Plakophilins 1a and 1b: widespread nuclear proteins recruited in specific epithelial cells as desmosomal plaque components

Paradoxical Roles of Desmosomal Components in Head and Neck Cancer

Desmosomes are intercellular adhesion complexes involved in various aspects of epithelial pathophysiology, including tissue homeostasis, morphogenesis, and disease development. Recent studies have reported that the abnormal expression of various desmosomal components correlates with tumor progression and poor survival. In addition, desmosomes have been shown to act as a signaling platform to regulate the proliferation, invasion, migration, morphogenesis, and apoptosis of cancer cells. The occurrence and progression of head and neck cancer (HNC) is accompanied by abnormal expression of desmosomal components and loss of desmosome structure. However, the role of desmosomal components in the progression of HNC remains controversial. This review aims to provide an overview of recent developments showing the paradoxical roles of desmosomal components in tumor suppression and promotion. It offers valuable insights for HNC diagnosis and therapeutics development.

Adhesive pads of gecko and anoline lizards utilize corneous and cytoskeletal proteins for setae development and renewal

The formation of the complex pattern of setae in adhesive pads of geckos and anoline lizards has been analyzed by ultrastructural, autoradiographic, and immunohistochemical methods. Setae terminate with spatulated ends responsible for adhesion that allow these lizards to climb vertical substrates and conquer arboreal niches. Setae derive from a complex interfaced molding between two specialized epidermal layers of the shedding complex that determines the cyclical skin molting, Oberhautchen and clear layers. Setae result from the action of setae cytoskeleton and the surrounding cytoplasm of clear cells. An intense protein synthesis, indicated by histidine and proline autoradiography, takes place during setae formation. Corneous and cytoskeletal proteins such as beta-proteins (CBPs), intermediate filament keratins (IFKs), actin, RhoV, tubulin, plakophilin-1, are produced during setae formation. Microfilaments of actin and microtubules of tubulin grow inside the elongating setae. Microtubules associated with filaments of unknown IKFs are produced in the cytoplasm of clear cells, forming a helical cytoskeleton that surrounds the growing setae. Oberhautchen and clear cells are tightly joined by numerous desmosomes and plakophilin-1 is mainly localized along the perimeter of these cells. These observations suggest that actin and tubulin are present in a dynamic form in the forming setae and in the surrounding cytoplasm of clear cells. Aside the localized micro-deformations of the cell membranes leading to setae formation the cytoskeleton determines the accumulation of CBPs inside the growing setae and the spatula. How the genome determines the specific pattern of cytoskeletal organization with the resulting species-specific setae branching remains unknown.

Plakophilin 1 enhances MYC translation, promoting squamous cell lung cancer

Plakophilin 1 (PKP1) is a member of the arm-repeat (armadillo) and plakophilin gene families and it is an essential component of the desmosomes. Although desmosomes have generally been associated with tumor suppressor functions, we have consistently observed that PKP1 is among the top overexpressed proteins in squamous cell lung cancer. To explore this paradox, we developed in vivo and in vitro functional models of PKP1 gain/loss in squamous cell lung cancer. CRISPR-Cas9 PKP1 knockout severely impaired cell proliferation, but it increased cell dissemination. In addition, PKP1 overexpression increased cell proliferation, cell survival, and in vivo xenograft engraftment. We further investigated the molecular mechanism of the mainly oncogenic function of PKP1 by combining transcriptomics, proteomics, and protein-nucleic acid interaction assays. Interestingly, we found that PKP1 enhances MYC translation in collaboration with the translation initiation complex by binding to the 5'-UTR of MYC mRNA. We propose PKP1 as an oncogene in SqCLC and a novel posttranscriptional regulator of MYC. PKP1 may be a valuable diagnostic biomarker and potential therapeutic target for SqCLC. Importantly, PKP1 inhibition may indirectly target MYC, a primary anticancer target.

Keratin 6 regulates collective keratinocyte migration by altering cell-cell and cell-matrix adhesion

Keratin 6 (K6) isoforms are induced in wound-proximal keratinocytes after injury to skin. Paradoxically, absence of K6 isoforms leads to faster directional cell migration. Wang et al. report that K6 promotes collective keratinocyte migration by interacting with desmoplakin and myosin IIA and stabilizing cell adhesion.

The a and b isoforms of keratin 6 (K6), a type II intermediate filament (IF) protein, are robustly induced upon injury to interfollicular epidermis. We previously showed that complete loss of K6a/K6b stimulates keratinocyte migration, correlating with enhanced Src activity. In this study, we demonstrate that this property is cell autonomous, depends on the ECM, and results from elevated speed, enhanced directionality, and an increased rate of focal adhesion disassembly. We show that myosin IIA interacts with K6a/K6b, that its levels are markedly reduced in Krt6a/Krt6b-null keratinocytes, and that inhibiting myosin ATPase activity normalizes the enhanced migration potential of Krt6a/Krt6b-null cells. Desmoplakin, which mediates attachment of IFs to desmosomes, is also expressed at reduced levels and is mislocalized to the nucleus in Krt6a/Krt6b-null cells, correlating with defects in cell adhesion. These findings reveal that K6a/K6b modulate keratinocyte migration by regulating cell–matrix and cell–cell adhesion and highlight a role for keratins in collective cell migration.

The role of desmosomes in carcinogenesis

Desmosomes, which are intercellular adhesive complexes, are essential for the maintenance of epithelial homeostasis. They are located at the cell membrane, where they act as anchors for intermediate filaments. Downregulation of desmosome proteins in various cancers promotes tumor progression. However, the role of desmosomes in carcinogenesis is still being elucidated. Recent studies revealed that desmosome family members play a crucial role in tumor suppression or tumor promotion. This review focuses on studies that provide insights into the role of desmosomes in carcinogenesis and address their molecular functions.

