Increased Internalization of p120-uncoupled E-cadherin and a Requirement for a Dileucine Motif in the Cytoplasmic Domain for Endocytosis of the Protein

CTNND1 is involved in germline predisposition to early-onset gastric cancer by affecting cell-to-cell interactions

BACKGROUND

CDH1 and CTNNA1 remain as the main genes for hereditary gastric cancer. However, they only explain a small fraction of gastric cancer cases with suspected inherited basis. In this study, we aimed to identify new hereditary genes for early-onset gastric cancer patients (EOGC; < 50 years old).

METHODS

After germline exome sequencing in 20 EOGC patients and replication of relevant findings by gene-panel sequencing in an independent cohort of 152 patients, CTNND1 stood out as an interesting candidate gene, since its protein product (p120ctn) directly interacts with E-cadherin. We proceeded with functional characterization by generating two knockout CTNND1 cellular models by gene editing and introducing the detected genetic variants using a lentiviral delivery system. We assessed β-catenin and E-cadherin levels, cell detachment, as well as E-cadherin localization and cell-to-cell interaction by spheroid modeling.

RESULTS

Three CTNND1 germline variants [c.28_29delinsCT, p.(Ala10Leu); c.1105C > T, p.(Pro369Ser); c.1537A > G, p.(Asn513Asp)] were identified in our EOGC cohorts. Cells encoding CTNND1 variants displayed altered E-cadherin levels and intercellular interactions. In addition, the p.(Pro369Ser) variant, located in a key region in the E-cadherin/p120ctn binding domain, showed E-cadherin mislocalization.

CONCLUSIONS

Defects in CTNND1 could be involved in germline predisposition to gastric cancer by altering E-cadherin and, consequently, cell-to-cell interactions. In the present study, CTNND1 germline variants explained 2% (3/172) of the cases, although further studies in larger external cohorts are needed.

N-cadherin directs the collective Schwann cell migration required for nerve regeneration through Slit2/3-mediated contact inhibition of locomotion

Collective cell migration is fundamental for the development of organisms and in the adult for tissue regeneration and in pathological conditions such as cancer. Migration as a coherent group requires the maintenance of cell-cell interactions, while contact inhibition of locomotion (CIL), a local repulsive force, can propel the group forward. Here we show that the cell-cell interaction molecule, N-cadherin, regulates both adhesion and repulsion processes during Schwann cell (SC) collective migration, which is required for peripheral nerve regeneration. However, distinct from its role in cell-cell adhesion, the repulsion process is independent of N-cadherin trans-homodimerisation and the associated adherens junction complex. Rather, the extracellular domain of N-cadherin is required to present the repulsive Slit2/Slit3 signal at the cell surface. Inhibiting Slit2/Slit3 signalling inhibits CIL and subsequently collective SC migration, resulting in adherent, nonmigratory cell clusters. Moreover, analysis of ex vivo explants from mice following sciatic nerve injury showed that inhibition of Slit2 decreased SC collective migration and increased clustering of SCs within the nerve bridge. These findings provide insight into how opposing signals can mediate collective cell migration and how CIL pathways are promising targets for inhibiting pathological cell migration.

Chemoproteomic strategy identified p120-catenin glutathionylation regulates E-cadherin degradation and cell migration

Identification of cysteines with high oxidation susceptibility is important for understanding redox-mediated biological processes. In this report, we report a chemical proteomic strategy that finds cysteines with high susceptibility to S-glutathionylation. Our proteomic strategy, named clickable glutathione-based isotope-coded affinity tag (G-ICAT), identified 1,518 glutathionylated cysteines while determining their relative levels of glutathionylated and reduced forms upon adding hydrogen peroxide. Among identified cysteines, we demonstrated that CTNND1 (p120) C692 has high susceptibility to glutathionylation. Also, p120 wild type (WT), compared to C692S, induces its dissociation from E-cadherin under oxidative stress, such as glucose depletion. p120 and E-cadherin dissociation correlated with E-cadherin destabilization via its proteasomal degradation. Lastly, we showed that p120 WT, compared to C692S, increases migration and invasion of MCF7 cells under glucose depletion, supporting a model that p120 C692 glutathionylation increases cell migration and invasion by destabilization of E-cadherin, a core player in cell-cell adhesion.

Deficiency of AP1 Complex Ap1g1 in Zebrafish Model Led to Perturbation of Neurodevelopment, Female and Male Fertility; New Insight to Understand Adaptinopathies

In vertebrates, two homologous heterotetrameric AP1 complexes regulate the intracellular protein sorting via vesicles. AP-1 complexes are ubiquitously expressed and are composed of four different subunits: γ, β1, μ1 and σ1. Two different complexes are present in eukaryotic cells, AP1G1 (contains γ1 subunit) and AP1G2 (contains γ2 subunit); both are indispensable for development. One additional tissue-specific isoform exists for μ1A, the polarized epithelial cells specific to μ1B; two additional tissue-specific isoforms exist for σ1A: σ1B and σ1C. Both AP1 complexes fulfil specific functions at the trans-Golgi network and endosomes. The use of different animal models demonstrated their crucial role in the development of multicellular organisms and the specification of neuronal and epithelial cells. Ap1g1 (γ1) knockout mice cease development at the blastocyst stage, while Ap1m1 (μ1A) knockouts cease during mid-organogenesis. A growing number of human diseases have been associated with mutations in genes encoding for the subunits of adaptor protein complexes. Recently, a new class of neurocutaneous and neurometabolic disorders affecting intracellular vesicular traffic have been referred to as adaptinopathies. To better understand the functional role of AP1G1 in adaptinopathies, we generated a zebrafish ap1g1 knockout using CRISPR/Cas9 genome editing. Zebrafish ap1g1 knockout embryos cease their development at the blastula stage. Interestingly, heterozygous females and males have reduced fertility and showed morphological alterations in the brain, gonads and intestinal epithelium. An analysis of mRNA profiles of different marker proteins and altered tissue morphologies revealed dysregulated cadherin-mediated cell adhesion. These data demonstrate that the zebrafish model organism enables us to study the molecular details of adaptinopathies and thus also develop treatment strategies.

Arvcf Dependent Adherens Junction Stability is Required to Prevent Age-Related Cortical Cataracts

The etiology of age-related cortical cataracts is not well understood but is speculated to be related to alterations in cell adhesion and/or the changing mechanical stresses occurring in the lens with time. The role of cell adhesion in maintaining lens transparency with age is difficult to assess because of the developmental and physiological roles that well-characterized adhesion proteins have in the lens. This report demonstrates that Arvcf, a member of the p120-catenin subfamily of catenins that bind to the juxtamembrane domain of cadherins, is an essential fiber cell protein that preserves lens transparency with age in mice. No major developmental defects are observed in the absence of Arvcf, however, cortical cataracts emerge in all animals examined older than 6-months of age. While opacities are not obvious in young animals, histological anomalies are observed in lenses at 4-weeks that include fiber cell separations, regions of hexagonal lattice disorganization, and absence of immunolabeled membranes. Compression analysis of whole lenses also revealed that Arvcf is required for their normal biomechanical properties. Immunofluorescent labeling of control and Arvcf-deficient lens fiber cells revealed a reduction in membrane localization of N-cadherin, β-catenin, and αN-catenin. Furthermore, super-resolution imaging demonstrated that the reduction in protein membrane localization is correlated with smaller cadherin nanoclusters. Additional characterization of lens fiber cell morphology with electron microscopy and high resolution fluorescent imaging also showed that the cellular protrusions of fiber cells are abnormally elongated with a reduction and disorganization of cadherin complex protein localization. Together, these data demonstrate that Arvcf is required to maintain transparency with age by mediating the stability of the N-cadherin protein complex in adherens junctions.

