Elimination of epiplakin by gene targeting results in acceleration of keratinocyte migration in mice

Retinal regions shape human and murine Müller cell proteome profile and functionality

The human macula is a highly specialized retinal region with pit-like morphology and rich in cones. How Müller cells, the principal glial cell type in the retina, are adapted to this environment is still poorly understood. We compared proteomic data from cone- and rod-rich retinae from human and mice and identified different expression profiles of cone- and rod-associated Müller cells that converged on pathways representing extracellular matrix and cell adhesion. In particular, epiplakin (EPPK1), which is thought to play a role in intermediate filament organization, was highly expressed in macular Müller cells. Furthermore, EPPK1 knockout in a human Müller cell-derived cell line led to a decrease in traction forces as well as to changes in cell size, shape, and filopodia characteristics. We here identified EPPK1 as a central molecular player in the region-specific architecture of the human retina, which likely enables specific functions under the immense mechanical loads in vivo.

A Ca2+-Mediated Switch of Epiplakin from a Diffuse to Keratin-Bound State Affects Keratin Dynamics

Keratins exert important structural but also cytoprotective functions. They have to be adaptable to support cellular homeostasis. Epiplakin (EPPK1) has been shown to decorate keratin filaments in epithelial cells and to play a protective role under stress, but the mechanism is still unclear. Using live-cell imaging of epithelial cells expressing fluorescently tagged EPPK1 and keratin, we report here an unexpected dynamic behavior of EPPK1 upon stress. EPPK1 was diffusely distributed throughout the cytoplasm and not associated with keratin filaments in living cells under standard culture conditions. However, ER-, oxidative and UV-stress, as well as cell fixation, induced a rapid association of EPPK1 with keratin filaments. This re-localization of EPPK1 was reversible and dependent on the elevation of cytoplasmic Ca²⁺ levels. Moreover, keratin filament association of EPPK1 led to significantly reduced keratin dynamics. Thus, we propose that EPPK1 stabilizes the keratin network in stress conditions, which involve increased cytoplasmic Ca²⁺.

Elevated Expressions of BTN3A1 and RhoB in Psoriasis Vulgaris Lesions by an Immunohistochemical Study

Psoriasis is a chronic, immune-mediated inflammatory disease which pathogenesis is closely linked to γδ T cells. Recently, a critical role for butyrophilin 3A1 (BTN3A1) in mediating the activation of Vγ9Vδ2 T cells, which are reported to redistribute from blood to the perturbed skin lesions in psoriasis, has been proposed. Additional molecular partners, including RhoB and periplakin, have also been speculated to interact with BTN3A1 in modulating Vγ9Vδ2 T-cell activation. Immunohistochemical staining was performed to examine the expressions of BTN3A1, RhoB, and the plakin family members, including periplakin, epiplakin, and envoplakin in the psoriasis vulgaris lesions as compared with the normal control. The expressions of BTN3A1 and RhoB were found significantly upregulated in the psoriatic lesions. Besides, a downregulation of periplakin and an upregulation of epiplakin were noticed in the psoriasis vulgaris lesions. Our data suggest that BTN3A1 and RhoB might participate in the pathogenesis of psoriasis through Vγ9Vδ2 T-cell responses. In addition, a potential involvement of the plakin protein family, especially periplakin and epiplakin, in psoriasis pathology was proposed.

Comparative genomics reveals evolutionary loss of epiplakin in cetaceans

The adaptation of vertebrates to different environments was associated with changes in the molecular composition and regulation of epithelia. Whales and dolphins, together forming the clade cetaceans, have lost multiple epithelial keratins during or after their evolutionary transition from life on land to life in water. It is unknown whether the changes in keratins were accompanied by gain or loss of cytoskeletal adapter proteins of the plakin family. Here we investigated whether plakin proteins are conserved in cetaceans and other vertebrates. Comparative analysis of genome sequences showed conservation of dystonin, microtubule actin crosslinking factor 1 (MACF1), plectin, desmoplakin, periplakin and envoplakin in cetaceans. By contrast, EPPK1 (epiplakin) was disrupted by inactivating mutations in all cetaceans investigated. Orthologs of EPPK1 are present in bony and cartilaginous fishes and tetrapods, indicating an evolutionary origin of EPPK1 in a common ancestor of jawed vertebrates (Gnathostomes). In many vertebrates, EPPK1 is flanked by an as-yet uncharacterized gene that encodes protein domains homologous to the carboxy-terminal segment of MACF1. We conclude that epiplakin, unlike other plakins, was lost in cetaceans.

