Plakophilin-3, a novel armadillo-like protein present in nuclei and desmosomes of epithelial cells

Unveiling the Binding between the Armadillo-Repeat Domain of Plakophilin 1 and the Intrinsically Disordered Transcriptional Repressor RYBP

Plakophilin 1 (PKP1), a member of the p120ctn subfamily of the armadillo (ARM)-repeat-containing proteins, is an important structural component of cell-cell adhesion scaffolds although it can also be ubiquitously found in the cytoplasm and the nucleus. RYBP (RING 1A and YY1 binding protein) is a multifunctional intrinsically disordered protein (IDP) best described as a transcriptional regulator. Both proteins are involved in the development and metastasis of several types of tumors. We studied the binding of the armadillo domain of PKP1 (ARM-PKP1) with RYBP by using in cellulo methods, namely immunofluorescence (IF) and proximity ligation assay (PLA), and in vitro biophysical techniques, namely fluorescence, far-ultraviolet (far-UV) circular dichroism (CD), and isothermal titration calorimetry (ITC). We also characterized the binding of the two proteins by using in silico experiments. Our results showed that there was binding in tumor and non-tumoral cell lines. Binding in vitro between the two proteins was also monitored and found to occur with a dissociation constant in the low micromolar range (~10 μM). Finally, in silico experiments provided additional information on the possible structure of the binding complex, especially on the binding ARM-PKP1 hot-spot. Our findings suggest that RYBP might be a rescuer of the high expression of PKP1 in tumors, where it could decrease the epithelial-mesenchymal transition in some cancer cells.

Plakophilin-3 Binds the Membrane and Filamentous Actin without Bundling F-Actin

Plakophilin-3 is a ubiquitously expressed protein found widely in epithelial cells and is a critical component of desmosomes. The plakophilin-3 carboxy-terminal domain harbors nine armadillo repeat motifs with largely unknown functions. Here, we report the 5 Å cryogenic electron microscopy (cryoEM) structure of the armadillo repeat motif domain of plakophilin-3, one of the smaller cryoEM structures reported to date. We find that this domain is a monomer or homodimer in solution. In addition, using an in vitro actin co-sedimentation assay, we show that the armadillo repeat domain of plakophilin-3 directly interacts with F-actin. This feature, through direct interactions with actin filaments, could be responsible for the observed association of extra-desmosomal plakophilin-3 with the actin cytoskeleton directly attached to the adherens junctions in A431 epithelial cells. Further, we demonstrate, through lipid binding analyses, that plakophilin-3 can effectively be recruited to the plasma membrane through phosphatidylinositol-4,5-bisphosphate-mediated interactions. Collectively, we report on novel properties of plakophilin-3, which may be conserved throughout the plakophilin protein family and may be behind the roles of these proteins in cell-cell adhesion.

The armadillo-repeat domain of Plakophilin 1 binds to human enzyme PADI4

Plakophilin 1 (PKP1), a member of the armadillo repeat family of proteins, is a key structural component of cell-cell adhesion scaffolds, although it can also be found in other cell locations, including the cytoplasm and the nucleus. PADI4 (peptidyl-arginine deiminase 4) is one of the human isoforms of a family of enzymes engaged in the conversion of arginine to citrulline, and is present in monocytes, macrophages, granulocytes, and in several types of cancer cells. It is the only family member observed both within the nucleus and the cytoplasm under ordinary conditions. We studied the binding of the armadillo domain of PKP1 (ARM-PKP1) with PADI4, by using several biophysical methods, namely fluorescence, far-ultraviolet (far-UV) circular dichroism (CD), isothermal titration calorimetry (ITC), and molecular simulations; furthermore, binding was also tested by Western-blot (WB) analyses. Our results show that there was binding between the two proteins, with a dissociation constant in the low micromolar range (∼ 1 μM). Molecular modelling provided additional information on the possible structure of the binding complex, and especially on the binding hot-spot predicted for PADI4. This is the first time that the interaction between these two proteins has been described and studied. Our findings could be of importance to understand the development of tumors, where PKP1 and PADI4 are involved. Moreover, our findings pave the way to describe the formation of neutrophil extracellular traps (NETs), whose construction is modulated by PADI4, and which mediate the proteolysis of cell-cell junctions where PKP1 intervenes.

