Hornerin, a novel profilaggrin-like protein and differentiation-specific marker isolated from mouse skin

Development-Associated Genes of the Epidermal Differentiation Complex (EDC)

The epidermal differentiation complex (EDC) is a cluster of genes that encode protein components of the outermost layers of the epidermis in mammals, reptiles and birds. The development of the stratified epidermis from a single-layered ectoderm involves an embryo-specific superficial cell layer, the periderm. An additional layer, the subperiderm, develops in crocodilians and over scutate scales of birds. Here, we review the expression of EDC genes during embryonic development. Several EDC genes are expressed predominantly or exclusively in embryo-specific cell layers, whereas others are confined to the epidermal layers that are maintained in postnatal skin. The S100 fused-type proteins scaffoldin and trichohyalin are expressed in the avian and mammalian periderm, respectively. Scaffoldin forms the so-called periderm granules, which are histological markers of the periderm in birds. Epidermal differentiation cysteine-rich protein (EDCRP) and epidermal differentiation protein containing DPCC motifs (EDDM) are expressed in the avian subperiderm where they are supposed to undergo cross-linking via disulfide bonds. Furthermore, a histidine-rich epidermal differentiation protein and feather-type corneous beta-proteins, also known as beta-keratins, are expressed in the subperiderm. The accumulating evidence for roles of EDC genes in the development of the epidermis has implications on the evolutionary diversification of the skin in amniotes.

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.

A proximity proteomics screen in three-dimensional spheroid cultures identifies novel regulators of lumen formation

Apical-basal cell polarity and lumen formation are essential features of many epithelial tissues, which are disrupted in diseases like cancer. Here, we describe a proteomics-based screen to identify proteins involved in lumen formation in three-dimensional spheroid cultures. We established a suspension-based culture method suitable for generating polarized cysts in sufficient quantities for proteomic analysis. Using this approach, we identified several known and unknown proteins proximally associated with PAR6B, an apical protein involved in lumen formation. Functional analyses of candidates identified PARD3B (a homolog of PARD3), RALB, and HRNR as regulators of lumen formation. We also identified PTPN14 as a component of the Par-complex that is required for fidelity of apical-basal polarity. Cells transformed with KRAS^G12V exhibit lumen collapse/filling concomitant with disruption of the Par-complex and down-regulation of PTPN14. Enforced expression of PTPN14 maintained the lumen and restricted the transformed phenotype in KRAS^G12V-expressing cells. This represents an applicable approach to explore protein–protein interactions in three-dimensional culture and to identify proteins important for lumen maintenance in normal and oncogene-expressing cells.

Protein profile of MCF-7 breast cancer cell line treated with lectin delivered by CaCO3NPs revealed changes in molecular chaperones, cytoskeleton, and membrane-associated proteins

The second most predominant cancer in the world and the first among women is breast cancer. We aimed to study the protein abundance profiles induced by lectin purified from the Agaricus bisporus mushroom (ABL) and conjugated with CaCO₃NPs in the MCF-7 breast cancer cell line. Two-dimensional electrophoresis (2-DE) and orbitrap mass spectrometry techniques were used to reveal the protein abundance pattern induced by lectin. Flow cytometric analysis showed the accumulation of ABL-CaCO₃NPs treated cells in the G1 phase than the positive control. Thirteen proteins were found different in their abundance in breast cancer cells after 24 h exposure to lectin conjugated with CaCO₃NPs. Most of the identified proteins were showing a low abundance in ABL-CaCO₃NPs treated cells in comparison to the positive and negative controls, including V-set and immunoglobulin domain, serum albumin, actin cytoplasmic 1, triosephosphate isomerase, tropomyosin alpha-4 chain, and endoplasmic reticulum chaperone BiP. Hornerin, tropomyosin alpha-1 chain, annexin A2, and protein disulfide-isomerase were up-regulated in comparison to the positive. Bioinformatic analyses revealed the regulation changes of these proteins mainly affected the pathways of 'Bcl-2-associated athanogene 2 signalling pathway', 'Unfolded protein response', 'Caveolar-mediated endocytosis signalling', 'Clathrin-mediated endocytosis signalling', 'Calcium signalling' and 'Sucrose degradation V', which are associated with breast cancer. We concluded that lectin altered the abundance in molecular chaperones/heat shock proteins, cytoskeletal, and metabolic proteins. Additionally, lectin induced a low abundance of MCF-7 cancer cell proteins in comparison to the positive and negative controls, including; V-set and immunoglobulin domain, serum albumin, actin cytoplasmic 1, triosephosphate isomerase, tropomyosin alpha-4 chain, and endoplasmic reticulum chaperone BiP.