Serum Epiplakin Might Be a Potential Serodiagnostic Biomarker for Bladder Cancer

Tumor markers that can be detected at an early stage are needed. Here, we evaluated the epiplakin expression levels in sera from patients with bladder cancer (BC). Using a micro-dot blot array, we evaluated epiplakin expression levels in 60 patients with BC, 20 patients with stone disease, and 28 healthy volunteers. The area under the curve (AUC) and best cut-off point were calculated using receiver-operating characteristic (ROC) analysis. Serum epiplakin levels were significantly higher in patients with BC than in those with stone disease (p = 0.0013) and in healthy volunteers (p < 0.0001). The AUC-ROC level for BC was 0.78 (95% confidence interval (CI) = 0.69-0.87). Using a cut-off point of 873, epiplakin expression levels exhibited 68.3% sensitivity and 79.2% specificity for BC. However, the serum epiplakin levels did not significantly differ by sex, age, pathological stage and grade, or urine cytology. We performed immunohistochemical staining using the same antibody on another cohort of 127 patients who underwent radical cystectomy. Univariate and multivariate analysis results showed no significant differences between epiplakin expression, clinicopathological findings, and patient prognoses. Our results showed that serum epiplakin might be a potential serodiagnostic biomarker in patients with BC.

Molecular mechanism of intermediate filament recognition by plakin proteins

The plakin family of cytolinkers interacts with intermediate filaments (IFs) through plakin repeat domain (PRD) and linker modules. Recent structure/function studies have established the molecular basis of envoplakin-PRD and periplakin-linker interactions with vimentin. Both plakin modules share a broad basic groove which recognizes acidic rod elements on IFs, a mechanism that is applicable to other plakin family members. This review postulates a universal IF engagement mechanism that illuminates the specific effects of pathogenic mutations associated with diseases including arrhythmogenic right ventricular cardiomyopathy, and reveals how diverse plakin proteins offer tailored IF tethering to ensure stable, dynamic and regulated cellular structures.

Keratin intermediate filaments: intermediaries of epithelial cell migration

Migration of epithelial cells is fundamental to multiple developmental processes, epithelial tissue morphogenesis and maintenance, wound healing and metastasis. While migrating epithelial cells utilize the basic acto-myosin based machinery as do other non-epithelial cells, they are distinguished by their copious keratin intermediate filament (KF) cytoskeleton, which comprises differentially expressed members of two large multigene families and presents highly complex patterns of post-translational modification. We will discuss how the unique mechanophysical and biochemical properties conferred by the different keratin isotypes and their modifications serve as finely tunable modulators of epithelial cell migration. We will furthermore argue that KFs together with their associated desmosomal cell-cell junctions and hemidesmosomal cell-extracellular matrix (ECM) adhesions serve as important counterbalances to the contractile acto-myosin apparatus either allowing and optimizing directed cell migration or preventing it. The differential keratin expression in leaders and followers of collectively migrating epithelial cell sheets provides a compelling example of isotype-specific keratin functions. Taken together, we conclude that the expression levels and specific combination of keratins impinge on cell migration by conferring biomechanical properties on any given epithelial cell affecting cytoplasmic viscoelasticity and adhesion to neighboring cells and the ECM.

Tracing the destiny of mesenchymal stem cells from embryo to adult bone marrow and white adipose tissue via Pdgfrα expression

Mesenchymal stem cells (MSCs) are somatic stem cells that can be derived from adult bone marrow (BM) and white adipose tissue (WAT), and that display multipotency and self-renewal capacity. Although MSCs are essential for tissue formation and have already been used in clinical therapy, the origins and markers of these cells remain unknown. In this study, we first investigated the developmental process of MSCs in mouse embryos using the gene encoding platelet-derived growth factor receptor α (Pdgfra) as a marker. We then traced cells expressing Pdgfra and other genes (brachyury, Sox1 and Pmx1) in various mutant mouse embryos until the adult stage. This tracing of MSC origins and destinies indicates that embryonic MSCs emerge in waves and that almost all adult BM MSCs and WAT MSCs originate from mesoderm and embryonic Pdgfrα-positive cells. Furthermore, we demonstrate that adult Pdgfrα-positive cells are involved in some pathological conditions.

Integration of pan-cancer transcriptomics with RPPA proteomics reveals mechanisms of epithelial-mesenchymal transition

Integrating data from multiple regulatory layers across cancer types could elucidate additional mechanisms of oncogenesis. Using antibody-based protein profiling of 736 cancer cell lines, along with matching transcriptomic data, we show that pan-cancer bimodality in the amounts of mRNA, protein, and protein phosphorylation reveals mechanisms related to the epithelial-mesenchymal transition (EMT). Based on the bimodal expression of E-cadherin, we define an EMT signature consisting of 239 genes, many of which were not previously associated with EMT. By querying gene expression signatures collected from cancer cell lines after small-molecule perturbations, we identify enrichment for histone deacetylase (HDAC) inhibitors as inducers of EMT, and kinase inhibitors as mesenchymal-to-epithelial transition (MET) promoters. Causal modeling of protein-based signaling identifies putative drivers of EMT. In conclusion, integrative analysis of pan-cancer proteomic and transcriptomic data reveals key regulatory mechanisms of oncogenic transformation.