Beyond β-catenin: prospects for a larger catenin network in the nucleus

β-catenin is widely regarded as the primary transducer of canonical WNT signals to the nucleus. In most vertebrates, there are eight additional catenins that are structurally related to β-catenin, and three α-catenin genes encoding actin-binding proteins that are structurally related to vinculin. Although these catenins were initially identified in association with cadherins at cell-cell junctions, more recent evidence suggests that the majority of catenins also localize to the nucleus and regulate gene expression. Moreover, the number of catenins reported to be responsive to canonical WNT signals is increasing. Here, we posit that multiple catenins form a functional network in the nucleus, possibly engaging in conserved protein-protein interactions that are currently better characterized in the context of actin-based cell junctions.

Inherited desmosomal disorders

Desmosomes serve as intercellular junctions in various tissues including the skin and the heart where they play a crucial role in cell-cell adhesion, signalling and differentiation. The desmosomes connect the cell surface to the keratin cytoskeleton and are composed of a transmembranal part consisting mainly of desmosomal cadherins, armadillo proteins and desmoplakin, which form the intracytoplasmic desmosomal plaque. Desmosomal genodermatoses are caused by mutations in genes encoding the various desmosomal components. They are characterized by skin, hair and cardiac manifestations occurring in diverse combinations. Their classification into a separate and distinct clinical group not only recognizes their common pathogenesis and facilitates their diagnosis but might also in the future form the basis for the design of novel and targeted therapies for these occasionally life-threatening diseases.

Plakophilins 1 and 3 bind to FXR1 and thereby influence the mRNA stability of desmosomal proteins

Plakophilins 1 and 3 (PKP1/3) are members of the arm repeat family of catenin proteins and serve as structural components of desmosomes, which are important for cell-cell-adhesion. In addition, PKP1/3 occur as soluble proteins outside desmosomes, yet their role in the cytoplasm is not known. We found that cytoplasmic PKP1/3 coprecipitated with the RNA-binding proteins FXR1, G3BP, PABPC1, and UPF1, and these PKP1/3 complexes also comprised desmoplakin and PKP2 mRNAs. Moreover, we showed that the interaction of PKP1/3 with G3BP, PABPC1, and UPF1 but not with FXR1 was RNase sensitive. To address the cytoplasmic function of PKP1/3, we performed gain-and-loss-of-function studies. Both PKP1 and PKP3 knockdown cell lines showed reduced protein and mRNA levels for desmoplakin and PKP2. Whereas global rates of translation were unaffected, desmoplakin and PKP2 mRNA were destabilized. Furthermore, binding of PKP1/3 to FXR1 was RNA independent, and both PKP3 and FXR1 stabilized PKP2 mRNA. Our results demonstrate that cytoplasmic PKP1/3 are components of mRNA ribonucleoprotein particles and act as posttranscriptional regulators of gene expression.

Plakophilin 3 mediates Rap1-dependent desmosome assembly and adherens junction maturation

Desmosomal Armadillo family member Pkp3 is established as a coordinator of desmosome and adherens junction assembly and maturation through its physical and functional association with Rap1. It thus functions in a manner distinct from the closely related Pkp2.

The pathways driving desmosome and adherens junction assembly are temporally and spatially coordinated, but how they are functionally coupled is poorly understood. Here we show that the Armadillo protein plakophilin 3 (Pkp3) mediates both desmosome assembly and E-cadherin maturation through Rap1 GTPase, thus functioning in a manner distinct from the closely related plakophilin 2 (Pkp2). Whereas Pkp2 and Pkp3 share the ability to mediate the initial phase of desmoplakin (DP) accumulation at sites of cell–cell contact, they play distinct roles in later steps: Pkp3 is required for assembly of a cytoplasmic population of DP-enriched junction precursors, whereas Pkp2 is required for transfer of the precursors to the membrane. Moreover, Pkp3 forms a complex with Rap1 GTPase, promoting its activation and facilitating desmosome assembly. We show further that Pkp3 deficiency causes disruption of an E-cadherin/Rap1 complex required for adherens junction sealing. These findings reveal Pkp3 as a coordinator of desmosome and adherens junction assembly and maturation through its functional association with Rap1.

In VivoFunction of Desmosomes

Desmosomes are morphologically and biochemically defined cell-cell junctions that are required for maintaining the mechanical integrity of skin and the heart in adult mammals. Furthermore, since mice with null mutations in desmosomal plaque proteins (plakoglobin and desmoplakin) die in utero, it is also evident that desmosomes are indispensable for normal embryonic development. This review focuses on the role of desmosomes in vivo. We will summarize the effects of mutations in desmosomal genes on pre- and post-embryonic development of mouse and man and discuss recent findings relating to the specific role of desmosomal cadherins in skin differentiation and homeostasis.