Effect of SUV39H1 Histone Methyltransferase Knockout on Expression of Differentiation-Associated Genes in HaCaT Keratinocytes

Keratinocytes undergo a complex differentiation process, coupled with extensive changes in gene expression through which they acquire distinctive features indispensable for cells that form the external body barrier—epidermis. Disturbed epidermal differentiation gives rise to multiple skin diseases. The involvement of epigenetic factors, such as DNA methylation or histone modifications, in the regulation of epidermal gene expression and differentiation has not been fully recognized yet. In this work we performed a CRISPR/Cas9-mediated knockout of SUV39H1, a gene-encoding H3K9 histone methyltransferase, in HaCaT cells that originate from spontaneously immortalized human keratinocytes and examined changes in the expression of selected differentiation-specific genes located in the epidermal differentiation complex (EDC) and other genomic locations by RT-qPCR. The studied genes revealed a diverse differentiation state-dependent or -independent response to a lower level of H3K9 methylation. We also show, by means of chromatin immunoprecipitation, that the expression of genes in the LCE1 subcluster of EDC was regulated by the extent of trimethylation of lysine 9 in histone H3 bound to their promoters. Changes in gene expression were accompanied by changes in HaCaT cell morphology and adhesion.

Trichohyalin-like 1 protein plays a crucial role in proliferation and anti-apoptosis of normal human keratinocytes and squamous cell carcinoma cells

Epidermal differentiation is a complex process that requires the regulated and sequential expression of various genes. Most fused-type S100 proteins are expressed in the granular layer and it is hypothesized that these proteins may be associated with cornification and barrier formation. We previously identified a member of the fused-type S100 proteins, Trichohyalin-like 1 (TCHHL1) protein. TCHHL1 is distributed in the basal layer of the normal epidermis. Furthermore, the expression is markedly increased in cancerous/non-cancerous skin samples with the hyperproliferation of keratinocytes. We herein examined the role of TCHHL1 in normal human keratinocytes (NHKs) and squamous cell carcinoma (SCC). The knockdown of TCHHL1 by transfection with TCHHL1 siRNA significantly inhibited proliferation and induced the early apoptosis of NHKs. In TCHHL1-knockdown NHKs, the level of extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation was markedly decreased. In addition, the slight inhibition of v-akt murine thymoma viral oncogene homolog (AKT) phosphorylation and upregulation of forkhead box-containing protein O1(FOXO1), B-cell lymphoma2 (BCL2) and Bcl2-like protein 11 (BCL2L11) was observed. Skin-equivalent models built by TCHHL1-knockdown NHKs showed a markedly hypoplastic epidermis. These findings highlight that TCHHL1 plays an important role in homeostasis of the normal epidermis. TCHHL1 was expressed in the growing cells of cutaneous SCC; therefore, we next examined an association with the cell growth in HSC-1 cells (a human SCC line). In HSC-1 cells, the knockdown of TCHHL1 also suppressed cell proliferation and induced apoptosis. These cells showed an inhibition of phosphorylation of ERK1/2, AKT and signal transducers and activator of transcription 3, and the significant upregulation of FOXO1, BCL2, and BCL2L11. Accordingly, TCHHL1 is associated with survival of cutaneous SCC. In addition, we hypothesize that TCHHL1 may be a novel therapeutic target in cutaneous SCC.

Characterization and molecular evolution of claudin genes in the Pungitius sinensis

Claudins are a family of integrated membrane-bound proteins involving in paracellular tightness, barrier forming, ion permeability, and substrate selection at tight junctions of chordate epithelial and endothelial cells. Here, 39 putative claudin genes were identified in the Pungitius sinensis based on the high throughput RNA-seq. Conservative motif distribution in each group suggested functional relevance. Divergence of duplicated genes implied the species' adaptation to the environment. In addition, selective pressure analyses identified one site, which may accelerate functional divergence in this protein family. Pesticides cause environmental pollution and have a serious impact on aquatic organisms when entering the water. The expression pattern of most claudin genes was affected by organophosphorus pesticide, indicating that they may be involved in the immune regulation of organisms and the detoxification of xenobiotics. Protein-protein network analyses also exhibited 439 interactions, which implied the functional diversity. It will provide some references for the functional study on claudin genes.

Liquid-liquid phase separation drives skin barrier formation

At the body surface, skin's stratified squamous epithelium is challenged by environmental extremes. The surface of the skin is composed of enucleated, flattened surface squames. They derive from underlying, transcriptionally active keratinocytes that display filaggrin-containing keratohyalin granules (KGs) whose function is unclear. Here, we found that filaggrin assembles KGs through liquid-liquid phase separation. The dynamics of phase separation governed terminal differentiation and were disrupted by human skin barrier disease-associated mutations. We used fluorescent sensors to investigate endogenous phase behavior in mice. Phase transitions during epidermal stratification crowded cellular spaces with liquid-like KGs whose coalescence was restricted by keratin filament bundles. We imaged cells as they neared the skin surface and found that environmentally regulated KG phase dynamics drive squame formation. Thus, epidermal structure and function are driven by phase-separation dynamics.

Hornerin promotes tumor progression and is associated with poor prognosis in hepatocellular carcinoma

Background

The function of hornerin (HRNR), a member of the S100 protein family, is poorly clarified in the development of human tumors. The role of HRNR in hepatocellular carcinoma (HCC) progression is investigated in the study.