Plakophilin3 loss leads to increased adenoma formation and rectal prolapse in APCmin mice

Plakophilin3 (PKP3) loss leads to tumor progression and metastasis of colon cancer cells. The goal of this report was to determine if PKP3 loss led to increased disease progression in mice. We generated a colonocyte-specific knockout of PKP3 in APC^min mice, which led to increased adenoma formation, the formation of rectal prolapse, and a significant decrease in survival. The observed increase in rectal prolapse formation and decrease in survival correlated with an increase in the expression of Lipocalin2 (LCN2). Increased disease progression was observed even upon treatment with 5-fluorouracil (5FU). These results suggest that an increase in LCN2 expression might lead to therapy resistance and that LCN2 might serve as a potential therapeutic target in colorectal cancer.

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.

A catenin of the plakophilin-subfamily, Pkp3, responds to canonical-Wnt pathway components and signals

Vertebrate beta-catenin plays a key role as a transducer of canonical-Wnt signals. We earlier reported that, similar to beta-catenin, the cytoplasmic signaling pool of p120-catenin-isoform1 is stabilized in response to canonical-Wnt signals. To obtain a yet broader view of the Wnt-pathway's impact upon catenin proteins, we focused upon plakophilin3 (plakophilin-3; Pkp3) as a representative of the plakophilin-catenin subfamily. Promoting tissue integrity, the plakophilins assist in linking desmosomal cadherins to intermediate filaments at desmosome junctions, and in common with other catenins they perform additional functions including in the nucleus. In this report, we test whether canonical-Wnt pathway components modulate Pkp3 protein levels. We find that in common with beta-catenin and p120-catenin-isoform1, Pkp3 is stabilized in the presence of a Wnt-ligand or a dominant-active form of the LRP6 receptor. Pkp3's levels are conversely lowered upon expressing destruction-complex components such as GSK3β and Axin, and in further likeness to beta-catenin and p120-isoform1, Pkp3 associates with GSK3beta and Axin. Finally, we note that Pkp3-catenin trans-localizes into the nucleus in response to Wnt-ligand and its exogenous expression stimulates an accepted Wnt reporter. These findings fit an expanded model where context-dependent Wnt-signals or pathway components modulate Pkp3-catenin levels. Future studies will be needed to assess potential gene regulatory, cell adhesive, or cytoskeletal effects.

The armadillo-repeat domain of plakophilin 1 binds the C-terminal sterile alpha motif (SAM) of p73

BACKGROUND

Plakophilin 1 (PKP1) is a component of desmosomes, which are key structural components for cell-cell adhesion, and can also be found in other cell locations. The p53, p63 and p73 proteins belong to the p53 family of transcription factors, playing crucial roles in tumour suppression. The α-splice variant of p73 (p73α) has at its C terminus a sterile alpha motif (SAM); such domain, SAMp73, is involved in the interaction with other macromolecules.

METHODS

We studied the binding of SAMp73 with the armadillo domain of PKP1 (ARM-PKP1) in the absence and the presence of 100 mM NaCl, by using several biophysical techniques, namely fluorescence, far-ultraviolet circular dichroism (CD), nuclear magnetic resonance (NMR), isothermal titration calorimetry (ITC), and molecular docking and simulations.

RESULTS

Association was observed between the two proteins, with a dissociation constant of ~5 μM measured by ITC and fluorescence in the absence of NaCl. The binding region of SAMp73 involved residues of the so-called "middle-loop-end-helix" binding region (i.e., comprising the third helix, together with the C terminus of the second one, and the N-cap of the fourth), as shown by ¹⁵N, ¹H- HSQC-NMR spectra. Molecular modelling provided additional information on the possible structure of the binding complex.

CONCLUSIONS

This newly-observed interaction could have potential therapeutic relevance in the tumour pathways where PKP1 is involved, and under conditions when there is a possible inactivation of p53.

GENERAL SIGNIFICANCE

The discovery of the binding between SAMp73 and ARM-PKP1 suggests a functional role for their interaction, including the possibility that SAMp73 could assist PKP1 in signalling pathways.

