Proprotein convertase expression and localization in epidermis: evidence for multiple roles and substrates

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

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

Receptors and Cofactors That Contribute to SARS-CoV-2 Entry: Can Skin Be an Alternative Route of Entry?

To prevent the spread of SARS-CoV-2, all routes of entry of the virus into the host must be mapped. The skin is in contact with the external environment and thus may be an alternative route of entry to transmission via the upper respiratory tract. SARS-CoV-2 cell entry is primarily dependent on ACE2 and the proteases TMPRSS2 or cathepsin L but other cofactors and attachment receptors have been identified that may play a more important role in specific tissues such as the skin. The continued emergence of new variants may also alter the tropism of the virus. In this review, we summarize current knowledge on these receptors and cofactors, their expression profile, factors modulating their expression and their role in facilitating SARS-CoV-2 infection. We discuss their expression in the skin and their possible involvement in percutaneous infection since the presence of the virus has been detected in the skin.

Investigations into the filaggrin null phenotype: showcasing the methodology for CRISPR/Cas9 editing of human keratinocytes

Ever since the association between filaggrin (FLG) loss-of-function mutations and ichthyosis vulgaris and atopic dermatitis disease onset was identified, filaggrins function has been under investigation. Intra-individual genomic predisposition, immunological confounders, and environmental interactions complicate the comparison between FLG genotypes and related causal effects. Using CRISPR/Cas9, we generated human FLG knockout (ΔFLG) N/TERT-2G keratinocytes. Filaggrin deficiency was demonstrated by immunohistochemistry of human epidermal equivalent (HEE) cultures. Next to (partial) loss of structural proteins (IVL, HRNR, KRT2, and TGM1), the stratum corneum was more dense and lacked the typical basket weave appearance. In addition, electrical impedance spectroscopy and transepidermal water loss analyses highlighted a compromised epidermal barrier in ΔFLG-HEEs. Correction of FLG reinstated the presence of keratohyalin granules in the stratum granulosum, filaggrin protein expression, and expression of aforementioned proteins. The beneficial effects on stratum corneum formation were reflected by normalization of EIS and TEWL. This study demonstrates the causal phenotypical and functional consequences of filaggrin deficiency, indicating filaggrin is not only central in epidermal barrier function but also vital for epidermal differentiation by orchestrating the expression of other important epidermal proteins. These observations pave the way to fundamental investigations into the exact role of filaggrin in skin biology and disease.

The Role of Proprotein Convertases in Upper Airway Remodeling

Chronic rhinosinusitis (CRS) is a multifactorial, heterogeneous disease characterized by persistent inflammation of the sinonasal mucosa and tissue remodeling, which can include basal/progenitor cell hyperplasia, goblet cell hyperplasia, squamous cell metaplasia, loss or dysfunction of ciliated cells, and increased matrix deposition. Repeated injuries can stimulate airway epithelial cells to produce inflammatory mediators that activate epithelial cells, immune cells, or the epithelial-mesenchymal trophic unit. This persistent inflammation can consequently induce aberrant tissue remodeling. However, the molecular mechanisms driving disease within the different molecular CRS subtypes remain inadequately characterized. Numerous secreted and cell surface proteins relevant to airway inflammation and remodeling are initially synthesized as inactive precursor proteins, including growth/differentiation factors and their associated receptors, enzymes, adhesion molecules, neuropeptides, and peptide hormones. Therefore, these precursor proteins require post-translational cleavage by proprotein convertases (PCs) to become fully functional. In this review, we summarize the roles of PCs in CRS-associated tissue remodeling and discuss the therapeutic potential of targeting PCs for CRS treatment.

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.

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.

Assessing SPP1/Osteopontin (OPN) Splice Variants and Their Association to Nonmelanoma Skin Cancer by Absolute Quantification: Identification of OPN-5 Subvariants and Their Protein Coding Potential

The study evaluated whether SPP1/osteopontin (OPN) splice variants are differentially expressed in nonmelanoma skin cancer compared to normal skin. The absolute number of mRNA molecules of OPN-a predominated in normal skin and nonmelanoma skin cancer compared to OPN-b, OPN-c, and OPN-5. However, mRNAs of OPN-a, OPN-b, and OPN-c were expressed in higher levels in cutaneous squamous cell carcinomas (cSCCs) and basal cell carcinomas relative to normal skin. Additionally, OPN-5 expression was higher than OPN-b and OPN-c, and OPN-c, in normal skin and nonmelanoma skin cancer, respectively. Furthermore, we identified four OPN-5 splice variants, which were cloned and analyzed for protein expression.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

The role of kallikreins in inflammatory skin disorders and their potential as therapeutic targets

The skin is a vital organ of the human body, serving numerous protective and functional roles that are essential for survival. Residing in the epidermis are various epidermal proteases responsible for the establishment and regulation of barrier function. The human tissue kallikrein-related peptidase family conserves homeostasis of the skin barrier through their roles in desquamation, antimicrobial defense, innate immune response, and barrier maintenance. The activity of kallikreins is tightly regulated and dysregulation of kallikrein activity is seen to contribute to the formation of several inflammatory skin disorders. This review highlights the roles of kallikreins in skin homeostasis and pathologies. Due to their part in these skin disorders, inhibitors of the skin kallikreins have become attractive therapeutics. Over the past few years, both natural and synthetic inhibitors of several kallikreins have been identified and are undergoing further development as treatments to restore compromised barrier function. This review summarizes the kallikrein inhibitors under development for this purpose. These inhibitors remain promising therapeutics in cases of severe skin inflammation not well managed by current therapies.