Methods

The expression levels of HRNR were assessed in tumor samples from a cohort of 271 HCC patients. The effect of HRNR on proliferation, colony formation and invasion of tumor cells was examined. We further determined the role of HRNR in tumor growth in vivo by using xenograft HCC tumor models. The possible mechanism of the HRNR promotion of HCC progression was explored.

Results

We found that HRNR was overexpressed in HCC tissues. The high expression of HRNR in HCCs was significantly associated with vascular invasion, poor tumor differentiation, and advanced TNM stage. The disease-free survival (DFS) and overall survival (OS) of HCC patients with high HRNR expression were poorer than those in the low HRNR expression group. HRNR expression was an independent risk factor linked to both poor DFS (HR = 2.209, 95% CI = 1.627–2.998,P <  0.001) and OS (HR = 2.459,95% CI = 1.736–3.484, P <  0.001). In addition, the knockdown of HRNR by shRNAs significantly inhibited the proliferation, colony formation, migration and invasion of HCC tumor cells. HRNR silencing led to the decreased phosphorylation of AKT signaling. Notably, tumor growth was markedly inhibited by HRNR silencing in a xenograft model of HCC.

Conclusions

HRNR promotes tumor progression and is correlated with a poor HCC prognosis. HRNR may contribute to HCC progression via the regulation of the AKT pathway.

Peptidylarginine deiminases and deiminated proteins at the epidermal barrier

Deimination or citrullination is a post-translational modification catalysed by a family of calcium-dependent enzymes called peptidylarginine deiminases (PADs). It corresponds to the transformation of arginine residues within a peptide sequence into citrulline residues. Deimination induces a decreased net charge of targeted proteins; therefore, it alters their folding and changes intra- and intermolecular ionic interactions. Deimination is involved in several physiological processes (inflammation, gene regulation, etc.) and human diseases (rheumatoid arthritis, neurodegenerative diseases, cancer, etc.). Here, we describe the PADs expressed in the epidermis and their known substrates, focusing on their role in the epidermal barrier function.

Identification of the S100 fused-type protein hornerin as a regulator of tumor vascularity

Sustained angiogenesis is essential for the development of solid tumors and metastatic disease. Disruption of signaling pathways that govern tumor vascularity provide a potential avenue to thwart cancer progression. Through phage display-based functional proteomics, immunohistochemical analysis of human pancreatic ductal carcinoma (PDAC) specimens, and in vitro validation, we reveal that hornerin, an S100 fused-type protein, is highly expressed on pancreatic tumor endothelium in a vascular endothelial growth factor (VEGF)-independent manner. Murine-specific hornerin knockdown in PDAC xenografts results in tumor vessels with decreased radii and tortuosity. Hornerin knockdown tumors have significantly reduced leakiness, increased oxygenation, and greater apoptosis. Additionally, these tumors show a significant reduction in growth, a response that is further heightened when therapeutic inhibition of VEGF receptor 2 (VEGFR2) is utilized in combination with hornerin knockdown. These results indicate that hornerin is highly expressed in pancreatic tumor endothelium and alters tumor vessel parameters through a VEGF-independent mechanism.Angiogenesis is essential for solid tumor progression. Here, the authors interrogate the proteome of pancreatic cancer endothelium via phage display and identify hornerin as a critical protein whose expression is essential to maintain the pancreatic cancer vasculature through a VEGF-independent mechanism.

Differential regulation of vaginal lipocalins (OBP, MUP) during the estrous cycle of the house mouse

Female house mice produce pheromone-carrying major urinary proteins (MUPs) in a cycling manner, thus reaching the maximum urinary production just before ovulation. This is thought to occur to advertise the time of ovulation via deposited urine marks. This study aimed to characterize the protein content from the house mouse vaginal flushes to detect putative vaginal-advertising molecules for a direct identification of reproductive states. Here we show that the mouse vaginal discharge contains lipocalins including those from the odorant binding (OBP) and major urinary (MUP) protein families. OBPs were highly expressed but only slightly varied throughout the cycle, whilst several MUPs were differentially abundant. MUP20 or 'darcin', was thought to be expressed only by males. However, in females it was significantly up-regulated during estrus similarly as the recently duplicated central/group-B MUPs (sMUP17 and highly expressed sMUP9), which in the mouse urine are male biased. MUPs rise between proestrus and estrus, remain steady throughout metestrus, and are co-expressed with antimicrobial proteins. Thus, we suggest that MUPs and potentially also OBPs are important components of female vaginal advertising of the house mouse.

Hornerin Is Involved in Breast Cancer Progression

Purpose

The S100 gene family, which comprises over 20 members, including S100A1, S100A2, S100A8, S100A9, profilaggrin, and hornerin encodes low molecular weight calcium-binding proteins with physiological and pathological roles in keratinization. Recent studies have suggested a link between S100 proteins and human cancer progression. The purpose of the present study was to determine the expression levels of hornerin, S100A8, and S100A9 and evaluate their roles in the progression of invasive ductal carcinoma (IDC).

