Characterization of a cDNA clone encoding human filaggrin and localization of the gene to chromosome region 1q21

Epidermal barrier function in dry, flaky and sensitive skin: A narrative review

The stratum corneum (SC)-the outermost layer of the epidermis-is the principal permeability and protective barrier of the skin. Different components of the SC, including corneocytes, natural moisturizing factor, a variety of enzymes and their inhibitors, antimicrobial peptides and lipids, work interactively to maintain barrier function. The main barrier properties of the SC are the limitation of water loss and the prevention of infection and contact with potentially harmful exogenous factors. Although the SC functions consistently as a protective barrier throughout the body, variations in functions and morphology occur across body sites with age and skin type. Healthy SC function also depends on the interplay between the chemosensory barrier, the skin's microbiome and the innate immune system. Dysregulation of SC barrier function can lead to the development of skin disorders, such as dry, flaky or sensitive skin, but the complete underlying pathophysiology of these are not fully understood. This review provides insight into the current literature and emerging themes related to epidermal barrier changes that occur in the context of dry, flaky and sensitive skin. Additional studies are needed to further elucidate the underlying aetiology of dry, flaky and sensitive skin and to provide tailored treatment.

The Discovery and Function of Filaggrin

Keratohyalin granules were discovered in the mid-19th century in cells that terminally differentiate to form the outer, cornified layer of the epidermis. The first indications of the composition of these structures emerged in the 1960s from a histochemical stain for histidine, followed by radioautographic evidence of a high incidence of histidine incorporation into newly synthesized proteins in cells containing the granules. Research during the next three decades revealed the structure and function of a major protein in these granules, which was initially called the ‘histidine-rich protein’. Steinert and Dale named the protein ‘filaggrin’ in 1981 because of its ability to aggregate keratin intermediate filaments. The human gene for the precursor, ‘profilaggrin,’ was reported in 1991 to encode 10, 11 or 12 nearly identical repeats. Remarkably, the mouse and rat genes encode up to 20 repeats. The lifetime of filaggrin is the time required for keratinocytes in the granular layer to move into the inner cornified layer. During this transition, filaggrin facilitates the collapse of corneocytes into ‘building blocks’ that become an impermeable surface barrier. The subsequent degradation of filaggrin is as remarkable as its synthesis, and the end-products aid in maintaining moisture in the cornified layer. It was apparent that ichthyosis vulgaris and atopic dermatitis were associated with the absence of this protein. McLean’s team in 2006 identified the cause of these diseases by discovering loss-of-function mutations in the profilaggrin gene, which led to dysfunction of the surface barrier. This story illustrates the complexity in maintaining a healthy, functional epidermis.

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.

Refined Immunochemical Characterization in Healthy Dog Skin of the Epidermal Cornification Proteins, Filaggrin, and Corneodesmosin

Filaggrin (FLG) and corneodesmosin (CDSN) are two key proteins of the human epidermis. FLG loss-of-function mutations are the strongest genetic risk factors for human atopic dermatitis. Studies of the epidermal distribution of canine FLG and CDSN are limited. Our aim was to better characterize the distribution of FLG and CDSN in canine skin. Using immunohistochemistry on beagle skin, we screened a series of monoclonal antibodies (mAbs) specific for human FLG and CDSN. The cross-reactive mAbs were further used using immunoelectron microscopy and Western blotting. The structure of canine CDSN and FLG was determined using publicly available databases. In the epidermis, four anti-FLG mAbs stained keratohyalin granules in the granular keratinocytes and corneocyte matrix of the lower cornified layer. In urea-extracts of dog epidermis, several bands corresponding to proFLG and FLG monomers were detected. One anti-CDSN mAb stained the cytoplasm of granular keratinocytes and cells of both the inner root sheath and medulla of hair follicles. Dog CDSN was located in lamellar bodies, in the extracellular parts of desmosomes and in corneodesmosomes. A protein of 52 kDa was immunodetected. Genomic DNA analysis revealed that the amino acid sequence and structure of canine and human CDSN were highly similar.

Identification of a panel of serum protein markers in early stage of sepsis and its validation in a cohort of patients

BACKGROUND

Sepsis is a life-threatening illness with a challenging diagnosis. Current serum biomarkers are not sensitive enough for diagnosis. With the aim of finding proteins associated with sepsis, serum protein profile was compared between patients and healthy donors and serum classical inflammatory proteins were analyzed in both groups.

METHODS

Serum protein profiles were characterized by two-dimensional electrophoresis (2DE). Identification of the proteins was carried out by mass spectrophotometry and their validation was performed by Enzyme-Linked-ImmunoSorbent Assay (ELISA) in a cohort of 85 patients and 67 healthy donors. Seven classical inflammatory proteins were analyzed in the same cohort by ELISA: interleukin-2 receptor α-chain (sCD25), scavenger receptor cysteine-rich-type-1 (sCD163), tumor-necrosis factor receptor superfamily-member-6 (sFas), hemeoxigenase-1 decycling (HO-1), interleukin-6 (IL-6), interleukin-18 (IL-18) and intercellular adhesion-molecule-1 (sICAM-1).

RESULTS

After 2DE, 20 significantly differently expressed spots were identified by mass spectrometry analysis, revealing deregulation of six different proteins upon sepsis and 50% were validated by ELISA: Antithrombin-III (AT-III), Clusterin (CLUS) and Serum amyloid A-1 (SAA-1). Serum concentration of AT-III and CLUS was significantly lower in patients' serum, whereas SAA-1 showed higher values in septic patients. Serum concentration of the seven inflammatory proteins was significantly increased in septic patients. Functional analysis of the ten deregulated proteins revealed an enrichment of proteins related mainly to the activation of the immune response.

CONCLUSION

We have identified a panel of ten potential sepsis marker proteins biologically connected and validated in a large number of patients, whose analysis could be considered as a complementary tool for the diagnosis of sepsis.

Proteomic analysis of filaggrin deficiency identifies molecular signatures characteristic of atopic eczema

BACKGROUND

Atopic eczema (AE) is characterized by skin barrier and immune dysfunction. Null mutations in filaggrin (FLG), a key epidermal barrier protein, strongly predispose to AE; however, the precise role of FLG deficiency in AE pathogenesis remains incompletely understood.

OBJECTIVES

We sought to identify global proteomic changes downstream of FLG deficiency in human epidermal living skin-equivalent (LSE) models and validate findings in skin of patients with AE.

METHODS

Differentially expressed proteins from paired control (nontargeting control short hairpin RNA [shNT]) and FLG knockdown (FLG knockdown short hairpin RNA [shFLG]) LSEs were identified by means of proteomic analysis (liquid chromatography-mass spectrometry) and Ingenuity Pathway Analysis. Expression of key targets was validated in independent LSE samples (quantitative RT-PCR and Western blotting) and in normal and AE skin biopsy specimens (immunofluorescence).