Profiling molecular and phenotypic characteristics of large collections of cancer cell lines can be used to identify distinct and common oncogenic pathways across cancer types. So far, most large-scale data obtained from cancer cell lines have been at the genomic, transcriptomic, and phenotypic levels. Recently, high-quality data at the level of cell signaling through protein abundances and phosphorylation sites has become available. By integrating this newly generated protein data with prior transcriptomic data, and by visualizing all cancer cell lines using dimensionality reduction techniques, pan-cancer cell lines are strikingly shown to organize into a gradient of epithelial to mesenchymal types. Interestingly, many of the measured proteins and transcripts display bimodality; the expression of genes, proteins, and protein phosphorylations is either high or low, strongly suggesting that they act as molecular switches. Focusing on further characterizing molecular switches of epithelial-mesenchymal transitions, we identify candidate regulators and small molecules that can induce or reverse such transition, as well as potential causal relationships between proteins. Since the mesenchymal state of tumors is known to be associated with metastasis and later-stage cancer development, better understanding the regulatory mechanisms of epithelial-to-mesenchymal transition can lead to improved targeted therapeutics.

Effects of epiplakin-knockdown in cultured corneal epithelial cells

BACKGROUND

To investigate effects of knockdown of epiplakin gene expression on the homeostasis of cultured corneal epithelial cell line. We previously reported acceleration of corneal epithelial wound healing in an epiplakin-null mouse.

METHODS

Gene expression of epiplakin was knockdowned by employing siRNA transfection in SV40-immortalized human corneal epithelial cell line. Protein expression of E-cadherin, keratin 6 and vimentin was examined by western blotting. Cell migration and proliferation were examined by using scratch assay and Alamar blue assay, respectively.

RESULTS

Scratch assay and Alamar blue assay showed migration and proliferation of the cells was accelerated by epiplakin knockdown. siRNA-knockdown of epiplakin suppressed protein expression of E-cadherin, keratin 6 and vimentin.

CONCLUSIONS

Decreased expression of E-cadherin, keratin 6 and vimentin might be included in the mechanisms of cell migration acceleration in the absence of epiplakin. The mechanism of cell proliferation stimulation by epiplakin knockdown is to be investigated.

Functional and Genetic Analysis of Epiplakin in Epithelial Cells

Epiplakin is a large member (>700 kDa) of the plakin protein family and exclusively expressed in epithelial cell types. Compared to other plakin proteins epiplakin exhibits an unusual structure as it consists entirely of a variable number of consecutive plakin repeat domains (13 in humans, 16 in mice). The only binding partners of epiplakin identified so far are keratins of simple as well as of stratified epithelia. Epiplakin-deficient mice show no obvious spontaneous phenotype. However, ex vivo studies using epiplakin-deficient primary cells indicated protective functions of epiplakin in response to stress. Recent studies using stress models for organs of the gastrointestinal tract revealed that epiplakin-deficient mice develop more pronounced pancreas and liver injuries than their wild-type littermates. In addition, impaired stress-induced keratin network reorganization was observed in the affected organs, and primary epiplakin-deficient hepatocytes showed reduced tolerance for forced keratin overexpression which could be rescued by a chemical chaperone. These findings indicate protective functions of epiplakin in chaperoning disease-induced keratin reorganization. In this review, we describe some of the methods we used to analyze epiplakin's function with the focus on biochemical and ex vivo techniques.

Epiplakin attenuates experimental mouse liver injury by chaperoning keratin reorganization

BACKGROUND & AIMS

Epiplakin is a member of the plakin protein family and exclusively expressed in epithelial tissues where it binds to keratins. Epiplakin-deficient (Eppk1(-/-)) mice displayed no obvious spontaneous phenotype, but their keratinocytes showed a faster keratin network breakdown in response to stress. The role of epiplakin in the stressed liver remained to be elucidated.

METHODS

Wild-type (WT) and Eppk1(-/-) mice were subjected to common bile duct ligation (CBDL) or fed with a 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-containing diet. The importance of epiplakin during keratin reorganization was assessed in primary hepatocytes.