The isolated armadillo-repeat domain of Plakophilin 1 is a monomer in solution with a low conformational stability

Plakophilin 1 (PKP1) is a member of the armadillo repeat family of proteins. It serves as a scaffold component of desmosomes, which are key structural components for cell-cell adhesion. We have embarked on the biophysical and conformational characterization of the ARM domain of PKP1 (ARM-PKP1) in solution by using several spectroscopic (namely, fluorescence and circular dichroism (CD)) and biophysical techniques (namely, analytical ultracentrifugation (AUC), dynamic light scattering (DLS) and differential scanning calorimetry (DSC)). ARM-PKP1 was a monomer in solution at physiological pH, with a low conformational stability, as concluded from DSC experiments and thermal denaturations followed by fluorescence and CD. The presence or absence of disulphide bridges did not affect its low stability. The protein unfolded through an intermediate which has lost native-like secondary structure. ARM-PKP1 acquired a native-like structure in a narrow pH range (between pH 6.0 and 8.0), indicating that its adherent properties might only work in a very narrow pH range.

The Barrier Molecules Junction Plakoglobin, Filaggrin, and Dystonin Play Roles in Melanoma Growth and Angiogenesis

OBJECTIVE

To understand role of barrier molecules in melanomas.

BACKGROUND

We have reported poor patient survival and low immune infiltration of melanomas that overexpress a set of genes that include filaggrin (FLG), dystonin (DST), junction plakoglobin (JUP), and plakophilin-3 (PKP3), and are involved in cell-cell adhesions. We hypothesized that these associations are causal, either by interfering with immune cell infiltration or by enhancing melanoma cell growth.

METHODS

FLG and DST were knocked out by CRISPR/Cas9 in human DM93 and murine B16-F1 melanoma cells. PKP3 and JUP were overexpressed in murine B16-AAD and human VMM39 melanoma cells by lentiviral transduction. These cell lines were evaluated in vitro for cell proliferation and in vivo for tumor burden, immune composition, cytokine expression, and vascularity.

RESULTS

Immune infiltrates were not altered by these genes. FLG/DST knockout reduced proliferation of human DM93 melanoma in vitro, and decreased B16-F1 tumor burden in vivo. Overexpression of JUP, but not PKP3, in B16-AAD significantly increased tumor burden, increased VEGF-A, reduced IL-33, and enhanced vascularity.

CONCLUSIONS

FLG and DST support melanoma cell growth in vitro and in vivo. Growth effects of JUP were only evident in vivo, and may be mediated, in part, by enhancing angiogenesis. In addition, growth-promoting effects of FLG and DST in vitro suggest that these genes may also support melanoma cell proliferation through angiogenesis-independent pathways. These findings identify FLG, DST, and JUP as novel therapeutic targets whose down-regulation may provide clinical benefit to patients with melanoma.

Armc8 is an evolutionarily conserved armadillo protein involved in cell-cell adhesion complexes through multiple molecular interactions

Armadillo-repeat-containing protein 8 (Armc8) belongs to the family of armadillo-repeat containing proteins, which have been found to be involved in diverse cellular functions including cell-cell contacts and intracellular signaling. By comparative analyses of armadillo repeat protein structures and genomes from various premetazoan and metazoan species, we identified orthologs of human Armc8 and analyzed in detail the evolutionary relationship of Armc8 genes and their encoded proteins. Armc8 is a highly ancestral armadillo protein although not present in yeast. Consequently, Armc8 is not the human ortholog of yeast Gid5/Vid28.Further, we performed a candidate approach to characterize new protein interactors of Armc8. Interactions between Armc8 and specific δ-catenins (plakophilins-1, -2, -3 and p0071) were observed by the yeast two-hybrid approach and confirmed by co-immunoprecipitation and co-localization. We also showed that Armc8 interacts specifically with αE-catenin but neither with αN-catenin nor with αT-catenin. Degradation of αE-catenin has been reported to be important in cancer and to be regulated by Armc8. A similar process may occur with respect to plakophilins in desmosomes. Deregulation of desmosomal proteins has been considered to contribute to tumorigenesis.

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.