RESULTS

Proteomic analysis identified 17 (P ≤ .05) differentially expressed proteins after FLG knockdown, including kallikrein-7 (KLK7; 2.2-fold), cyclophilin A (PPIA; 0.9-fold), and cofilin-1 (CFL1, 1.3-fold). Differential protein expression was confirmed in shNT/shFLG LSEs; however, only KLK7 was transcriptionally dysregulated. Molecular pathways overrepresented after FLG knockdown included inflammation, protease activity, cell structure, and stress. Furthermore, KLK7 (1.8-fold) and PPIA (0.65-fold) proteins were differentially expressed in lesional biopsy specimens from patients with AE relative to normal skin.

CONCLUSIONS

For the first time, we show that loss of FLG in the absence of inflammation is sufficient to alter the expression level of proteins relevant to the pathogenesis of AE. These include proteins regulating inflammatory, proteolytic, and cytoskeletal functions. We identify PPIA as a novel protein with levels that are decreased in clinically active AE skin and show that the characteristic upregulation of KLK7 expression in patients with AE occurs downstream of FLG loss. Importantly, we highlight disconnect between the epidermal proteome and transcriptome, emphasizing the utility of global proteomic studies.

The Molecular Revolution in Cutaneous Biology: EDC and Locus Control

The epidermal differentiation complex (EDC) locus consists of a cluster of genes important for the terminal differentiation of the epidermis. While early studies identified the functional importance of individual EDC genes, the recognition of the EDC genes as a cluster with its shared biology, homology, and physical linkage was pivotal to later studies that investigated the transcriptional regulation of the locus. Evolutionary conservation of the EDC and the transcriptional activation during epidermal differentiation suggested a cis-regulatory mechanism via conserved noncoding elements or enhancers. This line of pursuit led to the identification of CNE 923, an epidermal-specific enhancer that was found to mediate chromatin remodeling of the EDC in an AP-1 dependent manner. These genomic studies, as well as the advent of high-throughput sequencing and genome engineering techniques, have paved the way for future investigation into enhancer-mediated regulatory networks in cutaneous biology.

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.

The Cornified Cell Envelope: Loricrin and Transglutaminases

The cornified cell envelope (CE) of terminally differentiated human epidermis is a complex structure consisting of several defined protein constituents. The CE is the most insoluble component of the epidermis due to crosslinking by disulfide bonds as well as isodipeptide bonds that are formed by the action of transglutaminases (TGases). We have recently determined that loricrin is the major component of CE. We now have isolated and characterized its gene and showed that it has a simple structure with a single intron. We also show that the loricrin gene maps to position 1q21, which, coincidentally, is similar to the location of the profilaggrin and involucrin genes. Human loricrin in 26 kDa and consists of three long glycine-serine-cysteine rich sequence domains that contain quasi-repeating peptides and which form the novel glycine loop motif. These are interspersed by lysine+glutamine rich domains involved in isodipeptide crosslinks. The glycine loops are thought to be involved in organization of epidermal proteins and maintenance of the flexibility of the epidermis. By use of PCR analyses, we have found that human loricrin consists of two allelic size variants, due to sequence variations in the second glycine loop domain only, and these variants segregate in the human population by normal Mendelian mechanisms. Furthermore, there are multiple sequence variants within these two size class alleles due to various deletions of 12 bp (4 amino acids) in the major loop of this glycine loop domain. In order to study the expression and role of TGases in the formation of CE, we have isolated and sequenced cDNA and genomic clones encoding the TGase1 enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

The stratum corneum: the rampart of the mammalian body

BACKGROUND

The stratum corneum (SC) is the outermost region of the epidermis and plays key roles in cutaneous barrier function in mammals. The SC is composed of 'bricks', represented by flattened, protein-enriched corneocytes, and 'mortar', represented by intercellular lipid-enriched layers. As a result of this 'bricks and mortar' structure, the SC can be considered as a 'rampart' that encloses water and solutes essential for physiological homeostasis and that protects mammals from physical, chemical and biological assaults.

STRUCTURES AND FUNCTIONS

The corneocyte cytoskeleton contains tight bundles of keratin intermediate filaments aggregated with filaggrin monomers, which are subsequently degraded into natural moisturizing compounds by various proteases, including caspase 14. A cornified cell envelope is formed on the inner surface of the corneocyte plasma membrane by transglutaminase-catalysed cross-linking of involucrin and loricrin. Ceramides form a lipid envelope by covalently binding to the cornified cell envelope, and extracellular lamellar lipids play an important role in permeability barrier function. Corneodesmosomes are the main adhesive structures in the SC and are degraded by certain serine proteases, such as kallikreins, during desquamation.

CLINICAL RELEVANCE

The roles of the different SC components, including the structural proteins in corneocytes, extracellular lipids and some proteins associated with lipid metabolism, have been investigated in genetically engineered mice and in naturally occurring hereditary skin diseases, such as ichthyosis, ichthyosis syndrome and atopic dermatitis in humans, cattle and dogs.

NOT THAT RIGID MIDGETS AND NOT SO FLEXIBLE GIANTS: ON THE ABUNDANCE AND ROLES OF INTRINSIC DISORDER IN SHORT AND LONG PROTEINS

Intrinsically disordered proteins or proteins with disordered regions are very common in nature. These proteins have numerous biological functions which are complementary to the biological activities of traditional ordered proteins. A noticeable difference in the amino acid sequences encoding long and short disordered regions was found and this difference was used in the development of length-dependent predictors of intrinsic disorder. In this study, we analyze the scaling of intrinsic disorder in eukaryotic proteins and investigate the presence of length-dependent functions attributed to proteins containing long disordered regions.

Quantitative phosphoproteomics identifies filaggrin and other targets of ionizing radiation in a human skin model

Our objective here was to perform a quantitative phosphoproteomic study on a reconstituted human skin tissue to identify low- and high-dose ionizing radiation-dependent signalling in a complex three-dimensional setting. Application of an isobaric labelling strategy using sham and three radiation doses (3, 10, 200 cGy) resulted in the identification of 1052 unique phosphopeptides. Statistical analyses identified 176 phosphopeptides showing significant changes in response to radiation and radiation dose. Proteins responsible for maintaining skin structural integrity including keratins and desmosomal proteins (desmoglein, desmoplakin, plakophilin 1, 2 and 3) had altered phosphorylation levels following exposure to both low and high doses of radiation. Altered phosphorylation of multiple sites in profilaggrin linker domains coincided with altered profilaggrin processing suggesting a role for linker phosphorylation in human profilaggrin regulation. These studies demonstrate that the reconstituted human skin system undergoes a coordinated response to both low and high doses of ionizing radiation involving multiple layers of the stratified epithelium that serve to maintain tissue integrity and mitigate effects of radiation exposure.