Methods

Seventy cases of ductal carcinoma in situ (DCIS), IDC, and metastatic carcinoma in lymph nodes (MCN) were included. Tissue microarrays were constructed from lesions of DCIS, IDC, and MCN from the same patients. Expression of hornerin, S100A8, and S100A9 was analyzed using immunohistochemistry.

Results

The expression of hornerin was associated with the estrogen receptor-negative (p=0.003) and the human epidermal growth factor receptor 2-positive (p=0.002) groups. The expression of S100A8 was associated with a higher pT stage (p=0.017). A significant (p<0.001) correlation between the expression of S100A9 and S100A8 was also found. The mean percentages of hornerin-positive tumor cells in DCIS, IDC, and MCN were 1.0%±3.3% (mean±standard deviation), 12.0%±24.0%, and 75.3%± 27.6%, respectively. The expression of hornerin significantly (p<0.001) increased with the progression of carcinoma. The mean levels of S100A8 and S100A9 in DCIS, IDC, and MCN were not significantly (p>0.050) different. The expression of hornerin increased in a stepwise manner (DCIS<IDC<MCN).

Conclusion

Our data suggest that hornerin is involved in breast cancer progression and malignant transformation from preinvasive lesions.

Mechanisms of abnormal lamellar body secretion and the dysfunctional skin barrier in patients with atopic dermatitis

I review how diverse inherited and acquired abnormalities in epidermal structural and enzymatic proteins converge to produce defective permeability barrier function and antimicrobial defense in patients with atopic dermatitis (AD). Although best known are mutations in filaggrin (FLG), mutations in other member of the fused S-100 family of proteins (ie, hornerin [hrn] and filaggrin 2 [flg-2]); the cornified envelope precursor (ie, SPRR3); mattrin, which is encoded by TMEM79 and regulates the assembly of lamellar bodies; SPINK5, which encodes the serine protease inhibitor lymphoepithelial Kazal-type trypsin inhibitor type 1; and the fatty acid transporter fatty acid transport protein 4 have all been linked to AD. Yet these abnormalities often only predispose to AD; additional acquired stressors that further compromise barrier function, such as psychological stress, low ambient humidity, or high-pH surfactants, often are required to trigger disease. T(H)2 cytokines can also compromise barrier function by downregulating expression of multiple epidermal structural proteins, lipid synthetic enzymes, and antimicrobial peptides. All of these inherited and acquired abnormalities converge on the lamellar body secretory system, producing abnormalities in lipid composition, secretion, and/or extracellular lamellar membrane organization, as well as antimicrobial defense. Finally, I briefly review therapeutic options that address this new pathogenic paradigm.

Ultraviolet B irradiation induces the expression of hornerin in xenotransplanted human skin

Ultraviolet (UV) irradiation exerts numerous effects on the skin. Exposure of human skin to UVB at doses that induce mild sunburn reactions causes epidermal hyperproliferation and alterations in the expression of several epidermal differentiation markers. This study investigated the effects of UVB irradiation on the expression of hornerin, a member of the S100 fused-type protein family, using the xenotransplantation of normal human skin onto nude mice. Hornerin mRNA was detected in the UVB-irradiated skin on day 2 using RT-PCR. In accordance with the results of the RT-PCR, the expression of hornerin was induced in the granular layers of the UVB-exposed skin beginning two days after UVB irradiation and occurred in parallel with the expressions of cytokeratin 6 and Ki67. This finding suggests that hornerin induction in UVB-irradiated skin might be associated with epidermal hyperproliferation. This study demonstrated that hornerin is a protein whose expression is changed by UVB irradiation and suggests that the expression of hornerin might be a useful marker of acute UV damage in skin.

Trichohyalin-like 1 protein, a member of fused S100 proteins, is expressed in normal and pathologic human skin

Trichohyalin-like 1 (TCHHL1) protein is a novel member of the fused-type S100 protein gene family. The deduced amino acid sequence of TCHHL1 contains an EF-hand domain in the N-terminus, one trans-membrane domain and a nuclear localization signal. We generated specific antibodies against the C-terminus of the TCHHL1 protein and examined the expression of TCHHL1 proteins in normal and pathological human skin. An immunohistochemical study showed that TCHHL1 proteins were expressed in the basal layer of the normal epidermis. In addition, signals of TCHHL1 proteins were observed around the nuclei of cultured growing keratinocytes. Accordingly, TCHHL1 mRNA has been detected in normal skin and cultured growing keratinocytes. Furthermore, TCHHL1 proteins were strongly expressed in the peripheral areas of tumor nests in basal cell carcinomas and squamous cell carcinomas. A dramatic increase in the number of Ki67 positive cells was observed in TCHHL1-expressing areas. The expression of TCHHL1 proteins also increased in non-cancerous hyperproliferative epidermal tissues such as those of psoriasis vulgaris and lichen planus. These findings highlight the possibility that TCHHL1 proteins are expressed in growing keratinocytes of the epidermis and might be associated with the proliferation of keratinocytes.

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.