The PLEKHA7-PDZD11 complex regulates the localization of the calcium pump PMCA and calcium handling in cultured cells

The plasma membrane calcium ATPase (PMCA) extrudes calcium from the cytosol to the extracellular space to terminate calcium-dependent signaling. Although the distribution of PMCA is crucial for its function, the molecular mechanisms that regulate the localization of PMCA isoforms are not well understood. PLEKHA7 is implicated by genetic studies in hypertension and the regulation of calcium handling. PLEKHA7 recruits the small adapter protein PDZD11 to adherens junctions, and together they control the trafficking and localization of plasma membrane associated proteins, including the Menkes copper ATPase. Since PDZD11 binds to the C-terminal domain of b-isoforms of PMCA, PDZD11 and its interactor PLEKHA7 could control the localization and activity of PMCA. Here, we test this hypothesis using cultured cell model systems. We show using immunofluorescence microscopy and a surface biotinylation assay that KO of either PLEKHA7 or PDZD11 in mouse kidney collecting duct epithelial cells results in increased accumulation of endogenous PMCA at lateral cell–cell contacts and PDZ-dependent ectopic apical localization of exogenous PMCA4x/b isoform. In HeLa cells, coexpression of PDZD11 reduces membrane accumulation of overexpressed PMCA4x/b, and analysis of cytosolic calcium transients shows that PDZD11 counteracts calcium extrusion activity of overexpressed PMCA4x/b, but not PMCA4x/a, which lacks the PDZ-binding motif. Moreover, KO of PDZD11 in either endothelial (bEnd.3) or epithelial (mouse kidney collecting duct) cells increases the rate of calcium extrusion. Collectively, these results suggest that the PLEKHA7–PDZD11 complex modulates calcium homeostasis by regulating the localization of PMCA.

Cell-cell adhesions in embryonic stem cells regulate the stability and transcriptional activity of β-catenin

E-cadherin (CDH1) is involved in maintaining cell-cell adhesions in embryonic stem cells (ESCs). However, its function in the context of cell fate decisions is largely unknown. Using mouse ESCs (mESCs), we demonstrate that E-cadherin and β-catenin interact at the membrane and continue to do so upon internalization within the cell. Cdh1^-/- mESCs failed to form tight colonies, with altered differentiation marker expression, and retention of pluripotency factors during differentiation. Interestingly, Cdh1^-/- mESCs showed dramatically reduced β-catenin levels. Transcriptional profiling of Cdh1^-/- mESCs displayed a significant alteration in the expression of a subset of β-catenin targets in a cell state- and GSK3β-dependent manner. Our findings hint at hitherto unknown roles played by E-cadherin in regulating the activity of β-catenin in ESCs.

Calmodulin-dependent KCNE4 dimerization controls membrane targeting

The voltage-dependent potassium channel Kv1.3 participates in the immune response. Kv1.3 is essential in different cellular functions, such as proliferation, activation and apoptosis. Because aberrant expression of Kv1.3 is linked to autoimmune diseases, fine-tuning its function is crucial for leukocyte physiology. Regulatory KCNE subunits are expressed in the immune system, and KCNE4 specifically tightly regulates Kv1.3. KCNE4 modulates Kv1.3 currents slowing activation, accelerating inactivation and retaining the channel at the endoplasmic reticulum (ER), thereby altering its membrane localization. In addition, KCNE4 genomic variants are associated with immune pathologies. Therefore, an in-depth knowledge of KCNE4 function is extremely relevant for understanding immune system physiology. We demonstrate that KCNE4 dimerizes, which is unique among KCNE regulatory peptide family members. Furthermore, the juxtamembrane tetraleucine carboxyl-terminal domain of KCNE4 is a structural platform in which Kv1.3, Ca²⁺/calmodulin (CaM) and dimerizing KCNE4 compete for multiple interaction partners. CaM-dependent KCNE4 dimerization controls KCNE4 membrane targeting and modulates its interaction with Kv1.3. KCNE4, which is highly retained at the ER, contains an important ER retention motif near the tetraleucine motif. Upon escaping the ER in a CaM-dependent pattern, KCNE4 follows a COP-II-dependent forward trafficking mechanism. Therefore, CaM, an essential signaling molecule that controls the dimerization and membrane targeting of KCNE4, modulates the KCNE4-dependent regulation of Kv1.3, which in turn fine-tunes leukocyte physiology.

P120 catenin potentiates constitutive E-cadherin dimerization at the plasma membrane and regulates trans binding

Cadherins are essential adhesion proteins that regulate tissue cohesion and paracellular permeability by assembling dense adhesion plaques at cell-to-cell contacts. Adherens junctions are central to a wide range of tissue functions; identifying protein interactions that potentiate their assembly and regulation has been the focus of research for over 2 decades. Here, we present evidence for a new, unexpected mechanism of cadherin oligomerization on cells. Fully quantified spectral imaging fluorescence resonance energy transfer (FSI-FRET) and fluorescence intensity fluctuation (FIF) measurements directly demonstrate that E-cadherin forms constitutive lateral (cis) dimers at the plasma membrane. Results further show that binding of the cytosolic protein p120ctn binding to the intracellular region is required for constitutive E-cadherin dimerization. This finding differs from a model that attributes lateral (cis) cadherin oligomerization solely to extracellular domain interactions. The present, novel findings are further supported by studies of E-cadherin mutants that uncouple p120ctn binding or with cells in which p120ctn was knocked out using CRISPR-Cas9. Quantitative affinity measurements further demonstrate that uncoupling p120ctn binding reduces the cadherin trans binding affinity and cell adhesion. These findings transform the current model of cadherin assembly at cell surfaces and identify the core building blocks of cadherin-mediated intercellular adhesions. They also identify a new role for p120ctn and reconcile findings that implicate both the extracellular and intracellular cadherin domains in cadherin clustering and intercellular cohesion.

PLEKHA7, an Apical Adherens Junction Protein, Suppresses Inflammatory Breast Cancer in the Context of High E-Cadherin and p120-Catenin Expression

Inflammatory breast cancer is a highly aggressive form of breast cancer that forms clusters of tumor emboli in dermal lymphatics and readily metastasizes. These cancers express high levels of E-cadherin, the major mediator of adherens junctions, which enhances formation of tumor emboli. Previous studies suggest that E-cadherin promotes cancer when the balance between apical and basolateral cadherin complexes is disrupted. Here, we used immunohistochemistry of inflammatory breast cancer patient samples and analysis of cell lines to determine the expression of PLEKHA7, an apical adherens junction protein. We used viral transduction to re-express PLEKHA7 in inflammatory breast cancer cells and examined their aggressiveness in 2D and 3D cultures and in vivo. We determined that PLEKHA7 was deregulated in inflammatory breast cancer, demonstrating improper localization or lost expression in most patient samples and very low expression in cell lines. Re-expressing PLEKHA7 suppressed proliferation, anchorage independent growth, spheroid viability, and tumor growth in vivo. The data indicate that PLEKHA7 is frequently deregulated and acts to suppress inflammatory breast cancer. The data also promote the need for future inquiry into the imbalance between apical and basolateral cadherin complexes as driving forces in inflammatory breast cancer.

