In situ phosphorylation of immobilized receptors on biosensor surfaces: application to E-cadherin/beta-catenin interactions

Src- and confinement-dependent FAK activation causes E-cadherin relaxation and β-catenin activity

β-Catenin is a transcription cofactor proposed to be released from E-cadherin upon mechanically induced phosphorylation. However, evidence for this mechanism is lacking. Gayrard et al. show instead that during epithelial-to-mesenchymal transition, Src- and multicellular confinement–dependent FAK-induced cytoskeleton remodeling causes E-cadherin tension relaxation and phosphorylation-independent β-catenin nuclear translocation from the membrane.

In epithelia, E-cadherin cytoplasmic tail is under cytoskeleton-generated tension via a link that contains β-catenin. A cotranscription factor, β-catenin, is also active in morphogenetic processes associated with epithelial-to-mesenchymal transition. β-Catenin signaling appears mechanically inducible and was proposed to follow phosphorylation-induced β-catenin release from E-cadherin. Evidence for this mechanism is lacking, and whether E-cadherin tension is involved is unknown. To test this, we combined quantitative fluorescence microscopies with genetic and pharmacological perturbations of epithelial-to-mesenchymal transition–induced cells in culture. We showed that β-catenin nuclear activity follows a substantial release from the membrane specific to migrating cells and requires multicellular deconfinement and Src activity. Selective nuclear translocation occurs downstream of focal adhesion kinase activation, which targets E-cadherin tension relaxation through actomyosin remodeling. In contrast, phosphorylations of the cadherin/catenin complex are not substantially required. These data demonstrate that E-cadherin acts as a sensor of intracellular mechanics in a crosstalk with cell-substrate adhesions that target β-catenin signaling.

CK2-regulated schwannomin-interacting protein IQCJ-SCHIP-1 association with AnkG contributes to the maintenance of the axon initial segment

The axon initial segment (AIS) plays a central role in electrogenesis and in the maintenance of neuronal polarity. Its molecular organization is dependent on the scaffolding protein ankyrin (Ank) G and is regulated by kinases. For example, the phosphorylation of voltage-gated sodium channels by the protein kinase CK2 regulates their interaction with AnkG and, consequently, their accumulation at the AIS. We previously showed that IQ motif containing J-Schwannomin-Interacting Protein 1 (IQCJ-SCHIP-1), an isoform of the SCHIP-1, accumulated at the AIS in vivo. Here, we analyzed the molecular mechanisms involved in IQCJ-SCHIP-1-specific axonal location. We showed that IQCJ-SCHIP-1 accumulation in the AIS of cultured hippocampal neurons depended on AnkG expression. Pull-down assays and surface plasmon resonance analysis demonstrated that AnkG binds to CK2-phosphorylated IQCJ-SCHIP-1 but not to the non-phosphorylated protein. Surface plasmon resonance approaches using IQCJ-SCHIP-1, SCHIP-1a, another SCHIP-1 isoform, and their C-terminus tail mutants revealed that a segment including multiple CK2-phosphorylatable sites was directly involved in the interaction with AnkG. Pharmacological inhibition of CK2 diminished both IQCJ-SCHIP-1 and AnkG accumulation in the AIS. Silencing SCHIP-1 expression reduced AnkG cluster at the AIS. Finally, over-expression of IQCJ-SCHIP-1 decreased AnkG concentration at the AIS, whereas a mutant deleted of the CK2-regulated AnkG interaction site did not. Our study reveals that CK2-regulated IQJC-SCHIP-1 association with AnkG contributes to AIS maintenance. The axon initial segment (AIS) organization depends on ankyrin (Ank) G and kinases. Here we showed that AnkG binds to CK2-phosphorylated IQCJ-SCHIP-1, in a segment including 12 CK2-phosphorylatable sites. In cultured neurons, either pharmacological inhibition of CK2 or IQCJ-SCHIP-1 silencing reduced AnkG clustering. Overexpressed IQCJ-SCHIP-1 decreased AnkG concentration at the AIS whereas a mutant deleted of the CK2-regulated AnkG interaction site did not. Thus, CK2-regulated IQJC-SCHIP-1 association with AnkG contributes to AIS maintenance.

Looking beyond the Wnt pathway for the deep nature of β-catenin

After two decades of stardom, one would think that β-catenin has revealed all of its most intimate details. Yet the essence of its duality has remained mysterious--how can a single protein both be the core link between cadherins and the cytoskeleton, and the nuclear messenger for Wnt signalling? On the basis of the available evidence and on molecular and evolutionary considerations, I propose that β-catenin was a born nuclear transport receptor, which by interacting with adhesion molecules acquired the property to coordinate nuclear functions with cell-cell adhesion. While Wnt signalling diverted this activity, the original pathway might still function in modern eukaryotes.

