Transient expression of epidermal filaggrin in cultured cells causes collapse of intermediate filament networks with alteration of cell shape and nuclear integrity

Immunolabeling for filaggrin and acidic keratins in the granular layer of mammalian epidermis indicates that an acidic-basic interaction is involved in cornification

The soft epidermis of mammals derives from the accumulation of keratohyaline granules in the granular layer, before maturing into corneocytes. Main proteins accumulated in the granular layer are pro-filaggrin and filaggrin that determine keratin clumping and later moisturization of the stratum corneum that remains flexible. This soft epidermis allows the high sensitivity of mammalian skin. Presence and thickness of the stratum granulosum varies among different species of mammals and even between different body regions of the same animal, from discontinuous to multilayered. These variations are evident using antibodies for filaggrin, a large protein that share common epitopes among placentals. Here we have utilized filaggrin antibodies (8959 and 466) and an acidic keratin antibody (AK2) for labeling placental, marsupial and monotreme epidermis. AK2 labeling appears mainly to detect K24 keratin, and less likely other acidic keratins. Immunoreactivity for filaggrin is absent in platypus, discontinuous in Echidna and in the tested marsupials. In placentals, it is inconstantly or hardly detected in the thin epidermis of bat, rodents, and lagomorphs with a narrow, mono-stratified and/or discontinuous granular layer. In contrast, where the granular layer is continuous or even stratified, both filaggrin and AK2 antibodies decorate granular cells. The ultrastructural analysis using the AK2 antibody on human epidermis reveals that a weak labeling is associated with keratohyalin granules and filamentous keratins of transitional keratinocytes and corneocytes. This observation suggests that basophilic filaggrin interacts with acidic keratins like K24 and determines keratin condensation into corneocytes of the stratum corneum.

In silico analysis of the profilaggrin sequence indicates alterations in the stability, degradation route, and intracellular protein fate in filaggrin null mutation carriers

Background: Loss of function mutation in FLG is the major genetic risk factor for atopic dermatitis (AD) and other allergic manifestations. Presently, little is known about the cellular turnover and stability of profilaggrin, the protein encoded by FLG. Since ubiquitination directly regulates the cellular fate of numerous proteins, their degradation and trafficking, this process could influence the concentration of filaggrin in the skin. Objective: To determine the elements mediating the interaction of profilaggrin with the ubiquitin-proteasome system (i.e., degron motifs and ubiquitination sites), the features responsible for its stability, and the effect of nonsense and frameshift mutations on profilaggrin turnover. Methods: The effect of inhibition of proteasome and deubiquitinases on the level and modifications of profilaggrin and processed products was assessed by immunoblotting. Wild-type profilaggrin sequence and its mutated variants were analysed in silico using the DEGRONOPEDIA and Clustal Omega tool. Results: Inhibition of proteasome and deubiquitinases stabilizes profilaggrin and its high molecular weight of presumably ubiquitinated derivatives. In silico analysis of the sequence determined that profilaggrin contains 18 known degron motifs as well as multiple canonical and non-canonical ubiquitination-prone residues. FLG mutations generate products with increased stability scores, altered usage of the ubiquitination marks, and the frequent appearance of novel degrons, including those promoting C-terminus-mediated degradation routes. Conclusion: The proteasome is involved in the turnover of profilaggrin, which contains multiple degrons and ubiquitination-prone residues. FLG mutations alter those key elements, affecting the degradation routes and the mutated products' stability.

Novel Therapeutic Targets for the Treatment of Atopic Dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease that significantly impacts quality of life. The pathogenesis of AD is a complex combination of skin barrier dysfunction, type II immune response, and pruritus. Progress in the understanding of the immunological mechanisms of AD has led to the recognition of multiple novel therapeutic targets. For systemic therapy, new biologic agents that target IL-13, IL-22, IL-33, the IL-23/IL-17 axis, and OX40-OX40L are being developed. Binding of type II cytokines to their receptors activates Janus kinase (JAK) and its downstream signal, namely signal transduction and activator of transcription (STAT). JAK inhibitors block the activation of the JAK-STAT pathway, thereby blocking the signaling pathways mediated by type II cytokines. In addition to oral JAK inhibitors, histamine H4 receptor antagonists are under investigation as small-molecule compounds. For topical therapy, JAK inhibitors, aryl hydrocarbon receptor modulators, and phosphodiesterase-4 inhibitors are being approved. Microbiome modulation is also being examined for the treatment of AD. This review outlines current and future directions for novel therapies of AD that are currently being investigated in clinical trials, focusing on their mechanisms of action and efficacy. This supports the accumulation of data on advanced treatments for AD in the new era of precision medicine.

Filaggrin Defect at Atopic Dermatitis: Modern Treatment Options

Atopic dermatitis is a common chronic skin disease, its pathogenesis is associated with congenital or acquired deficiency of filaggrin protein. In recent years, extensive evidence on the causes of filaggrin deficiency has been obtained. The structure and functions of this protein are described, that opens new approaches for atopic dermatitis management.

Revisiting the Roles of Filaggrin in Atopic Dermatitis

The discovery in 2006 that loss-of-function mutations in the filaggrin gene (FLG) cause ichthyosis vulgaris and can predispose to atopic dermatitis (AD) galvanized the dermatology research community and shed new light on a skin protein that was first identified in 1981. However, although outstanding work has uncovered several key functions of filaggrin in epidermal homeostasis, a comprehensive understanding of how filaggrin deficiency contributes to AD is still incomplete, including details of the upstream factors that lead to the reduced amounts of filaggrin, regardless of genotype. In this review, we re-evaluate data focusing on the roles of filaggrin in the epidermis, as well as in AD. Filaggrin is important for alignment of keratin intermediate filaments, control of keratinocyte shape, and maintenance of epidermal texture via production of water-retaining molecules. Moreover, filaggrin deficiency leads to cellular abnormalities in keratinocytes and induces subtle epidermal barrier impairment that is sufficient enough to facilitate the ingress of certain exogenous molecules into the epidermis. However, although FLG null mutations regulate skin moisture in non-lesional AD skin, filaggrin deficiency per se does not lead to the neutralization of skin surface pH or to excessive transepidermal water loss in atopic skin. Separating facts from chaff regarding the functions of filaggrin in the epidermis is necessary for the design efficacious therapies to treat dry and atopic skin.

Cracking the Skin Barrier: Liquid-Liquid Phase Separation Shines under the Skin

Central to forming and sustaining the skin’s barrier, epidermal keratinocytes (KCs) fluxing to the skin surface undergo a rapid and enigmatic transformation into flat, enucleated squames. At the crux of this transformation are intracellular keratohyalin granules (KGs) that suddenly disappear as terminally differentiating KCs transition to the cornified skin surface. Defects in KGs have long been linked to skin barrier disorders. Through the biophysical lens of liquid-liquid phase separation (LLPS), these enigmatic KGs recently emerged as liquid-like membraneless organelles whose assembly and subsequent pH-triggered disassembly drive squame formation. To stimulate future efforts toward cracking the complex process of skin barrier formation, in this review, we integrate the key concepts and foundational work spanning the fields of LLPS and epidermal biology. We review the current progress in the skin and discuss implications in the broader context of membraneless organelles across stratifying epithelia. The discovery of environmentally sensitive LLPS dynamics in the skin points to new avenues for dissecting the skin barrier and for addressing skin barrier disorders. We argue that skin and its appendages offer outstanding models to uncover LLPS-driven mechanisms in tissue biology.

