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

Revisiting the Roles of Filaggrin in Atopic Dermatitis

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

Type 2 Inflammation Contributes to Skin Barrier Dysfunction in Atopic Dermatitis

Skin barrier dysfunction, a defining feature of atopic dermatitis (AD), arises from multiple interacting systems. In AD, skin inflammation is caused by host-environment interactions involving keratinocytes as well as tissue-resident immune cells such as type 2 innate lymphoid cells, basophils, mast cells, and T helper type 2 cells, which produce type 2 cytokines, including IL-4, IL-5, IL-13, and IL-31. Type 2 inflammation broadly impacts the expression of genes relevant for barrier function, such as intracellular structural proteins, extracellular lipids, and junctional proteins, and enhances Staphylococcus aureus skin colonization. Systemic anti‒type 2 inflammation therapies may improve dysfunctional skin barrier in AD.

Skin barrier dysfunction and filaggrin

Skin barrier dysfunction caused by endogenous or exogenous factors can lead to various disorders such as xerosis cutis, ichthyoses, and atopic dermatitis. Filaggrin is a pivotal structural protein of the stratum corneum (SC) and provides natural moisturizing factors that play a role in skin barrier functions. Filaggrin aggregates keratin filaments, resulting in the formation of a keratin network, which binds cornified envelopes and collapse keratinocytes to flattened corneocytes. This complex network contributes to the physical strength of the skin. Filaggrin is degraded by caspase-14, calpain 1, and bleomycin hydrolases into amino acids and amino acid metabolites such as trans-urocanic acid and pyrrolidone carboxylic acid, which are pivotal natural moisturizing factors in the SC. Accordingly, filaggrin is important for the pathophysiology of skin barrier disorders, and its deficiency or dysfunction leads to a variety of skin disorders. Here, the roles and biology of filaggrin, related skin diseases, and a therapeutic strategy targeting filaggrin are reviewed. In addition, several drug candidates of different mode of actions targeting filaggrin, along with their clinical efficacy, are discussed.

Cationic Intrinsically Disordered Antimicrobial Peptides (CIDAMPs) Represent a New Paradigm of Innate Defense with a Potential for Novel Anti-Infectives

In the search for potential mechanisms underlying the remarkable resistance of healthy skin against infection by soil bacteria like Pseudomonas (P.) aeruginosa we identified fragments of the intrinsically disordered protein hornerin as potent microbicidal agents in the stratum corneum. We found that, independent of the amino acid (AA)-sequence, any tested linear cationic peptide containing a high percentage of disorder-promoting AA and a low percentage of order-promoting AA is a potent microbicidal antimicrobial. We further show that the antimicrobial activity of these cationic intrinsically disordered antimicrobial peptides (CIDAMPs) depends on the peptide chain length, its net charge, lipidation and environmental conditions. The ubiquitous presence of latent CIDAMP sources in nature suggests a common and yet overlooked adapted innate disinfection system of body surfaces. The simple structure and virtually any imaginable sequence or composition of disorder-promoting AA allow the generation of a plethora of CIDAMPs. These are potential novel microbicidal anti-infectives for various bacterial pathogens, including P. aeruginosa, methicillin-resistant Staphylococcus aureus (MRSA) and fungal pathogens like Candida albicans and Cryptococcus neoformans.

Down-regulated SHARPIN may accelerate the development of atopic dermatitis through activating interleukin-33/ST2 signalling

SHARPIN is an important component of the linear ubiquitin chain assembly complex (LUBAC). Loss of function of SHARPIN results in eosinophilic inflammation in multiple organs including skin with Th₂ -dominant cytokines and dysregulated development of lymphoid tissues in mice. The clinicopathological features are similar to atopic dermatitis (AD) in humans. In order to investigate the potential role of SHARPIN in the pathogenesis of AD, we performed genetic association study of the genotypes and haplotypes as well as SHARPIN's expression between AD cases and controls. We found three mutations (g.480G>A, g.4576A>G and g.5070C>T) in patient group, and significantly decreased expression in AD lesions, suggesting a primary role of SHARPIN during AD development. Lentivirus-mediated in vitro assays identified that knockdown of SHARPIN can induce elevated expression of IL-33 and its orphan receptor ST2, FLG and STAT3 and NF-κB inactivation in HaCaT keratinocytes, which has been widely evidenced in regulating AD development. ST2 expression was highly induced in SHARPIN-silenced HaCaT keratinocytes after the combined stimulation of IL-4 and IL-13. Our in vivo and in vitro findings implicated that SHARPIN may be a novel participant in the pathogenesis and/or new therapeutic target of AD.