RESULTS

Our experiments revealed that epiplakin is expressed in hepatocytes and cholangiocytes, and binds to keratin 8 (K8) and K18 via multiple domains. In several liver stress models epiplakin and K8 genes displayed identical expression patterns and transgenic K8 overexpression resulted in elevated hepatic epiplakin levels. After CBDL and DDC treatment, Eppk1(-/-) mice developed a more pronounced liver injury and their livers contained larger amounts of hepatocellular keratin granules, indicating impaired disease-induced keratin network reorganization. In line with these findings, primary Eppk1(-/-) hepatocytes showed increased formation of keratin aggregates after treatment with the phosphatase inhibitor okadaic acid, a phenotype which was rescued by the chemical chaperone trimethylamine N-oxide (TMAO). Finally, transfection experiments revealed that Eppk1(-/-) primary hepatocytes were less able to tolerate forced K8 overexpression and that TMAO treatment rescued this phenotype.

CONCLUSION

Our data indicate that epiplakin plays a protective role during experimental liver injuries by chaperoning disease-induced keratin reorganization.

Epiplakin deficiency aggravates murine caerulein-induced acute pancreatitis and favors the formation of acinar keratin granules

Epiplakin, a member of the plakin protein family, is exclusively expressed in epithelial tissues and was shown to bind to keratins. Epiplakin-deficient (EPPK^−/−) mice showed no obvious spontaneous phenotype, however, EPPK^−/− keratinocytes displayed faster keratin network breakdown in response to stress. The role of epiplakin in pancreas, a tissue with abundant keratin expression, was not yet known. We analyzed epiplakin’s expression in healthy and inflamed pancreatic tissue and compared wild-type and EPPK^−/− mice during caerulein-induced acute pancreatitis. We found that epiplakin was expressed primarily in ductal cells of the pancreas and colocalized with apicolateral keratin bundles in murine pancreatic acinar cells. Epiplakin’s diffuse subcellular localization in keratin filament-free acini of K8-deficient mice indicated that its filament-associated localization in acinar cells completely depends on its binding partner keratin. During acute pancreatitis, epiplakin was upregulated in acinar cells and its redistribution closely paralleled keratin reorganization. EPPK^−/− mice suffered from aggravated pancreatitis but showed no obvious regeneration phenotype. At the most severe stage of the disease, EPPK^−/− acinar cells displayed more keratin aggregates than those of wild-type mice. Our data propose epiplakin to be a protective protein during acute pancreatitis, and that its loss causes impaired disease-associated keratin reorganization.

Plakins, a versatile family of cytolinkers: roles in skin integrity and in human diseases

The plakin family consists of giant proteins involved in the cross-linking and organization of the cytoskeleton and adhesion complexes. They further modulate several fundamental biological processes, such as cell adhesion, migration, and polarization or signaling pathways. Inherited and acquired defects of plakins in humans and in animal models potentially lead to dramatic manifestations in the skin, striated muscles, and/or nervous system. These observations unequivocally demonstrate the key role of plakins in the maintenance of tissue integrity. Here we review the characteristics of the mammalian plakin members BPAG1 (bullous pemphigoid antigen 1), desmoplakin, plectin, envoplakin, epiplakin, MACF1 (microtubule-actin cross-linking factor 1), and periplakin, highlighting their role in skin homeostasis and diseases.

Networking galore: intermediate filaments and cell migration

Intermediate filaments (IFs) are assembled from a diverse group of evolutionarily conserved proteins and are specified in a tissue-dependent, cell type-dependent, and context-dependent fashion in the body. IFs are involved in multiple cellular processes that are crucial for the maintenance of cell and tissue integrity and the response and adaptation to various stresses, as conveyed by the broad array of crippling clinical disorders caused by inherited mutations in IF coding sequences. Accordingly, the expression, assembly, and organization of IFs are tightly regulated. Migration is a fitting example of a cell-based phenomenon in which IFs participate as both effectors and regulators. With a particular focus on vimentin and keratin, we here review how the contributions of IFs to the cell's mechanical properties, to cytoarchitecture and adhesion, and to regulatory pathways collectively exert a significant impact on cell migration.

De novo microduplications at 1q41, 2q37.3, and 8q24.3 in patients with VATER/VACTERL association

The acronym VATER/VACTERL association describes the combination of at least three of the following congenital anomalies: vertebral defects (V), anorectal malformations (A), cardiac defects (C), tracheoesophageal fistula with or without esophageal atresia (TE), renal malformations (R), and limb defects (L). We aimed to identify highly penetrant de novo copy number variations (CNVs) that contribute to VATER/VACTERL association. Array-based molecular karyotyping was performed in a cohort of 41 patients with VATER/VACTERL association and 6 patients with VATER/VACTERL-like phenotype including all of the patients' parents. Three de novo CNVs were identified involving chromosomal regions 1q41, 2q37.3, and 8q24.3 comprising one (SPATA17), two (CAPN10, GPR35), and three (EPPK1, PLEC, PARP10) genes, respectively. Pre-existing data from the literature prompted us to choose GPR35 and EPPK1 for mouse expression studies. Based on these studies, we prioritized GPR35 for sequencing analysis in an extended cohort of 192 patients with VATER/VACTERL association and VATER/VACTERL-like phenotype. Although no disease-causing mutation was identified, our mouse expression studies suggest GPR35 to be involved in the development of the VATER/VACTERL phenotype. Follow-up of GPR35 and the other genes comprising the identified duplications is warranted.