The crosstalk between microtubules, actin and membranes shapes cell division

Mitotic progression is orchestrated by morphological and mechanical changes promoted by the coordinated activities of the microtubule (MT) cytoskeleton, the actin cytoskeleton and the plasma membrane (PM). MTs assemble the mitotic spindle, which assists sister chromatid separation, and contact the rigid and tensile actomyosin cortex rounded-up underneath the PM. Here, we highlight the dynamic crosstalk between MTs, actin and cell membranes during mitosis, and discuss the molecular connections between them. We also summarize recent views on how MT traction forces, the actomyosin cortex and membrane trafficking contribute to spindle positioning in isolated cells in culture and in epithelial sheets. Finally, we describe the emerging role of membrane trafficking in synchronizing actomyosin tension and cell shape changes with cell-substrate adhesion, cell-cell contacts and extracellular signalling events regulating proliferation.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

Destination and consequences of Panx1 and mutant expression in polarized MDCK cells

The channel-forming membrane glycoprotein pannexin 1 (Panx1) is best characterized as an ATP release channel. To investigate the trafficking and sorting of Panx1, we used polarized MDCK cells and non-polarized BICR-M1R_k cells to track the fate of GFP-tagged Panx1. In non-polarized cells, Panx1 was found throughout the plasma membrane, including the lamellipodia of the tumor cells and the cell surface-targeting domain was mapped to residues 307-379. Panx1 was preferentially enriched at the apical membrane domain of polarized MDCK cells grown as monolayer sheets or as spheroids. Residual Panx1 localized within basolateral membranes of polarized MDCK cells was independent of a putative dileucine sorting motif LL365/6 found within the C-terminal of Panx1. Unexpectedly, stable expression of a Panx1 mutant, where a putative tyrosine-based basolateral sorting motif (YxxØ) was mutated (Y308F), or a truncated Δ379 Panx1 mutant, caused MDCK cells to lose cell-cell contacts and their ability to polarize as they underwent a switch to a more fibroblast-like phenotype. We conclude that Panx1 is preferentially delivered to the apical domain of polarized epithelial cells, and Panx1 mutants drive phenotypic changes to MDCK cells preventing their polarization.

Endocytosis in proliferating, quiescent and terminally differentiated cells

Endocytosis mediates nutrient uptake, receptor internalization and the regulation of cell signaling. It is also hijacked by many bacteria, viruses and toxins to mediate their cellular entry. Several endocytic routes exist in parallel, fulfilling different functions. Most studies on endocytosis have used transformed cells in culture. However, as the majority of cells in an adult body have exited the cell cycle, our understanding is biased towards proliferating cells. Here, we review the evidence for the different pathways of endocytosis not only in dividing, but also in quiescent, senescent and terminally differentiated cells. During mitosis, residual endocytosis is dedicated to the internalization of caveolae and specific receptors. In non-dividing cells, clathrin-mediated endocytosis (CME) functions, but the activity of alternative processes, such as caveolae, macropinocytosis and clathrin-independent routes, vary widely depending on cell types and functions. Endocytosis supports the quiescent state by either upregulating cell cycle arrest pathways or downregulating mitogen-induced signaling, thereby inhibiting cell proliferation. Endocytosis in terminally differentiated cells, such as skeletal muscles, adipocytes, kidney podocytes and neurons, supports tissue-specific functions. Finally, uptake is downregulated in senescent cells, making them insensitive to proliferative stimuli by growth factors. Future studies should reveal the molecular basis for the differences in activities between the different cell states.

Peptides derived from cadherin juxtamembrane region inhibit platelet function

The juxtamembrane domains (JMD) of transmembrane proteins are rich in critical peptide sequences that participate in dynamic cell signalling events. Synthetic JMD peptides derived from cadherin cell adhesion proteins have previously been shown to modulate platelet function. In this study, we aimed to develop functional bioactive agents from bioinformatically identified critical peptide sequences. We synthesized overlapping 12-15 amino acid peptides from E- and N-cadherin JMD and assessed their effect on platelet aggregation and platelet ATP secretion. Peptides derived from close to the membrane proximal region inhibit platelet function. Sequential deletion of amino acids from the N- and C-termini of the inhibitory E-cadherin peptides identified the short K756EPLLP763 motif as a critical bioactive sequence. Alanine scanning studies further identified that the di-leucine (LL) motif and positively charged lysine (K) are crucial for peptide activity. Moreover, scrambled peptides failed to show any effect on platelet activity. We conclude that peptides derived from JMD of E-cadherin provide potential lead peptides for the development of anti-thrombotic agents and to enable further understanding of the role of cadherins in platelet function.

Role of glutamine in the mediation of E-cadherin, p120-catenin and inflammation in ventilator-induced lung injury

Background:

Ventilator-induced lung injury (VILI) is commonly associated with barrier dysfunction and inflammation reaction. Glutamine could ameliorate VILI, but its role has not been fully elucidated. This study examined the relationship between inflammatory cytokines (interleukin [IL]-6, tumor necrosis factor [TNF]-α, and IL-10) and adherens junctions (E-cadherin, p120-catenin), which were ameliorated by glutamine in VILI, both in vitro and in vivo.

Methods:

For the in vivo study, 30 healthy C57BL/6 mice weighing 25–30 g were randomly divided into five groups with random number table (n = 6 in each group): control (Group C); low tidal volume (Group L); low tidal volume + glutamine (Group L + G); high tidal volume (Group H); and high tidal volume + glutamine (Group H + G). Mice in all groups, except Group C, underwent mechanical ventilation for 4 h. For the in vitro study, mouse lung epithelial 12 (MLE-12) cells pretreated with glutamine underwent cyclic stretching at 20% for 4 h. Cell lysate and lung tissue were obtained to detect the junction proteins, inflammatory cytokines, and lung pathological changes by the Western blotting, cytokine assay, hematoxylin and eosin staining, and immunofluorescence.