Dinitrosopiperazine-decreased PKP3 through upregulating miR-149 participates in nasopharyngeal carcinoma metastasis

Nasopharyngeal carcinoma (NPC) has a high metastatic clinicopathological feature. As a carcinogen factor, N,N'-dinitrosopiperazine (DNP) is involved in NPC metastasis, but its precise mechanism has not been fully elucidated. Herein, we showed that DNP promotes NPC metastasis through upregulating miR-149. DNP was found to decrease Plakophilin3 (PKP3) expression, further DNP-decreased PKP3 was verified to be through upregulating miR-149. We also found that DNP induced proliferation, adhesion, migration and invasion of NPC cell, which was inhibited by miR-149-inhibitor. DNP may promote NPC metastasis through miR-149-decreased PKP3 expression. Therefore, DNP-increased miR-149 expression may be an important factor of NPC high metastasis, and miR-149 may serve as a molecular target for anti-metastasis therapy of NPC.

Plakophilin3 increases desmosome assembly, size and stability by increasing expression of desmocollin2

Previous reports show that the desmosomal plaque protein plakophilin3 (PKP3) is essential for desmosome formation. Here, we report that PKP3 over-expression decreases calcium dependency for de novo desmosome formation and makes existing cell-cell adhesion junctions more resilient in low calcium medium due to an increase in desmocollin2 expression. PKP3 overexpression increases the stability of other desmosomal proteins independently of the increase in DSC2 levels and regulates desmosome formation and stability by a multimodal mechanism affecting transcription, protein stability and cell border localization of desmosomal proteins.

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.

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.

γ-Catenin at adherens junctions: mechanism and biologic implications in hepatocellular cancer after β-catenin knockdown

β-Catenin is important in liver homeostasis as a part of Wnt signaling and adherens junctions (AJs), while its aberrant activation is observed in hepatocellular carcinoma (HCC). We have reported hepatocyte-specific β-catenin knockout (KO) mice to lack adhesive defects as γ-catenin compensated at AJ. Because γ-catenin is a desmosomal protein, we asked if its increase in KO might deregulate desmosomes. No changes in desmosomal proteins or ultrastructure other than increased plakophilin-3 were observed. To further elucidate the role and regulation of γ-catenin, we contemplate an in vitro model and show γ-catenin increase in HCC cells upon β-catenin knockdown (KD). Here, γ-catenin is unable to rescue β-catenin/T cell factor (TCF) reporter activity; however, it sufficiently compensates at AJs as assessed by scratch wound assay, centrifugal assay for cell adhesion (CAFCA), and hanging drop assays. γ-Catenin increase is observed only after β-catenin protein decrease and not after blockade of its transactivation. γ-Catenin increase is associated with enhanced serine/threonine phosphorylation and abrogated by protein kinase A (PKA) inhibition. In fact, several PKA-binding sites were detected in γ-catenin by in silico analysis. Intriguingly γ-catenin KD led to increased β-catenin levels and transactivation. Thus, γ-catenin compensates for β-catenin loss at AJ without affecting desmosomes but is unable to fulfill functions in Wnt signaling. γ-Catenin stabilization after β-catenin loss is brought about by PKA. Catenin-sensing mechanism may depend on absolute β-catenin levels and not its activity. Anti-β-catenin therapies for HCC affecting total β-catenin may target aberrant Wnt signaling without negatively impacting intercellular adhesion, provided mechanisms leading to γ-catenin stabilization are spared.

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.

Plakophilin3 loss leads to an increase in PRL3 levels promoting K8 dephosphorylation, which is required for transformation and metastasis

The desmosome anchors keratin filaments in epithelial cells leading to the formation of a tissue wide IF network. Loss of the desmosomal plaque protein plakophilin3 (PKP3) in HCT116 cells, leads to an increase in neoplastic progression and metastasis, which was accompanied by an increase in K8 levels. The increase in levels was due to an increase in the protein levels of the Phosphatase of Regenerating Liver 3 (PRL3), which results in a decrease in phosphorylation on K8. The increase in PRL3 and K8 protein levels could be reversed by introduction of an shRNA resistant PKP3 cDNA. Inhibition of K8 expression in the PKP3 knockdown clone S10, led to a decrease in cell migration and lamellipodia formation. Further, the K8 PKP3 double knockdown clones showed a decrease in colony formation in soft agar and decreased tumorigenesis and metastasis in nude mice. These results suggest that a stabilisation of K8 filaments leading to an increase in migration and transformation may be one mechanism by which PKP3 loss leads to tumor progression and metastasis.

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.

