Envoplakin, a novel precursor of the cornified envelope that has homology to desmoplakin

Liquid biopsy: an examination of platelet RNA obtained from head and neck squamous cell carcinoma patients for predictive molecular tumor markers

Aim

Recently, a tumor cell-platelet interaction was identified in different tumor entities, resulting in a transfer of tumor-derived RNA into platelets, named further "tumor-educated platelets (TEP)". The present pilot study aims to investigate whether such a tumor-platelet transfer of RNA occurs also in patients suffering from head and neck squamous cell carcinoma (HNSCC).

Methods

Sequencing analysis of RNA derived from platelets of tumor patients (TPs) and healthy donors (HDs) were performed. Subsequently, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was used for verification of differentially expressed genes in platelets from TPs and HDs in a second cohort of patients and HDs. Data were analyzed by applying bioinformatic tools.

Results

Sequencing of RNA derived from the tumor as well as from platelets of TPs and HDs revealed 426 significantly differentially existing RNA, at which 406 RNA were more and 20 RNA less abundant in platelets from TPs in comparison to that of HDs. In TPs' platelets, abundantly existing RNA coding for 49 genes were detected, characteristically expressed in epithelial cells and RNA, the products of which are involved in tumor progression. Applying bioinformatic tools and verification on a second TP/HD cohort, collagen type I alpha 1 chain (COL1A1) and zinc finger protein 750 (ZNF750) were identified as the strongest potentially platelet-RNA-sequencing (RNA-seq)-based biomarkers for HNSCC.

Conclusions

These results indicate a transfer of tumor-derived messenger RNA (mRNA) into platelets of HNSCC patients. Therefore, analyses of a patient's platelet RNA could be an efficient option for liquid biopsy in order to diagnose HNSCC or to monitor tumorigenesis as well as therapeutic responses at any time and in real time.

Proteomic Characterization of Urinary Extracellular Vesicles from Kidney-Transplanted Patients Treated with Calcineurin Inhibitors

Use of immunosuppressive drugs is still unavoidable in kidney-transplanted patients. Since their discovery, calcineurin inhibitors (CNI) have been considered the first-line immunosuppressive agents, in spite of their known nephrotoxicity. Chronic CNI toxicity (CNIT) may lead to kidney fibrosis, a threatening scenario for graft survival. However, there is still controversy regarding CNIT diagnosis, monitoring and therapeutic management, and their specific effects at the molecular level are not fully known. Aiming to better characterize CNIT patients, in the present study, we collected urine from kidney-transplanted patients treated with CNI who (i) had a normal kidney function, (ii) suffered CNIT, or (iii) presented interstitial fibrosis and tubular atrophy (IFTA). Urinary extracellular vesicles (uEV) were enriched and the proteome was analyzed to get insight into changes happening during CNI. Members of the uroplakin and plakin families were significantly upregulated in the CNIT group, suggesting an important role in CNIT processes. Although biomarkers cannot be asserted from this single pilot study, our results evidence the potential of uEV as a source of non-invasive protein biomarkers for a better detection and monitoring of this renal alteration in kidney-transplanted patients.

Keratins and the plakin family cytolinker proteins control the length of epithelial microridge protrusions

Actin filaments and microtubules create diverse cellular protrusions, but intermediate filaments, the strongest and most stable cytoskeletal elements, are not known to directly participate in the formation of protrusions. Here we show that keratin intermediate filaments directly regulate the morphogenesis of microridges, elongated protrusions arranged in elaborate maze-like patterns on the surface of mucosal epithelial cells. We found that microridges on zebrafish skin cells contained both actin and keratin filaments. Keratin filaments stabilized microridges, and overexpressing keratins lengthened them. Envoplakin and periplakin, plakin family cytolinkers that bind F-actin and keratins, localized to microridges, and were required for their morphogenesis. Strikingly, plakin protein levels directly dictate microridge length. An actin-binding domain of periplakin was required to initiate microridge morphogenesis, whereas periplakin-keratin binding was required to elongate microridges. These findings separate microridge morphogenesis into distinct steps, expand our understanding of intermediate filament functions, and identify microridges as protrusions that integrate actin and intermediate filaments.

Mammalian Plakins, Giant Cytolinkers: Versatile Biological Functions and Roles in Cancer

Cancer is a highly lethal disease that is characterized by aberrant cell proliferation, migration, and adhesion, which are closely related to the dynamic changes of cytoskeletons and cytoskeletal-adhesion. These will further result in cell invasion and metastasis. Plakins are a family of giant cytolinkers that connect cytoskeletal elements with each other and to junctional complexes. With various isoforms composed of different domain structures, mammalian plakins are broadly expressed in numerous tissues. They play critical roles in many cellular processes, including cell proliferation, migration, adhesion, and signaling transduction. As these cellular processes are key steps in cancer development, mammalian plakins have in recent years attracted more and more attention for their potential roles in cancer. Current evidence shows the importance of mammalian plakins in various human cancers and demonstrates mammalian plakins as potential biomarkers for cancer. Here, we introduce the basic characteristics of mammalian plakins, review the recent advances in understanding their biological functions, and highlight their roles in human cancers, based on studies performed by us and others. This will provide researchers with a comprehensive understanding of mammalian plakins, new insights into the development of cancer, and novel targets for cancer diagnosis and therapy.

Microtubule-Actin Crosslinking Factor 1 and Plakins as Therapeutic Drug Targets

Plakins are a family of seven cytoskeletal cross-linker proteins (microtubule-actin crosslinking factor 1 (MACF), bullous pemphigoid antigen (BPAG1) desmoplakin, envoplakin, periplakin, plectin, epiplakin) that network the three major filaments that comprise the cytoskeleton. Plakins have been found to be involved in disorders and diseases of the skin, heart, nervous system, and cancer that are attributed to autoimmune responses and genetic alterations of these macromolecules. Despite their role and involvement across a spectrum of several diseases, there are no current drugs or pharmacological agents that specifically target the members of this protein family. On the contrary, microtubules have traditionally been targeted by microtubule inhibiting agents, used for the treatment of diseases such as cancer, in spite of the deleterious toxicities associated with their clinical utility. The Research Collaboratory for Structural Bioinformatics (RCSB) was used here to identify therapeutic drugs targeting the plakin proteins, particularly the spectraplakins MACF1 and BPAG1, which contain microtubule-binding domains. RCSB analysis revealed that plakin proteins had 329 ligands, of which more than 50% were MACF1 and BPAG1 ligands and 10 were documented, clinically or experimentally, to have several therapeutic applications as anticancer, anti-inflammatory, and antibiotic agents.