Plakophilin-3 catenin associates with the ETV1/ER81 transcription factor to positively modulate gene activity

Members of the plakophilin-catenin sub-family (Pkp-1, -2, and -3) facilitate the linkage of desmosome junctional components to each other (e.g. desmosomal cadherins to desmoplakin) and the intermediate-filament cytoskeleton. Pkps also contribute to desmosomal stabilization and the trafficking of its components. The functions of Pkps outside of the desmosome are less well studied, despite evidence suggesting their roles in mRNA regulation, small-GTPase modulation (e.g. mid-body scission) during cell division, and cell survival following DNA damage. Pkp-catenins are further believed to have roles in the nucleus given their nuclear localization in some contexts and the known nuclear roles of structurally related catenins, such as beta-catenin and p120-catenin. Further, Pkp-catenin activities in the nuclear compartment have become of increased interest with the identification of interactions between Pkp2-catenin and RNA Pol III and Pkp1 with single-stranded DNA. Consistent with earlier reports suggesting possible nuclear roles in development, we previously demonstrated prominent nuclear localization of Pkp3 in Xenopus naïve ectoderm (“animal cap”) cells and recently resolved a similar localization in mouse embryonic stem cells. Here, we report the association and positive functional interaction of Pkp3 with a transcription factor, Ets variant gene 1 (ETV1), which has critical roles in neural development and prominent roles in human genetic disease. Our results are the first to report the interaction of a sequence-specific transcription factor with any Pkp. Using Xenopus laevis embryos and mammalian cells, we provide evidence for the Pkp3:ETV1 complex on both biochemical and functional levels.

Beta-catenin versus the other armadillo catenins: assessing our current view of canonical Wnt signaling

The prevailing view of canonical Wnt signaling emphasizes the role of beta-catenin acting downstream of Wnt activation to regulate transcriptional activity. However, emerging evidence indicates that other armadillo catenins in vertebrates, such as members of the p120 subfamily, convey parallel signals to the nucleus downstream of canonical Wnt pathway activation. Their study is thus needed to appreciate the networked mechanisms of canonical Wnt pathway transduction, especially as they may assist in generating the diversity of Wnt effects observed in development and disease. In this chapter, we outline evidence of direct canonical Wnt effects on p120 subfamily members in vertebrates and speculate upon these catenins' roles in conjunction with or aside from beta-catenin.

Structural and functional diversity of desmosomes

Desmosomes anchor intermediate filaments at sites of cell contact established by the interaction of cadherins extending from opposing cells. The incorporation of cadherins, catenin adaptors, and cytoskeletal elements resembles the closely related adherens junction. However, the recruitment of intermediate filaments distinguishes desmosomes and imparts a unique function. By linking the load-bearing intermediate filaments of neighboring cells, desmosomes create mechanically contiguous cell sheets and, in so doing, confer structural integrity to the tissues they populate. This trait and a well-established role in human disease have long captured the attention of cell biologists, as evidenced by a publication record dating back to the mid-1860s. Likewise, emerging data implicating the desmosome in signaling events pertinent to organismal development, carcinogenesis, and genetic disorders will secure a prominent role for desmosomes in future biological and biomedical investigations.

Stratifin (14-3-3 σ) limits plakophilin-3 exchange with the desmosomal plaque

Desmosomes are prominent cell-cell adhesive junctions in stratified squamous epithelia and disruption of desmosomal adhesion has been shown to have dramatic effects on the function and integrity of these tissues. During normal physiologic processes, such as tissue development and wound healing, intercellular adhesion must be modified locally to allow coordinated cell movements. The mechanisms that control junction integrity and adhesive strength under these conditions are poorly understood. We utilized a proteomics approach to identify plakophilin-3 associated proteins and identified the 14-3-3 family member stratifin. Stratifin interacts specifically with plakophilin-3 and not with other plakophilin isoforms and mutation analysis demonstrated the binding site includes serine 285 in the amino terminal head domain of plakophilin-3. Stratifin interacts with a cytoplasmic pool of plakophilin-3 and is not associated with the desmosome in cultured cells. FRAP analysis revealed that decreased stratifin expression leads to an increase in the exchange rate of cytoplasmic plakophilin-3/GFP with the pool of plakophilin-3/GFP in the desmosome resulting in decreased desmosomal adhesion and increased cell migration. We propose a model by which stratifin plays a role in regulating plakophilin-3 incorporation into the desmosomal plaque by forming a plakophilin-3 stratifin complex in the cytosol and thereby affecting desmosome dynamics in squamous epithelial cells.

Ultrastructural localization of desmoglein and plakophilin in the human hair suggests that the cell membrane complex is a long desmosomal remnant

Unlike the superficial part of the corneous layer of the epidermis (Stratum corneum) where desmosomes are degraded and corneocytes flake away, the trichocytes in the hair remain attached to each other after cornification. The permanence and fine localization of cell junctions, in particular of desmosomal proteins in the cornifying and mature human hair, is not known. The present electron microscope immunolocalization study indicates that two protein markers for desmosomes such as desmoglein 4 and plakophilins 1 and 3 are still present in mature cortical and cuticle cells. These proteins remain mainly localized in the cornified cytoplasmic side of desmosomal remnants of cortical cells, but also in the delta layer of the extracellular region of the membrane complex. This suggests that the delta layer represents an extensive desmosomal remnant formed between mature cortical cells and in cuticle cells. The endocuticle appears to be the site of accumulation of desmosomal proteins and degraded nuclear material. The cornification of desmosomal junctions in both cortical and cuticle cells likely contributes to stabilize the integrity of the hair shaft.

Mechanistic basis of desmosome-targeted diseases

Desmosomes are dynamic junctions between cells that maintain the structural integrity of skin and heart tissues by withstanding shear forces. Mutations in component genes cause life-threatening conditions including arrhythmogenic right ventricular cardiomyopathy, and desmosomal proteins are targeted by pathogenic autoantibodies in skin blistering diseases such as pemphigus. Here, we review a set of newly discovered pathogenic alterations and discuss the structural repercussions of debilitating mutations on desmosomal proteins. The architectures of native desmosomal assemblies have been visualized by cryo-electron microscopy and cryo-electron tomography, and the network of protein domain interactions is becoming apparent. Plakophilin and desmoplakin mutations have been discovered to alter binding interfaces, structures, and stabilities of folded domains that have been resolved by X-ray crystallography and NMR spectroscopy. The flexibility within desmoplakin has been revealed by small-angle X-ray scattering and fluorescence assays, explaining how mechanical stresses are accommodated. These studies have shown that the structural and functional consequences of desmosomal mutations can now begin to be understood at multiple levels of spatial and temporal resolution. This review discusses the recent structural insights and raises the possibility of using modeling for mechanism-based diagnosis of how deleterious mutations alter the integrity of solid tissues.