Epiplakin modifies the motility of the HeLa cells and accumulates at the outer surfaces of 3-D cell clusters

Elimination of epiplakin (EPPK) by gene targeting in mice results in acceleration of keratinocyte migration during wound healing, suggesting that epithelial cellular EPPK may be important for the regulation of cellular motility. To study the function of EPPK, we developed EPPK knock-down (KD) and EPPK-overexpressing HeLa cells and analyzed cellular phenotypes and motility by fluorescence/differential interference contrast time-lapse microscopy and immunolocalization of actin and vimentin. Cellular motility of EPPK-KD cells was significantly elevated, but that of EPPK-overexpressing cells was obviously depressed. Many spike-like projections were observed on EPPK-KD cells, with fewer such structures on overexpressing cells. By contrast, in EPPK-KD cells, expression of E-cadherin was unchanged but vimentin fibers were thinner and sparser than in controls, and they were more concentrated at the peri-nucleus, as observed in migrating keratinocytes at wound edges in EPPK(-/-) mice. In Matrigel 3-D cultures, EPPK co-localized on the outer surface of cell clusters with zonula occludens-1 (ZO-1), a marker of tight junctions. Our results suggest that EPPK is associated with the machinery for cellular motility and contributes to tissue architecture via the rearrangement of intermediate filaments.

The expanding significance of keratin intermediate filaments in normal and diseased epithelia

Intermediate filaments are assembled from a diverse group of evolutionary conserved proteins and are specified in a tissue-dependent, cell type-dependent, and context-dependent fashion in the body. Genetic mutations in intermediate filament proteins account for a large number of diseases, ranging from skin fragility conditions to cardiomyopathies and premature aging. Keratins, the epithelial-specific intermediate filaments, are now recognized as multi-faceted effectors in their native context. In this review, we emphasize the recent progress made in defining the role of keratins towards the regulation of cytoarchitecture, cell growth and proliferation, apoptosis, and cell motility during embryonic development, in normal adult tissues, and in select diseases such as cancer.

A wound-induced keratin inhibits Src activity during keratinocyte migration and tissue repair

Keratin 6 negatively regulates Src kinase activity and the migratory potential of skin keratinocytes during wound repair.

Injury to the epidermis triggers an elaborate homeostatic response resulting in tissue repair and recovery of the vital barrier function. The type II keratins 6a and 6b (K6a and K6b) are among the genes induced early on in wound-proximal keratinocytes and maintained during reepithelialization. Paradoxically, genetic ablation of K6a and K6b results in enhanced keratinocyte migration. In this paper, we show that this trait results from activation of Src kinase and key Src substrates that promote cell migration. Endogenous Src physically associated with keratin proteins in keratinocytes in a K6-dependent fashion. Purified Src bound K6-containing filaments via its SH2 domain in a novel phosphorylation-independent manner, resulting in kinase inhibition. K6 protein was enriched in the detergent-resistant membrane (DRM), a key site of Src inhibition, and DRMs from K6-null keratinocytes were depleted of both keratin and Src. We conclude that K6 negatively regulates Src kinase activity and the migratory potential of skin keratinocytes during wound repair. Our findings may also be important in related contexts such as cancer.

Transient expression of mouse pro-α3(V) collagen gene (Col5a3) in wound healing

The α3(V) chain is poorly characterized among type V collagen chains. Pro-α3(V) collagen is expressed in newly synthesized bone as well as in the superficial fascia of developing muscle. Present study examined the expression in a mouse model of wound healing. Real-time reverse transcriptase polymerase chain reaction and in situ hybridization revealed transient expression of pro-α3(V) chain at a lower level than other fibrillar collagen genes after injury. Immunohistochemistry showed a similar expression pattern in the injured skin. In addition, electron microscopy showed that pro-α3(V) chain was localized in the amorphous nonfibrillar region, but not in fine or dense fibrils. The pro-α3(V) chain co-localized with heparan sulfate, which appeared in the skin after injury and might bind via an acidic segment of the pro-α3(V) chain. The matrix containing the pro-α3(V) chain may therefore be needed for the initiation of wound healing.