Akt1-associated actomyosin remodelling is required for nuclear lamina dispersal and nuclear shrinkage in epidermal terminal differentiation

Keratinocyte cornification and epidermal barrier formation are tightly controlled processes, which require complete degradation of intracellular organelles, including removal of keratinocyte nuclei. Keratinocyte nuclear destruction requires Akt1-dependent phosphorylation and degradation of the nuclear lamina protein, Lamin A/C, essential for nuclear integrity. However, the molecular mechanisms that result in complete nuclear removal and their regulation are not well defined. Post-confluent cultures of rat epidermal keratinocytes (REKs) undergo spontaneous and complete differentiation, allowing visualisation and perturbation of the differentiation process in vitro. We demonstrate that there is dispersal of phosphorylated Lamin A/C to structures throughout the cytoplasm in differentiating keratinocytes. We show that the dispersal of phosphorylated Lamin A/C is Akt1-dependent and these structures are specific for the removal of Lamin A/C from the nuclear lamina; nuclear contents and Lamin B were not present in these structures. Immunoprecipitation identified a group of functionally related Akt1 target proteins involved in Lamin A/C dispersal, including actin, which forms cytoskeletal microfilaments, Arp3, required for actin filament nucleation, and Myh9, a component of myosin IIa, a molecular motor that can translocate along actin filaments. Disruption of actin filament polymerisation, nucleation or myosin IIa activity prevented formation and dispersal of cytoplasmic Lamin A/C structures. Live imaging of keratinocytes expressing fluorescently tagged nuclear proteins showed a nuclear volume reduction step taking less than 40 min precedes final nuclear destruction. Preventing Akt1-dependent Lamin A/C phosphorylation and disrupting cytoskeletal Akt1-associated proteins prevented nuclear volume reduction. We propose keratinocyte nuclear destruction and differentiation requires myosin II activity and the actin cytoskeleton for two intermediate processes: Lamin A/C dispersal and rapid nuclear volume reduction.

Deimination, Intermediate Filaments and Associated Proteins

Deimination (or citrullination) is a post-translational modification catalyzed by a calcium-dependent enzyme family of five peptidylarginine deiminases (PADs). Deimination is involved in physiological processes (cell differentiation, embryogenesis, innate and adaptive immunity, etc.) and in autoimmune diseases (rheumatoid arthritis, multiple sclerosis and lupus), cancers and neurodegenerative diseases. Intermediate filaments (IF) and associated proteins (IFAP) are major substrates of PADs. Here, we focus on the effects of deimination on the polymerization and solubility properties of IF proteins and on the proteolysis and cross-linking of IFAP, to finally expose some features of interest and some limitations of citrullinomes.

Protective effect of Aquaphilus dolomiae extract-G1, ADE-G1, on tight junction barrier function in a Staphylococcus aureus-infected atopic dermatitis model

BACKGROUND

Atopic dermatitis (AD) is a common skin disease characterized by recurrent pruritic inflammatory skin lesions and defects of the skin barrier. Bacterial infection with Staphylococcus aureus contributes to increased severity of AD by compromising the barrier further. A microorganism component of Avène Thermal Spring Water, Aquaphilus dolomiae, is thought to contribute to some of its beneficial effects to skin, eg AD alleviation.

AIMS

Here, we have investigated the effects of an extract of A. dolomiae, A. dolomiae extract-G1 (ADE-G1), on the structural barrier function of keratinocytes, tight junction (TJ) protein expression and the expression of several genes altered in AD patients.

METHODS

An epidermal cell culture model mimicking the AD environment and phenotype was used, in which S. aureus-infected cell cultures of normal human epidermal keratinocytes were exposed to a proinflammatory environment. Endpoints measured included the transepithelial electrical resistance (TER) and immunohistological staining of the epidermal TJ proteins, claudin and occludin. Additional analysis was made of several genes known to be differentially regulated in skin from AD patients (C-C motif chemokine ligand 20 (CCL20), interleukin-8 (IL-8), S100 calcium binding protein A7 (S100A7), defensin beta 4 (DEFB4) and filaggrin).

RESULTS

Aquaphilus dolomiae extract-G1 strongly increased TER in non-infected cells and provided protection against infection by overcoming the decrease in TER induced by the infection with S. aureus. In infected cells exposed to a pro-inflammatory environment - depicting AD-like conditions - TER protection by ADE-G1 was still observed. Gene expression analysis of infected and pro-inflammatory stimulated cells indicated that ADE-G1 modulated the inflammatory response (induced IL-8 and attenuated CCL20 expression), increased antimicrobial activities (induced DEFB4 and A100A7) and strengthened barrier function (restored filaggrin expression).

CONCLUSIONS

ADE-G1 reinforces barrier function and strongly protects TJ barrier disruption induced by bacterial infection and inflammation.

Structural properties of target binding by profilaggrin A and B domains and other S100 fused-type calcium-binding proteins

BACKGROUND

Profilaggrin belongs to the S100 fused-type protein family expressed in keratinocytes and is important for skin barrier integrity. Its N-terminus contains an S100 ("A") domain and a unique "B" domain with a nuclear localization sequence.

OBJECTIVE

To determine whether profilaggrin B domain cooperates with the S100 domain to bind macromolecules. To characterize the biochemical and structural properties of the profilaggrin N-terminal "AB" domain and compare it to other S100 fused-type proteins.

METHODS

We used biochemical (protease protection, light scattering, fluorescence spectroscopy, pull-down assays) and computational techniques (sequence analysis, molecular modeling with crystallographic structures) to examine human profilaggrin and S100 fused-type proteins.

RESULTS

Comparing profilaggrin S100 crystal structure with models of the other S100 fused-type proteins demonstrated each has a unique chemical composition of solvent accessible surface around the hydrophobic binding pocket. S100 fused-type proteins exhibit higher pocket hydrophobicity than soluble S100 proteins. The inter-EF-hand linker in S100 fused-type proteins contains conserved hydrophobic residues involved in binding substrates. Profilaggrin B domain cooperates with the S100 domain to bind annexin II and keratin intermediate filaments in a calcium-dependent manner using exposed cationic surface. Using molecular modeling we demonstrate profilaggrin B domain likely interacts with annexin II domains I and II. Steric clash analysis shows annexin II N-terminal peptide is favored to bind profilaggrin among S100 fused-type proteins.

CONCLUSION

The N-terminal S100 and B domains of profilaggrin cooperate to bind substrate molecules in granular layer keratinocytes to provide epidermal barrier functions.

Regulation of Filaggrin, Loricrin, and Involucrin by IL-4, IL-13, IL-17A, IL-22, AHR, and NRF2: Pathogenic Implications in Atopic Dermatitis

Atopic dermatitis (AD) is an eczematous, pruritic skin disorder with extensive barrier dysfunction and elevated interleukin (IL)-4 and IL-13 signatures. The barrier dysfunction correlates with the downregulation of barrier-related molecules such as filaggrin (FLG), loricrin (LOR), and involucrin (IVL). IL-4 and IL-13 potently inhibit the expression of these molecules by activating signal transducer and activator of transcription (STAT)6 and STAT3. In addition to IL-4 and IL-13, IL-22 and IL-17A are probably involved in the barrier dysfunction by inhibiting the expression of these barrier-related molecules. In contrast, natural or medicinal ligands for aryl hydrocarbon receptor (AHR) are potent upregulators of FLG, LOR, and IVL expression. As IL-4, IL-13, IL-22, and IL-17A are all capable of inducing oxidative stress, antioxidative AHR agonists such as coal tar, glyteer, and tapinarof exert particular therapeutic efficacy for AD. These antioxidative AHR ligands are known to activate an antioxidative transcription factor, nuclear factor E2-related factor 2 (NRF2). This article focuses on the mechanisms by which FLG, LOR, and IVL expression is regulated by IL-4, IL-13, IL-22, and IL-17A. The author also summarizes how AHR and NRF2 dual activators exert their beneficial effects in the treatment of AD.

Protein kinases involved in epidermal barrier formation: The AKT family and other animals

Formation of a stratified epidermis is required for the performance of the essential functions of the skin; to act as an outside-in barrier against the access of microorganisms and other external factors, to prevent loss of water and solutes via inside-out barrier functions and to withstand mechanical stresses. Epidermal barrier function is initiated during embryonic development and is then maintained throughout life and restored after injury. A variety of interrelated processes are required for the formation of a stratified epidermis, and how these processes are both temporally and spatially regulated has long been an aspect of dermatological research. In this review, we describe the roles of multiple protein kinases in the regulation of processes required for epidermal barrier formation.