Plakophilin-associated RNA-binding proteins in prostate cancer and their implications in tumor progression and metastasis

Both plakophilins (PKP) 1 and 3 play a role in the progression of prostate cancer. The RNA-binding proteins (RBPs) GAP-SH3-binding protein (G3BP), fragile-X-related protein 1 (FXR1), poly(A)-binding protein, cytoplasmic 1 (PABPC1), and up-frameshift factor 1 (UPF1) are associated with PKP3. All these RBPs have an impact on RNA metabolism. Until recently, the PKP-associated RBPs have not been analyzed in prostate cancer. In the current study, we showed by affinity purification that the PKP3-associated RBPs were also binding partners of PKP1. We examined the expression of PKP1/3-associated RBPs and PKP1/3 in prostate cell lines, tumor-free prostate, and 136 prostatic adenocarcinomas by immunofluorescence and immunoblot. All four RBPs G3BP, FXR1, UPF1, and PABPC1 were expressed in the glandular epithelium of the normal prostate. PKP1 and FXR1 were strongly reduced in tumor tissues with Gleason score >7 and diminished expression of PKP1 and FXR1 also appeared to be associated with a metastatic phenotype. Additionally, the predominant nuclear localization of UPF1 in normal glandular cells and low grade tumors was switched to a more cytoplasmic pattern in carcinomas with Gleason score >7. Our findings suggest that PKP1 and FXR1 may have a tumor-suppressive function and are downregulated in more aggressive tumors. Collectively, PKP1/3-associated RBPs FXR1 and UPF1 may have a functional role in prostate cancer progression and metastasis and highlight the potential importance of posttranscriptional regulation of gene expression and nonsense-mediated decay in cancer.

Immunolocalization of junctional proteins in human hairs indicates that the membrane complex stabilizes the inner root sheath while desmosomes contact the companion layer through specific keratins

The inner root sheath (IRS) sustains and addresses the hair shaft outside the follicle. Ultrastructural analysis of immunolabeling for beta-catenin, plakophilin-1, desmoglein-4 and keratin-17 in human hairs has indicated that adherens junctions and desmosomes initially connect cells in mature IRS and the companion layer. Beta-catenin immunolabeling for adherens junctions is only seen in sparse regions of differentiating Huxley cells, Flugelzellen cells and Henle cells, but disappears in cornified cells of the IRS. Desmoglein-4 and plakophilin-1 immunolabeling are observed in differentiating and cornified desmosomes of the Huxley and Henle layers and in the membrane complex joining these cells. Desmoglein-4 and plakophilin-1 are more frequently immunolocalized in the intracellular side of the junctions, but some labeling is also present in the delta-layer of the membrane complex. The labeling indicates a prevalent intracellular redistribution of desmoglein-4 and plakophilin-1 when the final cornification of the IRS occurs. Intense keratin-17 immunolabeling is observed in tonofilaments of the companion layer joining the plakophilin-1 rich desmosomes of the Henle layer. This suggests that this elastic type of keratin is present at desmosome junctions during the movements of the companion layer along the slippage plane of the hair shaft.

Epidermal growth factor-induced modulation of cytokeratin expression levels influences the morphological phenotype of head and neck squamous cell carcinoma cells

The migratory ability of tumor cells requires cytoskeletal rearrangement processes. Epidermal growth factor receptor (EGFR)-signaling tightly correlates with tumor progression in head and neck squamous cell carcinomas (HNSCCs), and has previously been implicated in the regulation of cytokeratin (CK) expression. In this study, HNSCC cell lines were treated with EGF, and CK expression levels were monitored by Western blot analysis. Changes in cellular morphology were documented by fluorescence- and atomic force microscopy. Some of the cell lines demonstrated an EGF-dependent modulation of CK expression levels. Interestingly, regression of some CK subtypes or initial up-regulation followed by downregulation at higher EGF-levels could also be observed in the tested cell lines. Overall, the influence of EGF on CK expression levels appeared variable and cell-type-dependent. Real-time cellular analysis of EGF-treated and -untreated HNSCC cell lines demonstrated a rise over time in cellular impedance. In three of the EGF-treated HNSCC cell lines, this rise was markedly higher than in untreated controls, whereas in one of the cell lines the gain of cellular impedance was paradoxically reduced after EGF treatment, which was found to correlate with changes in cellular morphology rather than with relevant changes in cellular viability or proliferation. After treating HNSCC cells with EGF, CK filaments frequently appeared diffusely distributed throughout the cytoplasm, and in some cases were found in a perinuclear localization, the latter being reminiscent to observations by other groups. In summary, the data points to a possible role of EGFR in modulating HNSCC cell morphology.

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

PKCα activation of p120-catenin serine 879 phospho-switch disassembles VE-cadherin junctions and disrupts vascular integrity

RATIONALE

Adherens junctions (AJs) are the primary intercellular junctions in microvessels responsible for endothelial barrier function. Homophilic adhesion of vascular endothelial (VE) cadherin forms AJs, which are stabilized by binding of p120-catenin (p120). p120 dissociation from VE-cadherin results in loss of VE-cadherin homotypic interaction and AJ disassembly; however, the signaling mechanisms regulating p120 dissociation from VE-cadherin are not understood.