[EXPRESSION OF PLAKIN FAMILY IN UROTHELIAL CARCINOMA OF THE UPPER URINARY TRACT]

(Objective) To determine whether the plakin family proteins periplakin, desmoplakin, plectin, and envoplakin could be markers of urothelial carcinoma of the upper urinary tract. (Materials and methods) Fifty-seven surgical specimens were obtained from patients with urothelial carcinoma of the upper urinary tract, who were admitted to the Jikei University Hospital between April 2000 and December 2005. The expression of plakin family proteins in cancerous and normal tissues was investigated using immunohistochemistry, and its association with clinicopathological parameters was analyzed. (Results) The expression of periplakin, envoplakin, and desmoplakin was significantly lower in cancerous tissue than in normal urothelium (P < 0.0001, P < 0.0001, and P < 0.0001, respectively). Strong desmoplakin expression in cancerous tissue was significantly associated with poor cancer-specific survival and overall survival (P = 0.023 and P = 0.034, respectively, compared with cancerous tissue with slight or less desmoplakin expression). Furthermore, strong plectin expression was significantly associated with poor metastasis-free survival (P = 0.034, compared with cancerous tissue with slight or less plectin expression). (Conclusion) Plakin family, particularly desmoplakin was suggested to be a prognostic marker of urothelial carcinoma of the upper urinary tract.

Sequence-specific 1H, 13C and 15N backbone resonance assignments of the plakin repeat domain of human envoplakin

The plakin repeat domain is a distinctive hallmark of the plakin superfamily of proteins, which are found within all epithelial tissues. Plakin repeat domains mediate the interactions of these proteins with the cell cytoskeleton and are critical for the maintenance of tissue integrity. Despite their biological importance, no solution state resonance assignments are available for any homologue. Here we report the essentially complete (1)H, (13)C and (15)N backbone chemical shift assignments of the singular 22 kDa plakin repeat domain of human envoplakin, providing the means to investigate its interactions with ligands including intermediate filaments.

The polarity protein Scrib mediates epidermal development and exerts a tumor suppressive function during skin carcinogenesis

BACKGROUND

The establishment and maintenance of polarity is vital for embryonic development and loss of polarity is a frequent characteristic of epithelial cancers, however the underlying molecular mechanisms remain unclear. Here, we identify a novel role for the polarity protein Scrib as a mediator of epidermal permeability barrier acquisition, skeletal morphogenesis, and as a potent tumor suppressor in cutaneous carcinogenesis.

METHODS

To explore the role of Scrib during epidermal development, we compared the permeability of toluidine blue dye in wild-type, Scrib heterozygous and Scrib KO embryonic epidermis at E16.5, E17.5 and E18.5. Mouse embryos were stained with alcian blue and alizarin red for skeletal analysis. To establish whether Scrib plays a tumor suppressive role during skin tumorigenesis and/or progression, we evaluated an autochthonous mouse model of skin carcinogenesis in the context of Scrib loss. We utilised Cre-LoxP technology to conditionally deplete Scrib in adult epidermis, since Scrib KO embryos are neonatal lethal.

RESULTS

We establish that Scrib perturbs keratinocyte maturation during embryonic development, causing impaired epidermal barrier formation, and that Scrib is required for skeletal morphogenesis in mice. Analysis of conditional transgenic mice deficient for Scrib specifically within the epidermis revealed no skin pathologies, indicating that Scrib is dispensable for normal adult epidermal homeostasis. Nevertheless, bi-allelic loss of Scrib significantly enhanced tumor multiplicity and progression in an autochthonous model of epidermal carcinogenesis in vivo, demonstrating Scrib is an epidermal tumor suppressor. Mechanistically, we show that apoptosis is the critical effector of Scrib tumor suppressor activity during skin carcinogenesis and provide new insight into the function of polarity proteins during DNA damage repair.

CONCLUSIONS

For the first time, we provide genetic evidence of a unique link between skin carcinogenesis and loss of the epithelial polarity regulator Scrib, emphasizing that Scrib exerts a wide-spread tumor suppressive function in epithelia.

Functional Analysis of Periplakin and Envoplakin, Cytoskeletal Linkers, and Cornified Envelope Precursor Proteins

Envoplakin and periplakin are the two smallest plakin family cytoskeletal linker proteins that connect intermediate filaments to cellular junctions and other membrane locations. These two plakins have a structural role in the assembly of the cornified envelope (CE), the terminal stage of epidermal differentiation. Analysis of gene-targeted mice lacking both these plakins and the third initial CE scaffold protein, involucrin, demonstrate the importance of the structural integrity of CE for a proper epidermal barrier function. It has emerged that periplakin, which also has a wider tissue distribution than envoplakin, has additional, independent roles. Periplakin participates in the cytoskeletal organization also in other tissues and interacts with a wide range of membrane-associated proteins such as kazrin and butyrophilin BTN3A1. This review covers methods used to understand periplakin and envoplakin functions in cell culture models, including siRNA ablation of periplakin expression and the use of tagged protein domain constructs to study localization and interactions. In addition, assays that can be used to analyze CEs and epidermal barrier function in gene-targeted mice are described and discussed.

Oxidative stress in aging human skin

Oxidative stress in skin plays a major role in the aging process. This is true for intrinsic aging and even more for extrinsic aging. Although the results are quite different in dermis and epidermis, extrinsic aging is driven to a large extent by oxidative stress caused by UV irradiation. In this review the overall effects of oxidative stress are discussed as well as the sources of ROS including the mitochondrial ETC, peroxisomal and ER localized proteins, the Fenton reaction, and such enzymes as cyclooxygenases, lipoxygenases, xanthine oxidases, and NADPH oxidases. Furthermore, the defense mechanisms against oxidative stress ranging from enzymes like superoxide dismutases, catalases, peroxiredoxins, and GSH peroxidases to organic compounds such as L-ascorbate, α-tocopherol, beta-carotene, uric acid, CoQ10, and glutathione are described in more detail. In addition the oxidative stress induced modifications caused to proteins, lipids and DNA are discussed. Finally age-related changes of the skin are also a topic of this review. They include a disruption of the epidermal calcium gradient in old skin with an accompanying change in the composition of the cornified envelope. This modified cornified envelope also leads to an altered anti-oxidative capacity and a reduced barrier function of the epidermis.