Role of anti-citrullinated protein antibodies in diagnosis and prognosis of rheumatoid arthritis

Antibodies to citrullinated proteins/peptides (ACPAs) are the second serological marker to have recently been included in the 2010 ACR/EULAR Rheumatoid Arthritis (RA) Classification Criteria, which are focused on early diagnosis and therapy. This review discusses their history and some clinical aspects of ACPAs, focusing on the diagnostic utility of anti-cyclic citrullinated peptide (anti-CCP) antibodies as a marker of RA as compared to the widely used rheumatoid factor (RF). Simultaneously, this review aims to raise physician awareness and interest in anti-citrullinated vimentin antibody (anti-Sa/anti-MCV), another member of the ACPA family, which appears to have a better predictive value as a marker of RA than anti-CCP or RF and correlates closely with disease activity and therapeutic response among patients with RA.

Neonatal skin maturation--vernix caseosa and free amino acids

Neonatal skin hydration decreases rapidly postnatally and then increases, indicating adaptive changes in stratum corneum water handling properties. Transition from high to low humidity at birth may initiate filaggrin proteolysis to free amino acids. Neonatal skin with vernix caseosa retained is more hydrated than skin with vernix removed. This study examines the potential roles of free amino acids and vernix in postnatal adaptation of infant stratum corneum in vivo. Specifically, the ontogeny of free amino acid generation in neonatal stratum corneum and the role of vernix caseosa in postnatal adaptation were examined using high performance liquid chromatography. Free amino acids were quantified for infant skin samples collected at (i) birth and 1 month and (ii) birth and 24 hours after vernix caseosa retention or removal and compared to neonatal foreskin, vernix caseosa, and adult stratum corneum using t-tests, analysis of variance, or univariate procedures. Free amino acids were extremely low at birth, significantly higher 1 month later but lower than in adults. Vernix caseosa retention led to significantly higher free amino acids 24 hours after birth compared to infants with vernix caseosa removed, and it paralleled the higher stratum corneum hydration of vernix caseosa-retained skin. Vernix caseosa contained free amino acids, with glutamic acid and histidine levels higher than in infants. Free amino acids in vernix caseosa-retained skin appear to originate from vernix caseosa. Free amino acids were lower in neonatal foreskin than adult forearm stratum corneum. Arginine was higher than citrulline at birth, but levels were comparable in older infants. The free amino acid increase at 1 month may be initiated by the humidity transition at birth and supports results in animals. The findings have implications for infant skin care practices.

Eczema in early life: genetics, the skin barrier, and lessons learned from birth cohort studies

Eczema is a chronic inflammatory disorder of the skin that affects up to 30% of children. It often afflicts infants in the first few months of life and can be the first indicator of the atopic march. Recent results from birth cohort studies have uncovered novel information regarding genetic and environmental factors that promote the development of eczema. Birth cohort studies provide an optimal study design to elucidate these associations and prospectively track longitudinal data including exposure assessment and health outcomes from birth into early life and childhood. This is especially relevant for eczema given the age specific emergence of this disease. In this review, we will provide a general overview of pediatric eczema and discuss the important findings in the literature with respect to genetics and environmental exposures, highlighting those derived from birth cohort studies. Additionally, we will review how these relate to the atopic march, the hygiene hypothesis and the integrity of the skin barrier.

Raman profiles of the stratum corneum define 3 filaggrin genotype-determined atopic dermatitis endophenotypes

BACKGROUND

Filaggrin (FLG) has a central role in the pathogenesis of atopic dermatitis (AD). FLG is a complex repetitive gene; highly population-specific mutations and multiple rare mutations make routine genotyping complex. Furthermore, the mechanistic pathways through which mutations in FLG predispose to AD are unclear.

OBJECTIVES

We sought to determine whether specific Raman microspectroscopic natural moisturizing factor (NMF) signatures of the stratum corneum could be used as markers of FLG genotype in patients with moderate-to-severe AD.

METHODS

The composition and function of the stratum corneum in 132 well-characterized patients with moderate-to-severe AD were assessed by means of confocal Raman microspectroscopy and measurement of transepidermal water loss (TEWL). These parameters were compared with FLG genotype and clinical assessment.

RESULTS

Three subpopulations closely corresponding with FLG genotype were identified by using Raman spectroscopy. The Raman signature of NMF discriminated between FLG-associated AD and non-FLG-associated AD (area under the curve, 0.94; 95% CI, 0.91-0.99). In addition, within the subset of FLG-associated AD, NMF distinguished between patients with 1 versus 2 mutations. Five novel FLG mutations were found on rescreening outlying patients with Raman signatures suggestive of undetected mutations (R3418X, G1138X, S1040X, 10085delC, and L2933X). TEWL did not associate with FLG genotype subgroups.

CONCLUSIONS

Raman spectroscopy permits rapid and highly accurate stratification of FLG-associated AD. FLG mutations do not influence TEWL within established moderate-to-severe AD.

Purification and characterization of active caspase-14 from human epidermis and development of the cleavage site-directed antibody

Restricted expression of caspase-14 in differentiating keratinocytes suggests the involvement of caspase-14 in terminal differentiation. We purified active caspase-14 from human cornified cells with sequential chromatographic procedures. Specific activity increased 764-fold with a yield of 9.1%. Purified caspase-14 revealed the highest activity on WEHD-methylcoumaryl-amide (MCA), although YVAD-MCA, another caspase-1 substrate, was poorly hydrolyzed. The purified protein was a heterodimer with 17 and 11 kDa subunits. N-terminal and C-terminal analyses demonstrated that the large subunit consisted of Ser(6)-Asp(146) and N-terminal of small subunit was identified as Lys(153). We successfully developed an antiserum (anti-h14D146) directed against the Asp(146) cleavage site, which reacted only with active caspase-14 but not with procaspase-14. Furthermore we confirmed that anti-h14D146 did not show any reactivity to the active forms of other caspases. Immunohistochemical analysis demonstrated that anti-h14D146 staining was mostly restricted to the cornified layer and co-localized with some of the TUNEL positive-granular cells in the normal human epidermis. UV radiation study demonstrated that caspase-3 was activated and co-localized with TUNEL-positive cells in the middle layer of human epidermis. In contrast, we could not detect caspase-14 activation in response to UV. Our study revealed tightly regulated action of caspase-14, in which only the terminal differentiation of keratinocytes controls its activation process.

