be exploited fully for drug development. Receptor protein tyrosine phosphatase δ (RPTPδ/PTPRD) has been shown to elicit tumor-promoting functions, including elevating SRC activity and promoting metastasis in certain cell contexts. Dimerization has been implicated in the inhibition of receptor protein tyrosine phosphatases (RPTPs). We have generated antibodies targeting PTPRD ectodomains with the goal of manipulating their dimerization status ectopically, thereby regulating intracellular signaling. We have validated antibody binding to endogenous PTPRD in a metastatic breast cancer cell line, CAL51, and demonstrated that a monoclonal antibody, RD-43, inhibited phosphatase activity and induced the degradation of PTPRD. Similar effects were observed following chemically induced dimerization of its phosphatase domain. Mechanistically, RD-43 triggered the formation of PTPRD dimers in which the phosphatase activity was impaired. Subsequently, the mAb-PTPRD dimer complex was degraded through lysosomal and proteasomal pathways, independently of secretase cleavage. Consequently, treatment with RD-43 inhibited SRC signaling and suppressed PTPRD-dependent cell invasion. Together, these findings demonstrate that manipulating RPTP function via antibodies to the extracellular segments has therapeutic potential.

A novel function for the sperm adhesion protein IZUMO1 in cell-cell fusion

Mammalian sperm-egg adhesion depends on the trans-interaction between the sperm-specific type I glycoprotein IZUMO1 and its oocyte-specific GPI-anchored receptor JUNO. However, the mechanisms and proteins (fusogens) that mediate the following step of gamete fusion remain unknown. Using live imaging and content mixing assays in a heterologous system and structure-guided mutagenesis, we unveil an unexpected function for IZUMO1 in cell-to-cell fusion. We show that IZUMO1 alone is sufficient to induce fusion, and that this ability is retained in a mutant unable to bind JUNO. On the other hand, a triple mutation in exposed aromatic residues prevents this fusogenic activity without impairing JUNO interaction. Our findings suggest a second function for IZUMO1 as a unilateral mouse gamete fusogen.

Protein tyrosine phosphatases in cell adhesion

Adhesive structures between cells and with the surrounding matrix are essential for the development of multicellular organisms. In addition to providing mechanical integrity, they are key signalling centres providing feedback on the extracellular environment to the cell interior, and vice versa. During development, mitosis and repair, cell adhesions must undergo extensive remodelling. Post-translational modifications of proteins within these complexes serve as switches for activity. Tyrosine phosphorylation is an important modification in cell adhesion that is dynamically regulated by the protein tyrosine phosphatases (PTPs) and protein tyrosine kinases. Several PTPs are implicated in the assembly and maintenance of cell adhesions, however, their signalling functions remain poorly defined. The PTPs can act by directly dephosphorylating adhesive complex components or function as scaffolds. In this review, we will focus on human PTPs and discuss their individual roles in major adhesion complexes, as well as Hippo signalling. We have collated PTP interactome and cell adhesome datasets, which reveal extensive connections between PTPs and cell adhesions that are relatively unexplored. Finally, we reflect on the dysregulation of PTPs and cell adhesions in disease.