T Helper 1 and T Helper 2 Cytokines Differentially Modulate Expression of Filaggrin and its Processing Proteases in Human Keratinocytes

Background:

Atopic dermatitis (AD) is characterized by defective skin barrier and imbalance in T helper 1/T helper 2 (Th1/Th2) cytokine expression. Filaggrin (FLG) is the key protein to maintaining skin barrier function. Recent studies indicated that Th1/Th2 cytokines influence FLG expression in keratinocytes. However, the role of Th1/Th2 cytokines on FLG processing is not substantially documented. Our aim was to investigate the impact of Th1/Th2 cytokines on FLG processing.

Methods:

HaCaT cells and normal human keratinocytes were cultured in low and high calcium media and stimulated by either interleukin (IL)-4, 13 or interferon-γ (IFN-γ). FLG, its major processing proteases and key protease inhibitor lymphoepithelial Kazal-type-related inhibitor (LEKTI) were measured by both real-time quantitative polymerase chain reaction and Western blotting. Their expression was also evaluated in acute and chronic AD lesions by immunohistochemistry.

Results:

IL-4/13 significantly reduced, while IFN-γ significantly up-regulated FLG expression. IL-4/13 significantly increased, whereas IFN-γ significantly decreased the expression of kallikreins 5 and 7, matriptase and channel-activating serine protease 1. On the contrary, IL-4/13 significantly decreased, while IFN-γ increased the expression of LEKTI and caspase-14. Similar trends were observed in AD lesions.

Conclusions:

Our results suggested that Th1/Th2 cytokines differentially regulated the expression of major FLG processing enzymes. The imbalance between Th1 and Th2 polarized immune response seems to extend to FLG homeostasis, through the network of FLG processing enzymes.

Dissecting the formation, structure and barrier function of the stratum corneum

The skin is the largest organ of the mammalian body. The outermost layer of mammalian skin, the stratum corneum (SC) of the epidermis, consists of piles of dead corneocytes that are the end-products of terminal differentiation of epidermal keratinocytes. The SC performs a crucial barrier function of epidermis. Langerhans cells, when activated, extend their dendrites through tight junctions just beneath the SC to capture external antigens. Recently, knowledge of the biology of corneocytes ('corneobiology') has progressed rapidly and many key factors that modulate its barrier function have been identified and characterized. In this review article on the SC, we summarize its evolution, formation, structure and function. Cornification is an important step of SC formation at the conversion of living epithelial cells to dead corneocytes, and consists of three major steps: formation of the intracellular keratin network, cornified envelopes and intercellular lipids. After cornification, the SC undergoes chemical reactions to form the mature SC with different functional layers. Finally, the SC is shed off at the surface ('desquamation'), mediated by a cascade of several proteases. This review will be helpful to understand our expanding knowledge of the biology of the SC, where immunity meets external antigens.

Crystal Structure of Human Profilaggrin S100 Domain and Identification of Target Proteins Annexin II, Stratifin, and HSP27

The fused-type S100 protein profilaggrin and its proteolytic products including filaggrin are important in the formation of a normal epidermal barrier; however, the specific function of the S100 calcium-binding domain in profilaggrin biology is poorly understood. To explore its molecular function, we determined a 2.2Å-resolution crystal structure of the N-terminal fused-type S100 domain of human profilaggrin with bound calcium ions. The profilaggrin S100 domain formed a stable dimer, which contained two hydrophobic pockets that provide a molecular interface for protein interactions. Biochemical and molecular approaches demonstrated that three proteins, annexin II/p36, stratifin/14-3-3 sigma, and Hsp27, bind to the N-terminal domain of human profilaggrin; one protein (stratifin) co-localized with profilaggrin in the differentiating granular cell layer of human skin. Together, these findings suggest a model where the profilaggrin N-terminus uses calcium-dependent and calcium-independent protein-protein interactions to regulate its involvement in keratinocyte terminal differentiation and incorporation into the cornified cell envelope.