An update on the genetics of atopic dermatitis: scratching the surface in 2009

A genetic basis for atopic dermatitis (AD) has long been recognized. Historic documents allude to family history of disease as a risk factor. Before characterization of the human genome, heritability studies combined with family-based linkage studies supported the definition of AD as a complex trait in that interactions between genes and environmental factors and the interplay between multiple genes contribute to disease manifestation. A summary of more than 100 published reports on genetic association studies through mid-2009 implicates 81 genes, in 46 of which at least 1 positive association with AD has been demonstrated. Of these, the gene encoding filaggrin (FLG) has been most consistently replicated. Most candidate gene studies to date have focused on adaptive and innate immune response genes, but there is increasing interest in skin barrier dysfunction genes. This review examines the methods that have been used to identify susceptibility genes for AD and how the underlying pathology of this disease has been used to select candidate genes. Current challenges and the potential effect of new technologies are discussed.

Filaggrin in the frontline: role in skin barrier function and disease

Recently, loss-of-function mutations in FLG, the human gene encoding profilaggrin and filaggrin, have been identified as the cause of the common skin condition ichthyosis vulgaris (which is characterised by dry, scaly skin). These mutations, which are carried by up to 10% of people, also represent a strong genetic predisposing factor for atopic eczema, asthma and allergies. Profilaggrin is the major component of the keratohyalin granules within epidermal granular cells. During epidermal terminal differentiation, the approximately 400 kDa profilaggrin polyprotein is dephosphorylated and rapidly cleaved by serine proteases to form monomeric filaggrin (37 kDa), which binds to and condenses the keratin cytoskeleton and thereby contributes to the cell compaction process that is required for squame biogenesis. Within the squames, filaggrin is citrullinated, which promotes its unfolding and further degradation into hygroscopic amino acids, which constitute one element of natural moisturising factor. Loss of profilaggrin or filaggrin leads to a poorly formed stratum corneum (ichthyosis), which is also prone to water loss (xerosis). Recent human genetic studies strongly suggest that perturbation of skin barrier function as a result of reduction or complete loss of filaggrin expression leads to enhanced percutaneous transfer of allergens. Filaggrin is therefore in the frontline of defence, and protects the body from the entry of foreign environmental substances that can otherwise trigger aberrant immune responses.

Carrier status for the common R501X and 2282del4 filaggrin mutations is not associated with hearing phenotypes in 5,377 children from the ALSPAC cohort

BACKGROUND

Filaggrin is a major protein in the epidermis. Several mutations in the filaggrin gene (FLG) have been associated with a number of conditions. Filaggrin is expressed in the tympanic membrane and could alter its mechanical properties, but the relationship between genetic variation in FLG and hearing has not yet been tested.

METHODOLOGY/PRINCIPAL FINDINGS

We examined whether loss-of function mutations R501X and 2282del4 in the FLG gene affected hearing in children. Twenty eight hearing variables representing five different aspects of hearing at age nine years in 5,377 children from the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort were tested for association with these mutations. No evidence of association was found between R501X or 2282del4 (or overall FLG mutation carrier status) and any of the hearing phenotypes analysed.

CONCLUSIONS/SIGNIFICANCE

In conclusion, carrier status for common filaggrin mutations does not affect hearing in children.

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.

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

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

LEDGF/DFS70, a major autoantigen of atopic dermatitis, is a component of keratohyalin granules

Lens epithelium-derived growth factor/dense fine speckles 70 kDa protein (LEDGF/DFS70) is a transcriptional cofactor, a transcriptional activator, survival factor, and HIV-1 transporter. It is also a major autoantigen in patients with atopic dermatitis (AD), because autoantibodies to this protein are found in approximately 30% of AD patients. To better understand the role of autoantibodies and autoantigens in the pathogenesis of AD, we examined the distribution of LEDGF/DFS70 in the epidermis of normal human skin by light and electron microscopic immunocytochemistry. Increased amounts of LEDGF/DFS70 were located in the nuclei of cells in the basal layer, whereas the cytoplasm of cells in the granular layer stained for LEDGF/DFS70 by light microscopy. Using immunoelectron microscopy, we observed the accumulation of LEDGF/DFS70 in keratohyalin granules (KGs) in the cytoplasm of cells in the granular layer. In addition, Ig heavy chain-binding protein/glucose-regulated protein, 78-kDa (Bip/GRP78), a stress sensing protein in the endoplasmic reticulum, colocalized with LEDGF/DFS70 in the KGs. These results suggest that LEDGF/DFS70 is predominantly located in the nucleus of the basal epidermal cells and translocates into the cytoplasm during differentiation. Once in the cytoplasm, LEDGF/DFS70 accumulates in the KGs in the granular layer. Finally, LEDGF/DFS70, a "nuclear" autoantigen in AD, may play a functional role in the KGs.

The cornified envelope: a model of cell death in the skin

The epidermis functions as a barrier against the environment by means of several layers of terminally differentiated, dead keratinocytes - the cornified layer, which forms the endpoint of epidermal differentiation and death. The cornified envelope replaces the plasma membrane of differentiating keratinocytes and consists of keratins that are enclosed within an insoluble amalgam of proteins, which are crosslinked by transglutaminases and surrounded by a lipid envelope. New insights into the molecular mechanisms and the physiological endpoints of cornification are increasing our understanding of the pathological defects of this unique form of programmed cell death, which is associated with barrier malfunctions and ichthyosis.

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.

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

A novel mouse cDNA named hornerin was isolated by RNA differential display applied to developing mouse skin. Hornerin, which has 2,496 amino acids, comprises EF-hand domains at the N terminus followed by a spacer sequence and a large repetitive domain, indicating that hornerin is a novel member of the "fused gene"-type cornified envelope precursor protein family. The repetitive domain of hornerin was found to be rich in glycine, serine, and glutamine. Hornerin was expressed in the tongue, esophagus, forestomach, and skin among the adult mouse tissues examined, all of them cornifying stratified epithelium. In the embryonic mouse skin, hornerin mRNA was first detected on gestational day 15.5 in the epidermis coincidentally with the formation of a granular layer. In accordance with this, hornerin was detected in the granular and cornified layers of the mature epidermis. In the granular cells of the epidermis, the hornerin protein was detected in keratohyalin granules together with profilaggrin. Furthermore, Western blot analysis of the mouse skin showed that the hornerin protein was cleaved during the process of epidermal differentiation, indicating possible posttranslational proteolytic processing as is observed in profilaggrin. Differentiation of primary mouse epidermal keratinocytes with 0.12 mm Ca(2+) resulted in the induction of hornerin. These results indicate that hornerin is structurally as well as functionally most similar to profilaggrin among the family members and possibly plays pleiotropic roles, including a role in cornification.