Epidermal cell junctions and their regulation by p63 in health and disease

As the outermost tissue of the body, the epidermis is the first physical barrier for any pressure, stress or trauma. Several specialized cell-matrix and cell-cell adhesion structures, together with an intracellular network of dedicated intermediate filaments, are required to confer critical resilience to mechanical stress. The transcription factor p63 is a master regulator of gene expression in the epidermis and in other stratified epithelia. It has been extensively demonstrated that p63 positively controls a large number of tissue-specific genes, including those encoding a large fraction of tissue-restricted cell adhesion molecules. Consistent with p63 functions in cell adhesion and in epidermal differentiation, heterozygous mutations clustered mainly in the p63 C-terminus are causative of AEC syndrome, an autosomal dominant disorder characterized by cleft palate, ankyloblepharon and ectodermal dysplasia associated with severe skin erosions, bleeding and infections. The molecular basis of skin erosions in AEC patients is not fully understood, although defects in desmosomes and in other cell junctions are likely to be involved. Here, we provide an extensive review of the different epidermal cell junctions that cooperate to withstand mechanical stress and on the mechanisms by which p63 regulates gene expression of their components in healthy skin and in AEC syndrome. Collectively, advancement in understanding the molecular mechanisms by which epidermal cell junctions precisely exert their functions and how p63 orchestrates their coordinated expression, will ultimately lead to insight into developing future strategies for the treatment of AEC syndrome and more in generally for diseases that share an overlapping phenotype.

Extracellularly Extruded Syntaxin-4 Is a Potent Cornification Regulator of Epidermal Keratinocytes

In the skin epidermis, keratinocytes undergo anchorage-dependent cornification, which gives rise to stratified multilayers, each with a distinct differentiation feature. The active formation of the cornified cell envelope (CCE), an important element in the skin barrier, occurs in keratinocytes of the upper epidermal layers and impacts their terminal differentiation. In the present study, we identified the extracellularly extruded syntaxin-4 as a potent differentiation regulator of epidermal keratinocytes. We found that differentiation stimuli led to the acceleration of syntaxin-4 exposure at the keratinocyte cell surface and that the artificial control of extracellular syntaxin-4, either by the forced expression of several syntaxin-4 mutants with structural alterations at the putative functional core site (AIEPQK), or by using antagonistic circular peptides containing this core sequence, dramatically influenced the CCE formation, with spatial misexpression of TGase1 and involucrin. We also found that the topical application of a peptide that exerted the most prominent antagonistic activity for syntaxin-4, named ST4n1, evidently prevented the formation of the hyperplastic and hyperkeratotic epidermis generated by physical irritation in HR-1 mice skin. Collectively, these results demonstrate that extracellularly extruded syntaxin-4 is a potent regulator of CCE differentiation, and that ST4n1 has potential as a clinically applicable reagent for keratotic skin lesions.

Two Antarctic penguin genomes reveal insights into their evolutionary history and molecular changes related to the Antarctic environment

BACKGROUND

Penguins are flightless aquatic birds widely distributed in the Southern Hemisphere. The distinctive morphological and physiological features of penguins allow them to live an aquatic life, and some of them have successfully adapted to the hostile environments in Antarctica. To study the phylogenetic and population history of penguins and the molecular basis of their adaptations to Antarctica, we sequenced the genomes of the two Antarctic dwelling penguin species, the Adélie penguin [Pygoscelis adeliae] and emperor penguin [Aptenodytes forsteri].

RESULTS

Phylogenetic dating suggests that early penguins arose ~60 million years ago, coinciding with a period of global warming. Analysis of effective population sizes reveals that the two penguin species experienced population expansions from ~1 million years ago to ~100 thousand years ago, but responded differently to the climatic cooling of the last glacial period. Comparative genomic analyses with other available avian genomes identified molecular changes in genes related to epidermal structure, phototransduction, lipid metabolism, and forelimb morphology.

CONCLUSIONS

Our sequencing and initial analyses of the first two penguin genomes provide insights into the timing of penguin origin, fluctuations in effective population sizes of the two penguin species over the past 10 million years, and the potential associations between these biological patterns and global climate change. The molecular changes compared with other avian genomes reflect both shared and diverse adaptations of the two penguin species to the Antarctic environment.

Skin aging, gene expression and calcium

The human epidermis provides a very effective barrier function against chemical, physical and microbial insults from the environment. This is only possible as the epidermis renews itself constantly. Stem cells located at the basal lamina which forms the dermoepidermal junction provide an almost inexhaustible source of keratinocytes which differentiate and die during their journey to the surface where they are shed off as scales. Despite the continuous renewal of the epidermis it nevertheless succumbs to aging as the turnover rate of the keratinocytes is slowing down dramatically. Aging is associated with such hallmarks as thinning of the epidermis, elastosis, loss of melanocytes associated with an increased paleness and lucency of the skin and a decreased barrier function. As the differentiation of keratinocytes is strictly calcium dependent, calcium also plays an important role in the aging epidermis. Just recently it was shown that the epidermal calcium gradient in the skin that facilitates the proliferation of keratinocytes in the stratum basale and enables differentiation in the stratum granulosum is lost in the process of skin aging. In the course of this review we try to explain how this calcium gradient is built up on the one hand and is lost during aging on the other hand. How this disturbed calcium homeostasis is affecting the gene expression in aged skin and is leading to dramatic changes in the composition of the cornified envelope will also be discussed. This loss of the epidermal calcium gradient is not only specific for skin aging but can also be found in skin diseases such as Darier disease, Hailey-Hailey disease, psoriasis and atopic dermatitis, which might be very helpful to get a deeper insight in skin aging.