Collagen IV significantly enhances migration and transplantation of embryonic stem cells: involvement of α2β1 integrin-mediated actin remodeling

Embryonic stem (ES) cell transplantation represents a potential means for the treatment of degenerative diseases and injuries. As appropriate distribution of transplanted ES cells in the host tissue is critical for successful transplantation, the exploration of efficient strategies to enhance ES cell migration is warranted. In this study we investigated ES cell migration under the influence of various extracellular matrix (ECM) proteins, which have been shown to stimulate cell migration in various cell models with unclear effects on ES cells. Using two mouse ES (mES) cell lines, ESC 26GJ9012-8-2 and ES-D3 GL, to generate embryoid bodies (EBs), we examined the migration of differentiating cells from EBs that were delivered onto culture surfaces coated with or without collagen I, collagen IV, Matrigel, fibronectin, and laminin. Among these ECM proteins, collagen IV exhibited maximal migration enhancing effect. mES cells expressed α2 and β1 integrin subunits and the migration enhancing effect of collagen IV was prevented by RGD peptides as well as antibodies against α2 and β1 integrins, indicating that the enhancing effect of collagen IV on cell migration was mediated by α2β1 integrin. Furthermore, staining of actin cytoskeleton that links to integrins revealed well-developed stress fibers and long filopodia in mES cells cultured on collagen IV, and the actin-disrupting cytochalasin D abolished the collagen IV-enhanced cell migration. In addition, pretreatment of undifferentiated or differentiated mES cells with collagen IV resulted in improved engraftment and growth after transplantation into the subcutaneous tissue of nude mice. Finally, collagen IV pretreatment of osteogenically differentiated mES cells increased osteogenic differentiation-like tissue and decreased undifferentiation-like tissue in the grafts grown after transplantation. Our results demonstrated that collagen IV significantly enhanced the migration of differentiating ES cells through α2β1 integrin-mediated actin remodeling and could promote ES cell transplantation efficiency, which may be imperative to stem cell therapy.

Epiplakin accelerates the lateral organization of keratin filaments during wound healing

BACKGROUND

Epiplakin (EPPK) belongs to the plakin family of cytolinker proteins and, resembling other members of the plakin family such as BPAG1 (an autoantigen of bullous pemphigoid) and plectin, EPPK has plakin repeat domains (PRDs) that bind to intermediate filaments. Elimination of EPPK by gene targeting in mice resulted in the acceleration of keratinocyte migration during wound healing. EPPK is expressed in proliferating keratinocytes at wound edges and, in view of its putative function in binding to keratin, we postulated that the keratin network in EPPK-null (EPPK(-/-)) mice might be disrupted during wound healing.

OBJECTIVE

To examine this hypothesis and to determine the precise localization of EPPK in relation to keratin filaments, we compared the non-wounded and wounded epidermis of wild-type and EPPK(-/-) mice.

METHODS

Non-wounded epidermis and wounded epidermis from wild-type and EPPK(-/-) mice were examined by immunofluorescence staining and electron microscopy before and after double immunostaining.

RESULTS

EPPK was colocalized with keratin 17 (K17) more extensively than with other keratins examined in wounded epidermis. The expression of K5, K10, K6, and K17 was the same in EPPK(-/-) mice after wounding as in normal mice, but diameters of keratin filaments were reduced in EPPK(-/-) keratinocytes. Electron microscopy after immunostaining revealed that EPPK colocalized with K5, K10 and K6 after wounding in wild-type mice.

CONCLUSION

Our data indicate that EPPK accelerates keratin bundling in proliferating keratinocytes during wound healing and suggest that EPPK might contribute to reinforcement of keratin networks under mechanical stress.

Potential risk factors associated with the use of cidofovir to treat benign human papillomavirus-related disease

BACKGROUND

Cidofovir is currently being used off-licence to treat different viral infections, such as benign low-risk human papillomavirus (HPV)-related recurrent respiratory papillomatosis (RRP). There are concerns over the safety of this practice as rat studies demonstrated a high malignant transformation rate. As yet, there are no clinical reports of cidofovir-induced malignant changes in humans.

METHODS

Telomerase immortalised human keratinocytes (hTert) stably expressing E6 proteins from either low-risk HPV6b or high-risk HPV16 and vector control cells were treated with either low-dose (5 microg/ml) or higher dose (30 microg/ml) cidofovir for 2 days and the effects evaluated by clonogenic survival assays. Based on these results, gene expression microarray analysis was performed on cidofovir-treated low-risk E6 and vector cells before, during and after drug treatment, and the results verified by real-time PCR.