OBJECTIVE

To address the mechanism of protein kinase C (PKC)-α function in increasing endothelial permeability, we determined the role of PKCα phosphorylation of p120 in mediating disruption of AJ integrity.

METHODS AND RESULTS

We showed that PKCα phosphorylation of p120 at serine (S)879 in response to thrombin or lipopolysaccharide challenge reduced p120 binding affinity for VE-cadherin and mediated AJ disassembly secondary to VE-cadherin internalization. In studies in mouse lung vessels, expression of the phosphodeficient S879A-p120 mutant prevented the increase in vascular permeability induced by activation of the thrombin receptor PAR-1.

CONCLUSIONS

PKCα phosphorylation of p120 at S879 is a critical phospho-switch mediating disassociation of p120 from VE-cadherin that results in AJ disassembly. Therefore, blocking PKCα-mediated p120 phosphorylation represents a novel targeted anti-inflammatory strategy to prevent disruption of vascular endothelial barrier function.

Plakophilin-2: a cell-cell adhesion plaque molecule of selective and fundamental importance in cardiac functions and tumor cell growth

Within the characteristic ensemble of desmosomal plaque proteins, the armadillo protein plakophilin-2 (Pkp2) is known as a particularly important regulatory component in the cytoplasmic plaques of various other cell-cell junctions, such as the composite junctions (areae compositae) of the myocardiac intercalated disks and in the variously-sized and -shaped complex junctions of permanent cell culture lines derived therefrom. In addition, Pkp2 has been detected in certain protein complexes in the nucleoplasm of diverse kinds of cells. Using a novel set of highly sensitive and specific antibodies, both kinds of Pkp2, the junctional plaque-bound and the nuclear ones, can also be localized to the cytoplasmic plaques of diverse non-desmosomal cell-cell junction structures. These are not only the puncta adhaerentia and the fasciae adhaerentes connecting various types of highly proliferative non-epithelial cells growing in culture but also some very proliferative states of cardiac interstitial cells and cardiac myxomata, including tumors growing in situ as well as fetal stages of heart development and cultures of valvular interstitial cells. Possible functions and assembly mechanisms of such Pkp2-positive cell-cell junctions as well as medical consequences are discussed.

Plakophilin-3 is required for late embryonic amphibian development, exhibiting roles in ectodermal and neural tissues

The p120-catenin family has undergone a significant expansion during the evolution of vertebrates, resulting in varied functions that have yet to be discerned or fully characterized. Likewise, members of the plakophilins, a related catenin subfamily, are found throughout the cell with little known about their functions outside the desmosomal plaque. While the plakophilin-3 (Pkp3) knockout mouse resulted in skin defects, we find larger, including lethal effects following its depletion in Xenopus. Pkp3, unlike some other characterized catenins in amphibians, does not have significant maternal deposits of mRNA. However, during embryogenesis, two Pkp3 protein products whose temporal expression is partially complimentary become expressed. Only the smaller of these products is found in adult Xenopus tissues, with an expression pattern exhibiting distinctions as well as overlaps with those observed in mammalian studies. We determined that Xenopus Pkp3 depletion causes a skin fragility phenotype in keeping with the mouse knockout, but more novel, Xenopus tailbud embryos are hyposensitive to touch even in embryos lacking outward discernable phenotypes, and we additionally resolved disruptions in certain peripheral neural structures, altered establishment and migration of neural crest, and defects in ectodermal multiciliated cells. The use of two distinct morpholinos, as well as rescue approaches, indicated the specificity of these effects. Our results point to the requirement of Pkp3 in amphibian embryogenesis, with functional roles in a number of tissue types.

Cell-cell connectivity: desmosomes and disease

Cell-cell connectivity is an absolute requirement for the correct functioning of cells, tissues and entire organisms. At the level of the individual cell, direct cell-cell adherence and communication is mediated by the intercellular junction complexes: desmosomes, adherens, tight and gap junctions. A broad spectrum of inherited, infectious and auto-immune diseases can affect the proper function of intercellular junctions and result in either diseases affecting specific individual tissues or widespread syndromic conditions. A particularly diverse group of diseases result from direct or indirect disruption of desmosomes--a consequence of their importance in tissue integrity, their extensive distribution, complex structure, and the wide variety of functions their components accomplish. As a consequence, disruption of desmosomal assembly, structure or integrity disrupts not only their intercellular adhesive function but also their functions in cell communication and regulation, leading to such diverse pathologies as cardiomyopathy, epidermal and mucosal blistering, palmoplantar keratoderma, woolly hair, keratosis, epidermolysis bullosa, ectodermal dysplasia and alopecia. Here, as well as describing the importance of the other intercellular junctions, we focus primarily on the desmosome, its structure and its role in disease. We will examine the various pathologies that result from impairment of desmosome function and thereby demonstrate the importance of desmosomes to tissues and to the organism as a whole.

Desmosomes: new perpetrators in tumour suppression

Adherens junctions, which are intercellular adhesive complexes that are crucial for maintaining epithelial homeostasis, are downregulated in many cancers to promote tumour progression. However, the role of desmosomes - adhesion complexes that are related to adherens junctions - in carcinogenesis has remained elusive. Recent studies using mouse genetic approaches have uncovered a role for desmosomes in tumour suppression, demonstrating that desmosome downregulation occurs before that of adherens junctions to drive tumour development and early invasion, suggesting a two-step model of adhesion dysfunction in cancer progression.

The molecular evolution of the p120-catenin subfamily and its functional associations

BACKGROUND

p120-catenin (p120) is the prototypical member of a subclass of armadillo-related proteins that includes δ-catenin/NPRAP, ARVCF, p0071, and the more distantly related plakophilins 1-3. In vertebrates, p120 is essential in regulating surface expression and stability of all classical cadherins, and directly interacts with Kaiso, a BTB/ZF family transcription factor.