Hornerin, an S100 family protein, is functional in breast cells and aberrantly expressed in breast cancer

Background

Recent evidence suggests an emerging role for S100 protein in breast cancer and tumor progression. These ubiquitous proteins are involved in numerous normal and pathological cell functions including inflammatory and immune responses, Ca²⁺ homeostasis, the dynamics of cytoskeleton constituents, as well as cell proliferation, differentiation, and death. Our previous proteomic analysis demonstrated the presence of hornerin, an S100 family member, in breast tissue and extracellular matrix. Hornerin has been reported in healthy skin as well as psoriatic and regenerating skin after wound healing, suggesting a role in inflammatory/immune response or proliferation. In the present study we investigated hornerin’s potential role in normal breast cells and breast cancer.

Methods

The expression levels and localization of hornerin in human breast tissue, breast tumor biopsies, primary breast cells and breast cancer cell lines, as well as murine mammary tissue were measured via immunohistochemistry, western blot analysis and PCR. Antibodies were developed against the N- and C-terminus of the protein for detection of proteolytic fragments and their specific subcellular localization via fluorescent immunocytochemisty. Lastly, cells were treated with H₂O₂ to detect changes in hornerin expression during induction of apoptosis/necrosis.

Results

Breast epithelial cells and stromal fibroblasts and macrophages express hornerin and show unique regulation of expression during distinct phases of mammary development. Furthermore, hornerin expression is decreased in invasive ductal carcinomas compared to invasive lobular carcinomas and less aggressive breast carcinoma phenotypes, and cellular expression of hornerin is altered during induction of apoptosis. Finally, we demonstrate the presence of post-translational fragments that display differential subcellular localization.

Conclusions

Our data opens new possibilities for hornerin and its proteolytic fragments in the control of mammary cell function and breast cancer.

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.

Postnatal development- and age-related changes in DNA-methylation patterns in the human genome

Alterations in DNA methylation have been reported to occur during development and aging; however, much remains to be learned regarding post-natal and age-associated epigenome dynamics, and few if any investigations have compared human methylome patterns on a whole genome basis in cells from newborns and adults. The aim of this study was to reveal genomic regions with distinct structure and sequence characteristics that render them subject to dynamic post-natal developmental remodeling or age-related dysregulation of epigenome structure. DNA samples derived from peripheral blood monocytes and in vitro differentiated dendritic cells were analyzed by methylated DNA Immunoprecipitation (MeDIP) or, for selected loci, bisulfite modification, followed by next generation sequencing. Regions of interest that emerged from the analysis included tandem or interspersed-tandem gene sequence repeats (PCDHG, FAM90A, HRNR, ECEL1P2), and genes with strong homology to other family members elsewhere in the genome (FZD1, FZD7 and FGF17). Our results raise the possibility that selected gene sequences with highly homologous copies may serve to facilitate, perhaps even provide a clock-like function for, developmental and age-related epigenome remodeling. If so, this would represent a fundamental feature of genome architecture in higher eukaryotic organisms.

Cytoskeleton in motion: the dynamics of keratin intermediate filaments in epithelia

Epithelia are exposed to multiple forms of stress. Keratin intermediate filaments are abundant in epithelia and form cytoskeletal networks that contribute to cell type–specific functions, such as adhesion, migration, and metabolism. A perpetual keratin filament turnover cycle supports these functions. This multistep process keeps the cytoskeleton in motion, facilitating rapid and protein biosynthesis–independent network remodeling while maintaining an intact network. The current challenge is to unravel the molecular mechanisms underlying the regulation of the keratin cycle in relation to actin and microtubule networks and in the context of epithelial tissue function.

Deimination of human filaggrin-2 promotes its proteolysis by calpain 1

Filaggrin-2 (FLG2), a member of the S100-fused type protein family, shares numerous features with filaggrin (FLG), a key protein implicated in the epidermal barrier functions. Both display a related structural organization, an identical pattern of expression and localization in human epidermis, and proteolytic processing of a large precursor. Here, we tested whether FLG2 was a substrate of calpain 1, a calcium-dependent protease directly involved in FLG catabolism. In addition, deimination being critical for FLG degradation, we analyzed whether FLG2 deimination interfered with its proteolytic processing. With this aim, we first produced a recombinant form of FLG2 corresponding to subunits B7 to B10 fused to a COOH-terminal His tag. Incubation with calpain 1 in the presence of calcium induced a rapid degradation of the recombinant protein and the production of several peptides, as shown by Coomassie Blue-stained gels and Western blotting with anti-FLG2 or anti-His antibodies. MALDI-TOF mass spectrometry confirmed this result and further evidenced the production of non-immunoreactive smaller peptides. The degradation was not observed when a calpain 1-specific inhibitor was added. The calpain cleavage sites identified by Edman degradation were regularly present in the B-type repeats of FLG2. Moreover, immunohistochemical analysis of normal human skin revealed colocalization of FLG2 and calpain 1 in the upper epidermis. Finally, the FLG2 deiminated by human peptidylarginine deiminases was shown to be more susceptible to calpain 1 than the unmodified protein. Altogether, these data demonstrate that calpain 1 is essential for the proteolytic processing of FLG2 and that deimination accelerates this process.