Results:

In vivo, compared with Group C, total cell counts (t = −28.182, P < 0.01), the percentage of neutrophils (t = −28.095, P < 0.01), IL-6 (t = −28.296, P < 0.01), and TNF-α (t = −19.812, P < 0.01) in bronchoalveolar lavage (BAL) fluid, lung injury scores (t = −6.708, P < 0.01), and the wet-to-dry ratio (t = −15.595, P < 0.01) were increased in Group H; IL-10 in BAL fluid (t = 9.093, P < 0.01) and the expression of E-cadherin (t = 10.044, P < 0.01) and p120-catenin (t = 13.218, P < 0.01) were decreased in Group H. Compared with Group H, total cell counts (t = 14.844, P < 0.01), the percentage of neutrophils (t = 18.077, P < 0.01), IL-6 (t = 18.007, P < 0.01), and TNF-α (t = 10.171, P < 0.01) in BAL fluid were decreased in Group H + G; IL-10 in BAL fluid (t = −7.531, P < 0.01) and the expression of E-cadherin (t = −14.814, P < 0.01) and p120-catenin (t = −9.114, P < 0.01) were increased in Group H + G. In vitro, compared with the nonstretching group, the levels of IL-6 (t = −21.111, P < 0.01) and TNF-α (t = −15.270, P < 0.01) were increased in the 20% cyclic stretching group; the levels of IL-10 (t = 5.450, P < 0.01) and the expression of E-cadherin (t = 17.736, P < 0.01) and p120-catenin (t = 16.136, P < 0.01) were decreased in the 20% cyclic stretching group. Compared with the stretching group, the levels of IL-6 (t = 11.818, P < 0.01) and TNF-α (t = 8.631, P < 0.01) decreased in the glutamine group; the levels of IL-10 (t = −3.203, P < 0.05) and the expression of E-cadherin (t = −13.567, P < 0.01) and p120-catenin (t = −10.013, P < 0.01) were increased in the glutamine group.

Conclusions:

High tidal volume mechanical ventilation and 20% cyclic stretching could cause VILI. Glutamine regulates VILI by improving cytokines and increasing the adherens junctions, protein E-cadherin and p120-catenin, to enhance the epithelial barrier function.

Junctional E-cadherin/p120-catenin Is Correlated with the Absence of Supporting Cells to Hair Cells Conversion in Postnatal Mice Cochleae

Notch inhibition is known to generate supernumerary hair cells (HCs) at the expense of supporting cells (SCs) in the mammalian inner ear. However, inhibition of Notch activity becomes progressively less effective at inducing SC-to-HC conversion in the postnatal cochlea and balance organs as the animal ages. It has been suggested that the SC-to-HC conversion capacity is inversely correlated with E-cadherin accumulation in postnatal mammalian utricles. However, whether E-cadherin localization is linked to the SC-to-HC conversion capacity in the mammalian inner ear is poorly understood. In the present study, we treated cochleae from postnatal day 0 (P0) with the Notch signaling inhibitor DAPT and observed apparent SC-to-HC conversion along with E-cadherin/p120ctn disruption in the sensory region. In addition, the SC-to-HC conversion capacity and E-cadherin/p120ctn disorganization were robust in the apex but decreased toward the base. We further demonstrated that the ability to regenerate HCs and the disruption of E-cadherin/p120ctn concomitantly decreased with age and ceased at P7, even after extended DAPT treatments. This timing is consistent with E-cadherin/p120ctn accumulation in the postnatal cochleae. These results suggest that the decreasing capacity of SCs to transdifferentiate into HCs correlates with E-cadherin/p120ctn localization in the postnatal cochleae, which might account for the absence of SC-to-HC conversion in the mammalian cochlea.

Functional roles of p120ctn family of proteins in central neurons

The cadherin-catenin complex in central neurons is associated with a variety of cytosolic partners, collectively called catenins. The p120ctn members are a family of catenins that are distinct from the more ubiquitously expressed α- and β-catenins. It is becoming increasingly clear that the functional roles of the p120ctn family of catenins in central neurons extend well beyond their functional roles in non-neuronal cells in partnering with cadherin to regulate adhesion. In this review, we will provide an overview of the p120ctn family in neurons and their varied functional roles in central neurons. Finally, we will examine the emerging roles of this family of proteins in neurodevelopmental disorders.

Adherens Junctions on the Move-Membrane Trafficking of E-Cadherin

Cadherin-based adherens junctions are conserved structures that mediate epithelial cell-cell adhesion in invertebrates and vertebrates. Despite their pivotal function in epithelial integrity, adherens junctions show a remarkable plasticity that is a prerequisite for tissue architecture and morphogenesis. Epithelial cadherin (E-cadherin) is continuously turned over and undergoes cycles of endocytosis, sorting and recycling back to the plasma membrane. Mammalian cell culture and genetically tractable model systems such as Drosophila have revealed conserved, but also distinct, mechanisms in the regulation of E-cadherin membrane trafficking. Here, we discuss our current knowledge about molecules and mechanisms controlling endocytosis, sorting and recycling of E-cadherin during junctional remodeling.

Cadherin tales: Regulation of cadherin function by endocytic membrane trafficking

Cadherins are the primary adhesion molecules in adherens junctions and desmosomes and play essential roles in embryonic development. Although significant progress has been made in understanding cadherin structure and function, we lack a clear vision of how cells confer plasticity upon adhesive junctions to allow for cellular rearrangements during development, wound healing and metastasis. Endocytic membrane trafficking has emerged as a fundamental mechanism by which cells confer a dynamic state to adhesive junctions. Recent studies indicate that the juxtamembrane domain of classical cadherins contains multiple endocytic motifs, or "switches," that can be used by cellular membrane trafficking machinery to regulate adhesion. The cadherin-binding protein p120-catenin (p120) appears to be the master regulator of access to these switches, thereby controlling cadherin endocytosis and turnover. This review focuses on p120 and other cadherin-binding proteins, ubiquitin ligases, and growth factors as key modulators of cadherin membrane trafficking.

p120-catenin regulates VE-cadherin endocytosis and degradation induced by the Kaposi sarcoma-associated ubiquitin ligase K5

Endocytosis of VE-cadherin in response to the Kaposi sarcoma E3 ubiquitin ligase K5 is dependent on two membrane-proximal lysine residues but independent of a constitutive endocytosis motif. p120-catenin blocks endocytosis mediated by both motifs, demonstrating that p120 is a master regulator of multiple context-dependent endocytic signals.

Vascular endothelial (VE)-cadherin undergoes constitutive internalization driven by a unique endocytic motif that also serves as a p120-catenin (p120) binding site. p120 binding masks the motif, stabilizing the cadherin at cell junctions. This mechanism allows constitutive VE-cadherin endocytosis and recycling to contribute to adherens junction dynamics without resulting in junction disassembly. Here we identify an additional motif that drives VE-cadherin endocytosis and pathological junction disassembly associated with the endothelial-derived tumor Kaposi sarcoma. Human herpesvirus 8, which causes Kaposi sarcoma, expresses the MARCH family ubiquitin ligase K5. We report that K5 targets two membrane-proximal VE-cadherin lysine residues for ubiquitination, driving endocytosis and down-regulation of the cadherin. K5-induced VE-cadherin endocytosis does not require the constitutive endocytic motif. However, K5-induced VE-cadherin endocytosis is associated with displacement of p120 from the cadherin, and p120 protects VE-cadherin from K5. Thus multiple context-dependent signals drive VE-cadherin endocytosis, but p120 binding to the cadherin juxtamembrane domain acts as a master regulator guarding cadherin stability.

Role of the PKCα-c-Src tyrosine kinase pathway in the mediation of p120-catenin degradation in ventilator-induced lung injury

BACKGROUND AND OBJECTIVE

Ventilator-induced lung injury (VILI) is commonly associated with respiratory barrier dysfunction; however, the mechanisms have not been fully elucidated. This study aimed to determine the order and components of the signalling pathway that mediates the degradation of adherin junction of p120-catenin in VILI.