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

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

Filaggrin - revisited

Profilaggrin (proFLG) and its processing products are critical to the health and appearance of skin. The recent identification of loss-of-function filaggrin (FLG) mutations as a predisposing factor in ichthyosis vulgaris and atopic dermatitis has lead to a resurgent interest in this enigmatic protein. Here, we review the literature on the structure and many functions of proFLG, from its role as a filament-aggregating protein and a source of natural moisturizing factor (NMF), to the more recent discoveries of its role in epidermal barrier formation and its more speculative functions as an antimicrobial and sunscreen. Finally, we discuss the relationship of proFLG with dry skin, the influence of moisturizers on NMF generation and speculate on next generation of FLG research.

Systems-level analysis of proteolytic events in increased vascular permeability and complement activation in skin inflammation

During inflammation, vascular permeability is increased by various proteolytic events, such as the generation of bradykinin, that augment local tissue responses by enabling tissue penetration of serum proteins, including complement and acute-phase proteins. Proteases also govern inflammatory responses by processing extracellular matrix proteins and soluble bioactive mediators. We quantified changes in the proteome and the nature of protein amino termini (the N-terminome) and the altered abundance of murine proteases and inhibitors during skin inflammation. Through analysis of the N-terminome by iTRAQ-TAILS, we identified cotranslational and posttranslational αN-acetylation motifs, quantitative increases in protein abundance, and qualitative changes in the proteolytic signature during inflammation. Of the proteins identified in normal skin, about half were cleaved, and phorbol ester-induced inflammation increased the proportion of cleaved proteins, including chemokines and complement proteins, that were processed at previously uncharacterized sites. In response to phorbol ester-induced inflammation, mice deficient in matrix metalloproteinase 2 (MMP2) showed reduced accumulation of serum proteins in the skin and exhibited different proteolytic networks from those of wild-type mice. We found that the complement 1 (C1) inhibitor attenuated the increase in serum protein accumulation in inflamed skin. Cleavage and inactivation of the C1 inhibitor by MMP2 increased complement activation and bradykinin generation in wild-type mice, leading to increased vessel permeability during inflammation, which was diminished in Mmp2(-/-) mice. Thus, our systems-level analysis of proteolysis dissected cleavage events associated with skin inflammation and demonstrated that loss of a single protease could perturb the proteolytic signaling network and enhance inflammation.

SASPase regulates stratum corneum hydration through profilaggrin-to-filaggrin processing

The stratum corneum (SC), the outermost layer of the epidermis, acts as a barrier against the external environment. It is hydrated by endogenous humectants to avoid desiccation. However, the molecular mechanisms of SC hydration remain unclear. We report that skin-specific retroviral-like aspartic protease (SASPase) deficiency in hairless mice resulted in dry skin and a thicker and less hydrated SC with an accumulation of aberrantly processed profilaggrin, a marked decrease of filaggrin, but no alteration in free amino acid composition, compared with control hairless mice. We demonstrated that recombinant SASPase directly cleaved a linker peptide of recombinant profilaggrin. Furthermore, missense mutations were detected in 5 of 196 atopic dermatitis (AD) patients and 2 of 28 normal individuals. Among these, the V243A mutation induced complete absence of protease activity in vitro, while the V187I mutation induced a marked decrease in its activity. These findings indicate that SASPase activity is indispensable for processing profilaggrin and maintaining the texture and hydration of the SC. This provides a novel approach for elucidating the complex pathophysiology of atopic dry skin.

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.

Optimization of filaggrin expression and processing in cultured rat keratinocytes

BACKGROUND

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

OBJECTIVE

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

METHOD

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

RESULTS

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

CONCLUSION

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

Increased matriptase zymogen activation in inflammatory skin disorders

Matriptase, a type 2 transmembrane serine protease, and its inhibitor hepatocyte growth factor activator inhibitor (HAI)-1 are required for normal epidermal barrier function, and matriptase activity is tightly regulated during this process. We therefore hypothesized that this protease system might be deregulated in skin disease. To test this, we examined the level and activation state of matriptase in examples of 23 human skin disorders. We first examined matriptase and HAI-1 protein distribution in normal epidermis. Matriptase was detected at high levels at cell-cell junctions in the basal layer and spinous layers but was present at minimal levels in the granular layer. HAI-1 was distributed in a similar pattern, except that high-level expression was retained in the granular layer. This pattern of expression was retained in most skin disorders. We next examined the distribution of activated matriptase. Although activated matriptase is not detected in normal epidermis, a dramatic increase is seen in keratinocytes at the site of inflammation in 16 different skin diseases. To gain further evidence that activation is associated with inflammatory stimuli, we challenged HaCaT cells with acidic pH or H(2)O(2) and observed matriptase activation. These findings suggest that inflammation-associated reactive oxygen species and tissue acidity may enhance matriptase activation in some skin diseases.