Profilaggrin requires both linker and filaggrin peptide sequences to form granules: implications for profilaggrin processing in vivo

Filaggrin is an intermediate filament associated protein that aids the packing of keratin filaments during terminal differentiation of keratinocytes. Premature aggregation of keratin filaments is prevented by filaggrin expression as the inactive precursor, profilaggrin, which is localized in keratohyalin granules in vivo. Profilaggrin is phosphorylated and contains multiple filaggrin repeats separated by a hydrophobic linker peptide. We have previously shown that filaggrin constructs containing the linker, when transiently transfected into epithelial cells, lead to expression of a protein that resembles keratohyalin (Dale et al. J Invest Dermatol 108:179-187 1997). To characterize further the region(s) of the linker and/or filaggrin that are necessary for granule formation, we generated several mutant constructs from Flag-FG-1, and generated fusions of filaggrin with green fluorescent protein. We also subjected profilaggrin to protein phosphatase 2A treatment and measured its subsequent solubility. We found that granular morphology is not dependent on the linker or conserved phosphorylation sites, nor is solubility affected by protein phosphatase 2A treatment. Granule morphology was abrogated only in a truncated construct, which still contains the linker. A construct consisting of 16 amino acids of filaggrin fused to green fluorescent protein led to rounded and bizarrely shaped transfected cells with compact keratin filaments, suggesting that very little filaggrin sequence is required for keratin filament interaction. Radiolabeled filaggrin-green fluorescent protein constructs specifically bound keratin in overlay assays confirming that the observed cytoskeletal collapse is due to filaggrin-keratin interaction. Our findings indicate that profilaggrin must be extensively processed before it loses both its granule forming ability as well as its insolubility, suggesting that granule formation in vivo correlates with insolubility in vitro. Further, filaggrin retains its ability to bind keratin as it is degraded to smaller peptides.

Mouse Sprr2 genes: a clustered family of genes showing differential expression in epithelial tissues

Small proline-rich (SPR) proteins are structural components of the cornified cell envelope of stratified squamous epithelia. They are subdivided into three families, i.e., SPR1, SPR2, and SPR3, of which the SPR2 family is the most complex. To understand the significance of this complexity, we have isolated 11 mouse Sprr2 genes, constructed a provisional physical map of the Sprr2 locus on mouse Chromosome 3, and examined the expression patterns of the Sprr2 genes in mouse epithelial tissues. The 11 Sprr2 sequences are highly conserved with a central domain containing a variable number of repeats. In situ hybridization showed the Sprr2 expression to be confined to epithelia. RT-PCR using primers specific for each of the 11 Sprr2 members demonstrated varying degrees of expression among the individual Sprr2 members in different tissues. The correlation between the physical location of the genes in the Sprr2 locus and their expression patterns suggests multiple levels of controlled expression.

The Epitopes Targeted by the Rheumatoid Arthritis-Associated Antifilaggrin Autoantibodies are Posttranslationally Generated on Various Sites of (Pro)Filaggrin by Deimination of Arginine Residues

Antifilaggrin autoantibodies (AFA) are a population of IgG autoantibodies associated to rheumatoid arthritis (RA), which includes the so-called “antikeratin” Abs and antiperinuclear factor. AFA are the most specific serological markers of RA. We previously showed that they recognize human epidermal filaggrin and other profilaggrin-related proteins of various epithelial tissues. Here, we report further characterization of the protein Ags and epitopes targeted by AFA. All the Ags that exhibit numerous neutral/acidic isoelectric variants were immunochemically demonstrated to be deiminated proteins. In vitro deimination of a recombinant human filaggrin by a peptidylarginine deiminase generated AFA epitopes on the protein. Moreover, two of three filaggrin-derived synthetic peptides with a citrulline in the central position were specifically and widely recognized by AFA affinity-purified from a series of RA sera. These results indicate that citrulline residues are constitutive of the AFA epitopes, but only in the context of specific amino acid sequences of filaggrin. In competition experiments, the two peptides abolished the AFA reactivity of RA sera, showing that they present major AFA epitopes. These data should help in the identification of a putative deiminated AFA-inducing or cross-reactive articular autoantigen and provide new insights into the pathogenesis of RA. They could also open the way toward specific immunosuppressive and/or preventive therapy of RA.

The major inherited disorders of cornification. New advances in pathogenesis

This article provides a synopsis of the major (most common) inherited disorders of cornification. It also reviews the recent advances that have been made for each disorder and their practical applications.

Reduced stability and bi-allelic, coequal expression of profilaggrin mRNA in keratinocytes cultured from subjects with ichthyosis vulgaris

Ichthyosis vulgaris (IV) is an inherited scaling skin disorder in which expression of profilaggrin is reduced. Previous studies have indicated that the reduction is caused by defective post-transcriptional control of gene expression. Here we present evidence that profilaggrin mRNA in keratinocytes cultured from subjects with IV is intrinsically unstable and has a shorter half-life compared with that in normal cells. When IV-affected keratinocytes were treated with the protein synthesis inhibitor cycloheximide, the steady-state level of profilaggrin mRNA was increased due to stabilization of the transcript. In addition, the number of filaggrin repeats within the profilaggrin gene was studied. The number of filaggrin repeats (10-12) in individuals with IV did not differ from that of unaffected subjects. Expression of the gene was bi-allelic and coequal in both control and affected individuals. Our results suggest a model in which a labile ribonuclease and a stabilizing factor may modulate the profilaggrin mRNA steady-state level in normal cells, whereas the stabilizing factor may be absent or functionally inactive in IV-affected keratinocytes.

Citrulline is an essential constituent of antigenic determinants recognized by rheumatoid arthritis-specific autoantibodies

Only a few autoantibodies that are more or less specific for RA have been described so far. The rheumatoid factor most often tested for is not very specific for RA, while the more specific antiperinuclear factor for several reasons is not routinely used as a serological parameter. Here we show that autoantibodies reactive with synthetic peptides containing the unusual amino acid citrulline, a posttranslationally modified arginine residue, are specifically present in the sera of RA patients. Using several citrulline-containing peptide variants in ELISA, antibodies could be detected in 76% of RA sera with a specificity of 96%. Sera showed a remarkable variety in the reactivity pattern towards different citrulline-containing peptides. Affinity-purified antibodies were shown to be positive in the immunofluorescence-based antiperinuclear factor test, and in the so-called antikeratin antibody test, and were reactive towards filaggrin extracted from human epidermis. The specific nature of these antibodies and the presence of these antibodies early in disease, even before other disease manifestations occur, are indicative for a possible role of citrulline-containing epitopes in the pathogenesis of RA.