Hornerin is a component of the epidermal cornified cell envelopes

A single-nucleotide polymorphism within the gene encoding hornerin (HRNR) has recently been linked with atopic dermatitis (AD) susceptibility. HRNR shares features with filaggrin, a key protein for keratinocyte differentiation, but conflicting reports have been published concerning its expression in the epidermis, and its role is still unknown. To analyze HRNR expression and function in the epidermis, anti-HRNR antibodies were produced and used in Western blot analysis and immunohistochemical, confocal, and immunoelectron microscopy analyses of human skin and of cornified cell envelopes purified from plantar stratum corneum. We also tested whether HRNR was a substrate of transglutaminases. In the epidermis, HRNR was detected at the periphery of keratohyalin granules in the upper granular layer and at the corneocyte periphery in the whole cornified layer. Detected in Western blot analysis as numerous bands, HRNR was relatively insoluble and only extracted from epidermis with urea and/or reducing agents. The presence of HRNR in the purified envelopes was confirmed by immunoelectron microscopy and by Western blot analysis after V8-protease digestion. HRNR was shown to be a substrate of transglutaminase 3. These data demonstrate that HRNR is a component of cornified cell envelopes of human epidermis. Its reduced expression in AD may contribute to the epidermal barrier defect observed in the disease.

Hematoxylin-stainability of keratohyalin granules is due to the novel component, fibrinogen γ-chain protein

Hematoxylin-stainability of keratohyalin granules (KHG) using biochemical and immunohistochemical techniques is due to the presence of a fibrinogen γ-chain protein. A protein with a molecular weight of 100 kDa was stained with anti-Ted-H-1 monoclonal antibody and hematoxylin solution (hematoxylin-stainable protein). Since the amino acid sequence of the hematoxylin-stainable protein was to that of fibrinogen γ-chain protein, a peptide was synthesized and an antibody against the peptide was produced. This antibody reacted with the hematoxylin-stainable protein and fibrinogen γ-chain protein on immunoblot analysis and with KHG on immunohistochemical examination. Furthermore, a commercial anti-fibrinogen γ-chain protein antibody (Ab) also reacted with the hematoxylin-stainable protein as well as fibrinogen. In contrast, anti-fibrinogen β-chain protein Ab did not react with the hematoxylin-stainable protein. The fibrinogen γ-chain protein also stained with hematoxylin. These findings suggested that fibrinogen γ-chain protein may be a novel component protein of KHG and may induce the hematoxylin-stainability of KHG.

Molecular identification and expression analysis of filaggrin-2, a member of the S100 fused-type protein family

Genes of the S100 fused-type protein (SFTP) family are clustered within the epidermal differentiation complex and encode essential components that maintain epithelial homeostasis and barrier functions. Recent genetic studies have shown that mutations within the gene encoding the SFTP filaggrin cause ichthyosis vulgaris and are major predisposing factors for atopic dermatitis. As a vital component of healthy skin, filaggrin is also a precursor of natural moisturizing factors. Here we present the discovery of a member of this family, designated as filaggrin-2 (FLG2) that is expressed in human skin. The FLG2 gene encodes a histidine- and glutamine-rich protein of approximately 248 kDa, which shares common structural features with other SFTP members, in particular filaggrin. We found that FLG2 transcripts are present in skin, thymus, tonsils, stomach, testis and placenta. In cultured primary keratinocytes, FLG2 mRNA expression displayed almost the same kinetics as that of filaggrin following Ca²⁺ stimulation, suggesting an important role in molecular regulation of epidermal terminal differentiation. We provide evidences that like filaggrin, FLG2 is initially expressed by upper granular cells, proteolytically processed and deposited in the stratum granulosum and stratum corneum (SC) layers of normal epidermis. Thus, FLG2 and filaggrin may have overlapping and perhaps synergistic roles in the formation of the epidermal barrier, protecting the skin from environmental insults and the escape of moisture by offering precursors of natural moisturizing factors.

Association of skin barrier genes within the PSORS4 locus is enriched in Singaporean Chinese with early-onset psoriasis

Psoriasis (OMIM#177900) is a common polygenic skin disorder affecting approximately 2% of the northern European population and 0.1% of the Han Chinese. Psoriasis patients suffer from chronic skin inflammation, manifested by erythematous scaly lesions. PSORS1-PSORS9 have been confirmed as psoriasis susceptibility loci in independent genetic studies on predominantly Caucasian populations, with psoriasis susceptibility loci (PSORS1, PSORS9) and additional loci at 9q33-34 and 2p22.3-11.2 reported in Han Chinese patients. In this study, we show the association of PSORS4 with psoriasis in Singaporean Chinese. Dense genotyping of single-nucleotide polymorphism-tagging candidate genes within the epidermal differentiation complex revealed significant association in the proximity of the involucrin gene (IVL); the strongest association was seen in early-onset psoriasis patients (P=0.0014). A follow-up genome-wide association screen localized the psoriasis susceptibility region to approximately 360 kb along chromosome 1 in the vicinity of IVL, small proline-rich region (SPRR) and proline-rich region 9 (PRR9) genes. The study of interactions between the causative variant(s) in this locus will provide insights into a possible role for epidermal barrier formation in the pathogenesis of psoriasis.