Up-regulation of plakophilin-2 and Down-regulation of plakophilin-3 are correlated with invasiveness in bladder cancer

OBJECTIVE

To examine plakophilin proteins (Pkp) and 3 expression levels in bladder cancer, in particular their levels during cellular growth and invasion. Pkp is associated with the binding of cadherin to intermediate filaments of the cytoskeleton.

METHODS

The relative mRNA and protein expression levels of Pkp2 and 3 in bladder cancer cell lines were determined using quantitative real-time polymerase chain reaction and Western blot analyses. The cellular localization of Pkp2 and 3 proteins in bladder cancer cells was also assayed using immunohistochemistry. The proliferation and invasive activities of bladder cancer cells were evaluated using cell growth and in vitro cell invasion assays, and were compared with those of bladder cancer cells treated with Pkp2 and 3 small interfering RNAs.

RESULTS

Pkp2 mRNA and protein levels were elevated, and those of Pkp3 were reduced, in bladder cancer cells that are known to exhibit increased proliferation and invasive activity. Pkp2/3 protein expression was predominantly observed in the cytoplasm of invasive bladder cancer cells and tissues. Pkp2 knockdown inhibited, and Pkp3 knockdown enhanced, invasion of bladder cancer cells, but these knockdowns did not alter cell proliferation.

CONCLUSION

We conclude that high Pkp2, and low Pkp3, expression is associated with bladder cancer cell invasion and that neither Pkp2 nor Pkp3 is associated with cell proliferation. We further hypothesize that accumulation of Pkp2 and 3 in the cell cytoplasm, rather than their recruitment to the cell membrane, is related to an increased ability of the tumor to invade and metastasize.

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.

PKP2 mutations in sudden death from arrhythmogenic right ventricular cardiomyopathy (ARVC) and sudden unexpected death with negative autopsy (SUDNA)

BACKGROUND

Plakophilin2 (PKP2) is a desmosome-related protein with numerous armadillo repeats and has been linked to arrhythmogenic right ventricular cardiomyopathy (ARVC). Fatal arrhythmias resulting in sudden death also occur in the absence of morphologic cardiac abnormalities at autopsy, and have been linked to ion channel mutations in a subset of cases, but so far not to PKP2.

METHODS AND RESULTS

We sequenced all 14 exons of PKP2 in DNA extracted from postmortem heart tissues of 25 patients dying from ARVC and 25 from sudden unexpected death with negative autopsy (SUDNA). The primers were designed using the Primer Express 3.0 software. Direct sequencing for both sense and antisense strands was performed with a BigDye Terminator DNA sequencing kit on a 3130XL Genetic Analyzer. Mutation damage prediction was made using Mutation Taster, Polyphen and SIFT software. In 6 of the 25 ARVC samples, 6 PKP2 mutations were identified, 4 of which were likely significant, and 3 of which were novel (p.N641del, p.L64PfsX22, p.G269R). In 6 of the 25 cases of SUDNA samples, 6 PKP2 mutations were identified, 3 of which were likely significant, and 4 of which were not previously described (p.P665S, p.Y217TfsX45, p.E540, p.S615T).

CONCLUSIONS

PKP2 mutations are not specific for ARVC and may result in SUDNA. The link between ARVC and desmosomal mutations may not be causal but related to an association between defective desmosomal proteins and arrhythmias.

Expression of Plakophilins (PKP1, PKP2, and PKP3) in breast cancers

Plakophilins (PKP) are desmosomal plague proteins, which belong to the p120ctn subfamily of armadillo repeat containing proteins. We aimed to analyze the role of plakophilins in breast cancer and its clinical progress. We have performed immunohistochemical study of the PKP1,2,3 in breast carcinoma. The study included 108 patients with breast cancer and 26 age- and sex-matched healthy controls. We investigated the associations between staining intensity and some clinicopathologic features like tumor size, axillary node status, stage, lymphovascular invasion, perineural invasion, grade, hormone receptor status, and c-erb B2. The mean age of patients was 46 years (22-78). In breast cancer, compared with normal tissue, PKP1 and PKP2 expressions were indifferent (P > 0.05), but PKP3 expression was significantly increased in breast cancer (P = 0.0014). Although PKP1 and PKP2 expression levels were not correlated with clinicopathological parameters, increased PKP3 expression was positively correlated with node positivity and grade (P = 0.000, P = 0.000).