Repetin (Rptn), a new member of the "fused gene" subgroup within the S100 gene family encoding a murine epidermal differentiation protein

We report the cloning and characterization of a murine epidermal differentiation gene, repetin (Rptn), exhibiting striking similarity to the genes of the intermediate filament-associated proteins profilaggrin and trichohyalin. The repetin gene consists of three exons and two introns. The first exon is short and untranslated. The deduced amino acid sequence distributed between exons II and III contains 1130 amino acids with a calculated molecular mass of 130 kDa and pI of 7.7. The amino terminus exhibits significant homology to the S100 proteins containing two calcium-binding motifs of the EF-hand type. The remainder coding sequence contains a central segment consisting of 49 tandem repeats of a 12-amino-acid sequence rich in glutamines. By fluorescence in situ hybridization the repetin gene was localized to chromosome band 3 F1-2. Expression of repetin mRNA is detectable in the stratified internal epithelia of forestomach and tongue and to a lesser degree in normal skin epidermis, where it is restricted to the differentiated suprabasal cell layers. Based on its chromosomal localization, its genomic organization, and its stage-specific expression during late epidermal differentiation, as well as on the structural features of the encoded protein, we conclude that the repetin gene represents a novel member of the "fused gene" subgroup of the S100 gene family encoding multifunctional epidermal matrix proteins.

Cytoplasmic processing of human profilaggrin by active mu-calpain

The differentiation of keratinocytes involves numerous steps including the formation of the cornified envelope and the aggregation of keratin filaments by filaggrin monomer molecules. In this study, we investigated whether mu-calpain is involved in the processing of profilaggrin to filaggrin monomers by using both an active mu-calpain specific antibody and a 27-mer synthetic calpastatin peptide, a cell-permeable calpain-specific inhibitor. Upon incubation of cultured keratinocytes with Ca2+ for 96 hours, active mu-calpain with a molecular mass of 76 kDa appeared preceded by an increase in mu-calpain mRNA. In synchrony with the appearance of active mu-calpain, the processing of profilaggrin occurred. Furthermore, the Ca2+-induced activation of mu-calpain and the processing of profilaggrin were affected by the addition of the synthetic calpastatin inhibitor. These results indicate that the activation of mu-calpain plays a major role in the profilaggrin processing.

S100A11, S100A10, annexin I, desmosomal proteins, small proline-rich proteins, plasminogen activator inhibitor-2, and involucrin are components of the cornified envelope of cultured human epidermal keratinocytes

The cornified envelope (CE) is an insoluble sheath of epsilon-(gamma-glutamyl)lysine cross-linked protein, which is deposited beneath the plasma membrane during keratinocyte terminal differentiation. We have probed the structure of the CE by proteolytic cleavage of purified CE fragments isolated from CEs formed spontaneously in cell culture. CNBr digestion, followed by trypsin and then proteinase K treatment released 25%, 42%, and 18%, respectively, of the CE protein. Purification and sequencing of released peptides has identified two novel CE precursors, S100A11 (S100C, calgizzarin) and S100A10 (calpactin light chain). We also sequenced peptides derived from annexin I and plasminogen activator inhibitor 2, two putative envelope precursors, as well as portions of the well established CE precursor proteins SPR1A, SPR1B, and involucrin. Many desmosomal components were identified (desmoglein 3, desmocolin A/B, desmoplakin I, plakoglobin, and plakophilin), indicating that desmosomes become cross-linked into the CE. Fragments derived from envoplakin, the recently sequenced 210-kDa membranous CE precursor protein, which also appears to be a desmosomal component, were also identified. Analysis of the pattern of peptide release following the sequential digestion indicates that S100A11 is anchored to the envelope via Gln102 and/or Lys103 at the carboxyl terminus and at Lys3, Lys23, and/or Gln22 in the amino terminus. A similar type of analysis indicates that small proline-rich proteins 1A and 1B (SPR1A and SPR1B) become cross-linked at the amino terminus (residues 1-23) and the carboxyl terminus (residues 86-89). No loricrin, cystatin A, or elafin peptides were detected.

Evidence for specific proteolytic cleavage of the N-terminal domain of human profilaggrin during epidermal differentiation

Profilaggrin is a large phosphoprotein that is expressed in the granular cells of epidermis where it is localized in keratohyalin. It consists of multiple copies of single filaggrin units plus N- and C-terminal sequences that differ from filaggrin. Profilaggrin is dephosphorylated and proteolytically processed during terminal differentiation to yield filaggrin, which associates with keratin intermediate filaments to form macrofibrils in the lower layers of the stratum corneum. The N-terminal sequence of human profilaggrin comprises two distinct domains; an acidic A domain of 81 amino acids that binds Ca2+, and a cationic B domain of 212 residues. In this report, we further characterize the N-terminal domain by immunohistochemistry and immunoblot analysis using anti-peptide antibodies raised to the A and B regions. All of these antibodies (n = 4) immunostained keratohyalin in the granular layer of human epidermis and also showed some reaction with the lower stratum corneum. In immunoblot studies, the high molecular weight human profilaggrin reacted with both B domain antibodies whereas it showed a weak and variable reaction with A domain antibodies. In addition to profilaggrin, a cationic 32-kDa protein was detected with all N-terminal antibodies. A similar-sized N-terminal peptide was also produced by in vitro proteolysis of human profilaggrin with endoproteinase 1 (PEP1), a protease involved in processing of mouse profilaggrin, and in cultured rat epidermal keratinocytes transfected with a human profilaggrin cDNA construct. Evidence for at least one additional cleavage within the N-terminal domain is shown by immunoreactivity of smaller (16-20 kDa) acidic and basic proteins with A and B domain antibodies, respectively. These results demonstrate that the N-terminal domain is an integral part of profilaggrin in keratohyalin but is proteolytically cleaved from profilaggrin during the terminal differentiation of keratinocytes to yield a 32-kDa peptide.

Activator protein 1 activity is involved in the regulation of the cell type-specific expression from the proximal promoter of the human profilaggrin gene

The human profilaggrin gene is expressed in the granular layer during the late stages of terminal differentiation of the epidermis. In in vitro transcription experiments we show that the abundance of the mRNA and the specificity of the expression are regulated primarily at the level of transcription. We found that the 5'-flanking sequences control the transcription in a keratinocyte-specific mode and that as little as 116 base pairs preceding the mRNA initiation site is sufficient to restrict the transcription to epidermal cells in vitro. This specificity depends critically on the presence of an activator protein 1 (AP1) motif at position -77. Binding of c-jun/c-fos heterodimers to this sequence confers high levels of expression to the reporter constructs in cultured epidermal keratinocytes, while having little effect in HeLa cells. The transactivating properties of c-jun are essential in this process. On the other hand, junB and junD, which are involved in transactivating the transcription of earlier epidermal differentiation markers, control profilaggrin expression through a pathway which does not depend on a direct binding at the AP1 site and is not cell-type specific. These data indicate that AP1 factors are involved in a complex, multipathway regulation of the profilaggrin gene expression.