Par-3 family proteins in cell polarity & adhesion

The Par‐3/Baz family of polarity determinants is highly conserved across metazoans and includes C. elegans PAR‐3, Drosophila Bazooka (Baz), human Par‐3 (PARD3), and human Par‐3‐like (PARD3B). The C. elegans PAR‐3 protein localises to the anterior pole of asymmetrically dividing zygotes with cell division cycle 42 (CDC42), atypical protein kinase C (aPKC), and PAR‐6. The same C. elegans ‘PAR complex’ can also localise in an apical ring in epithelial cells. Drosophila Baz localises to the apical pole of asymmetrically dividing neuroblasts with Cdc42‐aPKC‐Par6, while in epithelial cells localises both in an apical ring with Cdc42‐aPKC‐Par6 and with E‐cadherin at adherens junctions. These apical and junctional localisations have become separated in human PARD3, which is strictly apical in many epithelia, and human PARD3B, which is strictly junctional in many epithelia. We discuss the molecular basis for this fundamental difference in localisation, as well as the possible functions of Par‐3/Baz family proteins as oligomeric clustering agents at the apical domain or at adherens junctions in epithelial stem cells. The evolution of Par‐3 family proteins into distinct apical PARD3 and junctional PARD3B orthologs coincides with the emergence of stratified squamous epithelia in vertebrates, where PARD3B, but not PARD3, is strongly expressed in basal layer stem cells – which lack a typical apical domain. We speculate that PARD3B may contribute to clustering of E‐cadherin, signalling from adherens junctions via Src family kinases or mitotic spindle orientation by adherens junctions in response to mechanical forces.

Clathrin regulates Wnt/β-catenin signaling by affecting Golgi to plasma membrane transport of transmembrane proteins

The canonical Wnt/β-catenin signaling pathway regulates cell proliferation in development and adult tissue homeostasis. Dysregulated signaling contributes to human diseases, in particular cancer. Growing evidence suggests a role for clathrin and/or endocytosis in the regulation of this pathway, but conflicting results exist and demand a deeper mechanistic understanding. We investigated the consequences of clathrin depletion on Wnt/β-catenin signaling in cell lines and found a pronounced reduction in β-catenin protein levels, which affects the amount of nuclear β-catenin and β-catenin target gene expression. Although we found no evidence that clathrin affects β-catenin levels via endocytosis or multivesicular endosome formation, an inhibition of protein transport through the biosynthetic pathway led to reduced levels of a Wnt co-receptor, low-density lipoprotein receptor-related protein 6 (LRP6), and cell adhesion molecules of the cadherin family, thereby affecting steady-state levels of β-catenin. We conclude that clathrin impacts on Wnt/β-catenin signaling by controlling exocytosis of transmembrane proteins, including cadherins and Wnt co-receptors that together control the membrane-bound and soluble pools of β-catenin.

PTPRA Phosphatase Regulates GDNF-Dependent RET Signaling and Inhibits the RET Mutant MEN2A Oncogenic Potential

The RET proto-oncogene encodes receptor tyrosine kinase, expressed primarily in tissues of neural crest origin. De-regulation of RET signaling is implicated in several human cancers. Recent phosphatome interactome analysis identified PTPRA interacting with the neurotrophic factor (GDNF)-dependent RET-Ras-MAPK signaling-axis. Here, by identifying comprehensive interactomes of PTPRA and RET, we reveal their close physical and functional association. The PTPRA directly interacts with RET, and using the phosphoproteomic approach, we identify RET as a direct dephosphorylation substrate of PTPRA both in vivo and in vitro. The protein phosphatase domain-1 is indispensable for the PTPRA inhibitory role on RET activity and downstream Ras-MAPK signaling, whereas domain-2 has only minor effect. Furthermore, PTPRA also regulates the RET oncogenic mutant variant MEN2A activity and invasion capacity, whereas the MEN2B is insensitive to PTPRA. In sum, we discern PTPRA as a novel regulator of RET signaling in both health and cancer.

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PTPRA inhibits ligand (GDNF-GFRα1)-mediated RET activity on Ras-MAPK signaling axis

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PTPRA dephosphorylate RET on key functional phosphotyrosine sites

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PTPRA catalytic (PTPase) domain 1 regulates RET-driven signaling

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PTPRA suppresses RET oncogenic mutant MEN2A in both Ras-MAPK and cell invasion models

Protein tyrosine phosphatase alpha inhibits hypothalamic leptin receptor signaling and regulates body weight in vivo