CONCLUSION

Overexpressed PKP3 is likely to be an essential contributor to a growth-promoting pathway and to aggressive features of breast cancer.

E-cadherin and plakoglobin recruit plakophilin3 to the cell border to initiate desmosome assembly

A decrease in the levels of the desmosomal plaque protein, plakophilin3 (PKP3), leads to a decrease in desmosome size and cell-cell adhesion. To test the hypothesis that PKP3 is required for desmosome formation, the recruitment of desmosomal components to the cell surface was studied in the PKP3 knockdown clones. The PKP3 knockdown clones showed decreased cell border staining for multiple desmosomal proteins, when compared to vector controls, and did not form desmosomes in a calcium switch assay. Further analysis demonstrated that PKP3, plakoglobin (PG) and E-cadherin are present at the cell border at low concentrations of calcium. Loss of either PG or E-cadherin led to a decrease in the levels of PKP3 and other desmosomal proteins at the cell border. The results reported here are consistent with the model that PG and E-cadherin recruit PKP3 to the cell border to initiate desmosome formation.

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.

Evaluation of plakophilin-3 mRNA as a biomarker for detection of circulating tumor cells in gastrointestinal cancer patients

BACKGROUND

This study aims to assess Plakophilin-3 (PKP3) as a surrogate biomarker of circulating tumor cells in patients with gastrointestinal cancer.

METHODS

The primary aim is to estimate the diagnostic accuracy of PKP3 real-time reverse transcriptase-PCR in blood. Receiver operating characteristic curves were constructed. Correlations between the blood PKP3 levels and the clinicopathologic features of the study subjects were analyzed. Logistic regression was used to predict outcomes based on PKP3.

RESULTS

Sixty-four patients with gastrointestinal cancer and 23 controls were included. The mean relative PKP3 mRNA expression was 48.45 in cancer patients and 2.8 in controls (P < 0.0001). Comparing the PKP3 levels in patients and controls, the area under the curve was 0.852 (95% confidence interval, 0.76-0.94; P < 0.0001) in receiver operating characteristic analysis. A higher blood level of PKP3 mRNA was associated with a more advanced stage (P = 0.025), pT(3-4) tumors (P = 0.028), metastasis (P = 0.021), and residual (R2) disease (P = 0.037). Higher PKP3 mRNA was associated with the risk of cancer progression and death (odds ratio, 3.875; 95% confidence interval, 1.781-8.430; P = 0.001).

CONCLUSIONS

Increased PKP3 mRNA was detected in the blood of gastrointestinal cancer patients. Significant correlations were found with advanced stage, pT(3-4), metastatic disease, and the residual disease status. PKP3 mRNA in blood was associated with the risk of cancer progression and death.

IMPACT

PKP3 mRNA can be used as a marker of subclinical disease in gastrointestinal cancer and thus holds potential clinical relevance as a predictor for disease outcome.

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.

Upregulation of plakophilin-2 and its acquisition to adherens junctions identifies a novel molecular ensemble of cell-cell-attachment characteristic for transformed mesenchymal cells

In contrast to the desmosome-containing epithelial and carcinoma cells, normal and malignantly transformed cells derived from mesenchymal tissues and tumors are connected only by adherens junctions (AJs) containing N-cadherins and/or cadherin-11, anchored in a cytoplasmic plaque assembled by alpha- and beta-catenin, plakoglobin, proteins p120 and p0071. Here, we report that the AJs of many malignantly transformed cell lines are characterized by the additional presence of plakophilin-2 (Pkp2), a protein hitherto known only as a major component of desmosomal plaques, i.e., AJs of epithelia and carcinomatous cells. This massive acquisition of Pkp2 and its integration into AJ plaques of a large number of transformed cell lines is demonstrated with biochemical and immunolocalization techniques. Upregulation of Pkp2 and its integration into AJs has also been noted in some soft tissue tumors insitu and some highly proliferative colonies of cultured mesenchymal stem cells. As Pkp2 has recently been identified as a functionally important major regulatory organizer in AJs and related junctions in epithelial cells and cardiomyocytes, we hypothesize that the integration of Pkp2 into AJs of "soft tissue tumor" cells also can serve functions in the upregulation of proliferation, the promotion of malignant growth in general as well as the close-packing of diverse kinds of cells and the metastatic behavior of such tumors. We propose to examine its presence in transformed mesenchymal cells and related tumors and to use it as an additional diagnostic criterion.