Protease and protease-activated receptor-2 signaling in the pathogenesis of atopic dermatitis

Proteases in the skin are essential to epidermal permeability barrier homeostasis. In addition to their direct proteolytic effects, certain proteases signal to cells by activating protease-activated receptors (PARs), the G-protein-coupled receptors. The expression of functional PAR-2 on human skin and its role in inflammation, pruritus, and skin barrier homeostasis have been demonstrated. Atopic dermatitis (AD) is a multifactorial inflammatory skin disease characterized by genetic barrier defects and allergic inflammation, which is sustained by gene-environmental interactions. Recent studies have revealed aberrant expression and activation of serine proteases and PAR-2 in the lesional skin of AD patients. The imbalance between proteases and protease inhibitors associated with genetic defects in the protease/protease inhibitor encoding genes, increase in skin surface pH, and exposure to proteolytically active allergens contribute to this aberrant protease/ PAR-2 signaling in AD. The increased protease activity in AD leads to abnormal desquamation, degradation of lipid-processing enzymes and antimicrobial peptides, and activation of primary cytokines, thereby leading to permeability barrier dysfunction, inflammation, and defects in the antimicrobial barrier. Moreover, up-regulated proteases stimulate PAR-2 in lesional skin of AD and lead to the production of cytokines and chemokines involved in inflammation and immune responses, itching sensation, and sustained epidermal barrier perturbation with easier allergen penetration. In addition, PAR-2 is an important sensor for exogenous danger molecules, such as exogenous proteases from various allergens, and plays an important role in AD pathogenesis. Together, these findings suggest that protease activity or PAR-2 may be a future target for therapeutic intervention for the treatment of AD.

Elastase 2 is expressed in human and mouse epidermis and impairs skin barrier function in Netherton syndrome through filaggrin and lipid misprocessing

The human epidermis serves 2 crucial barrier functions: it protects against water loss and prevents penetration of infectious agents and allergens. The physiology of the epidermis is maintained by a balance of protease and antiprotease activities, as illustrated by the rare genetic skin disease Netherton syndrome (NS), in which impaired inhibition of serine proteases causes severe skin erythema and scaling. Here, utilizing mass spectrometry, we have identified elastase 2 (ELA2), which we believe to be a new epidermal protease that is specifically expressed in the most differentiated layer of living human and mouse epidermis. ELA2 localized to keratohyalin granules, where it was found to directly participate in (pro-)filaggrin processing. Consistent with the observation that ELA2 was hyperactive in skin from NS patients, transgenic mice overexpressing ELA2 in the granular layer of the epidermis displayed abnormal (pro-)filaggrin processing and impaired lipid lamellae structure, which are both observed in NS patients. These anomalies led to dehydration, implicating ELA2 in the skin barrier defect seen in NS patients. Thus, our work identifies ELA2 as a major new epidermal protease involved in essential pathways for skin barrier function. These results highlight the importance of the control of epidermal protease activity in skin homeostasis and designate ELA2 as a major protease driving the pathogenesis of NS.

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.

Neutral cysteine protease bleomycin hydrolase is essential for the breakdown of deiminated filaggrin into amino acids

Filaggrin is a component of the cornified cell envelope and the precursor of free amino acids acting as a natural moisturizing factor in the stratum corneum. Deimination is critical for the degradation of filaggrin into free amino acids. In this study, we tried to identify the enzyme(s) responsible for the cleavage of deiminated filaggrin in vitro. First, we investigated citrulline aminopeptidase activity in the extract of newborn rat epidermis by double layer fluorescent zymography and detected strong activity at neutral pH. Monitoring the citrulline-releasing activity, we purified an enzyme of 280 kDa, comprised of six identical subunits of 48 kDa. The NH(2) terminus of representative tryptic peptides perfectly matched the sequence of rat bleomycin hydrolase (BH). The enzyme released various amino acids except Pro from beta-naphthylamide derivatives and hydrolyzed citrulline-beta-naphthylamide most effectively. Thus, to break down deiminated filaggrin, another protease would be required. Among proteases tested, calpain I degraded the deiminated filaggrin effectively into many peptides of different mass on the matrix-assisted laser desorption/ionization-time of flight mass spectrum. We confirmed that various amino acids including citrulline were released by BH from those peptides. On the other hand, caspase 14 degraded deiminated filaggrin into a few peptides of limited mass. Immunohistochemical analysis of normal human skin revealed co-localization of BH and filaggrin in the granular layer. Collectively, our results suggest that BH is essential for the synthesis of natural moisturizing factors and that calpain I would play a role as an upstream protease in the degradation of filaggrin.