RESULTS

Both low-risk and high-risk E6-expressing cells show significantly improved long-term survival compared with vector control cells when exposed to 5 microg/ml cidofovir for 2 days, (hTert T6E6 P=0.0007, hTert T16E6 P=0.00023 and hTert vector control P=0.62). Microarray and real-time PCR analyses of low-dose cidofovir-treated low-risk E6-expressing cells revealed changes in gene expression that are known to be associated with malignant progression, which were not observed in drug-treated vector control cells.

CONCLUSIONS

This is the first report that cidofovir can both increase cell survival and induce alterations in gene expression that are known to be associated with malignant transformation in cells transduced only with the E6 gene from low-risk HPV. It is our belief that these data provide cause for concern over the off-license use of this drug to treat RRP.

Expression patterns of epiplakin1 in pancreas, pancreatic cancer and regenerating pancreas

Epiplakin1 (Eppk1) is a plakin family gene with its function remains largely unknown, although the plakin genes are known to function in interconnecting cytoskeletal filaments and anchoring them at plasma membrane-associated adhesive junction. Here we analyzed the expression patterns of Eppk1 in the developing and adult pancreas in the mice. In the embryonic pancreas, Eppk1+/Pdx1+ and Eppk1+/Sox9+ pancreatic progenitor cells were observed in early pancreatic epithelium. Since Pdx1 expression overlapped with that of Sox9 at this stage, these multipotent progenitor cells are Eppk1+/Pdx1+/Sox9+ cells. Then Eppk1 expression becomes confined to Ngn3+ or Sox9+ endocrine progenitor cells, and p48+ exocrine progenitor cells, and then restricted to the duct cells and a cells at birth. In the adult pancreas, Eppk1 is expressed in centroacinar cells (CACs) and in duct cells. Eppk1 is observed in pancreatic intraepithelial neoplasia (PanIN), previously identified as pancreatic ductal adenocarcinoma (PDAC) precursor lesions. In addition, the expansion of Eppk1-positive cells occurs in a caerulein-induced acute pancreatitis, an acinar cell regeneration model. Furthermore, in the partial pancreatectomy (Px) regeneration model using mice, Eppk1 is expressed in "ducts in foci", a tubular structure transiently induced. These results suggest that Eppk1 serves as a useful marker for detecting pancreatic progenitor cells in developing and regenerating pancreas.

Stress-induced recruitment of epiplakin to keratin networks increases their resistance to hyperphosphorylation-induced disruption

Epiplakin is a large (>725 kDa) cytoskeletal protein exclusively expressed in epithelial tissues. It has a unique structure, consisting entirely of plakin repeat domains (PRDs), one of the hallmarks of spectraplakin protein family members. Previous studies, including the phenotypic analyses of knockout mice, failed to reveal the biological function of epiplakin. Using in vitro binding assays, we show here that all but one of the 16 PRDs of mouse epiplakin bind to keratins of basal keratinocytes. Nevertheless, in primary keratinocyte cell cultures, epiplakin only partially colocalized with keratin intermediate filament networks. However, upon application of cellular stress in the form of keratin hyperphosphorylation, osmotic shock or UV irradiation, the entire cytoplasmic epiplakin pool became associated with keratin. In response to such types of stress, epiplakin initially translocated to the still-intact keratin filament network and remained associated with keratin after its disruption and transformation into granular aggregates. Time-course experiments revealed that serine/threonine (okadaic acid) and tyrosine (orthovanadate) phosphatase inhibitor-induced filament disruption in differentiated keratinocytes proceeded faster in epiplakin-deficient cells compared with wild-type cells. Our data suggest that epiplakin plays a role in keratin filament reorganization in response to stress, probably by protecting keratin filaments against disruption in a chaperone-like fashion.

Intermediate filament assembly: dynamics to disease

Intermediate filament (IF) proteins belong to a large and diverse gene family with broad representation in vertebrate tissues. Although considered the 'toughest' cytoskeletal fibers, studies in cultured cells have revealed that IF can be surprisingly dynamic and highly regulated. This review examines the diversity of IF assembly behaviors, and considers the ideas that IF proteins are co- or post-translationally assembled into oligomeric precursors, which can be delivered to different subcellular compartments by microtubules or actomyosin and associated motor proteins. Their interaction with other cellular elements via IF associated proteins (IFAPs) affects IF dynamics and also results in cellular networks with properties that transcend those of individual components. We end by discussing how mutations leading to defects in IF assembly, network formation or IF-IFAP association compromise in vivo functions of IF as protectors against environmental stress.