Large-scale identification of human genes implicated in epidermal barrier function

Identification of genes expressed in epidermal granular keratinocytes by ORESTES, including a number that are highly specific for these cells.

Background

During epidermal differentiation, keratinocytes progressing through the suprabasal layers undergo complex and tightly regulated biochemical modifications leading to cornification and desquamation. The last living cells, the granular keratinocytes (GKs), produce almost all of the proteins and lipids required for the protective barrier function before their programmed cell death gives rise to corneocytes. We present here the first analysis of the transcriptome of human GKs, purified from healthy epidermis by an original approach.

Results

Using the ORESTES method, 22,585 expressed sequence tags (ESTs) were produced that matched 3,387 genes. Despite normalization provided by this method (mean 4.6 ORESTES per gene), some highly transcribed genes, including that encoding dermokine, were overrepresented. About 330 expressed genes displayed less than 100 ESTs in UniGene clusters and are most likely to be specific for GKs and potentially involved in barrier function. This hypothesis was tested by comparing the relative expression of 73 genes in the basal and granular layers of epidermis by quantitative RT-PCR. Among these, 33 were identified as new, highly specific markers of GKs, including those encoding a protease, protease inhibitors and proteins involved in lipid metabolism and transport. We identified filaggrin 2 (also called ifapsoriasin), a poorly characterized member of the epidermal differentiation complex, as well as three new lipase genes clustered with paralogous genes on chromosome 10q23.31. A new gene of unknown function, C1orf81, is specifically disrupted in the human genome by a frameshift mutation.

Conclusion

These data increase the present knowledge of genes responsible for the formation of the skin barrier and suggest new candidates for genodermatoses of unknown origin.

Prediction of EF-hand calcium-binding proteins and analysis of bacterial EF-hand proteins

The EF-hand protein with a helix-loop-helix Ca(2+) binding motif constitutes one of the largest protein families and is involved in numerous biological processes. To facilitate the understanding of the role of Ca(2+) in biological systems using genomic information, we report, herein, our improvement on the pattern search method for the identification of EF-hand and EF-like Ca(2+)-binding proteins. The canonical EF-hand patterns are modified to cater to different flanking structural elements. In addition, on the basis of the conserved sequence of both the N- and C-terminal EF-hands within S100 and S100-like proteins, a new signature profile has been established to allow for the identification of pseudo EF-hand and S100 proteins from genomic information. The new patterns have a positive predictive value of 99% and a sensitivity of 96% for pseudo EF-hands. Furthermore, using the developed patterns, we have identified zero pseudo EF-hand motif and 467 canonical EF-hand Ca(2+) binding motifs with diverse cellular functions in the bacteria genome. The prediction results imply that pseudo EF-hand motifs are phylogenetically younger than canonical EF-hand motifs. Our prediction of Ca(2+) binding motifs provides not only an insight into the role of Ca(2+) and Ca(2+)-binding proteins in bacterial systems, but also a way to explore and define the role of Ca(2+) in other biological systems (calciomics).

Cornulin, a New Member of the “Fused Gene” Family, Is Expressed During Epidermal Differentiation

The protein encoded by the C1orf10 gene was described to be esophageal-specific and a marker for cancer development. This protein, however, has the previously unreported structural features of the "fused gene" family combining sequences and structural similarities of both the S100 proteins and precursor proteins of the cornified cell envelope as in profilaggrin, trichohyalin, and repetin. Since all members of this family are expressed in keratinocytes, we suspected a role in epidermal differentiation and named the protein cornulin. Here, we report that human cornulin mRNA is expressed primarily in the upper layers of differentiated squamous tissues including the epidermis. Using polyclonal peptide antibodies, we show that cornulin is expressed in the granular and lower cornified cell layers of scalp epidermis and foreskin, as well as in calcium-induced differentiated cultured keratinocytes. Ca(2+)-overlay assay indicated that EF-hand domains of cornulin are functional and bind calcium. In HeLa cells, cornulin, co-transfected with transglutaminase 1, was diffusely distributed throughout the cytoplasm in contrast to small proline-rich 4, which localized to the cell periphery. We conclude that cornulin is a new member of the "fused gene" family, does not appear to be a precursor of the cornified cell envelope by itself, and is a marker of late epidermal differentiation.

Isolation and Characterization of Human Repetin, a Member of the Fused Gene Family of the Epidermal Differentiation Complex

The human repetin gene is a member of the "fused" gene family and localized in the epidermal differentiation complex on chromosome 1q21. The "fused" gene family comprises profilaggrin, trichohyalin, repetin, hornerin, the profilaggrin-related protein and a protein encoded by c1orf10. Functionally, these proteins are associated with keratin intermediate filaments and partially crosslinked to the cell envelope (CE). Here, we report the isolation and characterization of the human repetin gene and of its protein product. The repetin protein of 784 amino acids contains EF (a structure resembling the E helix-calcium-binding loop-F helix domain of parvalbumin) hands of the S100 type and internal tandem repeats typical for CE precursor proteins, a combination which is characteristic for "fused" proteins. Repetin expression is scattered in the normal epidermis but strong in the acrosyringium, the inner hair root sheat and in the filiform papilli of the tongue. Ultrastructurally, repetin is a component of cytoplasmic non-membrane "keratohyalin" F-granules in the stratum granulosum of normal epidermis, similar to profilaggrin. Finally, we show that EF hands are functional and reversibly bind Ca(2+). Our results indicate that repetin is indeed a member of the fused gene family similar to the prototypical members profilaggrin and trichohyalin.