Decreased expression of filaggrin in atopic skin

The amounts of the epidermal proteins filaggrin, involucrin, cystatin A and Ted-H-1 antigen produced during the terminal differentiation of keratinocytes were immunohistochemically measured in lesional and nonlesional skin of atopic dermatitis (AD) patients. In addition, the amount of filaggrin in the skin of the inner surface of the upper arm of AD patients (nonlesional skin) and normal controls, obtained by punch biopsy, was measured by an enzyme-linked immunosorbent assay (ELISA) technique. The immunohistochemical study showed that all four proteins were decreased in lesional skin. By contrast, only filaggrin was decreased in nonlesional skin of AD patients. The ELISA showed that the amount of filaggrin in the skin of the inner surface of the upper arm was 2.48 +/- 0.45 microgram/7 mm2 (n = 8) in AD patients, which was 32% of that in the normal controls (7.7 +/- 0.55 microgram/7 mm2; n = 4). This decrease in filaggrin production in atopic skin may be one of the reasons why atopic skin can easily become dry, because filaggrin is thought to be the precursor protein of the emollient factors in the stratum corneum. The evidence that only the expression of filaggrin was suppressed in AD patients, though the genes of filaggrin and involucrin are localized to a very restricted portion of the same gene 1q21, indicates that the filaggrin gene does not share regulatory elements with the involucrin gene.

Microheterogeneity of human filaggrin: analysis of a complex peptide mixture using mass spectrometry

Filaggrin is the product of posttranslational processing of the large, epidermal protein profilaggrin, which consists of 10 or more tandem filaggrin domains plus an amino and a carboxyl domain. According to fragmentary cDNA sequences, the filaggrin domains in the human protein vary at 40% of the amino acid positions; hence, mature filaggrin is a population of homologous but heterogeneous proteins, even within one individual. Available gene sequences give only a limited picture of the heterogeneity of human filaggrin protein because no complete human profilaggrin gene has been sequenced. Questions about the extent of heterogeneity of filaggrin within and between individuals have not been answered, nor have questions concerning the limited proteolytic cleavage of human profilaggrin that generates filaggrin in vivo. In order to address these questions and to provide an analysis of the primary structure of human filaggrins, we employed various methods of mass spectrometry. The intact protein and a tryptic digest of the mixture of human filaggrins were examined by matrix-assisted laser desorption time-of-flight mass spectrometry. Tryptic digests of human filaggrin from single individuals were also separated and analyzed by liquid chromatography/mass spectrometry (LC/MS) (using electrospray mass spectrometry), and specific peptides were identified by tandem mass spectrometry (MS/MS). A robust data analysis program, Sherpa, was developed to facilitate the interpretation of both LC/MS and MS/MS. These experiments show that human filaggrin includes heterogeneity not yet seen in cDNA sequences, but that much structure is highly conserved. Interestingly, we found that the heterogeneity is conserved among individuals. An approximation of the regions linking filaggrins in human profilaggrin is developed. These investigations provide a unique test of the limits of tryptic mapping of complex mixtures using mass spectrometry.

Differentiation-associated localization of small proline-rich protein in normal and diseased human skin

The expression of SPRR (small proline-rich protein) was investigated in normal human skin and in diseased skin from patients with psoriasis, squamous cell carcinoma, basal cell epithelioma, naevus pigmentosus, ichthyosis vulgaris and several inflammatory skin diseases, by immunohistochemical staining. A polyclonal antibody was raised against a synthetic peptide for a C-terminal common region for SPRR1 and SPRR3. In immunoblot analysis, a positive band of 18 kDa was detected, which showed the presence of SPRR1 in human epidermal keratinocytes. In normal epidermis, positive staining for SPRR was observed in keratinocytes in the granular layer and the uppermost or two spinous cell layers, with no staining of the other spinous or basal layers. The staining was obvious at the cell periphery, weak at the cytoplasm, and absent in the nucleus. Staining was observed in several outer layers of the follicular infundibulum to the isthmus. No staining was detected in the inner root sheath of the hair follicles, hair matrix, sebaceous gland, eccrine gland, eccrine duct, melanocytes, Langerhans cells or fibroblasts. The arrectores pilorum, striated muscles, muscle layers of vessels, and myoepithelia of eccrine gland, were weakly stained. In psoriatic skin, stained keratinocytes were distributed in the spinous cell layers except for the basal layer. In ichthyosis vulgaris, SPRR was barely expressed in the uppermost living cell layers of the epidermis. In epidermolytic hyperkeratosis, degenerated squamous cells widely expressed SPRR. In Darier's disease, dyskeratotic cells were clearly stained. In squamous cell carcinoma, staining was observed in keratotic cells around horny pearls. In basal cell epithelioma, naevus pigmentosus, and malignant melanoma, the tumour cells or naevus cells were not stained. The distribution of SPRR was similar to that of involucrin in normal and several diseased skin, except for ichthyosis vulgaris. We conclude that SPRR is expressed in close association with epidermal differentiation in normal skin and skin diseases. The alteration of the expression of the proteins correlated to terminal differentiation, and differs from disease to disease.

Sequence and expression patterns of mouse SPR1: Correlation of expression with epithelial function

A final event in the terminal differentiation of stratified squamous epithelia is the formation of a cornified cell envelope, which is a complex of several proteins cross-linked together by transglutaminases. One set of proteins is the family of small proline rich (SPR) proteins. In human foreskin epidermal cell envelopes, SPRs serve as cross-bridging proteins among the more abundant loricrin. In order to study further their evolution and expression, we have isolated and sequenced cDNAs encoding two mouse SPR1 proteins, SPR1a and SPR1b Comparative sequence analysis showed the preservation of the overall structure of mammalian SPR1 proteins with highly conserved termini and a central peptide domain repeated 13 (SPE1a) or seven (SPR1b) times. Tissues obtained from mouse fetal, newborn, and adult skin were tested by Northern blot analyses, in situ hybridization and immunohistochemistry using an antibody raised to a synthetic peptide corresponding to the C terminus of the SPR1a protein. Skin expression was first detected in fetal periderm in anagen hair follicles of newborn and older mice, and in the thickened epidermis of the lip and footpad, but no signal was detected in interfollicular trunk epidermis. High levels of SPR1a expression were found in epithelia from the forestomach and penis, and in benign squamous papillomas. Other epithelia expressing SPR1a include the tongue, esophagus, and vagina. Whenever detected, SPR1a positive staining was present in the spinous and granular layers. In the forestomach and papillomas, the periphery of cells in the cornified layer was also stained. Our results suggest that SPR1a participates widely in the construction of cell envelopes in cornifying epithelia characterized by either increased thickness or a requirement for extreme flexibility. Based on its likely function as a cross-bridging protein in cell envelopes, we conclude that the mechanical attributes of cell envelopes may be determined in part by the SPR1 content, in accordance with the specific function of the epithelium.