Peptidylarginine deiminase is involved in maintaining the cornified oral mucosa of rats

BACKGROUND AND OBJECTIVE

Epithelial cells derived from different regions exhibit marked differences in their differentiation capacity, allowing them to provide a suitable protective barrier. We aimed to clarify the role of peptidylarginine deiminase (PAD) in modifying the key epidermal proteins filaggrin (FLG) and keratin 1 (K1) during stratification of the rat palate and buccal mucosa.

MATERIAL AND METHODS

We performed immunofluorescence, immunoblotting, PAD activity assays and 2-dimensional electrophoresis, and developed an organotypic culture model.

RESULTS

PAD1 expression was highest in the palate, whereas PAD2, PAD3 and PAD4 expression was highest in the skin, suggesting the tissue-specific expression of PAD isozymes that leads to differences in calcium dependency. Immunoblotting showed that the FLG monomer, as well as its degradation products and precursor (proFLG), were most abundantly expressed in the skin but had low expression in the palate, whereas only faint proFLG expression was detected in the buccal mucosa. FLG and K1 were colocalized with PAD1 and were likely to be citrullinated in the cornified layers of the skin; this colocalization was not detected on the palatal surface, and dot-like presence of proFLG that might be citrullinated and that of PAD1 were found in the granules of the palate. Organotypic models derived from the rat palate revealed that PAD inhibition reduced the breakdown of FLG, increased its association with K1 together with epithelial compaction, and decreased permeability in a dye permeability assay. Conversely, PAD stimulation had the opposite effects.

CONCLUSION

Citrullination is likely a protein modification that plays an important role in maintaining the structure and function of oral cornified mucosa in a way that is distinctly different from that of the skin.

Orchestrated control of filaggrin-actin scaffolds underpins cornification

Epidermal stratification critically depends on keratinocyte differentiation and programmed death by cornification, leading to formation of a protective skin barrier. Cornification is dynamically controlled by the protein filaggrin, rapidly released from keratohyalin granules (KHGs). However, the mechanisms of cornification largely remain elusive, partly due to limitations of the observation techniques employed to study filaggrin organization in keratinocytes. Moreover, while the abundance of keratins within KHGs has been well described, it is not clear whether actin also contributes to their formation or fate. We employed advanced (super-resolution) microscopy to examine filaggrin organization and dynamics in skin and human keratinocytes during differentiation. We found that filaggrin organization depends on the cytoplasmic actin cytoskeleton, including the role for α- and β-actin scaffolds. Filaggrin-containing KHGs displayed high mobility and migrated toward the nucleus during differentiation. Pharmacological disruption targeting actin networks resulted in granule disintegration and accelerated cornification. We identified the role of AKT serine/threonine kinase 1 (AKT1), which controls binding preference and function of heat shock protein B1 (HspB1), facilitating the switch from actin stabilization to filaggrin processing. Our results suggest an extended model of cornification in which filaggrin utilizes actins to effectively control keratinocyte differentiation and death, promoting epidermal stratification and formation of a fully functional skin barrier.

Co-stimulation of HaCaT keratinization with mechanical stress and air-exposure using a novel 3D culture device

Artificial skin or skin equivalents have been used for clinical purpose to skin graft and as substitutes for animal experiments. The culture of cell lines such as HaCaT has the potential to produce large amounts of artificial skin at a low cost. However, there is a limit to keratinization due to the restriction of differentiation in HaCaT. In this study, a culture device that mimics the in vivo keratinization mechanism, co-stimulated by air-exposure and mechanical stimulation, was developed to construct skin equivalents. The device can reconstruct the epidermal morphology, including the cornified layer, similar to its formation in vivo. Under the condition, epidermis was differentiated in the spinous and granular layers. Formation of the stratum corneum is consistent with the mRNA and protein expressions of differentiation markers. The device is the first of its kind to combine air-exposure with mechanical stress to co-stimulate keratinization, which can facilitate the economically viable production of HaCaT-based artificial skin substitutes.

An Analysis of the Filaggrin Gene Polymorphism in Korean Atopic Dermatitis Patients

Research of the FLG mutation in various ethnic groups revealed non-overlapping mutation patterns. In addition, Japanese and Chinese atopic patients showed somewhat different mutations. These ethnic differences make the research on Korean patients mandatory; however, no systematic research on Korean atopic dermatitis (AD) patients has been performed. This study aims to investigate the genetic polymorphism of FLG in Korean atopic dermatitis patients. The study was made up of three groups including 9 Ichthyosis vulgaris (IV) patients, 50 AD patients and 55 normal controls: the ichthyosis group was incorporated due to the reported association between the FLG mutation and IV. In comparison to other sequencing methods, the overlapping long-range PCR was used. We revealed the genetic polymorphism of filaggrin in Koreans, and at the same time, we discovered nonsense mutations in p.Y1767X and p.K4022X in Korean AD patients. By using FLG sequencing techniques confirmed in this study, new mutations or genetic polymorphisms with ethnic characteristics would be detected and further larger studies of repeat number polymorphisms could be performed.

Comparative genomics reveals conservation of filaggrin and loss of caspase-14 in dolphins

The expression of filaggrin and its stepwise proteolytic degradation are critical events in the terminal differentiation of epidermal keratinocytes and in the formation of the skin barrier to the environment. Here, we investigated whether the evolutionary transition from a terrestrial to a fully aquatic lifestyle of cetaceans, that is dolphins and whales, has been associated with changes in genes encoding filaggrin and proteins involved in the processing of filaggrin. We used comparative genomics, PCRs and re-sequencing of gene segments to screen for the presence and integrity of genes coding for filaggrin and proteases implicated in the maturation of (pro)filaggrin. Filaggrin has been conserved in dolphins (bottlenose dolphin, orca and baiji) but has been lost in whales (sperm whale and minke whale). All other S100 fused-type genes have been lost in cetaceans. Among filaggrin-processing proteases, aspartic peptidase retroviral-like 1 (ASPRV1), also known as saspase, has been conserved, whereas caspase-14 has been lost in all cetaceans investigated. In conclusion, our results suggest that filaggrin is dispensable for the acquisition of fully aquatic lifestyles of whales, whereas it appears to confer an evolutionary advantage to dolphins. The discordant evolution of filaggrin, saspase and caspase-14 in cetaceans indicates that the biological roles of these proteins are not strictly interdependent.

Skin Barrier Defects Caused by Keratinocyte-Specific Deletion of ADAM17 or EGFR Are Based on Highly Similar Proteome and Degradome Alterations

Keratinocyte-specific deletion of ADAM17 in mice impairs terminal differentiation of keratinocytes leading to severe epidermal barrier defects. Mice deficient for ADAM17 in keratinocytes phenocopy mice with a keratinocyte-specific deletion of epidermal growth factor receptor (EGFR), which highlights the role of ADAM17 as a "ligand sheddase" of EGFR ligands. In this study, we aim for the first proteomic/degradomic approach to characterize the disruption of the ADAM17-EGFR signaling axis and its consequences for epidermal barrier formation. Proteomic profiling of the epidermal proteome of mice deficient for either ADAM17 or EGFR in keratinocytes at postnatal days 3 and 10 revealed highly similar protein alterations for ADAM17 and EGFR deficiency. These include massive proteome alterations of structural and regulatory components important for barrier formation such as transglutaminases, involucrin, filaggrin, and filaggrin-2. Cleavage site analysis using terminal amine isotopic labeling of substrates revealed increased proteolytic processing of S100 fused-type proteins including filaggrin-2. Alterations in proteolytic processing are supported by altered abundance of numerous proteases upon keratinocyte-specific Adam17 or Egfr deletion, among them kallikreins, cathepsins, and their inhibitors. This study highlights the essential role of proteolytic processing for maintenance of a functional epidermal barrier. Furthermore, it suggests that most defects in formation of the postnatal epidermal barrier upon keratinocyte-specific ADAM17 deletion are mediated via EGFR.