Understanding how body weight is regulated at the molecular level is essential for treating obesity. We show that female mice genetically lacking protein tyrosine phosphatase (PTP) receptor type α (PTPRA) exhibit reduced weight and adiposity and increased energy expenditure, and are more resistant to diet-induced obesity than matched wild-type control mice. These mice also exhibit reduced levels of circulating leptin and are leptin hypersensitive, suggesting that PTPRA inhibits leptin signaling in the hypothalamus. Male and female PTPRA-deficient mice fed a high-fat diet were leaner and displayed increased metabolic rates and lower circulating leptin levels, indicating that the effects of loss of PTPRA persist in the obese state. Molecularly, PTPRA down-regulates leptin receptor signaling by dephosphorylating the receptor-associated kinase JAK2, with which the phosphatase associates constitutively. In contrast to the closely related tyrosine phosphatase ε, leptin induces only weak phosphorylation of PTPRA at its C-terminal regulatory site Y789, and this does not affect the activity of PTPRA toward JAK2. PTPRA is therefore an inhibitor of hypothalamic leptin signaling in vivo and may prevent premature activation of leptin signaling, as well as return signaling to baseline after exposure to leptin.-Cohen-Sharir, Y., Kuperman, Y., Apelblat, D., den Hertog, J., Spiegel, I., Knobler, H., Elson, A. Protein tyrosine phosphatase alpha inhibits hypothalamic leptin receptor signaling and regulates body weight in vivo.

Redistribution of Adhesive Forces through Src/FAK Drives Contact Inhibition of Locomotion in Neural Crest

Contact inhibition of locomotion is defined as the behavior of cells to cease migrating in their former direction after colliding with another cell. It has been implicated in multiple developmental processes and its absence has been linked to cancer invasion. Cellular forces are thought to govern this process; however, the exact role of traction through cell-matrix adhesions and tension through cell-cell adhesions during contact inhibition of locomotion remains unknown. Here we use neural crest cells to address this and show that cell-matrix adhesions are rapidly disassembled at the contact between two cells upon collision. This disassembly is dependent upon the formation of N-cadherin-based cell-cell adhesions and driven by Src and FAK activity. We demonstrate that the loss of cell-matrix adhesions near the contact leads to a buildup of tension across the cell-cell contact, a step that is essential to drive cell-cell separation after collision.

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Focal adhesions disassemble at cell-cell contacts in contact inhibition of locomotion

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FA disassembly at the cell contact during CIL requires N-cadherin/Src/FAK signaling

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Cell separation during CIL involves a buildup of tension across the cell contact

Cell-Cell Contact and Receptor Tyrosine Kinase Signaling

The behavior of cells within tissues is governed by the activities of adhesion receptors that provide spatial cues and transmit forces through intercellular junctions, and by growth-factor receptors, particularly receptor tyrosine kinases (RTKs), that respond to biochemical signals from the environment. Coordination of these two activities is essential for the patterning and polarized migration of cells during morphogenesis and for homeostasis in mature tissues; loss of this coordination is a hallmark of developing cancer and driver of metastatic progression. Although much is known about the individual functions of adhesion and growth factor receptors, we have a surprisingly superficial understanding of the mechanisms by which their activities are coordinated.

Cortactin promotes colorectal cancer cell proliferation by activating the EGFR-MAPK pathway

Cortactin (CTTN) is overexpressed in various tumors, including head and neck squamous cell carcinoma and colorectal cancer (CRC), and can serve as a biomarker of cancer metastasis. We observed that CTTN promotes cancer cell proliferation in vitro and increases CRC tumor xenograft growth in vivo. CTTN expression increases EGFR protein levels and enhances the activation of the MAPK signaling pathway. CTTN expression also inhibits the ubiquitin-mediated degradation of EGFR by suppressing the coupling of c-Cbl with EGFR. CoIP experiments indicate CTTN can interact with c-Cbl in CRC cells. These results demonstrate that CTTN promotes the proliferation of CRC cells and suppresses the degradation of EGFR.