METHODOLOGY/PRINCIPAL FINDINGS

To clarify functional relationships between these proteins and how they relate to the classical cadherins, we have examined the proteomes of 14 diverse vertebrate and metazoan species. The data reveal a single ancient δ-catenin-like p120 family member present in the earliest metazoans and conserved throughout metazoan evolution. This single p120 family protein is present in all protostomes, and in certain early-branching chordate lineages. Phylogenetic analyses suggest that gene duplication and functional diversification into "p120-like" and "δ-catenin-like" proteins occurred in the urochordate-vertebrate ancestor. Additional gene duplications during early vertebrate evolution gave rise to the seven vertebrate p120 family members. Kaiso family members (i.e., Kaiso, ZBTB38 and ZBTB4) are found only in vertebrates, their origin following that of the p120-like gene lineage and coinciding with the evolution of vertebrate-specific mechanisms of epigenetic gene regulation by CpG island methylation.

CONCLUSIONS/SIGNIFICANCE

The p120 protein family evolved from a common δ-catenin-like ancestor present in all metazoans. Through several rounds of gene duplication and diversification, however, p120 evolved in vertebrates into an essential, ubiquitously expressed protein, whereas loss of the more selectively expressed δ-catenin, p0071 and ARVCF are tolerated in most species. Together with phylogenetic studies of the vertebrate cadherins, our data suggest that the p120-like and δ-catenin-like genes co-evolved separately with non-neural (E- and P-cadherin) and neural (N- and R-cadherin) cadherin lineages, respectively. The expansion of p120 relative to δ-catenin during vertebrate evolution may reflect the pivotal and largely disproportionate role of the non-neural cadherins with respect to evolution of the wide range of somatic morphology present in vertebrates today.

Junctional music that the nucleus hears: cell-cell contact signaling and the modulation of gene activity

Cell-cell junctions continue to capture the interest of cell and developmental biologists, with an emerging area being the molecular means by which junctional signals relate to gene activity in the nucleus. Although complexities often arise in determining the direct versus indirect nature of such signal transduction, it is clear that such pathways are essential for the function of tissues and that alterations may contribute to many pathological outcomes. This review assesses a variety of cell-cell junction-to-nuclear signaling pathways, and outlines interesting areas for further study.

The desmosome

Desmosomes are intercellular junctions that tether intermediate filaments to the plasma membrane. Desmogleins and desmocollins, members of the cadherin superfamily, mediate adhesion at desmosomes. Cytoplasmic components of the desmosome associate with the desmosomal cadherin tails through a series of protein interactions, which serve to recruit intermediate filaments to sites of desmosome assembly. These desmosomal plaque components include plakoglobin and the plakophilins, members of the armadillo gene family. Linkage to the cytoskeleton is mediated by the intermediate filament binding protein, desmoplakin, which associates with both plakoglobin and plakophilins. Although desmosomes are critical for maintaining stable cell-cell adhesion, emerging evidence indicates that they are also dynamic structures that contribute to cellular processes beyond that of cell adhesion. This article outlines the structure and function of the major desmosomal proteins, and explores the contributions of this protein complex to tissue architecture and morphogenesis.

Desmosomes: adhesive strength and signalling in health and disease

Desmosomes are intercellular junctions whose primary function is strong intercellular adhesion, known as hyperadhesion. In the present review, we discuss how their structure appears to support this function as well as how they are assembled and down-regulated. Desmosomal components also have signalling functions that are important in tissue development and remodelling. Their adhesive and signalling functions are both compromised in genetic and autoimmune diseases that affect the heart, skin and mucous membranes. We conclude that much work is required on structure–function relationships within desmosomes in vivo and on how they participate in signalling processes to enhance our knowledge of tissue homoeostasis and human disease.

Plakophilin-1 localizes to the nucleus and interacts with single-stranded DNA

Plakophilins (Pkp-1, -2, and -3) comprise a family of armadillo repeat-containing proteins first identified as desmosomal plaque components, in which they link desmoplakin to the desmosomal cadherins. In addition to their role in desmosomal cell-cell adhesion, Pkps also localize to the nucleus, where they perform unknown functions. Of the three Pkps, Pkp-1 is most readily detected in the nucleus, where it is localized to the nucleoplasm. Pkp chimeras containing the Pkp-1 head domain and Pkp-3 armadillo repeat domain were localized to the nucleus in A431 cells, whereas Pkp chimeras containing the Pkp-3 head domain and Pkp-1 armadillo repeat domain localized to the desmosome and the cytosol. DNAse I digestion of chromatin in cultured cells results in loss of nuclear Pkp-1, suggesting that Pkp-1 associates specifically with nuclear components. In addition, in vitro assays revealed that the amino-terminal head domains of Pkps-1 and -2 were sufficient to bind single-stranded DNA. Induction of DNA damage induced a partial redistribution of Pkp-1 protein to the nucleolus, and depletion of Pkp-1 resulted in increased survival in response to DNA damage. These data suggest that in addition to mediating desmosome assembly, the nuclear pool of Pkp can influence cell survival by interactions with DNA.