Development of oral epithelial cell line ROE2 with differentiation potential from transgenic rats harboring temperature-sensitive simian virus40 large T-antigen gene

We have developed an immortalized oral epithelial cell line, ROE2, from fetal transgenic rats harboring temperature-sensitive simian virus 40 large T-antigen gene. The cells grew continuously at either a permissive temperature of 33°C or an intermediate temperature of 37°C. At the nonpermissive temperature of 39°C, on the other hand, growth decreased significantly, and the Sub-G1 phase of the cell cycle increased, indicating that the cells undergo apoptosis at a nonpermissive temperature. Histological and immunocytochemical analyses revealed that ROE2 cells at 37°C had a stratified epithelial-like morphology and expressed cytokeratins Krt4 and Krt13, marker proteins for oral nonkeratinized epithelial cells. Global-scale comprehensive microarray analysis, coupled with bioinformatics tools, demonstrated a significant gene network that was obtained from the upregulated genes. The gene network contained 16 genes, including Cdkn1a, Fos, Krt13, and Prdm1, and was associated mainly with the biological process of skin development in the category of biological functions, organ development. These four genes were validated by quantitative real-time polymerase chain reaction, and the results were nearly consistent with the microarray data. It is therefore anticipated that this cell line will be useful as an in vitro model for studies such as physiological functions, as well as for gene expression in oral epithelial cells.

Epidermal barrier defects link atopic dermatitis with altered skin cancer susceptibility

Atopic dermatitis can result from loss of structural proteins in the outermost epidermal layers, leading to a defective epidermal barrier. To test whether this influences tumour formation, we chemically induced tumours in EPI−/− mice, which lack three barrier proteins—Envoplakin, Periplakin, and Involucrin. EPI−/− mice were highly resistant to developing benign tumours when treated with 7,12-dimethylbenz(a)anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA). The DMBA response was normal, but EPI−/− skin exhibited an exaggerated atopic response to TPA, characterised by abnormal epidermal differentiation, a complex immune infiltrate and elevated serum thymic stromal lymphopoietin (TSLP). The exacerbated TPA response could be normalised by blocking TSLP or the immunoreceptor NKG2D but not CD4+ T cells. We conclude that atopy is protective against skin cancer in our experimental model and that the mechanism involves keratinocytes communicating with cells of the immune system via signalling elements that normally protect against environmental assaults.

DOI:http://dx.doi.org/10.7554/eLife.01888.001

Skin cancer is a common and growing problem—according to the World Health Organization, skin cancers account for one in every three cancers diagnosed world wide. There is some evidence from epidemiological studies that patients with certain allergies might be protected against cancer and, in particular, that the allergic skin condition atopic dermatitis is associated with reduced levels of various skin cancers. However, it is difficult to know if this reduction is due to the atopic dermatitis itself or to the drugs used to treat this allergy.

Genetically engineered mice that are lacking three proteins that are involved in the formation of the cornified envelope—the protective layer that replaces the normal plasma membrane in the cells of the outermost skin layers—can be used to study atopic dermatitis. These ‘triple knockout mice’ have a defective epidermal barrier and altered levels of immune T-cells in the skin.

Now Cipolat et al. have investigated whether defects in the epidermal barrier protect against skin cancer. Knockout mice and wild-type mice were treated with two chemicals: DMBA, which causes mutations in a gene called HRas, and TPA, which promotes the formation of tumours from cells that contain HRas mutations. After about 16 weeks almost all of the wild-type mice had at least one benign tumour, whereas half of the knockout mice had no tumours. Overall, the average number of benign tumours per mouse was six times higher in the wild-type mice. This shows that the mutations that cause the epidermal barrier defects in knockout mice also protect them against the tumours caused by the combined effects of DMBA and TPA.

Cipolat et al. then compared how the mice responded to DMBA or TPA alone. The knockout mice and the wild-type mice responded to DMBA in the same way; however, the knockout mice showed an exaggerated response to TPA, including a strong inflammatory reaction. This response comprised the production of higher levels of various proteins that are involved in communications between skin cells and the immune system. Cipolat et al. propose that the immune reaction caused by this exaggerated response could help to prevent tumour formation by eliminating tumour-forming cells in the skin.

DOI:http://dx.doi.org/10.7554/eLife.01888.002

Epidermal barriers

The epidermis functions as a physical barrier to the external environment and works to prevent loss of water from the skin. Numerous factors have been implicated in the formation of epidermal barriers, such as cornified envelopes, corneocytes, lipids, junctional proteins, proteases, protease inhibitors, antimicrobial peptides, and transcription factors. This review illustrates human diseases (ichthyoses) and animal models in which the epidermal barrier is disrupted or dysfunctional at steady state owing to ablation of one or more of the above factors. These diseases and animal models help us to understand the complicated mechanisms of epidermal barrier formation and give further insights on epidermal development.

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.

Detection of autoantibodies to periplakin and envoplakin in paraneoplastic pemphigus but not idiopathic pulmonary fibrosis using full-length recombinant proteins

BACKGROUND

Paraneoplastic pemphigus (PNP) serum preferentially reacts with periplakin and envoplakin, which are plakin family proteins localized to desmosomes and intermediate filaments. Recently, anti-periplakin antibodies were also detected in patients with idiopathic pulmonary fibrosis (IPF). Although previous epitope-mapping studies showed multiple epitopes in each protein, enzyme-linked immunosorbent assays have used several truncated, but not full-length, recombinant proteins.

METHODS

This study aimed to produce full-length biotinylated recombinant proteins of periplakin and envoplakin for detection of autoantibodies by immunoprecipitation and ELISA. Serum from a PNP patient who had been confirmed as carrying anti-periplakin and anti-envoplakin antibodies in our previous study was used as a positive control. Sera from 15 patients with IPF were analyzed for both antibodies by immunoprecipitation and by ELISA.

RESULTS

The PNP serum reacted strongly with the full-length recombinant proteins in immunoprecipitation and ELISA. Longitudinal serum samples from the PNP patient showed a clear decline of autoantibodies to both periplakin and envoplakin. None of the IPF sera showed both autoantibodies.

CONCLUSIONS

We found that the detection of anti-periplakin and anti-envoplakin antibodies using full-length recombinant proteins is useful immunoprecipitation and ELISA.