Epidermal growth factor receptor and integrins control force-dependent vinculin recruitment to E-cadherin junctions

This study reports novel findings that link E-cadherin (also known as CDH1)-mediated force-transduction signaling to vinculin targeting to intercellular junctions via epidermal growth factor receptor (EGFR) and integrins. These results build on previous findings that demonstrated that mechanically perturbed E-cadherin receptors activate phosphoinositide 3-kinase and downstream integrins in an EGFR-dependent manner. Results of this study show that this EGFR-mediated kinase cascade controls the force-dependent recruitment of vinculin to stressed E-cadherin complexes - a key early signature of cadherin-based mechanotransduction. Vinculin targeting requires its phosphorylation at tyrosine 822 by Abl family kinases (hereafter Abl), but the origin of force-dependent Abl activation had not been identified. We now present evidence that integrin activation, which is downstream of EGFR signaling, controls Abl activation, thus linking E-cadherin to Abl through a mechanosensitive signaling network. These findings place EGFR and integrins at the center of a positive-feedback loop, through which force-activated E-cadherin signals regulate vinculin recruitment to cadherin complexes in response to increased intercellular tension.This article has an associated First Person interview with the first author of the paper.

Regulated recruitment of SRGAP1 modulates RhoA signaling for contractility during epithelial junction maturation

Adherens junctions in epithelia are contractile structures, where coupling of adhesion to the actomyosin cytoskeleton generates mechanical tension for morphogenesis and homeostasis. In established monolayers, junctional contractility is supported by the interplay between cell signals and scaffolding proteins. However, less is known about how contractile junctions develop, especially during the establishment of epithelial monolayers. Here, we show that junctional tension increases concomitant with accumulation of actomyosin networks as Caco-2 epithelia become confluent. This is associated with development of a zone of RhoA signaling at junctions. Further, we find that the low levels of RhoA signaling and contractility found in subconfluent cultures reflect a mechanism for their active suppression. Specifically, the RhoA antagonist, SRGAP1, is present at subconfluent junctions to a greater extent than in confluent cultures and SRGAP1 RNAi restores RhoA signaling and contractility in subconfluent cultures to levels seen in confluent cells. Overall, these observations suggest that regulated changes in junctional contractility mediated by modulation of RhoA signaling occur as epithelial monolayers mature.

Live imaging molecular changes in junctional tension upon VE-cadherin in zebrafish

Forces play diverse roles in vascular development, homeostasis and disease. VE-cadherin at endothelial cell-cell junctions links the contractile acto-myosin cytoskeletons of adjacent cells, serving as a tension-transducer. To explore tensile changes across VE-cadherin in live zebrafish, we tailored an optical biosensor approach, originally established in vitro. We validate localization and function of a VE-cadherin tension sensor (TS) in vivo. Changes in tension across VE-cadherin observed using ratio-metric or lifetime FRET measurements reflect acto-myosin contractility within endothelial cells. Furthermore, we apply the TS to reveal biologically relevant changes in VE-cadherin tension that occur as the dorsal aorta matures and upon genetic and chemical perturbations during embryonic development.

Mechanical forces play a crucial role during morphogenesis, but how these are sensed and transduced in vivo is not fully understood. Here the authors apply a FRET tension sensor to live zebrafish and study changes in VE-cadherin tension at endothelial cell-cell junctions during arterial maturation.

Tyrosine dephosphorylated cortactin downregulates contractility at the epithelial zonula adherens through SRGAP1

Contractile adherens junctions support cell−cell adhesion, epithelial integrity, and morphogenesis. Much effort has been devoted to understanding how contractility is established; however, less is known about whether contractility can be actively downregulated at junctions nor what function this might serve. We now identify such an inhibitory pathway that is mediated by the cytoskeletal scaffold, cortactin. Mutations of cortactin that prevent its tyrosine phosphorylation downregulate RhoA signaling and compromise the ability of epithelial cells to generate a contractile zonula adherens. This is mediated by the RhoA antagonist, SRGAP1. We further demonstrate that this mechanism is co-opted by hepatocyte growth factor to promote junctional relaxation and motility in epithelial collectives. Together, our findings identify a novel function of cortactin as a regulator of RhoA signaling that can be utilized by morphogenetic regulators for the active downregulation of junctional contractility.