Ex vivo construction of a novel model of bioengineered bladder mucosa: A preliminary study

OBJECTIVE

To generate and to evaluate ex vivo a novel model of bioengineered human bladder mucosa based on fibrin-agarose biomaterials.

METHODS

We first established primary cultures of stromal and epithelial cells from small biopsies of the human bladder using enzymatic digestion and selective cell culture media. Then, a bioengineered substitute of the bladder lamina propria was generated using cultured stromal cells and fibrin-agarose scaffolds, and the epithelial cells were then subcultured on top to generate a complete bladder mucosa substitute. Evaluation of this substitute was carried out by cell viability and histological analyses, immunohistochemistry for key epithelial markers and transmission electron microscopy.

RESULTS

The results show a well-configured stroma substitute with a single-layer epithelium on top. This substitute was equivalent to the control bladder mucosa. After 7 days of ex vivo development, the epithelial layer expressed pancytokeratin, and cytokeratins CK7, CK8 and CK13, as well as filaggrin and ZO-2, with negative expression of CK4 and uroplakin III. A reduction of the expression of CK8, filaggrin and ZO-2 was found at day 14 of development. An immature basement membrane was detected at the transition between the epithelium and the lamina propria, with the presence of epithelial hemidesmosomes, interdigitations and immature desmosomes.

CONCLUSIONS

The present results suggest that this model of bioengineered human bladder mucosa shared structural and functional similarities with the native bladder mucosa, although the epithelial cells were not fully differentiated ex vivo. We hypothesize that this bladder mucosa substitute could have potential clinical usefulness after in vivo implantation.

AKT1-mediated Lamin A/C degradation is required for nuclear degradation and normal epidermal terminal differentiation

Nuclear degradation is a key stage in keratinocyte terminal differentiation and the formation of the cornified envelope that comprises the majority of epidermal barrier function. Parakeratosis, the retention of nuclear material in the cornified layer of the epidermis, is a common histological observation in many skin diseases, notably in atopic dermatitis and psoriasis. Keratinocyte nuclear degradation is not well characterised, and it is unclear whether the retained nuclei contribute to the altered epidermal differentiation seen in eczema and psoriasis. Loss of AKT1 function strongly correlated with parakeratosis both in eczema samples and in organotypic culture models. Although levels of DNAses, including DNase1L2, were unchanged, proteomic analysis revealed an increase in Lamin A/C. AKT phosphorylates Lamin A/C, targeting it for degradation. Consistent with this, Lamin A/C degradation was inhibited and Lamin A/C was observed in the cornified layer of AKT1 knockdown organotypic cultures, surrounding retained nuclear material. Using AKT-phosphorylation-dead Lamin A constructs we show that the retention of nuclear material is sufficient to cause profound changes in epidermal terminal differentiation, specifically a reduction in Loricrin, Keratin 1, Keratin 10, and filaggrin expression. We show that preventing nuclear degradation upregulates BMP2 expression and SMAD1 signalling. Consistent with these data, we observe both parakeratosis and evidence of increased SMAD1 signalling in atopic dermatitis. We therefore present a model that, in the absence of AKT1-mediated Lamin A/C degradation, DNA degradation processes, such as those mediated by DNAse 1L2, are prevented, leading to parakeratosis and changes in epidermal differentiation.

Causes of epidermal filaggrin reduction and their role in the pathogenesis of atopic dermatitis

The epidermis protects human subjects from exogenous stressors and helps to maintain internal fluid and electrolyte homeostasis. Filaggrin is a crucial epidermal protein that is important for the formation of the corneocyte, as well as the generation of its intracellular metabolites, which contribute to stratum corneum hydration and pH. The levels of filaggrin and its degradation products are influenced not only by the filaggrin genotype but also by inflammation and exogenous stressors. Pertinently, filaggrin deficiency is observed in patients with atopic dermatitis regardless of filaggrin mutation status, suggesting that the absence of filaggrin is a key factor in the pathogenesis of this skin condition. In this article we review the various causes of low filaggrin levels, centralizing the functional and morphologic role of a deficiency in filaggrin, its metabolites, or both in the etiopathogenesis of atopic dermatitis.

Unique Keratinization Process in Psoriasis: Late Differentiation Markers Are Abolished Because of the Premature Cell Death

The keratinization process in psoriasis is a unique phenomenon. We have proposed an organized system for keratinization in psoriasis based on the recognition of early and late differentiation markers combined with premature cell death. The early differentiation markers, such as involucrin, small proline-rich proteins (SPRR), cystatin A and transglutaminase l, are more conspicuously expressed in psoriasis, while the late differentiation markers, such as profilaggrin and loricrin, are abolished. Keratinization markers that are not observed in the normal epidermis are also detected; these include SKALP/elafin as well as K6 and K16. With a markedly diminished turnover time, the psoriatic epidermis rapidly synthesizes differentiation markers that are mostly under the control of the protein kinase C-AP1 transcriptional control system. Because of the premature cell death, however, the late differentiation markers are not expressed. During the improvement of the lesion and the therefore longer turnover time, the late differentiation markers rapidly catch up to reveal their expression. This explains the rapid appearance of keratohyalin granules (profilaggrin) in the healing lesion of psoriasis. Thus the keratinization process in psoriasis can be explained by the accelerated keratinization combined with premature cell death. The keratinization process in psoriasis is unique, because both accelerated keratinization and premature cell death co-exist, resulting in the disappearance of late differentiation markers such as profilaggrin and loricrin. It is interesting to note that the premature cell death is also under the control of protein kinase C signaling.

Spontaneous atopic dermatitis-like symptoms in a/a ma ft/ma ft/J flaky tail mice appear early after birth

Loss-of-function mutations in human profilaggrin gene have been identified as the cause of ichthyosis vulgaris (IV), and as a major predisposition factor for atopic dermatitis (AD). Similarly, flaky tail (a/a ma ft/ma ft/J) mice were described as a model for IV, and shown to be predisposed to eczema. The aim of this study was to correlate the flaky tail mouse phenotype with human IV and AD, in order to dissect early molecular events leading to atopic dermatitis in mice and men, suffering from filaggrin deficiency. Thus, 5-days old flaky tail pups were analyzed histologically, expression of cytokines was measured in skin and signaling pathways were investigated by protein analysis. Human biopsies of IV and AD patients were analyzed histologically and by real time PCR assays. Our data show acanthosis and hyperproliferation in flaky tail epidermis, associated with increased IL1β and thymic stromal lymphopoietin (TSLP) expression, and Th2-polarization. Consequently, NFκB and Stat pathways were activated, and IL6 mRNA levels were increased. Further, quantitative analysis of late epidermal differentiation markers revealed increased Small proline-rich protein 2A (Sprr2a) synthesis. Th2-polarization and Sprr2a increase may result from high TSLP expression, as shown after analysis of 5-days old K14-TSLP tg mouse skin biopsies. Our findings in the flaky tail mouse correlate with data obtained from patient biopsies of AD, but not IV. We propose that proinflammatory cytokines are responsible for acanthosis in flaky tail epidermis, and together with the Th2-derived cytokines lead to morphological changes. Accordingly, the a/a ma ft/ma ft/J mouse model can be used as an appropriate model to study early AD onset associated with profilaggrin deficiency.

Murine filaggrin-2 is involved in epithelial barrier function and down-regulated in metabolically induced skin barrier dysfunction

The S100 fused-type proteins (SFTPs) are thought to be involved in the barrier formation and function of the skin. Mutations in the profilaggrin gene, one of the best investigated members of this family, are known to be the major risk factors for ichthyosis vulgaris and atopic dermatitis. Recently, we identified human filaggrin-2 as a new member of the SFTP family. To achieve further insight into its function, here the murine filaggrin-2 was analysed as a possible orthologue. The 5' and 3' ends of the mouse filaggrin-2 cDNA of the BALB/c strain were sequenced and confirmed an organization typical for SFTPs. Murine filaggrin-2 showed an expression pattern mainly in keratinizing epithelia in the upper cell layers on both mRNA and protein levels. The expression in cultured mouse keratinocytes was increased upon elevated Ca(2+) levels. Immunoblotting experiments indicated an intraepidermal processing of the 250-kDa full-length protein. In metabolically (essential fatty acid-deficient diet) induced skin barrier dysfunction, filaggrin-2 expression was significantly reduced, whereas filaggrin expression was up-regulated. In contrast, mechanical barrier disruption with acetone treatment did not affect filaggrin-2 mRNA expression. These results suggest that filaggrin-2 may contribute to epidermal barrier function and its regulation differs, at least in parts, from that of filaggrin.