Ultraviolet radiation stimulates expression of Snail family transcription factors in keratinocytes

The related zinc finger transcription factors Slug and Snail modulate epithelial mesenchymal transformation (EMT), the conversion of sessile epithelial cells into migratory fibroblast-like cells. EMT occurs during development, wound healing, and tumor progression. Growth factors, acting through mitogen-activated protein kinase (MAPK) cascades, regulate expression of Slug and Snail. Expression of Snail family transcription factors appears to be elevated in UVR-induced murine squamous cell carcinomas (SCC). We report here that ultraviolet radiation (UVR), which activates MAPK cascades, also stimulates Snail and Slug expression in epidermal keratinocytes. UVR exposure transiently elevated Slug and Snail mRNA expression in human keratinocytes in vitro and mouse epidermis in vivo. This induction was mediated, at least in part, through the ERK and p38 MAPK cascades, as pharmacological inhibition of these cascades partially or completely blocked Slug and Snail induction by UVR. On the other hand, UVR induction of Slug and Snail was enhanced by inhibition of JNK. Slug appears to play a functional role in the acute response of keratinocytes to UVR, as UVR induction of keratin 6 in the epidermis of Slug knockout mice was markedly delayed compared to wild-type mice. Slug and Snail are known to regulate molecules important in the cytoskeleton, intercellular adhesion, cell motility, and apoptosis, thus it seems probable that transiently or persistently elevated expression of these factors fosters the progression of UVR-induced SCC.

Plakins in development and disease

Plakins are large multi-domain molecules that have various functions to link cytoskeletal elements together and to connect them to junctional complexes. Plakins were first identified in epithelial cells where they were found to connect the intermediate filaments to desmosomes and hemidesmosomes [Ruhrberg, C., and Watt, F.M. (1997). The plakin family: versatile organizers of cytoskeletal architecture. Curr Opin Genet Dev 7, 392-397.]. They were subsequently found to be important for the integrity of muscle cells. Most recently, they have been found in the nervous system, where their functions appear to be more complex, including cross-linking of microtubules (MTs) and actin filaments [Leung, C.L., Zheng, M., Prater, S.M., and Liem, R.K. (2001). The BPAG1 locus: Alternative splicing produces multiple isoforms with distinct cytoskeletal linker domains, including predominant isoforms in neurons and muscles. J Cell Biol 154, 691-697., Leung, C.L., Sun, D., Zheng, M., Knowles, D.R., and Liem, R.K. (1999). Microtubule actin cross-linking factor (MACF): a hybrid of dystonin and dystrophin that can interact with the actin and microtubule cytoskeletons. J Cell Biol 147, 1275-1286.]. These plakins have also indicated their relationship to the spectrin superfamily of proteins and the plakins appear to be evolutionarily related to the spectrins, but have diverged to perform different specialized functions. In invertebrates, a single plakin is present in both Drosophila melanogaster and Caenorhabditis elegans, which resemble the more complex plakins found in mammals [Roper, K., Gregory, S.L., and Brown, N.H. (2002). The 'spectraplakins': cytoskeletal giants with characteristics of both spectrin and plakin families. J Cell Sci 115, 4215-4225.]. In contrast, there are seven plakins found in mammals and most of them have alternatively spliced forms leading to a very complex group of proteins with potential tissue specific functions [Jefferson, J.J., Leung, C.L., and Liem, R.K. (2004). Plakins: goliaths that link cell junctions and the cytoskeleton. Nat Rev Mol Cell Biol 5, 542-553.]. In this review, we will first describe the plakins, desmoplakin, plectin, envoplakin and periplakin and then describe two other mammalian plakins, Bullous pemphigoid antigen 1 (BPAG1) and microtubule actin cross-linking factor 1 (MACF1), that are expressed in multiple isoforms in different tissues. We will also describe the relationship of these two proteins to the invertebrate plakins, shortstop (shot) in Drosophila and VAB-10 in C. elegans. Finally, we will describe an unusual mammalian plakin, called epiplakin.

Interactions between epiplakin and intermediate filaments

Epiplakin, a cytoskeletal linker protein, was originally identified as an autoantigen in a serum specimen obtained from a patient with subepidermal blistering disease. To examine the binding ability of epiplakin with intermediate filaments (IF), we performed slot-blot assays using fusion proteins that included various domains and subdomains of epiplakin. At least two of the 4.6 copies in the B domains of epiplakin were necessary for the binding of fusion proteins to keratin. The repeated structures of linker domains also played an important role in the binding of epiplakin to keratin in these assays while also increasing the repeated structure in the linker domain of epiplakin which is involved in the increased binding to IF. A similar but weaker binding to vimentin and desmin was also detected. These observations indicated that the highly repeated structures of epiplakin in both the B and the linker domains, which is the unique feature of this molecule in the plakin family, play an essential role in the functioning of this molecule.