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.

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.

Plakophilin3 downregulation leads to a decrease in cell adhesion and promotes metastasis

Plakophilin3 is a desmosomal plaque protein whose levels are reduced in poorly differentiated tumors of the oropharyngeal cavity and in invasive colon carcinomas. To test the hypothesis that plakophilin3 loss stimulates neoplastic progression, plakophilin3 expression was inhibited by DNA vector driven RNA interference in 3 epithelial cell lines, HCT116, HaCaT and fetal buccal mucosa. The plakophilin3-knockdown clones showed a decrease in cell-cell adhesion as assessed in a hanging drop assay, which was accompanied by an increase in cell migration. The HCT116 plakophilin3-knockdown clones showed a decrease in desmosome size as revealed by electron microscopy. These altered desmosomal properties were accompanied by colony formation in soft agar and growth to high density in culture. The HCT116-derived clones showed accelerated tumor formation in nude mice and increased metastasis to the lung, a phenotype consistent with the increased migration observed in vitro and is consistent with data from human tumors that suggests that plakophililn3 is lost in invasive and metastatic tumors. These data indicate that plakophilin3 loss leads to a decrease in cell-cell adhesion leading to the stimulation of neoplastic progression and metastasis.

Bioinformatics approach to mRNA markers discovery for detection of circulating tumor cells in patients with gastrointestinal cancer

BACKGROUND

Detection of tumor cells in the blood, or minimal deposits in distant organs as bone marrow, could be important to identify cancer patients at high risk of relapse or disease progression. Quantitative polymerase chain reaction (PCR) amplification of tissue or tumor selective mRNA is the most powerful tool for the detection of this circulating or occult metastatic cells. Our study aims to identify novel gastrointestinal cancer-specific markers for circulating tumor cell detection.

METHOD

Phase I preclinical study was performed by means of computational tools for expression analysis. In silico data were used to identify and prioritize molecular markers highly expressed in gastrointestinal cancers but absent in hematopoietic-derived libraries. Selected genes were evaluated by means of qRT-PCR in gastrointestinal cancer and hematopoietic cell-lines, normal human bone marrows and bloods, tumor tissue, and blood from cancer patients.

RESULTS

Novel and known mRNA markers for circulating tumor cell detection in gastrointestinal cancer have been identified. Among all the genes assessed, PKP3, AGR2, S100A16, S100A6, LGALS4, and CLDN3 were selected and assays based on blood qRT-PCR were developed. Reliably qRT-PCR assays for the novel targets plakophilin 3 (PKP3) and anterior gradient-2 (AGR2) to identify blood-borne cells in cancer patients were developed.

CONCLUSIONS

Novel and known gastrointestinal-specific mRNA markers for circulating tumor cells have been identified through in silico analysis and validated in clinical material. qRT-PCR assay targeted to PKP3 and AGR2 mRNAs might be helpful to detect circulating tumor cells in patients with gastrointestinal cancer.

Differential expression pattern of protein ARVCF in nephron segments of human and mouse kidney

The protein ARVCF is a member of the p120 subfamily of armadillo proteins whose members have been described to occur in junction-bound and non-junction-bound forms. Studies on ARVCF were constrained because the endogenous protein was difficult to detect with the available reagents. We have generated novel monoclonal and polyclonal antibodies usable for biochemical and localization studies. By systematic immunohistochemical analysis of various tissues protein ARVCF is prominently detected in mouse, bovine and human kidney. Using antibodies against specific markers of nephron segments protein ARVCF is localized in proximal tubules according to double label immunofluorescence. Besides its occurrence in proximal tubules of adult kidney and in renal cell carcinoma derived from proximal tubules ARVCF is also detected in maturing nephrons in early mouse developmental stages such as, for example, 15 days of gestation (E15). Immunoblotting of total extracts of cultured cells of renal origin showed that ARVCF is detected in all human and murine cultured cells analyzed. Upon immunolocalization ARVCF is mostly detected in the cytoplasm occurring in a fine granular form. This prominent cytoplasmic localization of ARVCF in cultured cells and its occurrence in proximal tubules implies an involvement of ARVCF in specific functional processes of proximal tubules of kidney.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.