The human epidermal differentiation complex: cornified envelope precursors, S100 proteins and the 'fused genes' family

  The skin is essential for survival and protects our body against biological attacks, physical stress, chemical injury, water loss, ultraviolet radiation and immunological impairment. The epidermal barrier constitutes the primordial frontline of this defense established during terminal differentiation. During this complex process proliferating basal keratinocytes become suprabasally mitotically inactive and move through four epidermal layers (basal, spinous, granular and layer, stratum corneum) constantly adapting to the needs of the respective cell layer. As a result, squamous keratinocytes contain polymerized keratin intermediate filament bundles and a water-retaining matrix surrounded by the cross-linked cornified cell envelope (CE) with ceramide lipids attached on the outer surface. These cells are concomitantly insulated by intercellular lipid lamellae and hold together by corneodesmosmes. Many proteins essential for epidermal differentiation are encoded by genes clustered on chromosomal human region 1q21. These genes constitute the 'epidermal differentiation complex' (EDC), which is divided on the basis of common gene and protein structures, in three gene families: (i) CE precursors, (ii) S100A and (iii) S100 fused genes. EDC protein expression is regulated in a gene and tissue-specific manner by a pool of transcription factors. Among them, Klf4, Grhl3 and Arnt are essential, and their deletion in mice is lethal. The importance of the EDC is further reflected by human diseases: FLG mutations are the strongest risk factor for atopic dermatitis (AD) and for AD-associated asthma, and faulty CE formation caused by TG1 deficiency causes life-threatening lamellar ichthyosis. Here, we review the EDC genes and the progress in this field.

Interaction of the profilaggrin N-terminal domain with loricrin in human cultured keratinocytes and epidermis

The relationship between the two coexpressed differentiation markers, profilaggrin and loricrin, is not clear right now. In this study, we explored the interaction of profilaggrin N-terminal domain (PND) with loricrin in keratinocytes and epidermis. Confocal immunofluorescence microscopic analysis of human epidermis showed that PND colocalized with loricrin. Loricrin nucleofected into HaCaT cells colocalized with PND in the nucleus and cytoplasm. The PND localizes to both the nucleus and cytoplasm of epidermal granular layer cells. Nucleofected PND also colocalized with keratin 10 (K10) in the nucleus and cytoplasm. Immunoelectron microscopic analysis of human epidermis confirmed the findings in nucleofected keratinocytes. Yeast two-hybrid assays showed that the B domain of human and mouse PND interacted with loricrin. The glutathione S-transferase (GST) pull-down analysis using recombinant GST-PND revealed that PND interacted with loricrin and K10. Knockdown of PND in an organotypic skin culture model caused loss of filaggrin expression and a reduction in both the size and number of keratohyalin granules, as well as markedly reduced expression of loricrin. Considering that expression of PND is closely linked to keratinocyte terminal differentiation, we conclude that PND interacts with loricrin and K10 in vivo and that these interactions are likely to be relevant for cornified envelope assembly and subsequent epidermal barrier formation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin increases the expression of genes in the human epidermal differentiation complex and accelerates epidermal barrier formation

Chloracne is commonly observed in people exposed to dioxins, yet the mechanism of toxicity is not well understood. The pathology of chloracne is characterized by hyperkeratinization of the interfollicular squamous epithelium, hyperproliferation and hyperkeratinization of hair follicle cells as well as a metaplastic response of the ductular sebum secreting sebaceous glands. In vitro studies using normal human epidermal keratinocytes to model interfollicular human epidermis demonstrate a 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated acceleration of differentiation and increase in gene expression of several prodifferentiation genes, including filaggrin (FLG). Here, we demonstrated that the TCDD-activated aryl hydrocarbon receptor (AHR) bound a small fragment of DNA upstream of the transcriptional start sites of the FLG gene, containing one of two candidate xenobiotic response elements (XREs). Reporter assays using the promoter region of FLG containing the two putative XREs indicated that the increase in this messenger RNA (mRNA) was due to TCDD-mediated enhanced transcription, which was lost when both XREs were mutated. As FLG is part of the human epidermal differentiation complex (EDC) found on chromosome 1, we measured mRNAs from an additional 18 EDC genes for their regulation by TCDD. Of these genes, 14 were increased by TCDD. Immunoblot assays demonstrated that the proteins of FLG as well as that of another prodifferentiation gene, small proline rich protein 2, were increased by TCDD. In utero exposure to TCDD accelerated the formation of the epidermal barrier in the developing mouse fetus by approximately 1 day. These results indicate that the epidermal permeability barrier is a functional target of the TCDD-activated AHR.

Filaggrin genotype in ichthyosis vulgaris predicts abnormalities in epidermal structure and function

Although it is widely accepted that filaggrin (FLG) deficiency contributes to an abnormal barrier function in ichthyosis vulgaris and atopic dermatitis, the pathomechanism of how FLG deficiency provokes a barrier abnormality in humans is unknown. We report here that the presence of FLG mutations in Caucasians predicts dose-dependent alterations in epidermal permeability barrier function. Although FLG is an intracellular protein, the barrier abnormality occurred solely via a paracellular route in affected stratum corneum. Abnormal barrier function correlated with alterations in keratin filament organization (perinuclear retraction), impaired loading of lamellar body contents, followed by nonuniform extracellular distribution of secreted organelle contents, and abnormalities in lamellar bilayer architecture. In addition, we observed reductions in corneodesmosome density and tight junction protein expression. Thus, FLG deficiency provokes alterations in keratinocyte architecture that influence epidermal functions localizing to the extracellular matrix. These results clarify how FLG mutations impair epidermal permeability barrier function.

Deconstructing the skin: cytoarchitectural determinants of epidermal morphogenesis

To provide a stable environmental barrier, the epidermis requires an integrated network of cytoskeletal elements and cellular junctions. Nevertheless, the epidermis ranks among the body's most dynamic tissues, continually regenerating itself and responding to cutaneous insults. As keratinocytes journey from the basal compartment towards the cornified layers, they completely reorganize their adhesive junctions and cytoskeleton. These architectural components are more than just rivets and scaffolds - they are active participants in epidermal morphogenesis that regulate epidermal polarization, signalling and barrier formation.

Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin

Caspase-14 is a protease that is mainly expressed in suprabasal epidermal layers and activated during keratinocyte cornification. Caspase-14-deficient mice display reduced epidermal barrier function and increased sensitivity to UVB radiation. In these mice, profilaggrin, a protein with a pivotal role in skin barrier function, is processed correctly to its functional filaggrin (FLG) repeat unit, but proteolytic FLG fragments accumulate in the epidermis. In wild-type stratum corneum, FLG is degraded into free amino acids, some of which contribute to generation of the natural moisturizing factors (NMFs) that maintain epidermal hydration. We found that caspase-14 cleaves the FLG repeat unit and identified two caspase-14 cleavage sites. These results indicate that accumulation of FLG fragments in caspase-14(-/-) mice is due to a defect in the terminal FLG degradation pathway. Consequently, we show that the defective FLG degradation in caspase-14-deficient skin results in substantial reduction in the amount of NMFs, such as urocanic acid and pyrrolidone carboxylic acid. Taken together, we identified caspase-14 as a crucial protease in FLG catabolism.