Hinged plakin domains provide specialized degrees of articulation in envoplakin, periplakin and desmoplakin

Envoplakin, periplakin and desmoplakin are cytoskeletal proteins that provide structural integrity within the skin and heart by resisting shear forces. Here we reveal the nature of unique hinges within their plakin domains that provides divergent degrees of flexibility between rigid long and short arms composed of spectrin repeats. The range of mobility of the two arms about the hinge is revealed by applying the ensemble optimization method to small-angle X-ray scattering data. Envoplakin and periplakin adopt 'L' shaped conformations exhibiting a 'helicopter propeller'-like mobility about the hinge. By contrast desmoplakin exhibits essentially unrestricted mobility by 'jack-knifing' about the hinge. Thus the diversity of molecular jointing that can occur about plakin hinges includes 'L' shaped bends, 'U' turns and fully extended 'I' orientations between rigid blocks of spectrin repeats. This establishes specialised hinges in plakin domains as a key source of flexibility that may allow sweeping of cellular spaces during assembly of cellular structures and could impart adaptability, so preventing irreversible damage to desmosomes and the cell cytoskeleton upon exposure to mechanical stress.

Cholestasis induces reversible accumulation of periplakin in mouse liver

BACKGROUND

Periplakin (PPL) is a rod-shaped cytolinker protein thought to connect cellular adhesion junctional complexes to cytoskeletal filaments. PPL serves as a structural component of the cornified envelope in the skin and interacts with various types of proteins in cultured cells; its level decreases dramatically during tumorigenic progression in human epithelial tissues. Despite these intriguing observations, the physiological roles of PPL, especially in non-cutaneous tissues, are still largely unknown. Because we observed a marked fluctuation of PPL expression in mouse liver in association with the bile acid receptor farnesoid X receptor (FXR) and cholestasis, we sought to characterize the role of PPL in the liver and determine its contributions to the etiology and pathogenesis of cholestasis.

METHODS

Time- and context-dependent expression of PPL in various mouse models of hepatic and renal disorders were examined by immunohistochemistry, western blotting, and quantitative real-time polymerase chain reactions.

RESULTS

The hepatic expression of PPL was significantly decreased in Fxr-/- mice. In contrast, the expression was dramatically increased during cholestasis, with massive PPL accumulation observed at the boundaries of hepatocytes in wild-type mice. Interestingly, the hepatic accumulation of PPL resulting from cholestasis was reversible. In addition, similar accumulation of PPL at cellular boundaries was found in epithelial cells around renal tubules upon ureteral obstruction.

CONCLUSIONS

PPL may be involved in the temporal accommodation to fluid stasis in different tissues. Further examination of the roles for PPL may lead to the discovery of a novel mechanism for cellular protection by cytolinkers that is applicable to many tissues and in many contexts.

Anti-α-2-macroglobulin-like-1 autoantibodies are detected frequently and may be pathogenic in paraneoplastic pemphigus

Paraneoplastic pemphigus (PNP) shows autoantibodies mainly to plakin and desmosomal cadherin family proteins. We have recently identified alpha-2-macroglobulin-like-1 (A2ML1), a broad range protease inhibitor, as a unique PNP antigen. In this study, we tested a large number of PNP sera by various methods. Forty (69.0%) of 58 PNP sera recognized A2ML1 recombinant protein expressed in COS7 cells by immunofluorescence (IF) and/or immunoprecipitation (IP)/immunoblotting (IB). IP/IB showed higher sensitivity than IF. In addition, 22 (37.9%) PNP sera reacted with A2ML1 by IB of cultured normal human keratinocytes (NHKs) under non-reducing conditions. Statistical analyses using various clinical and immunological data showed that the presence of anti-A2ML1 autoantibodies was associated with early disease onset and absence of ocular lesions. Next, to investigate the pathogenic role of anti-A2ML1 antibody, we performed additional functional studies. Addition of anti-A2ML1 polyclonal antibody to culture media decreased NHK cell adhesion examined by dissociation assay, and increased plasmin activity detected by casein zymography, suggesting that anti-A2ML1 antibody may decrease NHK cell adhesion through plasmin activation by inhibition of A2ML1. This study demonstrates that autoantibodies to A2ML1 are frequently and specifically detected and may have a pathogenic role in PNP.

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.

Spectraplakins: master orchestrators of cytoskeletal dynamics

The dynamics of different cytoskeletal networks are coordinated to bring about many fundamental cellular processes, from neuronal pathfinding to cell division. Increasing evidence points to the importance of spectraplakins in integrating cytoskeletal networks. Spectraplakins are evolutionarily conserved giant cytoskeletal cross-linkers, which belong to the spectrin superfamily. Their genes consist of multiple promoters and many exons, yielding a vast array of differential splice forms with distinct functions. Spectraplakins are also unique in their ability to associate with all three elements of the cytoskeleton: F-actin, microtubules, and intermediate filaments. Recent studies have begun to unveil their role in a wide range of processes, from cell migration to tissue integrity.

Characterization of rainbow trout gonad, brain and gill deep cDNA repertoires using a Roche 454-Titanium sequencing approach

Rainbow trout, Oncorhynchus mykiss, is an important aquaculture species worldwide and, in addition to being of commercial interest, it is also a research model organism of considerable scientific importance. Because of the lack of a whole genome sequence in that species, transcriptomic analyses of this species have often been hindered. Using next-generation sequencing (NGS) technologies, we sought to fill these informational gaps. Here, using Roche 454-Titanium technology, we provide new tissue-specific cDNA repertoires from several rainbow trout tissues. Non-normalized cDNA libraries were constructed from testis, ovary, brain and gill rainbow trout tissue samples, and these different libraries were sequenced in 10 separate half-runs of 454-Titanium. Overall, we produced a total of 3million quality sequences with an average size of 328bp, representing more than 1Gb of expressed sequence information. These sequences have been combined with all publicly available rainbow trout sequences, resulting in a total of 242,187 clusters of putative transcript groups and 22,373 singletons. To identify the predominantly expressed genes in different tissues of interest, we developed a Digital Differential Display (DDD) approach. This approach allowed us to characterize the genes that are predominantly expressed within each tissue of interest. Of these genes, some were already known to be tissue-specific, thereby validating our approach. Many others, however, were novel candidates, demonstrating the usefulness of our strategy and of such tissue-specific resources. This new sequence information, acquired using NGS 454-Titanium technology, deeply enriched our current knowledge of the expressed genes in rainbow trout through the identification of an increased number of tissue-specific sequences. This identification allowed a precise cDNA tissue repertoire to be characterized in several important rainbow trout tissues. The rainbow trout contig browser can be accessed at the following publicly available web site (http://www.sigenae.org/).