Autophosphorylation of serine 608 in the p85 regulatory subunit of wild type or cancer-associated mutants of phosphoinositide 3-kinase does not affect its lipid kinase activity

Background

The α-isoform of the Type 1A Phosphoinositide 3-kinases (PI3Kα) has protein kinase activity as well as phosphoinositide lipid kinase activity. The best described substrate for its protein kinase activity is its regulatory subunit, p85α, which becomes phosphorylated on Serine 608. Phosphorylation of Serine 608 has been reported to down-regulate its lipid kinase activity.

Results

We have assessed whether oncogenic mutants of PI3Kα, which have up-regulated lipid kinase activity, have altered levels of Serine 608 phosphorylation compared to wild type PI3Kα, and whether differential phosphorylation of Serine 608 contributes to increased activity of oncogenic forms of PI3Kα with point mutations in the helical or the kinase domains. Despite markedly increased lipid kinase activity, protein kinase activity was not altered in oncogenic compared to wild type forms of PI3Kα. By manipulating levels of phosphorylation of Serine 608 in vitro, we found no evidence that the protein kinase activity of PI3Kα affects its phosphoinositide lipid kinase activity in either wild-type or oncogenic mutants of PI3Kα.

Conclusions

Phosphorylation of p85α S608 is not a significant regulator of wild-type or oncogenic PI3Kα lipid kinase activity.

Solubilisation of the armadillo-repeat protein β-catenin using a zwitterionic detergent allows resolution of phosphorylated forms by 2DE

β-catenin is a member of the armadillo repeat family of proteins and has important functions in cell-cell adhesion and Wnt signalling. Different protein species of β-catenin have been shown to exist in the cell and the relative proportions of these species are altered upon stimulation of cells with Wnt-3a (Gottardi and Gumbiner, 2004). In order to determine whether posttranslational modifications (PTMs) of β-catenin underlie these different protein species, we have used 2DE separation and immunoblotting with an antibody specific for β-catenin. High-resolution separation of differentially modified species of β-catenin in 2DE required the addition of ASB-16, a zwitterionic detergent that can solubilise integral membrane proteins. ASB-16 was also necessary for focussing of other armadillo repeat proteins, such as γ-catenin and p120-catenin. 2DE using ASB-16 allowed detection of a previously unreported phosphorylation site in the transcriptionally active form of β-catenin that binds to GST-Tcf in response to Wnt signalling.

Label-free electrochemical impedance detection of kinase and phosphatase activities using carbon nanofiber nanoelectrode arrays

We demonstrate the feasibility of a label-free electrochemical method to detect the kinetics of phosphorylation and dephosphorylation of surface-attached peptides catalyzed by kinase and phosphatase, respectively. The peptides with a sequence specific to c-Src tyrosine kinase and protein tyrosine phosphatase 1B (PTP1B) were first validated with ELISA-based protein tyrosine kinase assay and then functionalized on vertically aligned carbon nanofiber (VACNF) nanoelectrode arrays (NEAs). Real-time electrochemical impedance spectroscopy (REIS) measurements showed reversible impedance changes upon the addition of c-Src kinase and PTP1B phosphatase. Only a small and unreliable impedance variation was observed during the peptide phosphorylation, but a large and fast impedance decrease was observed during the peptide dephosphorylation at different PTP1B concentrations. The REIS data of dephosphorylation displayed a well-defined exponential decay following the Michaelis-Menten heterogeneous enzymatic model with a specific constant, k(cat)/K(m), of (2.1±0.1)×10(7) M(-1)s(-1). Consistent values of the specific constant was measured at PTP1B concentration varying from 1.2 to 2.4 nM with the corresponding electrochemical signal decay constant varying from 38.5 to 19.1s. This electrochemical method can be potentially used as a label-free method for profiling enzyme activities in fast reactions.