Analysis of the individual and aggregate genetic contributions of previously identified serine peptidase inhibitor Kazal type 5 (SPINK5), kallikrein-related peptidase 7 (KLK7), and filaggrin (FLG) polymorphisms to eczema risk

BACKGROUND

Polymorphisms in the serine protease inhibitor gene serine peptidase inhibitor Kazal type 5 (SPINK5) and the serine protease kallikrein-related peptidase 7 gene (KLK7) appear to confer risk to eczema in some cohorts, but these findings have not been widely replicated. These genes encode proteins thought to be involved in the regulation of posttranslation processing of filaggrin (FLG), the strongest identified genetic risk factor for eczema to date.

OBJECTIVES

We sought to clarify the individual risk of eczema conferred by the SPINK5 polymorphism rs2303067 (Glu420Lys) and a previously described insertion in the 3' untranslated region of KLK7 and to examine potential epistatic effects between these variants and FLG mutations.

METHODS

Initially, we examined the effects of these polymorphisms and FLG in 486 unrelated patients from a German family-based study, an additional 287 German patients, and 418 unrelated Irish/English patients with eczema (n for 3 genes studied = 1191 vs 4544 control subjects). We then additionally studied the SPINK5 polymorphism and FLG mutations in 1583 patients with eczema from the Avon Longitudinal Study of Parents and Children cohort (sample size for 2 genes studied = 2774 vs 10,607 control subjects).

RESULTS

No association was seen with the SPINK5 or KLK7 variants in the case-control analysis; however, a weaker effect was observed for the SPINK5 variant with maternal transmission in the family-based study. No interactions were seen between the polymorphisms in KLK7, SPINK5, and FLG.

CONCLUSION

The SPINK5 420LysSer mutation confers a risk of eczema when maternally inherited but is not a major eczema risk factor. The KLK7 insertion appears to confer no risk of eczema. We found no interaction between the SPINK5 risk allele or the putative KLK7 risk allele and FLG mutations.

Crucial Roles of MZF1 and Sp1 in the Transcriptional Regulation of the Peptidylarginine Deiminase Type I Gene (PADI1) in Human Keratinocytes

Peptidylarginine deiminases (PADs) catalyze the conversion of protein-bound arginine residues into citrulline residues in a calcium-dependent manner. The PAD1 gene (PADI1) is expressed in a few tissues, including the epidermis, where the protein is detected with a higher level in the more differentiated keratinocytes. Using quantitative reverse transcription-PCR experiments, we show that PADI1 mRNAs are more abundant in keratinocytes cultured with 1.2 than 0.15 mM calcium. We cloned and characterized the promoter region using human keratinocytes transfected with variously deleted fragments of the 5'-upstream region of PADI1 coupled to the luciferase gene. We found that as few as 195 bp upstream from the transcription initiation site were sufficient to direct transcription of the reporter gene. Mutations of MZF1- or Sp1-binding sites markedly reduced PADI1 promoter activity. Chromatin immunoprecipitation assays revealed that MZF1 and Sp1/Sp3 bind to this region in vivo. Furthermore, MZF1 or Sp1 small interfering RNAs (siRNAs) effectively diminished PADI1 expression in keratinocytes cultured in both low- and high-calcium-containing medium. In addition, the expression of MZF1 and PAD1 increased in parallel when normal human epidermal keratinocytes underwent differentiation. These data indicate that MZF1 and Sp1/Sp3 binding to the promoter region drive the PADI1 expression.