The biology of the desmosome-like junction a versatile anchoring junction and signal transducer in the seminiferous epithelium

Mammalian spermatogenesis, a complex process that involves the movement of developing germ cells across the seminiferous epithelium, entails extensive restructuring of Sertoli-Sertoli and Sertoli-germ cell junctions. Presently, it is not entirely clear how zygotene spermatocytes gain entry into the adluminal compartment of the seminiferous epithelium, which is sealed off from the systemic circulation by the Sertoli cell component of the blood-testis barrier, without compromising barrier integrity. To begin to address this question, it is critical that we first have a good understanding of the biology and the regulation of different types of Sertoli-Sertoli and Sertoli-germ cell junctions in the testis. Supported by recent studies in the field, we discuss how crosstalk between different types of junctions contributes to their restructuring during germ cell movement across the blood-testis barrier. We place special emphasis on the emerging role of desmosome-like junctions as signal transducers during germ cell movement across the seminiferous epithelium.

Plectin gene defects lead to various forms of epidermolysis bullosa simplex

Plectin is an important organizer of the keratin filament cytoskeleton in basal keratinocytes. It is essential for anchoring these filaments to the extracellular matrix via hemidesmosomal integrins. Loss of plectin or incorrect function of the protein due to mutations in its gene can lead to various forms of the skin blistering disease, epidermolysis bullosa simplex. Severity and subtype of the disease is dependent on the specific mutation and can be associated with (late-onset) muscular dystrophy or pyloric atresia. Mouse models mimicking the human phenotypes allow detailed study of plectin function.

Ancient origin of the gene encoding involucrin, a precursor of the cross-linked envelope of epidermis and related epithelia

The cross-linked (cornified) envelope is a characteristic product of terminal differentiation in the keratinocyte of the epidermis and related epithelia. This envelope contains many proteins of which involucrin was the first to be discovered and shown to become cross-linked by a cellular transglutaminase. Involucrin has evolved greatly in placental mammals, but retains the glutamine repeats that make it a good substrate for the transglutaminase. Until recently, it has been impossible to detect involucrin outside the placental mammals, but analysis of the GenBank and Ensembl databases that have become available since 2006 reveals the existence of involucrin in marsupials and birds. We describe here the properties of these involucrins and the ancient history of their evolution.

Mice deficient in involucrin, envoplakin, and periplakin have a defective epidermal barrier

The cornified envelope is assembled from transglutaminase cross-linked proteins and lipids in the outermost epidermal layers and is essential for skin barrier function. Involucrin, envoplakin, and periplakin form the protein scaffold on which the envelope assembles. To examine their combined function, we generated mice deficient in all three genes. The triple knockouts have delayed embryonic barrier formation and postnatal hyperkeratosis (abnormal accumulation of cornified cells) resulting from impaired desquamation. Cornified envelopes form but are ultrastructurally abnormal, with reduced lipid content and decreased mechanical integrity. Expression of proteases is reduced and the protease inhibitor, serpina1b, is highly upregulated, resulting in defective filaggrin processing and delayed degradation of desmoglein 1 and corneodesmosin. There is infiltration of CD4⁺ T cells and a reduction in resident γδ⁺ T cells, reminiscent of atopic dermatitis. Thus, combined loss of the cornified envelope proteins not only impairs the epidermal barrier, but also changes the composition of T cell subpopulations in the skin.

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.

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.

Proteomic analysis of the autoantibody response following immunization with a single autoantigen

In most autoimmune diseases, the autoantibody response is directed against several antigens of the target organ whose identification is crucial for understanding the physiopathological process. Thus, technologies allowing a characterization of the whole autoantibody pattern of both human and experimental autoimmune diseases are required. Here we have used immunoproteomic analysis of human epidermal extracts to characterize the diversity of the anti-desmosome antibody response induced in normal mice immunized with desmoglein 1, the major autoantigen of pemphigus foliaceus, an autoimmune blistering skin disease. In particular, this analysis enables us to characterize the binding properties of anti-desmosome mAbs derived from these mice and to show that the autoantibody response induced upon immunization with a single autoantigen targets different epidermal autoantigens with a pattern similar to that observed in certain variety of human pemphigus.

Epitopes in the linker subdomain region of envoplakin recognized by autoantibodies in paraneoplastic pemphigus patients

Sera from paraneoplastic pemphigus (PNP) immunoprecipitate multiple antigens from human epidermal protein extract. In this study, we further characterized the autoantibodies in 12 PNP sera. Immunoblotting using recombinant linker subdomains of envoplakin, periplakin, desmoplakin, and bullous pemphigoid antigen I found that 11 of the 12 sera recognized linker subdomains of envoplakin and periplakin. We then synthesized 12 peptides covering the linker subdomain of envoplakin for ELISA. One of the peptides, peptide no. 8, was recognized by nine out of the 12 sera with a higher affinity. A method of ligand-receptor binding assay was designed and performed using this peptide labeled with fluorescence as the ligand. Peptide no. 8 bound to CD20+ cells in Castleman's tumors from the patients whose sera were positive to this peptide by ELISA. Our data suggest that the linker subdomain of plakin proteins may be one of the major areas recognized by PNP autoantibodies, and epitopes in the linker subdomain of envoplakin recognized by PNP autoantibodies with a high affinity are dispersed in several areas and are variable among PNP patients. We also demonstrate that B-lymphocyte clones specifically reacting to epidermal proteins exist in Castleman's tumors from PNP.

Inherited disorders of desmosomes

Desmosomes are highly organized intercellular junctions that provide mechanical integrity to tissues by anchoring intermediate filaments to sites of strong adhesion. These cell-cell adhesion junctions are found in skin, heart, lymph nodes and meninges. Over the last 8 years, several naturally occurring human gene mutations in structural components of desmosomes have been reported. These comprise autosomal dominant or recessive mutations in plakophilin 1, plakophilin 2, desmoplakin, plakoglobin, desmoglein 1, desmoglein 4 and corneodesmosin. These discoveries have often highlighted novel or unusual phenotypes, including abnormal skin fragility and differentiation, and developmental anomalies of various ectodermal appendages, especially hair. Some desmosomal gene mutations may also result in cardiac disease, notably cardiomyopathy. This article describes the spectrum of clinical features that may be found in the inherited disorders of desmosomes and highlights the key functions of several of the desmosomal proteins in tissue adhesion and cell biology.