Optimization of filaggrin expression and processing in cultured rat keratinocytes

BACKGROUND

In normal mammalian epidermis, cell division occurs primarily in the basal layer where cells are attached to the basement membrane. Upon release from the basement membrane, these basal cells stop dividing and begin to differentiate and stratify producing cornified cells expressing differentiation markers, including the keratin bundling protein filaggrin, and cornified envelope proteins. Little is understood about the regulatory mechanisms of these processes. A rat epidermal keratinocyte cell line synthesizing and processing profilaggrin at confluence in a synchronous manner for 4-5 days provides a useful culture model for epidermal differentiation. Profilaggrin expression in this cell line however decreases with passaging, and its processing involves extensive nonspecific proteolysis.

OBJECTIVE

Our objective was to identify culture conditions that effect the decrease in profilaggrin expression with passaging and nonspecific proteolysis of profilaggrin in order to study epidermal differentiation more closely.

METHOD

The large amount of nonspecific proteolysis suggested autophagocytosis. To test this, cells were cultured in the presence of 3-methyladenine (3-MA). Two known gradients in epidermis are decreasing serum components and increasing calcium concentrations in the upper cell layers. To determine whether these gradients effected processing, cells were cultured in serum/DMEM or in serum-free KGM and under varying external calcium concentrations. Cells were also cultured in presence of aminoguanidine in an attempt to maintain profilaggrin expression with passaging.

RESULTS

Profilaggrin expression was enhanced in the presence of 3-MA, with optimum around 6mM. In the absence of aminoguanidine, profilaggrin expression decreased as a function of increasing passage number; in its presence, profilaggrin expression remained high in some, but not in all of the independently maintained cell lines. Thus, culturing in aminoguanidine was necessary, but not sufficient, for sustained ability to express profilaggrin at confluence. Production of filaggrin from profilaggrin was maximized in a serum-free medium with [Ca(2+)] at 5mM. Filaggrin associates with phospholipid vesicles in vitro forming aggregates similar to those seen in vivo, suggesting that filaggrin release induces vesicular aggregation and autophagocytosis.

CONCLUSION

We have used a keratinocyte cell line that synthesizes and processes profilaggrin after confluence as a culture model to study epidermal differentiation. In this system profilaggrin processing must be preceded by inhibition of autophagosome formation and/or modulation of vesicular trafficking, and these processes are regulated by epidermal calcium and serum factor gradients.

Knockdown of filaggrin impairs diffusion barrier function and increases UV sensitivity in a human skin model

Loss-of-function mutations in the filaggrin gene are associated with ichthyosis vulgaris and atopic dermatitis. To investigate the impact of filaggrin deficiency on the skin barrier, filaggrin expression was knocked down by small interfering RNA (siRNA) technology in an organotypic skin model in vitro. Three different siRNAs each efficiently suppressed the expression of profilaggrin and the formation of mature filaggrin. Electron microscopy revealed that keratohyalin granules were reduced in number and size and lamellar body formation was disturbed. Expression of keratinocyte differentiation markers and the composition of lipids appeared normal in filaggrin-deficient models. The absence of filaggrin did not render keratins 1, 2, and 10 more susceptible to extraction by urea, arguing against a defect in aggregation. Despite grossly normal stratum corneum morphology, filaggrin-deficient skin models showed a disturbed diffusion barrier function in a dye penetration assay. Moreover, lack of filaggrin led to a reduction in the concentration of urocanic acid, and sensitized the organotypic skin to UVB-induced apoptosis. This study thus demonstrates that knockdown of filaggrin expression in an organotypic skin model reproduces epidermal alterations caused by filaggrin mutations in vivo. In addition, our results challenge the role of filaggrin in intermediate filament aggregation and establish a link between filaggrin and endogenous UVB protection.

Filaggrin deficiency confers a paracellular barrier abnormality that reduces inflammatory thresholds to irritants and haptens

BACKGROUND

Mutations in the human filaggrin gene (FLG) are associated with atopic dermatitis (AD) and are presumed to provoke a barrier abnormality. Yet additional acquired stressors might be necessary because the same mutations can result in a noninflammatory disorder, ichthyosis vulgaris.

OBJECTIVE

We examined here whether FLG deficiency alone suffices to produce a barrier abnormality, the basis for the putative abnormality, and its proinflammatory consequences.

METHODS

By using the flaky-tail mouse, which lacks processed murine filaggrin because of a frameshift mutation in the gene encoding profilaggrin that mimics some mutations in human AD, we assessed whether FLG deficiency provokes a barrier abnormality, further localized the defect, identified its subcellular basis, and assessed thresholds to irritant- and hapten-induced dermatitis.

RESULTS

Flaky-tail mice exhibit low-grade inflammation with increased bidirectional, paracellular permeability of water-soluble xenobiotes caused by impaired lamellar body secretion and altered stratum corneum extracellular membranes. This barrier abnormality correlates with reduced inflammatory thresholds to both topical irritants and haptens. Moreover, when exposed repeatedly to topical haptens at doses that produce no inflammation in wild-type mice, flaky-tail mice experience a severe AD-like dermatosis with a further deterioration in barrier function and features of a T(H)2 immunophenotype (increased CRTH levels plus inflammation, increased serum IgE levels, and reduced antimicrobial peptide [mBD3] expression).

CONCLUSIONS

FLG deficiency alone provokes a paracellular barrier abnormality in mice that reduces inflammatory thresholds to topical irritants/haptens, likely accounting for enhanced antigen penetration in FLG-associated AD.

Association of a genetic polymorphism (-44 C/G SNP) in the human DEFB1 gene with expression and inducibility of multiple beta-defensins in gingival keratinocytes

Background

Human β-defensins (hBDs) are antimicrobial peptides with a role in innate immune defense. Our laboratory previously showed that a single nucleotide polymorphism (SNP) in the 5' untranslated region of the hBD1 gene (DEFB1), denoted -44 (rs1800972), is correlated with protection from oral Candida. Because this SNP alters the putative mRNA structure, we hypothesized that it alters hBD1 expression.

Methods

Transfection of reporter constructs and evaluation of antimicrobial activity and mRNA expression levels in keratinocytes from multiple donors were used to evaluate the effect of this SNP on constitutive and induced levels of expression.

Results

Transfection of CAT reporter constructs containing the 5' untranslated region showed that the -44 G allele yielded a 2-fold increase in CAT protein compared to other common haplotypes suggesting a cis effect on transcription or translation. The constitutive hBD1 mRNA level in human oral keratinocytes was significantly greater in cells from donors with the -44 GG genotype compared to those with the common CC genotype. Surprisingly, the hBD3 mRNA level as well as antimicrobial activity of keratinocyte extracts also correlated with the -44 G allele. Induced levels of hBD1, hBD2, and hBD3 mRNA were evaluated in keratinocytes challenged with Toll-like receptor 2 and 4 ligands, interleukin-1β, TNFα, and interferon-γ (IFNγ). In contrast to constitutive expression levels, IFNγ-induced keratinocyte hBD1 and hBD3 mRNA expression was significantly greater in cells with the common CC genotype, but there was no clear correlation of genotype with hBD2 expression.

Conclusion

The DEFB1 -44 G allele is associated with an increase in overall constitutive antimicrobial activity and expression of hBD1 and hBD3 in a manner that is consistent with protection from candidiasis, while the more common C allele is associated with IFNγ inducibility of these β-defensins and is likely to be more protective in conditions that enhance IFNγ expression such as chronic periodontitis. These results suggest a complex relationship between genetics and defensin expression that may influence periodontal health and innate immune responses.