Regulation of the epithelial Na+ channel by peptidases

Recent investigations point to an important role for peptidases in regulating transcellular ion transport by the epithelial Na(+) channel, ENaC. Several peptidases, including furins and proteasomal hydrolases, modulate ENaC maturation and disposal. More idiosyncratically, apical Na(+) transport by ENaC in polarized epithelia of kidney, airway, and gut is stimulated constitutively by one or more trypsin-family serine peptidases, as revealed by inhibition of amiloride-sensitive Na(+) transport by broad-spectrum antipeptidases, including aprotinin and bikunin/SPINT2. In vitro, the transporting activity of aprotinin-suppressed ENaC can be restored by exposure to trypsin. The prototypical channel-activating peptidase (CAP) is a type 1 membrane-anchored tryptic peptidase first identified in Xenopus kidney cells. Frog CAP1 strongly upregulates Na(+) transport when coexpressed with ENaC in oocytes. The amphibian enzyme's apparent mammalian orthologue is prostasin, otherwise known as CAP1, which is coexpressed with ENaC in a variety of epithelia. In airway cells, prostasin is the major basal regulator of ENaC activity, as suggested by inhibition and knockdown experiments. Other candidate regulators of mature ENaC include CAP2/TMPRSS4 and CAP3/matriptase (also known as membrane-type serine protease 1/ST14). Mammalian CAPs are potential targets for treatment of ENaC-mediated Na(+) hyperabsorption by the airway in cystic fibrosis (CF) and by the kidney in hypertension. CAPs can be important for mammalian development, as indicated by embryonic lethality in mice with null mutations of CAP1/prostasin. Mice with selectively knocked out expression of CAP1/prostasin in the epidermis and mice with globally knocked out expression of CAP3/matriptase exhibit phenotypically similar defects in skin barrier function and neonatal death from dehydration. In rats, transgenic overexpression of human prostasin disturbs salt balance and causes hypertension. Thus, several converging lines of evidence indicate that ENaC function is regulated by peptidases, and that such regulation is critical for embryonic development and adult function of organs such as skin, kidney, and lung.

Evidence for a matriptase-prostasin proteolytic cascade regulating terminal epidermal differentiation

Recent gene ablation studies in mice have shown that matriptase, a type II transmembrane serine protease, and prostasin, a glycosylphosphatidylinositol-anchored membrane serine protease, are both required for processing of the epidermis-specific polyprotein, profilaggrin, stratum corneum formation, and acquisition of epidermal barrier function. Here we present evidence that matriptase acts upstream of prostasin in a zymogen activation cascade that regulates terminal epidermal differentiation and is required for prostasin zymogen activation. Enzymatic gene trapping of matriptase combined with prostasin immunohistochemistry revealed that matriptase was co-localized with prostasin in transitional layer cells of the epidermis and that the developmental onset of expression of the two membrane proteases was coordinated and correlated with acquisition of epidermal barrier function. Purified soluble matriptase efficiently converted soluble prostasin zymogen to an active two-chain form that formed SDS-stable complexes with the serpin protease nexin-1. Whereas two forms of prostasin with molecular weights corresponding to the prostasin zymogen and active prostasin were present in wild type epidermis, prostasin was exclusively found in the zymogen form in matriptase-deficient epidermis. These data suggest that matriptase, an autoactivating protease, acts upstream from prostasin to initiate a zymogen cascade that is essential for epidermal differentiation.

Prevalent and Rare Mutations in the Gene Encoding Filaggrin Cause Ichthyosis Vulgaris and Predispose Individuals to Atopic Dermatitis

Mutations in the filament aggregating protein (filaggrin) gene have recently been identified as the cause of the common genetic skin disorder ichthyosis vulgaris (IV), the most prevalent inherited disorder of keratinization. The main characteristics of IV are fine-scale on the arms and legs, palmar hyperlinearity, and keratosis pilaris. Here, we have studied six Irish families with IV for mutations in filaggrin. We have identified a new mutation, 3702delG, in addition to further instances of the reported mutations R501X and 2282del4, which are common in people of European origin. A case of a 2282del4 homozygote was also identified. Mutation 3702delG terminates protein translation in filaggrin repeat domain 3, whereas both recurrent mutations occur in repeat 1. These mutations are semidominant: heterozygotes have an intermediate phenotype most readily identified by palmar hyperlinearity and in some cases fine-scale and/or keratosis pilaris, whereas homozygotes or compound heterozygotes generally have more marked ichthyosis. Interestingly, the phenotypes of individuals homozygous for R501X, 2282del4, or compound heterozygous for R501X and 3702delG, were comparable, suggesting that mutations located centrally in the filaggrin repeats are also pathogenic.