Targeted deletion of the sciellin gene resulted in normal development and maturation

Sciellin, together with other precursor proteins, was cross-linked by transglutaminase 1 to form the cornified envelope, an essential component of the physical barrier of the epidermis and stratified squamous epithelia. To more fully understand the function of sciellin in cornified envelope formation, we generated sciellin null mice. The mice appeared normal in their development and maturation and there were no structural features that distinguished them from littermate controls. Isolated cornified envelopes appeared normal in structure and were not more fragile to mechanical stress. There was no evidence of decreased barrier function or altered expression of other cornified envelope components. Transgenic mice expressing the repeat domain appeared to have a normal phenotype, like the null, and did not alter endogenous sciellin expression. We conclude that sciellin null mice had no structural anomalies and the transgenic mice did not act as a dominant-negative mutation.

Human hair abnormalities resulting from inherited desmosome gene mutations

Over the last eight years, several naturally occurring human gene mutations in structural components of desmosomes, cell-cell adhesion junctions found in skin, heart and meninges, have been reported. These comprise dominant or recessive mutations in plakophilin 1, plakophilin 2, desmoplakin, desmoglein 1, desmoglein 4, plakoglobin and corneodesmosin. Of note, as well as compromising tissue integrity, many of the resulting phenotypes have been associated with visible changes in hair. This article describes the particular hair abnormalities resulting from these desmosome gene mutations. Collectively, the data demonstrate the surprising effects inherited desmosome gene/protein pathology may have on hair growth and development. Further analysis of these and other desmosome genes is likely to resolve more hair disease mysteries and provides several further intriguing new discoveries in years to come.

Differential recognition of response elements determines target gene specificity for p53 and p63

p63 is a member of the p53 tumor suppressor gene family, which regulates downstream target gene expression by binding to sequence-specific response elements similar to those of p53. By using oligonucleotide expression microarray analysis and analyzing the promoters of p63-induced genes, we have identified novel p63-specific response elements (p63-REs) in the promoter regions of EVPL and SMARCD3. These p63-REs exhibit characteristic differences from the canonical p53-RE (RRRCWWGYYY) in both the core-binding element (CWWG) as well as the RRR and/or YYY stretches. Luciferase assays on mutagenized promoter constructs followed by electromobility shift analysis showed that p53 preferentially activates and binds to the RRRCATGYYY sequence, whereas p63 preferentially activates RRRCGTGYYY. Whereas EVPL protein is highly expressed in epithelial cells of the skin and pharynx in the p63+/+ mouse, it is undetectable in these tissues in the p63-/- mouse. Our results indicate that p63 can regulate expression of specific target genes such as those involved in skin, limb, and craniofacial development by preferentially activating distinct p63-specific response elements.

Epidermal differentiation: the role of proteases and their inhibitors

Dermatological diseases range from minor cosmetic problems to life-threatening conditions, as seen in some severe disorders of keratinization and cornification. These disorders are commonly due to abnormal epidermal differentiation processes, which result in disturbed barrier function of human skin. Elucidation of the cellular differentiation programs that regulate the formation and homeostasis of the epidermis is therefore of great importance for the understanding and therapy of these disorders. Much of the barrier function of human epidermis against the environment is provided by the cornified cell envelope (CE), which is assembled by transglutaminase (TGase)-mediated cross-linking of several structural proteins and lipids during the terminal stages of normal keratinocyte differentiation. The major constituents of the stratum corneum and the current knowledge on the formation of the stratum corneum will be briefly reviewed here. The discovery of mutations that underlie several human diseases caused by genetic defects in the protein or lipid components of the CE, and recent analyses of mouse mutants with defects in the structural components of the CE, catalyzing enzymes, and lipid processing, have highlighted their essential function in establishing the epidermal barrier. In addition, recent findings have provided evidence that a disturbed protease-antiprotease balance could cause faulty differentiation processes in the epidermis and hair follicle. The importance of regulated proteolysis in epithelia is well demonstrated by the recent identification of the SPINK5 serine proteinase inhibitor as the defective gene in Netherton syndrome, cathepsin C mutations in Papillon-Lefevre syndrome, cathepsin L deficiency infurless mice, targeted ablation of the serine protease Matriptase/MTSP1, targeted ablation of the aspartate protease cathepsin D, and the phenotype of targeted epidermal overexpression of stratum corneum chymotryptic enzyme in mice. Notably, our recent findings on the role of cystatin M/E and legumain as a functional dyad in skin and hair follicle cornification, a paradigm example of the regulatory functions exerted by epidermal proteases, will be discussed.

Mammalian transglutaminases. Identification of substrates as a key to physiological function and physiopathological relevance

Transglutaminases form a large family of intracellular and extracellular enzymes that catalyse the Ca2+-dependent post-translational modification of proteins. Despite significant advances in our understanding of the biological role of most mammalian transglutaminase isoforms, recent findings suggest new scenarios, most notably for the ubiquitous tissue transglutaminase. It is becoming apparent that some transglutaminases, normally expressed at low levels in many tissue types, are activated and/or overexpressed in a variety of diseases, thereby resulting in enhanced concentrations of cross-linked proteins. As applies to all enzymes that exert their metabolic function by modifying the properties of target proteins, the identification and characterization of the modified proteins will cast light on the functions of transglutaminases and their involvement in human diseases. In this paper we review data on the properties of mammalian transglutaminases, particularly as regards their protein substrates and the relevance of transglutaminase-catalysed reactions in physiological and disease conditions.

Characterization of human epiplakin: RNAi-mediated epiplakin depletion leads to the disruption of keratin and vimentin IF networks

Epiplakin is a member of the plakin family with multiple copies of the plakin repeat domain (PRD). We studied the subcellular distribution and interactions of human epiplakin by immunostaining, overlay assays and RNAi knockdown. Epiplakin decorated the keratin intermediate filaments (IF) network and partially that of vimentin. In the binding assays, the repeat unit (PRD plus linker) showed strong binding and preferentially associated with assembled IF over keratin monomers. Epiplakin knockdown revealed disruption of IF networks in simple epithelial but not in epidermal cells. In rescue experiments, the repeat unit was necessary to prevent the collapse of IF networks in transient knockdown; however, it could only partially restore the keratin but not the vimentin IF network in stably knocked down HeLa cells. We suggest that epiplakin is a cytolinker involved in maintaining the integrity of IF networks in simple epithelial cells. Furthermore, we observed an increase of epiplakin expression in keratinocytes after the calcium switch, suggesting the involvement of epiplakin in the process of keratinocyte differentiation.