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.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

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.

Expression of hornerin in stratified squamous epithelium in the mouse: a comparative analysis with profilaggrin

We have recently identified a novel protein named hornerin, the structural features of which are most similar to those of profilaggrin, an essential protein for keratinization of epidermal tissues. In this study we examined the expression of hornerin compared with that of profilaggrin in various mouse tissues. Hornerin was expressed in the upper epidermis of newborn mouse skin, as was profilaggrin. In addition, both hornerin and profilaggrin were expressed in the tongue, esophagus, and forestomach. In all four tissues, immunostaining for hornerin and profilaggrin showed a granular pattern, and most of the signals for the two proteins were co-localized on keratohyalin granules. This was confirmed by double immunoelectron microscopy. Within keratohyalin granules, hornerin was detected more frequently in the periphery, whereas profilaggrin was equally distributed. A quantitative RT-PCR revealed that both genes were expressed at highest levels in the forestomach and at the next highest levels in skin. Profilaggrin mRNA was most abundant in the forestomach. In skin, the amount of hornerin mRNA was more than fourfold greater than the amount of profilaggrin mRNA. These results form the basis for a better understanding of possible overlapping and/or differential functions of hornerin and profilaggrin.

Suprabasin, a novel epidermal differentiation marker and potential cornified envelope precursor

The suprabasin gene is a novel gene expressed in mouse and human differentiating keratinocytes. We identified a partial cDNA encoding suprabasin using a suppression subtractive hybridization method between the proliferative basal and differentiating suprabasal populations of the mouse epidermis. A 3' gene-specific probe hybridized to transcripts of 0.7- and 2.2-kb pairs on Northern blots with specific detection in differentiated keratinocytes of stratified epithelia. The mouse gene was mapped to chromosome 7 by fluorescence in situ hybridization. This region is syntenic to human chromosome band 19q13.1, which contained the only region in the data bases with homology to the mouse suprabasin sequence. During embryonic mouse development, suprabasin mRNA was detected at day 15.5, coinciding with epidermal stratification. Suprabasin was detected in the suprabasal layers of the epithelia in the tongue, stomach, and epidermis. Differentiation of cultured primary epidermal keratinocytes with 0.12 mm Ca(2+) or 12-O-tetradecanoylphorbol-13-acetate treatment resulted in the induction of suprabasin. The 2.2-kb cDNA transcript encodes a protein of 72 kDa with a predicted isoelectric point of 6.85. The translated sequence has an amino-terminal domain, a central domain composed of repeats rich in glycine and alanine, and a carboxyl-terminal domain. The alternatively spliced 0.7-kb transcript encodes a smaller protein that shares the NH(2)- and COOH-terminal regions but lacks the repeat domain region. Cross-linking experiments indicate that suprabasin is a substrate for transglutaminase 2 and 3 activity. Altogether, these results indicate that the suprabasin protein potentially plays a role in the process of epidermal differentiation.

Characterization of mouse profilaggrin: evidence for nuclear engulfment and translocation of the profilaggrin B-domain during epidermal differentiation

Filaggrin is a keratin filament associated protein that is expressed in granular layer keratinocytes and derived by sequential proteolysis from a polyprotein precursor termed profilaggrin. Depending on the species, each profilaggrin molecule contains between 10 and 20 filaggrin subunits organized as tandem repeats with a calcium-binding domain at the N- terminal end. We now report the characterization of the complete mouse gene. The structural organization of the mouse gene is identical to the human profilaggrin gene and consists of three exons with a 4 kb intron within the 5' noncoding region and a 1.7 kb intron separating the sequences encoding the calcium-binding EF-hand motifs. A processed pseudogene was found embedded within the second intron. The third and largest exon encodes the second EF-hand, a basic domain (designated the B-domain) followed by 12 filaggrin repeats and a unique C-terminal tail domain. A polyclonal antibody raised against the conceptually translated sequence of the B-domain specifically stained keratohyalin granules and colocalized with a filaggrin antibody in granular layer cells. In upper granular layer cells, B-domain containing keratohyalin granules were in close apposition to the nucleus and, in some cells, appeared to be completely engulfed by the nucleus. In transition layer cells, B-domain staining was evident in the nucleus whereas filaggrin staining remained cytoplasmic. Nuclear staining of the B-domain was also observed in primary mouse keratinocytes induced to differentiate. This study has also revealed significant sequence homology between the mouse and human promoter sequences and in the calcium-binding domain but the remainder of the protein-coding region shows substantial divergence.

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.