The epidermal barrier function is dependent on the serine protease CAP1/Prss8

Serine proteases are proteolytic enzymes that are involved in the regulation of various physiological processes. We generated mice lacking the membrane-anchored channel-activating serine protease (CAP) 1 (also termed protease serine S1 family member 8 [Prss8] and prostasin) in skin, and these mice died within 60 h after birth. They presented a lower body weight and exhibited severe malformation of the stratum corneum (SC). This aberrant skin development was accompanied by an impaired skin barrier function, as evidenced by dehydration and skin permeability assay and transepidermal water loss measurements leading to rapid, fatal dehydration. Analysis of differentiation markers revealed no major alterations in CAP1/Prss8-deficient skin even though the epidermal deficiency of CAP1/Prss8 expression disturbs SC lipid composition, corneocyte morphogenesis, and the processing of profilaggrin. The examination of tight junction proteins revealed an absence of occludin, which did not prevent the diffusion of subcutaneously injected tracer (∼600 D) toward the skin surface. This study shows that CAP1/Prss8 expression in the epidermis is crucial for the epidermal permeability barrier and is, thereby, indispensable for postnatal survival.

Identification and Characterization of a Novel Retroviral-Like Aspartic Protease Specifically Expressed in Human Epidermis

Proteases play a pivotal role in epidermal differentiation and desquamation. Separation of a total protein extract from human reconstructed epidermis by two-dimensional gel electrophoresis and subsequent peptide analysis of a specific protein spot identified a new protein exhibiting similarities with the retroviral aspartic protease family. Cloning of the corresponding full-length cDNA revealed an open reading frame encoding for a new protease of 343 amino acids, containing a putative aspartic protease catalytic domain. We named this protein Skin ASpartic Protease (SASPase). RT-PCR and northern blot analysis of various human tissues revealed that SASPase was specifically expressed within the epidermis. Immunohistochemical analysis showed a particularly intense expression restricted to the granular layers, whereas in diseased skin, its expression was changed. Western blot analysis, using a monoclonal antibody, revealed the expression of two forms of the enzyme: a 28 kDa putative proform and the active 14 kDa form. Recombinant truncated SASPase (SASP28) was generated from a prokaryotic expression system in Escherichia coli as a fusion protein with GST. SASP28 degraded insulin and to a lesser extent casein with a pH optimum of 5. As seen for retroviral proteases, an auto-activation processing was evidenced, generating a 14 kDa protein (SASP14). Site-directed mutagenesis inhibited auto-activation of the enzyme. Indinavir, a potent HIV protease inhibitor used in AIDS therapy, had a significant inhibitory effect on rSASPase auto-activation, which could explain its side effects on skin.

Epidermal differentiation: the role of proteases and their inhibitors

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

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.

Intermediate filament associated proteins

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

Lethal, neonatal ichthyosis with increased proteolytic processing of filaggrin in a mouse model of Netherton syndrome

Netherton syndrome is an autosomal recessive multisystemic disorder characterized by congenital ichthyosiform erythroderma, hair shaft defects and atopy, caused by mutations within the human SPINK5 gene. To investigate the development of this disease, we have cloned mouse spink5 and created mice with a mutated premature stop codon at amino acid R820X, to produce an allele that closely mimics a point mutation (E827X) in human SPINK5. Newborn spink5(R820X/R820X) mice develop a lethal, severe ichthyosis with a loss of skin barrier function and dehydration, resulting in death within a few hours of birth, similar to that observed in patients with severe Netherton syndrome. Epidermal barrier function is compromised because of the stratum corneum becoming spontaneously detached in the newborn mice, and this is probably compounded by the reduced mechanical strength detected in the cornified envelopes. Biochemical analysis of skin from newborn wild-type and spink5(R820X/R820X) mice revealed a substantial increase in the proteolytic processing of profilaggrin into its constituent filaggrin monomers. Filaggrin functions to organize keratin filaments into highly ordered macrofibrils that crisscross the cornified cells of the stratum corneum imparting structural integrity, and defects in filaggrin processing occur in a number of forms of congenital ichthyosis. These data suggest that in the absence of the serine protease inhibitor spink5, there is an abnormal increase in the processing of profilaggrin, resulting in an overabundance of filaggrin monomers, and that this may play a direct role in the observed deficit in the adhesion of the stratum corneum and the severely compromised epidermal barrier function.