Desmosomal molecules in and out of adhering junctions: normal and diseased States of epidermal, cardiac and mesenchymally derived cells

Current cell biology textbooks mention only two kinds of cell-to-cell adhering junctions coated with the cytoplasmic plaques: the desmosomes (maculae adhaerentes), anchoring intermediate-sized filaments (IFs), and the actin microfilament-anchoring adherens junctions (AJs), including both punctate (puncta adhaerentia) and elongate (fasciae adhaerentes) structures. In addition, however, a series of other junction types has been identified and characterized which contain desmosomal molecules but do not fit the definition of desmosomes. Of these special cell-cell junctions containing desmosomal glycoproteins or proteins we review the composite junctions (areae compositae) connecting the cardiomyocytes of mature mammalian hearts and their importance in relation to human arrhythmogenic cardiomyopathies. We also emphasize the various plakophilin-2-positive plaques in AJs (coniunctiones adhaerentes) connecting proliferatively active mesenchymally-derived cells, including interstitial cells of the heart and several soft tissue tumor cell types. Moreover, desmoplakin has also been recognized as a constituent of the plaques of the complexus adhaerentes connecting certain lymphatic endothelial cells. Finally, we emphasize the occurrence of the desmosomal transmembrane glycoprotein, desmoglein Dsg2, out of the context of any junction as dispersed cell surface molecules in certain types of melanoma cells and melanocytes. This broadening of our knowledge on the diversity of AJ structures indicates that it may still be too premature to close the textbook chapters on cell-cell junctions.

The desmosomal plaque proteins of the plakophilin family

Three related proteins of the plakophilin family (PKP1_3) have been identified as junctional proteins that are essential for the formation and stabilization of desmosomal cell contacts. Failure of PKP expression can have fatal effects on desmosomal adhesion, leading to abnormal tissue and organ development. Thus, loss of functional PKP 1 in humans leads to ectodermal dysplasia/skin fragility (EDSF) syndrome, a genodermatosis with severe blistering of the epidermis as well as abnormal keratinocytes differentiation. Mutations in the human PKP 2 gene have been linked to severe heart abnormalities that lead to arrhythmogenic right ventricular cardiomyopathy (ARVC). In the past few years it has been shown that junctional adhesion is not the only function of PKPs. These proteins have been implicated in cell signaling, organization of the cytoskeleton, and control of protein biosynthesis under specific cellular circumstances. Clearly, PKPs are more than just cell adhesion proteins. In this paper we will give an overview of our current knowledge on the very distinct roles of plakophilins in the cell.

Desmosomal plakophilins in the prostate and prostatic adenocarcinomas: implications for diagnosis and tumor progression

The plakophilins, members of the armadillo-repeat family, consist of three different proteins (PKP1-3) that are specifically recruited to desmosomal plaques in a highly cell type-specific manner. Using immunofluorescence, immunoelectron microscopy, and immunoblot, we found that all three plakophilins occurred in luminal and basal cells of the pseudostratified prostate epithelium. The analysis of 135 cases of prostatic adenocarcinomas grouped into tumors with low (Gleason score < or = 6), intermediate (Gleason score 7), and high Gleason score (8 < or = Gleason score < or = 10) showed that the expression of PKP1 was reduced or lost in adenocarcinomas with high Gleason scores. The expression of PKP2 was unchanged in all prostatic adenocarcinomas analyzed. In contrast, PKP3 expression was increased in carcinomas with high Gleason scores in comparison with carcinomas with low Gleason scores. In DU 145 cell lines with either overexpression or knockdown of PKP3, both imbalances resulted in fewer desmosomal cell contacts. In addition, overexpression of PKP3 in DU 145 cells led to an augmentation in proliferation rate. Our data imply that both loss of PKP1 and up-regulation of PKP3 expression are biologically important events in prostate cancer and are associated with a more aggressive phenotype.

Desmosomal component expression in normal, dysplastic, and oral squamous cell carcinoma

Squamous cell carcinoma (oral SCC) is the most common oral cancer in the U.S., affecting nearly 30,000 Americans each year. Despite recent advances in detection and treatment, there has been little improvement in the five-year survival rate for this devastating disease. Oral cancer may be preceded by premalignant disease that appears histologically as dysplasia. Identification of molecular markers for cellular change would assist in determining the risk of dysplasia progressing to oral squamous cell carcinoma. The goal of this study was to determine if any correlation exists between histological diagnosed dysplasia and OSCC lesions and altered expression of desmosomal cell-cell adhesion molecules in the oral epithelium. Our data showed that oral SCC tissue samples showed decreased immunoreactivity of both desmoplakin and plakophilin-1 proteins compared to normal oral epithelium. Furthermore, significant decrease in desmoplakin immunoreactivity was observed in dysplastic tissue compared to normal oral epithelium. In contrast, the level of desmoglein-1 staining was unchanged between samples however desmoglein-1 was found localized to cell borders in oral SCC samples. These data suggest that changes in expression of desmoplakin and plakophilin-1 may prove to be a useful marker for changes in tissue morphology and provide a tool for identifying pre-neoplastic lesions of the oral cavity.

Plakophilin 1 stimulates translation by promoting eIF4A1 activity

p120 armadillo protein plakophilin 1 binds to eukaryotic translation factor eIF4A1, recruiting it into cap-binding complexes and stimulating translation.

Plakophilins 1–3 (PKP1–3) are desmosomal proteins of the p120^ctn family of armadillo-related proteins that are essential for organizing the desmosomal plaque. Recent findings identified PKPs in stress granules, suggesting an association with the translational machinery. However, a role of PKPs in controlling translation remained elusive so far. In this study, we show a direct association of PKP1 with the eukaryotic translation initiation factor 4A1 (eIF4A1). PKP1 stimulated eIF4A1-dependent translation via messenger RNA cap and encephalomyocarditis virus internal ribosomal entry site (IRES) structures, whereas eIF4A1-independent translation via hepatitis C virus IRES was not affected. PKP1 copurified with eIF4A1 in the cap complex, and its overexpression stimulated eIF4A1 recruitment into cap-binding complexes. At the molecular level, PKP1 directly promoted eIF4A1 adenosine triphosphatase activity. The stimulation of translation upon PKP1 overexpression correlated with the up-regulation of proliferation and cell size. In conclusion, these findings identify PKP1 as a regulator of translation and proliferation via modulation of eIF4A1 activity and suggest that PKP1 controls cell growth in physiological and pathological conditions.