Kazrin, a novel periplakin-interacting protein associated with desmosomes and the keratinocyte plasma membrane

Periplakin forms part of the scaffold onto which the epidermal cornified envelope is assembled. The NH₂-terminal 133 amino acids mediate association with the plasma membrane and bind a novel protein, kazrin. Kazrin is highly conserved and lacks homology to any known protein. There are four alternatively spliced transcripts, encoding three proteins with different NH₂ termini. Kazrin is expressed in all layers of stratified squamous epithelia; it becomes membrane associated in the suprabasal layers, coincident with up-regulation of periplakin, and is incorporated into the cornified envelope of cultured keratinocytes. Kazrin colocalizes with periplakin and desmoplakin at desmosomes and with periplakin at the interdesmosomal plasma membrane, but its subcellular distribution is independent of periplakin. On transfection, all three kazrin isoforms have similar subcellular distributions. We conclude that kazrin is a novel component of desmosomes that associates with periplakin.

Identification of human hornerin and its expression in regenerating and psoriatic skin

We previously isolated a new member of the fused-type S100 protein family (hornerin) from the mouse (Makino, T., Takaishi, M., Morohashi, M., and Huh, N.-h. (2001) J. Biol. Chem. 276, 47445-47452). Mouse hornerin shares structural features, expression profiles, and intracellular localization with profilaggrin, indicating possible involvement of hornerin in cornification. In this study, we identified and partially characterized a human ortholog of mouse hornerin. The human hornerin gene was mapped between trichohyalin and filaggrin on chromosome 1q21.3, the region being completely syntenic with the counterpart of the mouse. The deduced amino acid sequence of 2850 residues shows typical structural features of "fused-type" S100 protein family members. Mature transcripts and protein from human hornerin were not detected in normal stratified epithelium, including the trunk epidermis, tongue, and esophagus. After screening of various normal and pathological human tissues, we found that human hornerin was expressed in psoriatic skin. Hornerin protein was present in the keratinizing region, although at a lower level and in fewer cells compared with filaggrin. Mature transcripts and protein from hornerin were also detected in regenerating human skin after wounding. Hornerin mRNA was induced 5 days after wounding. The mRNA level remained almost constant until 15 days and declined at 30 days after wounding. Hornerin protein was detected in the proximal epidermis (but not in the distal epidermis) at 15 days after wounding. These results indicate that hornerin has a function similar to but distinct from that of filaggrin in cornification.

Ratio of immature cornified envelopes does not correlate with parakeratosis in inflammatory skin disorders

We have previously established a non-invasive method to evaluate the maturity of cornified envelopes (CEs), and have reported the appearance of immature CEs in the stratum corneum (SC) with poor barrier function, such as the SC of the face. The purpose of the present study was to evaluate CEs in inflammatory skin disorders, and to clarify the relationship between the appearance of the immature CEs and parakeratosis, which is often used as a marker for defective keratinization in inflammatory skin disorders. Cornified envelopes in the outermost SC of involved areas of psoriasis vulgaris (PV) and atopic dermatitis (AD) were strikingly heterogeneous, and consisted of immature CEs stained with anti-involucrin and mature CEs stained with Nile red, whereas CEs of the uninvolved areas were relatively homogeneous, exhibiting mature phenotype. The ratio of immature CEs was significantly higher in the involved areas of PV and AD than that in the corresponding uninvolved areas, suggesting that defective CE maturation may, at least in part, account for the inflammatory disorders. Simultaneous evaluation of CE maturity and parakeratosis was carried out by a combination of involucrin immunostaining and nuclear staining of detergent-dissociated corneocytes. In the involved area of PV, four types of corneocytes in regard to the combination of involucrin staining and nuclear remnant were observed, while both immature CEs and parakeratosis were more often detected in the involved areas of PV than in the uninvolved areas or the upper arm of healthy subjects as a normal control. Thus, corneocytes with involucrin-positive immature CEs were not always associated with parakeratosis at the cellular level. In the involved areas of PV, the ratio of immature CEs and that of parakeratosis were heterogeneous, depending on the cases, and no correlation between the ratios was observed. Inter-individual and intraindividual variations in CE maturity were also suggested by the heterogeneous localization of involucrin in the psoriatic epidermis as examined by immunohistochemistry. In addition, in the face of healthy subjects, four types of corneocytes were similarly detected, and the ratio of immature CEs was significantly higher than that of parakeratosis. These results obviously suggest that the maturation of CEs and disappearance of nuclei are differentially regulated in the epidermis.

Co-assembly of Envoplakin and Periplakin into Oligomers and Ca2+-dependent Vesicle Binding

Plakin family members envoplakin and periplakin have been shown to be part of the cornified cell envelope in terminally differentiating stratified squamous epithelia. In the present study, purified recombinant human envoplakin and periplakin were used to investigate their properties and interactions. We found that envoplakin was insoluble at physiological conditions in vitro, and co-assembly with periplakin was required for its solubility. Envoplakin and periplakin formed soluble complexes with equimolar stoichiometry. Chemical cross-linking revealed that the major soluble form of all periplakin constructs and of envoplakin/periplakin rod domains was a dimer, although co-assembly of the full-length proteins resulted in formation of higher order oligomers. Electron microscopy of rotary-shadowed periplakin demonstrated thin flexible molecules with an average contour length of 88 nm for the rod-plus-tail fragment, and immunolabeling EM confirmed the molecule as a parallel, in-register, dimer. Both periplakin and envoplakin/periplakin oligomers were able to bind synthetic lipid vesicles whose composition mimicked the cytoplasmic side of the plasma membrane of eukaryotic cells. This binding was dependent on anionic phospholipids and Ca(2+). These findings raise the possibility that envoplakin and periplakin bind to the plasma membrane upon elevation of intracellular [Ca(2+)] in differentiating keratinocytes, where they serve as a scaffold for cornified cell envelope assembly.