Epithelial cell-cell adhesions are contractile junctions, but whether contractility can be down-regulated is not known. Here the authors report how tyrosine dephosphorylation of the cytoskeletal scaffold, cortactin, recruits the RhoA antagonist SRGAP1 to relax adherens junctions in response to HGF.

Regulation of receptor-type protein tyrosine phosphatases by their C-terminal tail domains

Protein tyrosine phosphatases (PTPs) perform specific functions in vivo, despite being vastly outnumbered by their substrates. Because of this and due to the central roles PTPs play in regulating cellular function, PTP activity is regulated by a large variety of molecular mechanisms. We review evidence that indicates that the divergent C-terminal tail sequences (C-terminal domains, CTDs) of receptor-type PTPs (RPTPs) help regulate RPTP function by controlling intermolecular associations in a way that is itself subject to physiological regulation. We propose that the CTD of each RPTP defines an 'interaction code' that helps determine molecules it will interact with under various physiological conditions, thus helping to regulate and diversify PTP function.

Increased PTPRA expression leads to poor prognosis through c-Src activation and G1 phase progression in squamous cell lung cancer

PTPRA is reported to be involved in cancer development and progression through activating the Src family kinase (SFK) signaling pathways, however, the roles of PTPRA in the squamous cell lung cancer (SCC) development are unclear. The purpose of this study was to clarify the clinical relevance and biological roles of PTPRA in SCC. We found that PTPRA was upregulated in squamous cell lung cancer compared to matched normal tissues at the mRNA (N=20, P=0.004) and protein expression levels (N=75, P<0.001). Notably, high mRNA level of PTPRA was significantly correlated with poorer prognosis in 675 SCC patients from the Kaplan-Meier plotter database. With 75 cases, we found that PTPRA protein expression was significantly correlated with tumor size (P=0.002), lymph node metastasis (P=0.008), depth of tumor invasion (P<0.001) and clinical stage (P<0.001). The Kaplan-Meier plot suggested that high expression of PTPRA had poorer overall survival in SCC patients (P=0.009). Multivariate Cox regression analysis suggested that PTPRA expression was an independent prognostic factor in SCC patients. In the cellular models, PTPRA promotes SCC cell proliferation through modulating Src activation as well as cell cycle progression. In conclusion, higher PTPRA level was associated with worse prognosis of SCC patients and PTPRA could promote the cell cycle progression through stimulating the c-Src signaling pathways.

New Insights into VacA Intoxication Mediated through Its Cell Surface Receptors

Helicobacter pylori (H. pylori), a major cause of gastroduodenal diseases, produces VacA, a vacuolating cytotoxin associated with gastric inflammation and ulceration. The C-terminal domain of VacA plays a crucial role in receptor recognition on target cells. We have previously identified three proteins (i.e., RPTPα, RPTPβ, and LRP1) that serve as VacA receptors. These receptors contribute to the internalization of VacA into epithelial cells, activate signal transduction pathways, and contribute to cell death and gastric ulceration. In addition, other factors (e.g., CD18, sphingomyelin) have also been identified as cell-surface, VacA-binding proteins. Since we believe that, following interactions with its host cell receptors, VacA participates in events leading to disease, a better understanding of the cellular function of VacA receptors may provide valuable information regarding the mechanisms underlying the pleiotropic actions of VacA and the pathogenesis of H. pylori-mediated disease. In this review, we focus on VacA receptors and their role in events leading to cell damage.