Ectodermal dysplasias: an overview and update of clinical and molecular-functional mechanisms

The ectodermal dysplasias (EDs) are a large and complex group of disorders. In various combinations, they all share anomalies in hair, teeth, nails, and sweat gland function. The anomalies affecting the epidermis and epidermal appendages are extremely variable. Many are associated with malformations in other organs and systems. Clinical overlap is present among EDs. Few causative genes have been identified, to date. Most of the EDs present multisystem involvement with abnormal development of structures also derived from mesoderm. In the last few years, it has become evident that gene expression in the EDs is not limited to the ectoderm and that there is a concomitant effect on developing mesenchymal structures, with modification or abolition of ectodermal-mesenchymal signaling. It is possible to approach this group of diseases basing on functional and molecular findings and to begin to explain the complex clinical consequences of mutations affecting specific developmental pathways. We have reviewed the molecular basis of ectodermal dysplasias applying this new clinical-functional classification. For each subset of the identified ED, we will now describe the genes and related proteins involved in terms of: (1) structure of the genes and their role in differentiation of the epidermis and the ectodermal derivatives; (2) genotype-phenotype correlation.

Discovering the molecular components of intercellular junctions--a historical view

The organization of metazoa is based on the formation of tissues and on tissue-typical functions and these in turn are based on cell-cell connecting structures. In vertebrates, four major forms of cell junctions have been classified and the molecular composition of which has been elucidated in the past three decades: Desmosomes, which connect epithelial and some other cell types, and the almost ubiquitous adherens junctions are based on closely cis-packed glycoproteins, cadherins, which are associated head-to-head with those of the hemi-junction domain of an adjacent cell, whereas their cytoplasmic regions assemble sizable plaques of special proteins anchoring cytoskeletal filaments. In contrast, the tight junctions (TJs) and gap junctions (GJs) are formed by tetraspan proteins (claudins and occludins, or connexins) arranged head-to-head as TJ seal bands or as paracrystalline connexin channels, allowing intercellular exchange of small molecules. The by and large parallel discoveries of the junction protein families are reported.

Plakophilins: multifunctional scaffolds for adhesion and signaling

Armadillo family proteins known as plakophilins have been characterized as structural components of desmosomes that stabilize and strengthen adhesion by enhancing attachments with the intermediate filament cytoskeleton. However, plakophilins and their close relatives are emerging as versatile scaffolds for multiple signaling and metabolic processes that not only facilitate junction dynamics but also more globally regulate diverse cellular activities. While perturbation of plakophilin functions contribute to inherited diseases and cancer pathogenesis, the functional significance of the multiple PKP isoforms and the mechanisms by which their behaviors are regulated remain to be elucidated.

Decreased Plakophilin-1 Expression Promotes Increased Motility in Head and Neck Squamous Cell Carcinoma Cells

Desmosomes are prominent cell-cell adhesive junctions found in a variety of epithelial tissues, including the oral epithelium. The transmembrane core of the desmosome is composed of the desmosomal cadherins that interact extracellularly to mediate cell-cell adhesion. The cytoplasmic domain of desmosomal cadherins interact with plaque proteins that in turn interact with the keratin intermediate filament cytoskeleton. Plakophilin 1 is a major desmosomal plaque component that functions to recruit intermediate filaments to sites of cell-cell contact via interactions with desmoplakin. Decreased assembly of desmosomes has been reported in several epithelial cancers. We examined plakophilin-1 expression in an esophageal squamous cell carcinoma tissue microarray and found that plakophilin-1 expression inversely correlates with tumor grade. In addition, we sought to investigate the effect of plakophilin-1 expression on desmosome assembly and cell motility in oral squamous cell carcinoma cell lines. Cell lines expressing altered levels of plakophilin-1 were generated and the ability of these cells to recruit desmoplakin to sites of cell-cell contact was examined. Our results show that decreased expression of plakophilin-1 results in decreased desmosome assembly and increased cell motility and invasion. These data lead us to propose that loss of plakophilin-1 expression during head and neck squamous cell carcinoma (HNSCC) progression may contribute to an invasive phenotype.

Protein p0071, a major plaque protein of non-desmosomal adhering junctions, is a selective cell-type marker

Protein p0071, which originally was introduced as a member of the p120-subfamily of armadillo proteins, common to desmosomes and adhaerens junctions (AJs) and to several other cell structures (centrosomes, midbodies), has been localized by using a series of novel mono- and polyclonal antibodies generated against various domains of the molecule. By protein analysis and immunolocalization techniques, protein p0071 has been localized as a plaque protein in AJs of diverse epithelia and certain vascular endothelia, in the composite junctions (areal compositae) of the intercalated disks of cardiomyocytes, and in the punctate or more extended AJs of the vast majority of cell culture types examined, including mitotic states. Using these antibodies, we have also shown that this AJ protein occurs only rarely or is even absent in tissues such as skeletal and smooth muscles, in a series of mesenchymal tissue cells, and in specific desmosome-rich cells such as those of the upper layers of the epidermis and certain other stratified epithelia and Hassall corpuscles of the thymus. We have also demonstrated that p0071 is absent from desmosomes. The occurrence of two major subtypes of lymphatic endothelial cells, one with AJs containing p0071 and one without detectable p0071, is emphasized. Possible structural and functional roles of p0071 are discussed in light of these new findings regarding its localization, and the addition of p0071 to the armamentarium of cytodiagnostic cell-type markers is recommended.