METHODS

For the in vivo study, C57BL/6 mice were pre-treated with inhibitors for 60 min prior to 4 h of mechanical ventilation. For the in vitro study, mouse lung epithelial 12 (MLE-12) cells were pre-treated with inhibitors for 60 min or small interfering RNA (siRNA) for 48 h prior to cyclic stretch at 20% for 4 h. The protein levels of protein kinase Cα (PKCα), activated c-Src and p120-catenin were determined via western blot analysis. Lung injury was determined via HE staining, immunofluorescence, wet/dry ratio and lung injury scores.

RESULTS

High tidal volume mechanical ventilation and 20% cyclic stretch resulted in the degradation of p120-catenin. Inhibitors of PKCα blocked c-Src kinase activation and p120-catenin degradation in VILI. Inhibitors of c-Src kinase or PP2 or siRNA blocked p120-catenin degradation but not PKCα activation.

CONCLUSION

The current findings demonstrates that PKCα and c-Src kinase participate in VILI. PKCα activation phosphorylates c-Src kinase and further decreases p120-catenin in VILI.

Partitioning-Defective 1a/b Depletion Impairs Glomerular and Proximal Tubule Development

The kidney is a highly polarized epithelial organ that develops from undifferentiated mesenchyme, although the mechanisms that regulate the development of renal epithelial polarity are incompletely understood. Partitioning-defective 1 (Par1) proteins have been implicated in cell polarity and epithelial morphogenesis; however, the role of these proteins in the developing kidney has not been established. Therefore, we studied the contribution of Par1a/b to renal epithelial development. We examined the renal phenotype of newborn compound mutant mice carrying only one allele of Par1a or Par1b. Loss of three out of four Par1a/b alleles resulted in severe renal hypoplasia, associated with impaired ureteric bud branching. Compared with kidneys of newborn control littermates, kidneys of newborn mutant mice exhibited dilated proximal tubules and immature glomeruli, and the renal proximal tubular epithelia lacked proper localization of adhesion complexes. Furthermore, Par1a/b mutants expressed low levels of renal Notch ligand Jag1, activated Notch2, and Notch effecter Hes1. Together, these data demonstrate that Par1a/b has a key role in glomerular and proximal tubule development, likely via modulation of Notch signaling.

Microtubules Inhibit E-Cadherin Adhesive Activity by Maintaining Phosphorylated p120-Catenin in a Colon Carcinoma Cell Model

Tight regulation of cadherin-mediated intercellular adhesions is critical to both tissue morphogenesis during development and tissue homeostasis in adults. Cell surface expression of the cadherin-catenin complex is often directly correlated with the level of adhesion, however, examples exist where cadherin appears to be inactive and cells are completely non-adhesive. The state of p120-catenin phosphorylation has been implicated in regulating the adhesive activity of E-cadherin but the mechanism is currently unclear. We have found that destabilization of the microtubule cytoskeleton, independent of microtubule plus-end dynamics, dephosphorylates p120-catenin and activates E-cadherin adhesion in Colo 205 cells. Through chemical screening, we have also identified several kinases as potential regulators of E-cadherin adhesive activity. Analysis of several p120-catenin phosphomutants suggests that gross dephosphorylation of p120-catenin rather than that of specific amino acids may trigger E-cadherin adhesion. Uncoupling p120-catenin binding to E-cadherin at the membrane causes constitutive adhesion in Colo 205 cells, further supporting an inhibitory role of phosphorylated p120-catenin on E-cadherin activity.

Drosophila p120-catenin is crucial for endocytosis of the dynamic E-cadherin-Bazooka complex

The intracellular functions of classical cadherins are mediated through the direct binding of two catenins: β-catenin and p120-catenin (also known as CTNND1 in vertebrates, and p120ctn in Drosophila). Whereas β-catenin is crucial for cadherin function, the role of p120-catenin is less clear and appears to vary between organisms. We show here that p120-catenin has a conserved role in regulating the endocytosis of cadherins, but that its ancestral role might have been to promote endocytosis, followed by the acquisition of a new inhibitory role in vertebrates. In Drosophila, p120-catenin facilitates endocytosis of the dynamic E-cadherin-Bazooka subcomplex, which is followed by its recycling. The absence of p120-catenin stabilises this subcomplex at the membrane, reducing the ability of cells to exchange neighbours in embryos and expanding cell-cell contacts in imaginal discs.

Multifaceted plasma membrane Ca(2+) pumps: From structure to intracellular Ca(2+) handling and cancer

Plasma membrane Ca(2+) ATPases (PMCAs) are intimately involved in the control of intracellular Ca(2+) concentration. They reduce Ca(2+) in the cytosol not only by direct ejection, but also by controlling the formation of inositol-1,4,5-trisphosphate and decreasing Ca(2+) release from the endoplasmic reticulum Ca(2+) pool. In mammals four genes (PMCA1-4) are expressed, and alternative RNA splicing generates more than twenty variants. The variants differ in their regulatory characteristics. They localize into highly specialized membrane compartments and respond to the incoming Ca(2+) with distinct temporal resolution. The expression pattern of variants depends on cell type; a change in this pattern can result in perturbed Ca(2+) homeostasis and thus altered cell function. Indeed, PMCAs undergo remarkable changes in their expression pattern during tumorigenesis that might significantly contribute to the unbalanced Ca(2+) homeostasis of cancer cells. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

Cadherin-11 endocytosis through binding to clathrin promotes cadherin-11-mediated migration in prostate cancer cells

Cadherin-11 (Cad11) cell adhesion molecule plays a role in prostate cancer cell migration. Because disassembly of adhesion complexes through endocytosis of adhesion proteins has been shown to play a role in cell migration, we examined whether Cad11 endocytosis plays a role in Cad11-mediated migration. The mechanism by which Cad11 is internalized is unknown. Using a GST pulldown assay, we found that clathrin binds to the Cad11 cytoplasmic domain but not to that of E-cadherin. Using deletion analysis, we identified a unique sequence motif, VFEEE, in the Cad11 membrane proximal region (amino acid residues 11-15) that binds to clathrin. Endocytosis assays using K(+)-depletion buffer showed that Cad11 internalization is clathrin dependent. Proximity ligation assays showed that Cad11 colocalizes with clathrin, and immunofluorescence assays showed that Cad11 localizes in vesicles that stain for the early endosomal marker Rab5. Deletion of the VFEEE sequence from the Cad11 cytoplasmic domain (Cad11-cla-Δ5) leads to inhibition of Cad11 internalization and reduces Cad11-mediated cell migration in C4-2B and PC3-mm2 prostate cancer cells. These observations suggest that clathrin-mediated internalization of Cad11 regulates surface trafficking of Cad11 and that dynamic turnover of Cad11 regulates the migratory function of Cad11 in prostate cancer cells.