Identification of a shootin1 isoform expressed in peripheral tissues

Shootin1 is a brain-specific cytoplasmic protein involved in neuronal polarity formation and axon outgrowth. It accumulates at the leading edge of axonal growth cones, where it mediates the mechanical coupling between F-actin retrograde flow and cell adhesions as a clutch molecule, thereby producing force for axon outgrowth. In this study, we report a novel splicing isoform of shootin1 which is expressed not only in the brain but also in peripheral tissues. We have renamed the brain-specific shootin1 as shootin1a and termed the novel isoform as shootin1b. Immunoblot and immunohistochemical analyses with a shootin1b-specific antibody revealed that shootin1b is distributed in various mouse tissues including the lung, liver, stomach, intestines, spleen, pancreas, kidney and skin. Interestingly, shootin1b immunoreactivity was widely detected in epithelial cells that constitute simple and stratified epithelia; in some cells, it colocalized with E-cadherin and cortactin at cell-cell contact sites. Shootin1b also localized in dendritic cells in the spleen. These results suggest that shootin1b may function in various peripheral tissues including epithelial cells.

Regulation of Endothelial Adherens Junctions by Tyrosine Phosphorylation

Endothelial cells form a semipermeable, regulated barrier that limits the passage of fluid, small molecules, and leukocytes between the bloodstream and the surrounding tissues. The adherens junction, a major mechanism of intercellular adhesion, is comprised of transmembrane cadherins forming homotypic interactions between adjacent cells and associated cytoplasmic catenins linking the cadherins to the cytoskeleton. Inflammatory conditions promote the disassembly of the adherens junction and a loss of intercellular adhesion, creating openings or gaps in the endothelium through which small molecules diffuse and leukocytes transmigrate. Tyrosine kinase signaling has emerged as a central regulator of the inflammatory response, partly through direct phosphorylation and dephosphorylation of the adherens junction components. This review discusses the findings that support and those that argue against a direct effect of cadherin and catenin phosphorylation in the disassembly of the adherens junction. Recent findings indicate a complex interaction between kinases, phosphatases, and the adherens junction components that allow a fine regulation of the endothelial permeability to small molecules, leukocyte migration, and barrier resealing.

RhoGTPase signalling at epithelial tight junctions: Bridging the GAP between polarity and cancer

The establishment and maintenance of epithelial polarity must be correctly controlled for normal development and homeostasis. Tight junctions (TJ) in vertebrates define apical and basolateral membrane domains in polarized epithelia via bi-directional, complex signalling pathways between TJ themselves and the cytoskeleton they are associated with. RhoGTPases are central to these processes and evidence suggests that their regulation is coordinated by interactions between GEFs and GAPs with junctional, cytoplasmic adapter proteins. In this InFocus review we determine that the expression, localization or stability of a variety of these adaptor proteins is altered in various cancers, potentially representing an important mechanistic link between loss of polarity and cancer. We focus here, on two well characterized RhoGTPases Cdc42 and RhoA who's GEFs and GAPs are predominantly localized to TJ via cytoplasmic adaptor proteins.

An RPTPα/Src family kinase/Rap1 signaling module recruits myosin IIB to support contractile tension at apical E-cadherin junctions

Cell-cell adhesion couples the contractile cortices of epithelial cells together, generating tension to support a range of morphogenetic processes. E-cadherin adhesion plays an active role in generating junctional tension by promoting actin assembly and cortical signaling pathways that regulate myosin II. Multiple myosin II paralogues accumulate at mammalian epithelial cell-cell junctions. Earlier, we found that myosin IIA responds to Rho-ROCK signaling to support junctional tension in MCF-7 cells. Although myosin IIB is also found at the zonula adherens (ZA) in these cells, its role in junctional contractility and its mode of regulation are less well understood. We now demonstrate that myosin IIB contributes to tension at the epithelial ZA. Further, we identify a receptor type-protein tyrosine phosphatase alpha-Src family kinase-Rap1 pathway as responsible for recruiting myosin IIB to the ZA and supporting contractile tension. Overall these findings reinforce the concept that orthogonal E-cadherin-based signaling pathways recruit distinct myosin II paralogues to generate the contractile apparatus at apical epithelial junctions.