Identification of a Wnt-induced protein complex by affinity proteomics using an antibody that recognizes a sub-population of β-catenin

β-catenin is a signaling protein with diverse functions in cell adhesion and Wnt signaling. Although β-catenin has been shown to participate in many protein-protein interactions, it is not clear which combinations of β-catenin-interacting proteins form discrete complexes. We have generated a novel antibody, termed 4B3, which recognizes only a small subset of total cellular β-catenin. Affinity proteomics using 4B3, in combination with subcellular fractionation, has allowed us to define a discrete trimeric complex of β-catenin, α-catenin and the tumor suppressor APC, which forms in the cytoplasm in response to Wnt signaling. Depletion of the limiting component of this complex, APC, implicates the complex in mediating Wnt-induced changes in cell-cell adhesion. APC is also essential for N-terminal phosphorylation of β-catenin within this complex. Each component of β-catenin/APC/α-catenin complex co-exists in other protein complexes, thus use of a selective antibody for affinity proteomics has allowed us to go beyond the generation of a list of potential β-catenin-interacting proteins, and define when and where a specific complex forms.

Expression of EphA2 and E-cadherin in gastric cancer: correlated with tumor progression and lymphogenous metastasis

In this study, gastric cancer progression was correlated with the over-expression of erythropoietin-producing hepatocellular (Eph)A2 receptor and down-expression of epithelial cadherin (E-cadherin). Immunohistochemistry of EphA2 and E-cadherin were performed on these tumor samples from 165 primary lesions of gastric cancer. The results showed that expression of EphA2 was obviously increased in gastric cancer tissues (P < 0.01), which was positively correlated with the depth of cancer invasion, tumor-node-metastasis (TNM) stage and lymph node metastasis (P < 0.05). Meanwhile, the expression of E-cadherin was significantly reduced (P < 0.01), which was negatively correlated with the depth of cancer invasion, grade of tumor differentiation, TNM stage and lymph node metastasis (P < 0.05). The correlation between EphA2 and E-cadherin expression was negative (r = -0.198, P = 0.011). In conclusion, either the over-expression of EphA2 or the down-expression of E-cadherin is correlated with cancer progression and lymphogenous metastasis in gastric cancer, suggesting that both of them may play an important role in tumor progression and metastasis.

Threonine 393 of beta-catenin regulates interaction with Axin

CK2 is a regulatory kinase implicated in embryonic development and in cancer. Among the CK2 substrates is beta-catenin, a protein with dual function in Wnt signaling and cell adhesion. Previously, we reported that CK2 activity is required for beta-catenin stability and we identified threonine (T) 393 as a major CK2 phosphorylation site in beta-catenin. However, it is not known whether phosphorylation at T393 increases beta-catenin stability and if so, what is the mechanism. In this study we investigate the molecular mechanism of beta-catenin stabilization through phosphorylation at T393. We found that pseudophosphorylation of beta-catenin at T393 resulted in a stable activated form of beta-catenin with decreased affinity for Axin in vitro. This phosphomimetic mutant also displayed decreased regulation by Axin in vivo in a bioassay in Xenopus laevis embryos. In contrast, the binding of T393 pseudophosphorylated beta-catenin to E-cadherin was unaffected. Further analysis showed that pseudophosphorylation at T393 did not prevent beta-catenin phosphorylation by GSK3beta. Interestingly, we found that in the presence of pseudophophorylated beta-catenin and another activated form of beta-catenin, the recruitment of GSK3beta to Axin is enhanced. These findings indicate that phosphorylation of T393 by CK2 may affect the stability of beta-catenin through decreased binding to Axin. In addition, the increased recruitment of GSK3beta to the destruction complex in the presence of activated beta-catenin mutants could be a feedback mechanism to suppress overactive Wnt signaling.

Protein kinase CK2 in health and disease: CK2: a key player in cancer biology

Elevated levels of protein kinase CK2 (formerly casein kinase 2 or II) have long been associated with increased cell growth and proliferation both in normal and cancer cells. The ability of CK2 to also act as a potent suppressor of apoptosis offers an important link to its involvement in cancer since deregulation of both cell proliferation and apoptosis are among the key features of cancer cell biology. Dysregulated CK2 may impact both of these processes in cancer cells. All cancers that have been examined show increased CK2 expression, which may also relate to prognosis. The extensive involvement of CK2 in cancer derives from its impact on diverse molecular pathways controlling cell proliferation and cell death. Downregulation of CK2 by various approaches results in induction of apoptosis in cultured cell and xenograft cancer models suggesting its potential as a therapeutic target.