Phosphorylation and isoform use in p120-catenin during development and tumorigenesis

P120-catenin is essential to vertebrate development, modulating cadherin and small-GTPase functions, and growing evidence points also to roles in the nucleus. A complexity in addressing p120-catenin's functions is its many isoforms, including optional splicing events, alternative points of translational initiation, and secondary modifications. In this review, we focus upon how choices in the initiation of protein translation, or the earlier splicing of the RNA transcript, relates to primary sequences that harbor established or putative regulatory phosphorylation sites. While certain p120 phosphorylation events arise via known kinases/phosphatases and have defined outcomes, in most cases the functional consequences are still to be established. In this review, we provide examples of p120-isoforms as they relate to phosphorylation events, and thereby to isoform dependent protein-protein associations and downstream functions. We also provide a view of upstream pathways that determine p120's phosphorylation state, and that have an impact upon development and disease. Because other members of the p120 subfamily undergo similar processing and phosphorylation, as well as related catenins of the plakophilin subfamily, what is learned regarding p120 will by extension have wide relevance in vertebrates.

E-cadherin cytoplasmic domain inhibits cell surface localization of endogenous cadherins and fusion of C2C12 myoblasts

Myoblast fusion is a highly regulated process that is essential for skeletal muscle formation during muscle development and regeneration in mammals. Much remains to be elucidated about the molecular mechanism of myoblast fusion although cadherins, which are Ca(2+)-dependent cell-cell adhesion molecules, are thought to play a critical role in this process. Mouse myoblasts lacking either N-cadherin or M-cadherin can still fuse to form myotubes, indicating that they have no specific function in this process and may be functionally replaced by either M-cadherin or N-cadherin, respectively. In this study, we show that expressing the E-cadherin cytoplasmic domain ectopically in C2C12 myoblasts inhibits cell surface localization of endogenous M-cadherin and N-cadherin, as well as cell-cell fusion. This domain, however, does not inhibit myoblast differentiation according to microarray-based gene expression analysis. In contrast, expressing a dominant-negative β-catenin mutant ectopically, which suppresses Wnt/β-catenin signaling, did not inhibit cell-cell fusion. Therefore, the E-cadherin cytoplasmic domain inhibits cell-cell fusion by inhibiting cell surface localization of endogenous cadherins and not by inhibiting Wnt/β-catenin signaling.

Polarized E-cadherin endocytosis directs actomyosin remodeling during embryonic wound repair

Clathrin, dynamin, and ARF6 accumulate around wounds in Drosophila embryos in a calcium- and actomyosin-dependent manner and drive polarized E-cadherin endocytosis, which is necessary for actomyosin remodeling during wound repair.

Embryonic epithelia have a remarkable ability to rapidly repair wounds. A supracellular actomyosin cable around the wound coordinates cellular movements and promotes wound closure. Actomyosin cable formation is accompanied by junctional rearrangements at the wound margin. We used in vivo time-lapse quantitative microscopy to show that clathrin, dynamin, and the ADP-ribosylation factor 6, three components of the endocytic machinery, accumulate around wounds in Drosophila melanogaster embryos in a process that requires calcium signaling and actomyosin contractility. Blocking endocytosis with pharmacological or genetic approaches disrupted wound repair. The defect in wound closure was accompanied by impaired removal of E-cadherin from the wound edge and defective actomyosin cable assembly. E-cadherin overexpression also resulted in reduced actin accumulation around wounds and slower wound closure. Reducing E-cadherin levels in embryos in which endocytosis was blocked rescued actin localization to the wound margin. Our results demonstrate a central role for endocytosis in wound healing and indicate that polarized E-cadherin endocytosis is necessary for actomyosin remodeling during embryonic wound repair.

On the move: endocytic trafficking in cell migration

Directed cell migration is a fundamental process underlying diverse physiological and pathophysiological phenomena ranging from wound healing and induction of immune responses to cancer metastasis. Recent advances reveal that endocytic trafficking contributes to cell migration in multiple ways. (1) At the level of chemokines and chemokine receptors: internalization of chemokines by scavenger receptors is essential for shaping chemotactic gradients in tissue, whereas endocytosis of chemokine receptors and their subsequent recycling is key for maintaining a high responsiveness of migrating cells. (2) At the level of integrin trafficking and focal adhesion dynamics: endosomal pathways do not only modulate adhesion by delivering integrins to their site of action, but also by supplying factors for focal adhesion disassembly. (3) At the level of extracellular matrix reorganization: endosomal transport contributes to tumor cell migration not only by targeting integrins to invadosomes but also by delivering membrane type 1 matrix metalloprotease to the leading edge facilitating proteolysis-dependent chemotaxis. Consequently, numerous endocytic and endosomal factors have been shown to modulate cell migration. In fact key modulators of endocytic trafficking turn out to be also key regulators of cell migration. This review will highlight the recent progress in unraveling the contribution of cellular trafficking pathways to cell migration.

Cadherin-6B undergoes macropinocytosis and clathrin-mediated endocytosis during cranial neural crest cell EMT

The epithelial-to-mesenchymal transition (EMT) is important for the formation of migratory neural crest cells during development and is co-opted in human diseases such as cancer metastasis. Chick premigratory cranial neural crest cells lose intercellular contacts, mediated in part by Cadherin-6B (Cad6B), migrate extensively, and later form a variety of adult derivatives. Importantly, modulation of Cad6B is crucial for proper neural crest cell EMT. Although Cad6B possesses a long half-life, it is rapidly lost from premigratory neural crest cell membranes, suggesting the existence of post-translational mechanisms during EMT. We have identified a motif in the Cad6B cytoplasmic tail that enhances Cad6B internalization and reduces the stability of Cad6B upon its mutation. Furthermore, we demonstrate for the first time that Cad6B is removed from premigratory neural crest cells through cell surface internalization events that include clathrin-mediated endocytosis and macropinocytosis. Both of these processes are dependent upon the function of dynamin, and inhibition of Cad6B internalization abrogates neural crest cell EMT and migration. Collectively, our findings reveal the significance of post-translational events in controlling cadherins during neural crest cell EMT and migration.

Epithelial-to-Mesenchymal Plasticity Harnesses Endocytic Circuitries

The ability of cells to alter their phenotypic and morphological characteristics, known as cellular plasticity, is critical in normal embryonic development and adult tissue repair and contributes to the pathogenesis of diseases, such as organ fibrosis and cancer. The epithelial-to-mesenchymal transition (EMT) is a type of cellular plasticity. This transition involves genetic and epigenetic changes as well as alterations in protein expression and post-translational modifications. These changes result in reduced cell-cell adhesion, enhanced cell adhesion to the extracellular matrix, and altered organization of the cytoskeleton and of cell polarity. Among these modifications, loss of cell polarity represents the nearly invariable, distinguishing feature of EMT that frequently precedes the other traits or might even occur in their absence. EMT transforms cell morphology and physiology, and hence cell identity, from one typical of cells that form a tight barrier, like epithelial and endothelial cells, to one characterized by a highly motile mesenchymal phenotype. Time-resolved proteomic and phosphoproteomic analyses of cells undergoing EMT recently identified thousands of changes in proteins involved in many cellular processes, including cell proliferation and motility, DNA repair, and - unexpectedly - membrane trafficking (1). These results have highlighted a picture of great complexity. First, the EMT transition is not an all-or-none response but rather a gradual process that develops over time. Second, EMT events are highly dynamic and frequently reversible, involving both cell-autonomous and non-autonomous mechanisms. The net results is that EMT generates populations of mixed cells, with partial or full phenotypes, possibly accounting (at least in part) for the physiological as well as pathological cellular heterogeneity of some tissues. Endocytic circuitries have emerged as complex connectivity infrastructures for numerous cellular networks required for the execution of different biological processes, with a primary role in the control of polarized functions. Thus, they may be relevant for controlling EMT or certain aspects of it. Here, by discussing a few paradigmatic cases, we will outline how endocytosis may be harnessed by the EMT process to promote dynamic changes in cellular identity, and to increase cellular flexibility and adaptation to micro-environmental cues, ultimately impacting on physiological and pathological processes, first and foremost cancer progression.