Transcriptional analysis of the 5'-noncoding region of the human involucrin gene

Human involucrin whose gene transcription is directed by a 2456-nucleotide (nt) 5'-noncoding region is a structural component of the epithelial cornified layer. Transient transfection assays demonstrated that this region is transcriptionally active in multiplying keratinocytes and is enhanced by 2 mM CaCl2 treatment. Calcium-independent transcriptional activity and the interaction with the AP-1 transcriptional factor was located on the proximal part (nt -159 to -1) of the 5'-noncoding region. However, CaCl2 responsiveness was mapped to a distal 1185-nt fragment (nt -2456 to -1272). Moreover, this fragment potentiated the Herpes simplex thymidine kinase promoter in normal keratinocytes and is responsive to calcium treatment in a cell type-specific manner. Interestingly, the absence of a 491-nt fragment located between the two enhancer domains (nt -651 to -160) resulted in transcriptional activation in multiplying keratinocytes. This fragment interacts with AP-1 and the YY1 transcriptional silencer. It is concluded that human involucrin 5'-noncoding region contains at least three regulatory domains, a distal CaCl2-responsive enhancer, a putative transcriptional silencer (that interacts with AP-1 and YY1), and a proximal enhancer/promoter (that interacts with AP-1). Thus, this study demonstrates the presence of particular transcriptional factors can potentially regulate the human involucrin expression.

Identification of proteins that are abnormally regulated in differentiated cultured human keratinocytes

Comparison of the protein expression patterns of proliferating normal primary human keratinocytes plated in serum-free medium (SFKM), supplemented with epidermal growth factor (EGF) and bovine pituitary extract (BPE), and similar cultures induced to differentiate by the addition of Dulbecco's modified Eagle medium (DMEM), containing 10% fetal calf serum (FCS), revealed several known and unknown polypeptides that are abnormally regulated in the differentiated cells. Upregulated proteins included keratins (keratins 6, 10/11, 14 and 16), members of the S100 protein family psoriasin, MRP8, MRP14 and S100c), actin-binding proteins (gelsolin and tropomyosin 9220), annexins (annexins IV and VIII), hsp28, the fatty acid binding protein 5 (FABP5), the squamous cell carcinoma (SCC) antigen, members of the 14-3-3 family, involucrin, E-cadherin, cystatin A, desmoglein and integrins alpha 2 and beta 1, as well as several proteins of as yet unknown identity. The highest upregulated proteins corresponded to psoriasin (124.0 times), MRP8 (42.4 times), MRP14 (14.9 times), tropomyosin 9220 (11.5 times), involucrin (11.1 times), and FABP5 (9.1 times). FABP5, hsp28, and tropomyosin 9220 were also highly upregulated in quiescent keratinocytes indicating that their increased levels in the differentiated cells may be due to loss of proliferative activity. Highly downregulated proteins included PAI-2, tropomyosins 9213, 9121 and 9122, keratin 5, calnexin, 14-3-3 beta and eta, nucleoside diphosphate kinase A, Rho GDIs, hsp60, hnRNPs H and C2, alpha-enolase, eIF-4D, thioredoxin, annexins III and V, moesin, nucleolar protein B23, GST pi and PCNA/cyclin. Both the high expression of keratin 6 and 16--which are markers for an alternative pathway of keratinocyte differentiation--as well as the extremely high upregulation of some members of the S100 protein family indicate that the cells have differentiated via an abnormal pathway.

Characterization of profilaggrin endoproteinase 1. A regulated cytoplasmic endoproteinase of epidermis

Profilaggrin, an insoluble precursor of the intermediate filament-associated protein filaggrin, contains multiple internal repeats (PIRs). At terminal differentiation of epidermis, proteolytic processing within a "linker" region of each PIR releases soluble filaggrin in a two-stage process. The first stage endoproteinase (PEP1, profilaggrin endoproteinase 1) cleaves mouse profilaggrin at a subset of the linkers, yielding processing intermediates consisting of several filaggrin repeats. An epidermal endoproteinase that cleaves the requisite linker subset has been purified 4,966-fold from mouse epidermal extracts. SDS-polyacrylamide gel electrophoresis demonstrated a band of molecular mass of 29.5 kDa that correlated with the activity. Labeling with [3H]diisopropylfluorophosphate identified PEP1 as a serine protease; inhibitor studies suggest that it is similar to chymotrypsin, as expected from previous in vivo studies. The purified PEP1 cleaved a peptide derived from profilaggrin (P1) at three residues within and adjacent to a multiple tyrosine sequence, consistent with the in vivo processing sites. No exopeptidase activity was detected. PEP1 is only active toward insoluble profilaggrin, resulting in partial solubilization, consistent with a role in dispersal of profilaggrin during terminal differentiation. In contrast to the specific cleavage of mouse profilaggrin, PEP1 cleaved all linker regions of rat profilaggrin. Studies with phosphorylated P1 suggest that PEP1 specificity may be partly regulated by profilaggrin phosphorylation.

Refined mapping of the psoriasin gene S100A7 to chromosome 1cen-q21

Psoriasin is a low molecular weight protein of the S100 family, which is highly upregulated in psoriatic epidermis, and whose function is related to skin inflammatory responses. We have applied a cDNA probe from the corresponding psoriasin gene S100A7 in a refined localisation analysis. S100A7 was mapped physically by human/rodent somatic cell hybrid analysis, and more precisely genetically by multilocus linkage analysis of 40 CEPH (Centre d'Etude du Polymorphisme Humain) families. The resulting 12-point linkage map was supported by odds of at least 1000:1, where S100A7 could be placed with a multipoint lodscore of 27.4 in an approximately 7-cM interval. The order of the 12 loci was as follows (with the best estimates of recombination frequencies given in between): AMY2B-0.091-D1S73(0.039)-D1S11(-0.053)-D1S189(-0 .017)-D1S1252 (-0.017)-D1S13(-0.078)-D1Z5(-0.051)-S100A7(-0.022)- MUC1(-0.026)-SPTA1 (-0.066)-ATP1A2(-0.014)-APOA2. Furthermore, from this map S100A7 could be assigned to the regional position of chromosome 1cen-q21. The linkage information presented should be of great value in association and linkage studies of diseases where psoriasin, or some of the several other very closely linked and functionally related genes, are seen as candidate genes, e.g. in psoriasis.