Wnt-5a-CKI{alpha} signaling promotes {beta}-catenin/E-cadherin complex formation and intercellular adhesion in human breast epithelial cells

Wnt-5a is a non-transforming Wnt protein that is implicated in cell polarity, adhesion, and motility. We have previously shown that low expression of Wnt-5a is a predictor of shorter disease-free survival in human breast cancer. Here, we investigated whether beta-catenin/E-cadherin-mediated cell-cell adhesion was affected by loss of Wnt-5a in breast carcinomas, thereby promoting a metastatic behavior of the tumor. We show that Wnt-5a stimulation of human breast epithelial cells leads to an increased Ca(2+)-dependent cell-cell adhesion. Furthermore, Wnt-5a/casein kinase Ialpha (CKIalpha)-specific Ser-45 phosphorylation of beta-catenin is associated with an increased complex formation of beta-catenin/E-cadherin. Mutation of Ser-45 decreases the beta-catenin/E-cadherin association. Also, the inhibitory effect of Wnt-5a on breast epithelial cell invasion is reduced upon mutation of beta-catenin-Ser-45. The Wnt-5a-CKIalpha-induced Ser-45 phosphorylation does not lead to degradation of beta-catenin. Finally we show that human breast cancers lacking Wnt-5a protein have a significantly lower level of membrane-associated beta-catenin. Down-regulation of Wnt-5a expression and subsequent reduction of membrane-associated beta-catenin in invasive breast cancer, can therefore contribute to a decreased cell-cell adhesion and increased motility resulting in a higher probability for metastatic disease.

Protein kinase CK2 contributes to the organization of sodium channels in axonal membranes by regulating their interactions with ankyrin G

In neurons, generation and propagation of action potentials requires the precise accumulation of sodium channels at the axonal initial segment (AIS) and in the nodes of Ranvier through ankyrin G scaffolding. We found that the ankyrin-binding motif of Na(v)1.2 that determines channel concentration at the AIS depends on a glutamate residue (E1111), but also on several serine residues (S1112, S1124, and S1126). We showed that phosphorylation of these residues by protein kinase CK2 (CK2) regulates Na(v) channel interaction with ankyrins. Furthermore, we observed that CK2 is highly enriched at the AIS and the nodes of Ranvier in vivo. An ion channel chimera containing the Na(v)1.2 ankyrin-binding motif perturbed endogenous sodium channel accumulation at the AIS, whereas phosphorylation-deficient chimeras did not. Finally, inhibition of CK2 activity reduced sodium channel accumulation at the AIS of neurons. In conclusion, CK2 contributes to sodium channel organization by regulating their interaction with ankyrin G.

Phospho-regulation of Beta-catenin adhesion and signaling functions

Beta-catenin plays a critical structural role in cadherin-based adhesions and is also an essential co-activator of Wnt-mediated gene expression. The degree to which beta-catenin participates in these two functions is dictated by the availability of beta-catenin binding partners, and an emerging theme is that these binding interactions are regulated by phosphorylation. Inputs from various cell-signaling events can therefore impact beta-catenin function, which may be necessary for the finely tuned adhesive and signaling responses required for tissue morphogenesis.

Use of multidimensional separation protocols for the purification of trace components in complex biological samples for proteomics analysis

The routine detection of low abundance components in complex samples for detailed proteomics analysis continues to be a challenge. Whilst the potential of multidimensional chromatographic fractionation for this purpose has been proposed for some years, and was used effectively for the purification to homogeneity of trace components in bulk biological samples for N-terminal sequence analysis, its practical application in the proteomics arena is still limited. This article reviews some of the recent data using these approaches, including the use of microaffinity purification as part of multidimensional protocols for downstream proteomics analysis.

A microtiter assay for quantifying protein-protein interactions associated with cell-cell adhesion

Cell-cell adhesions are a hallmark of epithelial tissues, and the disruption of these contacts plays a critical role in both the early and late stages of oncogenesis. The interaction between the transmembrane protein E-cadherin and the intracellular protein beta-catenin plays a crucial role in the formation and maintenance of epithelial cell-cell contacts and is known to be downregulated in many cancers. The authors have developed a protein complex enzyme-linked immunosorbent assay (ELISA) that can quantify the amount of beta-catenin bound to E-cadherin in unpurified whole-cell lysates with a Z' factor of 0.74. The quantitative nature of the E-cadherin:beta-catenin ELISA represents a dramatic improvement over the low-throughput assays currently used to characterize endogenous E-cadherin:beta-catenin complexes. In addition, the protein complex ELISA format is compatible with standard sandwich ELISAs for parallel measurements of total levels of endogenous E-cadherin and beta-catenin. In 2 case studies closely related to cancer cell biology, the authors use the protein complex ELISA and traditional sandwich ELISAs to provide a detailed, quantitative picture of the molecular changes occurring within adherens junctions in vivo. Because the E-cadherin: beta-catenin protein complex plays a crucial role in oncogenesis, this protein complex ELISA may prove to be a valuable quantitative prognostic marker of tumor progression.