Cell adhesion in epidermal development and barrier formation

Cell-cell adhesions are necessary for structural integrity and barrier formation of the epidermis. Here, we discuss insights from genetic and cell biological studies into the roles of individual cell-cell junctions and their composite proteins in regulating epidermal development and function. In addition to individual adhesive functions, we will discuss emerging ideas on mechanosensation/transduction of junctions in the epidermis, noncanonical roles for adhesion proteins, and crosstalk/interdependencies between the junctional systems. These studies have revealed that cell adhesion proteins are connected to many aspects of tissue physiology including growth control, differentiation, and inflammation.

Tropomyosin isoforms support actomyosin biogenesis to generate contractile tension at the epithelial zonula adherens

Epithelial cells generate contractile forces at their cell-cell contacts. These are concentrated at the specialized apical junction of the zonula adherens (ZA), where a ring of stabilized E-cadherin lies adjacent to prominent actomyosin bundles. Coupling of adhesion and actomyosin contractility yields tension in the junction. The biogenesis of junctional contractility requires actin assembly at the ZA as well as the recruitment of nonmuscle myosin II, but the molecular regulators of these processes are not yet fully understood. We now report a role for tropomyosins 5NM1 (Tm5NM1) and 5NM2 (Tm5NM2) in their generation. Both these tropomyosin isoforms were found at the ZA and their depletion by RNAi or pharmacological inhibition reduced both F-actin and myosin II content at the junction. Photoactivation analysis revealed that the loss of F-actin was attributable to a decrease in filament stability. These changes were accompanied by a decrease in E-cadherin content at junctions. Ultimately, both long-term depletion of Tm5NM1/2 and acute inhibition with drugs caused junctional tension to be reduced. Thus these tropomyosin isoforms are novel contributors to junctional contractility and integrity.

Activation of human γδ T cells by cytosolic interactions of BTN3A1 with soluble phosphoantigens and the cytoskeletal adaptor periplakin

The three butyrophilin BTN3A molecules, BTN3A1, BTN3A2, and BTN3A3, are members of the B7/butyrophilin-like group of Ig superfamily receptors, which modulate the function of T cells. BTN3A1 controls activation of human Vγ9/Vδ2 T cells by direct or indirect presentation of self and nonself phosphoantigens (pAg). We show that the microbial metabolite (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate binds to the intracellular B30.2 domain of BTN3A1 with an affinity of 1.1 μM, whereas the endogenous pAg isopentenyl pyrophosphate binds with an affinity of 627 μM. Coculture experiments using knockdown cell lines showed that in addition to BTN3A1, BTN3A2 and BTN3A3 transmit activation signals to human γδ T cells in response to (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate and the aminobisphosphonate drug zoledronate that causes intracellular accumulation of isopentenyl pyrophosphate. The plakin family member periplakin, identified in yeast two-hybrid assays, interacted with a membrane-proximal di-leucine motif, located proximal to the B30.2 domain in the BTN3A1 cytoplasmic tail. Periplakin did not interact with BTN3A2 or BTN3A3, which do not contain the di-leucine motif. Re-expression into a BTN3A1 knockdown line of wild-type BTN3A1, but not of a variant lacking the periplakin binding motif, BTN3A1Δexon5, restored γδ T cell responses, demonstrating a functional role for periplakin interaction. These data, together with the widespread expression in epithelial cells, tumor tissues, and macrophages detected using BTN3A antiserum, are consistent with complex functions for BTN3A molecules in tissue immune surveillance and infection, linking the cell cytoskeleton to γδ T cell activation by indirectly presenting pAg to the Vγ9/Vδ2 TCR.

CD148 tyrosine phosphatase promotes cadherin cell adhesion

CD148 is a transmembrane tyrosine phosphatase that is expressed at cell junctions. Recent studies have shown that CD148 associates with the cadherin/catenin complex and p120 catenin (p120) may serve as a substrate. However, the role of CD148 in cadherin cell-cell adhesion remains unknown. Therefore, here we addressed this issue using a series of stable cells and cell-based assays. Wild-type (WT) and catalytically inactive (CS) CD148 were introduced to A431D (lacking classical cadherins), A431D/E-cadherin WT (expressing wild-type E-cadherin), and A431D/E-cadherin 764AAA (expressing p120-uncoupled E-cadherin mutant) cells. The effects of CD148 in cadherin adhesion were assessed by Ca²⁺ switch and cell aggregation assays. Phosphorylation of E-cadherin/catenin complex and Rho family GTPase activities were also examined. Although CD148 introduction did not alter the expression levels and complex formation of E-cadherin, p120, and β-catenin, CD148 WT, but not CS, promoted cadherin contacts and strengthened cell-cell adhesion in A431D/E-cadherin WT cells. This effect was accompanied by an increase in Rac1, but not RhoA and Cdc42, activity and largely diminished by Rac1 inhibition. Further, we demonstrate that CD148 reduces the tyrosine phosphorylation of p120 and β-catenin; causes the dephosphorylation of Y529 suppressive tyrosine residue in Src, a well-known CD148 site, increasing Src activity and enhancing the phosphorylation of Y228 (a Src kinase site) in p120, in E-cadherin contacts. Consistent with these findings, CD148 dephosphorylated both p120 and β-catenin in vitro. The shRNA-mediated CD148 knockdown in A431 cells showed opposite effects. CD148 showed no effects in A431D and A431D/E-cadherin 764AAA cells. In aggregate, these findings provide the first evidence that CD148 promotes E-cadherin adhesion by regulating Rac1 activity concomitant with modulation of p120, β-catenin, and Src tyrosine phosphorylation. This effect requires E-cadherin and p120 association.

Common themes in PrP signaling: the Src remains the same

The ability of the cellular prion protein (PrP^C) to trigger intracellular signals appears central to neurodegeneration pathways, yet the physiological significance of such signals is rather puzzling. For instance, PrP^C deregulation disrupts phenomena as diverse as synaptic transmission in mammals and cell adhesion in zebrafish. Although unrelated, the key proteins in these events -the NMDA receptor (NMDAR) and E-cadherin, respectively- are similarly modulated by the Src family kinase (SFK) Fyn. These observations highlight the importance of PrP^C-mediated Fyn activation, a finding reported nearly two decades ago. Given their complex functions and regulation, SFKs may hold the key to intriguing aspects of PrP biology such as its seemingly promiscuous functions and the lack of strong phenotypes in knockout mice. Here we provide a mechanistic perspective on how SFKs might contribute to the uncertain molecular basis of neuronal PrP phenotypes affecting ion channel activity, axon myelination and olfactory function. In particular, we discuss SFK target proteins involved in these processes and the role of tyrosine phosphorylation in the regulation of their activity and cell surface expression.