Association between P478S polymorphism of the filaggrin gene and risk of psoriasis in a Chinese population in Taiwan

Abnormal keratinocyte terminal differentiation is one of the important characteristics of psoriatic lesions. Filaggrin (FLG) is a key protein that facilitates the terminal differentiation of the epidermis. Thus, FLG genetic variants may modify the risk of psoriasis. In total, 314 patients with psoriasis and 611 control subjects were analyzed for the presence of FLG R501X, 2282del4 mutations, and P478S (rs11584340, C/T base change) polymorphism by polymerase chain reaction (PCR). The analysis revealed that both the R501X and 2282del4 mutations were not present in a subset of 200 patients (64%) with psoriasis. In contrast, a marginally significant difference (P = 0.020) was found in the distribution of rs11584340 genotype frequencies between psoriatic patients and controls. The frequency of the TT genotype in psoriasis patients was significantly higher than in controls (37.9% vs. 29.1%, respectively, P = 0.007). The T allele frequency of patients (60.5%) was also significantly higher than that of controls (53.9%) (P = 0.007). After adjusting for age and gender, carriers of the TT genotype were 1.46 (95% CI, 1.08-1.96) times more likely than non-carriers to have psoriasis (P = 0.013). In conclusion, our results suggest that FLG P478S polymorphism may confer susceptibility to the development of psoriasis among Taiwanese Chinese.

S100A2 gene is a direct transcriptional target of p53 homologues during keratinocyte differentiation

The p53 paralogues p73, p63 and their respective truncated isoforms have been shown to be critical regulators of developmental and differentiation processes. Indeed, both p73- and p63-deficient mice exhibit severe developmental defects. Here, we show that S100A2 gene, whose transcript and protein are induced during keratinocyte differentiation of HaCaT cells, is a direct transcriptional target of p73beta and DeltaNp63alpha and is required for proper keratinocyte differentiation. Transactivation assays reveal that p73beta and DeltaNp63alpha exert opposite transcriptional effects on S100A2 gene. While DeltaNp63alpha is found in vivo onto S100A2 regulatory regions predominantly in proliferating cells, p73beta is recruited in differentiating cells. Silencing of p73 impairs the induction of S100A2 during the differentiation of HaCaT cells. Moreover, silencing of p73 or S100A2 impairs the proper expression of keratinocyte differentiation markers. Of note, p53 family members do not trigger S100A2 gene expression in response to apoptotic doses of cisplatin and doxorubicin.

Interactions among stratum corneum defensive functions

With the exception of vitamin D production, virtually all epidermal functions can be considered as protective, or more specifically, as defensive in nature. Yet, the term "barrier function" of the stratum corneum (SC) is often used synonymously with only one such defensive function, although arguably its most important, i.e., permeability barrier homeostasis. Regardless of their hierarchy of relative importance, these critical protective functions largely reside in the SC. In this short review, we explore the ways in which the multiple defensive functions of the SC are linked and interrelated, either by their shared localization or by common biochemical processes.

Stratum corneum defensive functions: an integrated view

Most epidermal functions can be considered as protective, or more specifically, as defensive in nature. Yet, the term "barrier function" is often used synonymously with only one such defensive function, though arguably its most important, i.e., permeability barrier homeostasis. Regardless of their relative importance, these protective cutaneous functions largely reside in the stratum corneum (SC). In this review, I first explore the ways in which the multiple defensive functions of the SC are linked and interrelated, either by their shared localization or by common biochemical processes; how they are co-regulated in response to specific stressors; and how alterations in one defensive function impact other protective functions. Then, the structural and biochemical basis for these defensive functions is reviewed, including metabolic responses and signaling mechanisms of barrier homeostasis. Finally, the clinical consequences and therapeutic implications of this integrated perspective are provided.

Psoriatic architecture constructed by epidermal remodeling

Epidermal remodeling is the concept that epidermal architecture is determined by a simple self-organizing mechanism; epidermal hyperproliferation constructs typical psoriatic architecture. This is based on the assumption that the enlargements in both the two-dimensional proliferative compartment (basal cell layer) and three-dimensional whole epidermal volume coexist. During this process, the dermal papillae become markedly, but passively, expanded by enlargement of the proliferative compartment. This creates a considerable shrinkage force against the crowded basal cell layer, which is forced to lose adherence to the dermal extracellular matrix (ECM). This results in anoikis, a type of apoptosis characterized by cell detachment, and, consequently, a markedly diminished epidermal turnover time in psoriasis. The papillary shrinkage force also explains the fact that dermal papillary height does not exceed a certain limit. At the cessation of hyperproliferation a normalisation remodeling takes place toward normal tissue architecture. Thus the concept of epidermal remodeling explains the self-organizing mechanism of the architectural change in psoriasis, which is essentially a reversible disorder depending on epidermal hyperproliferation.

FOXN1 is critical for onycholemmal terminal differentiation in nude (Foxn1) mice

Nude mice have a mutation in the transcription factor Foxn1(nu), resulting in downregulation of hair keratins. Although hair follicles develop normally, the hair fibers become structurally weak, curl, and break off at the surface. Nails in nude mice are deformed, based on alterations of the onychocyte differentiation process. Elemental microanalysis of the nail plate reveals marked decreases in sulfur concentrations in the nude mouse nail plates. Immunohistochemistry shows a lack of keratin 1 expression in terminally differentiating keratinocytes of the nail matrix. Instead, the typical differentiation process of the matrix is altered toward an epidermis-like differentiation pattern, comprising the production of filaggrin-containing keratohyalin granules in cells resembling those of the stratum granulosum, which are never observed in normally haired mice. The nail plate has diffuse basophilic stippling. It is thinner than normal, weak, and in most Foxn1(nu)/Foxn1(nu) mice breaks where it separates from the hyponychium. These studies indicate that the Foxn1(nu) mutated gene has effects beyond downregulating keratin expression, including changes in filaggrin expression, and is critical for normal onycholemmal differentiation. The nails of nude mice provide new insights into the molecular controls of onychocyte differentiation, and they offer a useful model to investigate the pathogenesis of nail hypergranulosis, a common feature in human nail diseases.

Intermediate filament associated proteins

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Molecular cloning and expression of keratinocyte proline-rich protein, a novel squamous epithelial marker isolated during skin development

We describe a novel rat cDNA named keratinocyte proline-rich protein (KPRP) isolated by RNA differential display during skin development. We determine that KPRP is expressed in stratified squamous epithelium, and its approximately 2.8-kb cDNA encodes a 699-amino acid protein with high proline content (19%). KPRP is an insoluble protein, similar to most epidermal terminal differentiation-associated proteins. Immunoblot of the protein lysate from keratinocytes, using strong reducing conditions, demonstrates two KPRP bands of approximately 76 and 55 kDa size. KPRP is expressed in stratified squamous epithelia of skin, tongue, and esophagus. The initiation of KPRP expression in fetal rat skin at E17, E18, E19, E20, and E21 was analyzed by reverse transcription-PCR. Fetal skin at E19 and later expresses KPRP. In situ hybridization of skin from E18, E19, and 4-day-old neonatal rats demonstrates that interfollicular and follicular keratinocytes express KPRP. Anti-KPRP antibody demonstrates KPRP protein localizes to all layers of stratified epithelia in skin, tongue, and esophagus. In cultured dermal keratinocytes, KPRP is diffusely distributed throughout the cytoplasm with denser staining adjacent to the nuclear and plasma membranes. Additionally, immunoreactive intracellular granules are observed during keratinocyte detachment from their plastic substrate. Rat KPRP has 89% homology to a mouse genomic DNA sequence and 56% homology to a human hypothetical protein. We conclude that KPRP may be a new epidermal terminal differentiation-related protein expressed in stratified squamous epithelia. KPRP is expressed by fetal dermal keratinocytes during late gestation and is a new marker of maturing epidermis during fetal skin development.