Loss of proteolytically processed filaggrin caused by epidermal deletion of Matriptase/MT-SP1

Profilaggrin is a large epidermal polyprotein that is proteolytically processed during keratinocyte differentiation to release multiple filaggrin monomer units as well as a calcium-binding regulatory NH₂-terminal filaggrin S-100 protein. We show that epidermal deficiency of the transmembrane serine protease Matriptase/MT-SP1 perturbs lipid matrix formation, cornified envelope morphogenesis, and stratum corneum desquamation. Surprisingly, proteomic analysis of Matriptase/MT-SP1–deficient epidermis revealed the selective loss of both proteolytically processed filaggrin monomer units and the NH₂-terminal filaggrin S-100 regulatory protein. This was associated with a profound accumulation of profilaggrin and aberrant profilaggrin-processing products in the stratum corneum. The data identify keratinocyte Matriptase/MT-SP1 as an essential component of the profilaggrin-processing pathway and a key regulator of terminal epidermal differentiation.

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.

Epithelial structural proteins of the skin and oral cavity: function in health and disease

Epithelial tissues function to protect the organism from physical, chemical, and microbial damage and are essential for survival. To perform this role, epithelial keratinocytes undergo a well-defined differentiation program that results in the expression of structural proteins which maintain the integrity of epithelial tissues and function as a protective barrier. This review focuses on structural proteins of the epidermis and oral mucosa. Keratin proteins comprise the predominant cytoskeletal component of these epithelia. Keratin filaments are attached to the plasma membrane via desmosomes, and together these structural components form a three-dimensional array within the cytoplasm of epithelial cells and tissues. Desmosomes contain two types of transmembrane proteins, the desmogleins and desmocollins, that are members of the cadherin family. The desmosomal cadherins are linked to the keratin cytoskeleton via several cytoplasmic plaque proteins, including desmoplakin and plakoglobin (gamma-catenin). Epidermal and oral keratinocytes express additional differentiation markers, including filaggrin and trichohyalin, that associate with the keratin cytoskeleton during terminal differentiation, and proteins such as loricrin, small proline-rich proteins, and involucrin, that are cross-linked into the cornified envelope by transglutaminase enzymes. The importance of these cellular structures is highlighted by the large numbers of genetic and acquired (autoimmune) human disorders that involve mutations in, or autoantibodies to, keratins and desmosomal and cornified envelope proteins. While much progress has been made in the identification of the structural proteins and enzymes involved in epithelial differentiation, regulation of this process is less clear. Both calcium and retinoids influence epithelial differentiation by altering the transcription of target genes and by regulating activity of enzymes critical in epithelial differentiation, such as transglutaminases, proteinases, and protein kinases. These studies have furthered our understanding of how epithelial tissue and cell integrity is maintained and provide a basis for the future treatment of skin and oral disorders by gene therapy and other novel therapeutics.

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.

Proteinase inhibitor 6 (PI-6) expression in human skin: induction of PI-6 and a PI-6/proteinase complex during keratinocyte differentiation

Proteinase inhibitor 6 (PI-6) is a 42-kDa intracellular protein present in epithelial cells and endothelial cells. It is capable of inhibiting a number of serine proteinases, including trypsin and chymotrypsin. In this study we examined PI-6 expression in human skin and its primary cell type, the keratinocyte. By immunohistochemical analysis, PI-6 staining is absent from the basal cells, weak in the spinous layer, and strongest in the granulosa layer of human epidermis. Immunoblotting of cultured primary keratinocytes revealed that PI-6 production increases 24-fold on differentiation. Analysis of an immortalized keratinocyte cell line, HaCat, showed a 5-fold increase in PI-6 mRNA and a 7-fold increase in PI-6 protein upon differentiation, and indirect immunofluorescence revealed that this is due to an increase in the number of differentiated cells expressing high levels of PI-6. Of particular interest is the appearance of a preformed complex between PI-6 and an endogenous serine proteinase in differentiating HaCat cells, which was detected by a monoclonal antibody demonstrated to preferentially recognize PI-6 in complex with a proteinase. This identification of a PI-6/proteinase complex is the first example of a serpin bound to a proteinase in keratinocytes. We postulate that a physiological role of PI-6 is to regulate a serine proteinase associated with keratinocyte differentiation.

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.

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.