Minus end-directed motor KIFC3 suppresses E-cadherin degradation by recruiting USP47 to adherens junctions

KIFC3, a minus end–directed kinesin motor, recruits the ubiquitin-specific protease USP47, a protease that removes ubiquitin chains from substrates, to epithelial adherens junctions. This process suppresses the ubiquitination and resultant degradation of E-cadherin, thereby maintaining stable cell–cell adhesion in epithelial sheets.

The adherens junction (AJ) plays a crucial role in maintaining cell–cell adhesion in epithelial tissues. Previous studies show that KIFC3, a minus end–directed kinesin motor, moves into AJs via microtubules that grow from clusters of CAMSAP3 (also known as Nezha), a protein that binds microtubule minus ends. The function of junction-associated KIFC3, however, remains to be elucidated. Here we find that KIFC3 binds the ubiquitin-specific protease USP47, a protease that removes ubiquitin chains from substrates and hence inhibits proteasome-mediated proteolysis, and recruits it to AJs. Depletion of KIFC3 or USP47 promotes cleavage of E-cadherin at a juxtamembrane region of the cytoplasmic domain, resulting in the production of a 90-kDa fragment and the internalization of E-cadherin. This cleavage depends on the E3 ubiquitin protein ligase Hakai and is inhibited by proteasome inhibitors. E-cadherin ubiquitination consistently increases after depletion of KIFC3 or USP47. These findings suggest that KIFC3 suppresses the ubiquitination and resultant degradation of E-cadherin by recruiting USP47 to AJs, a process that may be involved in maintaining stable cell–cell adhesion in epithelial sheets.

Cadherin cytoplasmic domains inhibit the cell surface localization of endogenous E-cadherin, blocking desmosome and tight junction formation and inducing cell dissociation

The downregulation of E-cadherin function has fundamental consequences with respect to cancer progression, and occurs as part of the epithelial–mesenchymal transition (EMT). In this study, we show that the expression of the Discosoma sp. red fluorescent protein (DsRed)-tagged cadherin cytoplasmic domain in cells inhibited the cell surface localization of endogenous E-cadherin, leading to morphological changes, the inhibition of junctional assembly and cell dissociation. These changes were associated with increased cell migration, but were not accompanied by the down-regulation of epithelial markers and up-regulation of mesenchymal markers. Thus, these changes cannot be classified as EMT. The cadherin cytoplasmic domain interacted with β-catenin or plakoglobin, reducing the levels of β-catenin or plakoglobin associated with E-cadherin, and raising the possibility that β-catenin and plakoglobin sequestration by these constructs induced E-cadherin intracellular localization. Accordingly, a cytoplasmic domain construct bearing mutations that weakened the interactions with β-catenin or plakoglobin did not impair junction formation and adhesion, indicating that the interaction with β-catenin or plakoglobin was essential to the potential of the constructs. E-cadherin–α-catenin chimeras that did not require β-catenin or plakoglobin for their cell surface transport restored cell–cell adhesion and junction formation.

Cadherin juxtamembrane region derived peptides inhibit TGFβ1 induced gene expression

Bioactive peptides in the juxtamembrane regions of proteins are involved in many signaling events. The juxtamembrane regions of cadherins were examined for the identification of bioactive regions. Several peptides spanning the cytoplasmic juxtamembrane regions of E- and N-cadherin were synthesized and assessed for the ability to influence TGFβ responses in epithelial cells at the gene expression and protein levels. Peptides from regions closer to the membrane appeared more potent inhibitors of TGFβ signaling, blocking Smad3 phosphorylation. Thus inhibiting nuclear translocation of phosphorylated Smad complexes and subsequent transcriptional activation of TGFβ signal propagating genes. The peptides demonstrated a peptide-specific potential to inhibit other TGFβ superfamily members, such as BMP4.

p120-catenin in cancer - mechanisms, models and opportunities for intervention

The epithelial adherens junction is an E-cadherin-based complex that controls tissue integrity and is stabilized at the plasma membrane by p120-catenin (p120, also known as CTNND1). Mutational and epigenetic inactivation of E-cadherin has been strongly implicated in the development and progression of cancer. In this setting, p120 translocates to the cytosol where it exerts oncogenic properties through aberrant regulation of Rho GTPases, growth factor receptor signaling and derepression of Kaiso (also known as ZBTB33) target genes. In contrast, indirect inactivation of the adherens junction through conditional knockout of p120 in mice was recently linked to tumor formation, indicating that p120 can also function as a tumor suppressor. Supporting these opposing functions are findings in human cancer, which show that either loss or cytoplasmic localization of p120 is a common feature in the progression of several types of carcinoma. Underlying this dual biological phenomenon might be the context-dependent regulation of Rho GTPases in the cytosol and the derepression of Kaiso target genes. Here, we discuss past and present findings that implicate p120 in the regulation of cancer progression and highlight opportunities for clinical intervention.

Dynamic contacts: rearranging adherens junctions to drive epithelial remodelling

Epithelial cells display dynamic behaviours, such as rearrangement, movement and shape changes, particularly during embryonic development and in equivalent processes in adults. Accumulating evidence suggests that the remodelling of cell junctions, especially adherens junctions (AJs), has major roles in controlling these behaviours. AJs comprise cadherin adhesion receptors and cytoplasmic proteins that associate with them, including catenins and actin filaments, and exhibit various forms, such as linear or punctate. Remodelling of AJs induces epithelial reshaping in various ways, including by planar-polarized apical constriction that is driven by the contraction of AJ-associated actomyosin and that occurs during neural plate bending and germband extension. RHO GTPases and their effectors regulate actin polymerization and actomyosin contraction at AJs during the epithelial reshaping processes.

CXCR4 is highly expressed at the tumor front but not in the center of prostate cancers

OBJECTIVE

To evaluate the expression of CXCR4, its ligand SDF-1, β-catenin and E-cadherin throughout the local tumor microenvironment of prostate cancer.

PATIENTS AND METHODS

A total of 64 prostate cancer specimens, 24 frozen and 40 paraffin-embedded sections, were obtained from patients treated with radical prostatectomy for clinically localized cancer. Real-time RT-PCR was used for mRNA quantification of CXCR4 and SDF-1 in the tumor center (T), tumor front (F) and distant peritumoral tissue (D). Immunohistochemical analysis was used to investigate the expression patterns of CXCR4, E-cadherin and β-catenin. Clinical records of these patients were studied for follow-up data, and the prognostic value of these molecules' expression was statistically assessed.

RESULTS

CXCR4 mRNA and protein were significantly increased at the tumor front as compared to distant tissue or tumor center. In comparison, SDF-1 mRNA level gradually increased from the tumor center to the distant peritumoral tissue. High CXCR4 at the tumor front was associated with high Gleason score. Low SDF-1 at the tumor front was associated with locally advanced cancer and disease recurrence. Moreover, high CXCR4 staining at the tumor front and increased cytosolic E-cadherin expression in the same location was associated with locally advanced disease.

CONCLUSIONS

CXCR4 seems overexpressed at the tumor front of prostate tumors, where it potentially promotes cell migration toward the SDF-1 centrifugal attracting gradient, as well as epithelial-mesenchymal transition. High CXCR4 and low SDF-1 levels at tumor front were both associated with adverse histological features.