Functional analysis of the profilaggrin N-terminal peptide: identification of domains that regulate nuclear and cytoplasmic distribution

Profilaggrin is expressed in the differentiating granular layer of epidermis and other stratified epithelia, where it forms a major component of cytoplasmic keratohyalin granules. It consists of two distinct domains, an N-terminal S100-like Ca2+- binding domain containing two EF-hands and multiple filaggrin units that aggregate keratin filaments in the stratum corneum. Here, we report structure-function studies of the N-terminal peptide from mouse, human, and rat profilaggrin. The profilaggrin N- terminal peptides of all species contain two S100-like EF-hands, bipartite nuclear localization sequences, and proprotein convertase cleavage sites. The nuclear localization signals in human and mouse profilaggrin were shown to be functional by transfection of epithelial cells and depended on the absence of filaggrin sequences. The nuclear localization of the processed (free) N-terminal peptide of human profilaggrin is consistent with immunolocalization findings in normal human skin and in parakeratotic skin disorders, which exhibit nuclear staining of granular and/or cornified layers. The mouse profilaggrin N-terminus undergoes proteolytic processing in two steps, first releasing an N-terminal peptide containing some filaggrin sequence and finally the free N-terminus of 28-30 kDa; these peptides have cytoplasmic and nuclear distributions, respectively, when expressed in transfected cells. The N-terminal processing may occur prior to or simultaneously with the proteolytic processing of the polyfilaggrin domain. The nuclear accumulation of the profilaggrin N-terminal peptide in epidermis and in transfected cells strongly suggests a calcium-dependent nuclear function for the profilaggrin N-terminus during epidermal terminal differentia tion when the free N-terminus is released from profilaggrin by specific proteolysis.

Regulated expression of human filaggrin in keratinocytes results in cytoskeletal disruption, loss of cell-cell adhesion, and cell cycle arrest

Filaggrin is an intermediate filament (IF)-associated protein that aggregates keratin IFs in vitro and is thought to perform a similar function during the terminal differentiation of epidermal keratinocytes. To further explore the role of filaggrin in the cytoskeletal rearrangement that accompanies epidermal differentiation, we generated keratinocyte cell lines that express human filaggrin using a tetracycline-inducible promoter system. Filaggrin expression resulted in reduced keratinocyte proliferation and caused an alteration in cell cycle distribution consistent with a post-G1 phase arrest. Keratin filament distribution was disrupted in filaggrin-expressing lines, while the organization of actin microfilaments and microtubules was more mildly affected. Evidence for direct interaction of filaggrin and keratin IFs was seen by overlay assays of GFP-filaggrin with keratin proteins in vitro and by filamentous filaggrin distribution in cells with low levels of expression. Cells expressing moderate to high levels of filaggrin showed a rounded cell morphology, loss of cell-cell adhesion, and compacted cytoplasm. There was also partial or complete loss of the desmosomal proteins desmoplakin, plakoglobin, and desmogleins from cell-cell borders, while the distribution of the adherens junction protein E-cadherin was not affected. No alterations in keratin cytoskeleton, desmosomal protein distribution, or cell shape were observed in control cell lines expressing beta-galactosidase. Filaggrin altered the cell shape and disrupted the actin filament distribution in IF-deficient SW13 cells, demonstrating that filaggrin can affect cell morphology independent of the presence of a cytoplasmic IF network. These studies demonstrate that filaggrin, in addition to its known effects on IF organization, can affect the distribution of other cytoskeletal elements including actin microfilaments, which can occur in the absence of a cytoplasmic IF network. Further, filaggrin can disrupt the distribution of desmosome proteins, suggesting an additional role(s) for this protein in the cytoskeletal and desmosomal reorganization that occurs at the granular to cornified cell transition during terminal differentiation of epidermal keratinocytes.

Regulation of human profilaggrin promoter activity in cultured epithelial cells by retinoic acid and glucocorticoids

Vitamin A and other retinoids profoundly inhibit both morphological and biochemical aspects of epidermal differentiation in vitro. Profilaggrin, like most other markers of keratinocyte differentiation, is negatively regulated by retinoic acid in vitro, both at the level of mRNA synthesis and by inhibiting the activity of endoproteases that convert profilaggrin to filaggrin. Profilaggrin is an abundant component of keratohyalin granules and forms the precursor of filaggrin, the keratin associated protein of the stratum corneum. In this report, we identify a region of the human profilaggrin promoter that is involved in the transcriptional regulation of expression by retinoic acid (RA). A series of promoter deletions linked to the chloramphenicol acetyl transferase (CAT) reporter gene were prepared and analyzed by transfection into Hela cells and keratinocytes. We also cotransfected vectors expressing retinoic acid receptor and cultured the transfected cells in the presence and absence of ligand. The region responsive to retinoic acid was localized to a 53 bp sequence between -1109 and -1056 (relative to the mRNA start site at +1) that contains a cluster of five retinoic acid response elements with variable spacing and orientation. In vitro gel shift analysis demonstrated that nuclear retinoid receptors do not bind directly to the identified sequence, suggesting that the mode of regulation by RA may be indirect or that binding requires another cofactor in addition to retinoid receptors. Whereas in keratin genes retinoic acid and glucocorticoid responsive sequences frequently coincide, the glucocorticoid response element in the profilaggrin promoter was located downstream of the RARE cluster between -965 and -951. These studies demonstrate that RA and glucocorticoids regulate profilaggrin expression at least in part by transcriptional mechanisms, via a region of the promoter that contains both retinoid and glucocorticoid responsive elements.

Expression of a truncated keratin 5 may contribute to severe palmar--plantar hyperkeratosis in epidermolysis bullosa simplex patients

Epidermolysis bullosa simplex are dominant disorders of skin fragility characterized by intraepidermal blistering upon mild mechanical trauma. Skin fragility is caused by expression of either an abnormal keratin 5 or an abnormal keratin 14 protein, which compromises the structure and function of the keratin cytoskeleton of basal cells. We report an epidermolysis bullosa simplex patient with a novel single base substitution (A-->T1414) that changes the lysine residue at amino acid 472 to a non-sense codon (K472X). This change predicts the synthesis of a truncated keratin 5, missing 119 amino acids, including the entire tail domain and the highly conserved KLLEGE motif at the carboxy terminus of the 2B domain of the central rod. Expression of an altered keratin 5, of predicted mass and pI for the product of the K472X allele, was documented by one- and two-dimensional western blots of protein extracts from patient skin. Ultrastructural analysis of the patient's nonhyperkeratotic skin was remarkable for basal keratinocytes with dense and irregular keratin filaments proximal to the basement membrane. Keratinocytes, transfected with a cDNA carrying the A-->T1414 non-sense mutation, overexpressed a truncated keratin 5, and showed a disorganized and collapsed keratin filament cytoskeleton. This is the second epidermolysis bullosa simplex patient reported with a premature termination mutation in the KLLEGE motif. The remarkable occurrence of severe palmar--plantar hyperkeratosis in both patients suggests that the keratin 5 tail domain may have unrecognized, but important, normal functions in palmar-plantar tissues.

Programmed cell death of keratinocytes culminates in apoptotic secretion of a humectant upon secretagogue action of acetylcholine

The programmed cell death of the stratified squamous epithelial cells comprising human epidermis culminates in abrupt transition of viable granular keratinocytes (KC) into dead corneocytes sloughed by the skin. The granular cell-corneocyte transition is associated with a loss in volume and dry cell weight but the mechanism for and biological significance of this form of keratinocyte apoptosis remain obscure. We show that terminally differentiated KC extrude into the intercellular spaces of living epidermis the cytoplasmic buds containing randomly congregated components of the cytosol as well as filaggrin, a precursor of the natural moisturizing factor. The discharge of secretory product is reminiscent of holocrine secretion, suggesting the term ‘apoptotic secretion’ for this novel, essential step in the process of cornification. The secretory product may become a part of the glycocalyx (a.k.a. ‘intercellular cement substance’ of epidermis) and serve as a humectant that counterbalances the osmotic pressure imposed by the natural moisturizing factor located in the stratum corneum comprised by corneocytes. The apoptotic secretion commences upon secretagouge action of acetylcholine which is synthesized and released by KC. A combination of a cholinergic nicotinic agonist and a muscarinic antagonist which increases intracellular calcium levels is required to trigger the apoptotic secretion. Analysis of the relative amounts of cholinergic enzymes and receptors expressed by KC capable of secretion and the pharmacological profiles of secretion regulation revealed an upward concentration gradient of free acetylcholine in epidermis which may provide for its unopposed secretagogue action via the m1 muscarinic and the (α)7, and (α)9 nicotinic receptor types expressed by KC at the latest stage of their development in the epidermis.