The matriptase-prostasin proteolytic cascade in dermatologic diseases

The proper development and function of skin and hair are dependent on proteolytic activities. Specifically, the matriptase-prostasin cascade is a series of proteolytic reactions in the epidermis integral to normal regulation of desquamation. An increasing amount of research describing this pathway has recently become available, and the importance of this pathway is exhibited by the association of genetic defects in this pathway with human diseases of the skin and hair. Given the relevance of this pathway to dermatology, we provide a review of the current understanding of its relevance to distinct clinical entities, including ichthyosis-hypotrichosis and Netherton syndromes.

Proprotein convertases: Key players in inflammation-related malignancies and metastasis

Many cancers occur from locations of inflammation due to chronic irritation and/or infection. Tumor microenvironment contains various different inflammatory cells and mediators that orchestrate diverse neoplastic processes, including proliferation, survival, adhesion and migration. In parallel, tumor cells have adapted some of the signaling molecules used by inflammatory cells, such as selectins and chemokines as well as their receptors for invasion, extravasation and subsequently metastasis. Expression and/or activation of the majority of these molecules is mediated by the proprotein convertases (PCs); proteases expressed by both tumor cells and inflammatory cells. This review analyzes the potential role of these enzymatic system in inflammation-associated cancer impacting on the malignant and metastatic potential of cancer cells, describing the possible use of PCs as a new anti-inflammatory therapeutic approach to tumor progression and metastasis.

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Proteins maturation by the proprotein convertases plays important role in inflammation-related cancer and metastasis.

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Protein precursors require the proprotein convertases for the induction of inflammation.

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Understanding of the molecular mechanism linking the proprotein convertases to inflammation will allow novel therapies.

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Inhibitors of the proprotein convertases constitute great potential for cancer treatment.

In vivo Change of Keratin-Bound Molecules in the Human Stratum Corneum following Exposure to Ultraviolet Radiation

BACKGROUND/OBJECTIVES

Ultraviolet (UV) radiation damages the stratum corneum (SC) and disrupts the skin barrier. The damaged skin changes in the molecular composition of the SC, including its water content. However, it is difficult to examine the in vivo SC changes with existing methods, so those have not been well characterized. Therefore, we investigated in vivo changes of UV-induced SC damage using confocal Raman spectroscopy.

METHOD

We irradiated the volar forearm of 10 subjects with 0.5, 1, and 1.5 minimal erythemal doses of UV radiation. Then, we examined erythema, the transepidermal water loss (TEWL), the water content, the natural moisturizing factor (NMF), and the lipids of the skin.

RESULTS

After UV irradiation, erythema and TEWL of the skin were both increased. The bound water content of the SC was also increased following UV irradiation. The NMF of the SC revealed different tendencies. All free amino acids (FAAs) of the NMF were increased after UV irradiation, except proline. trans-urocanic acid, pyrrolidone carboxylic acid, lactate, and urea, which are NMF components produced by the subsequent catabolism of FAAs and sweat, were decreased after UV irradiation. The amount of ceramide in the SC was also decreased after UV exposure, while cholesterol was increased.

CONCLUSIONS

The bound water content of the SC was increased by UV exposure along with increasing TEWL, several NMF components, and cholesterol. These in vivo results for UV-damaged SC obtained via Raman spectroscopy could be applied to research with regard to protecting the SC from UV radiation and treating UV-damaged SC.

Peptidylarginine deiminases and deiminated proteins at the epidermal barrier

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

Cleavage of the HPV16 Minor Capsid Protein L2 during Virion Morphogenesis Ablates the Requirement for Cellular Furin during De Novo Infection

Infections by high-risk human papillomaviruses (HPV) are the causative agents for the development of cervical cancer. As with other non-enveloped viruses, HPVs are taken up by the cell through endocytosis following primary attachment to the host cell. Through studies using recombinant pseudovirus particles (PsV), many host cellular proteins have been implicated in the process. The proprotein convertase furin has been demonstrated to cleave the minor capsid protein, L2, post-attachment to host cells and is required for infectious entry by HPV16 PsV. In contrast, using biochemical inhibition by a furin inhibitor and furin-negative cells, we show that tissue-derived HPV16 native virus (NV) initiates infection independent of cellular furin. We show that HPV16 L2 is cleaved during virion morphogenesis in differentiated tissue. In addition, HPV45 is also not dependent on cellular furin, but two other alpha papillomaviruses, HPV18 and HPV31, are dependent on the activity of cellular furin for infection.

Proprotein convertase 5/6a is associated with bone morphogenetic protein-2-induced squamous cell differentiation

Squamous metaplasia in airway epithelium is a pathological process arising from abnormal remodeling/repair responses to injury. Proteolytic maturation of many growth and differentiation factors involved in tissue remodeling is controlled by proprotein convertases (PCs). However, the role of these convertases in airway remodeling remains poorly understood. Using a retinoic acid deficiency-induced squamous metaplasia model of cultured human nasal epithelial cells (HNECs), we observed a significant increase in the expression of PC5/6A, a PC member, and bone morphogenetic protein-2 (BMP-2), a candidate substrate for PC5/6A. Specific lentiviral short hairpin RNA-mediated PC5/6A knockdown decreased BMP-2 expression and maturation, decreased expression of squamous cell markers, and increased expression of ciliated cell markers. Decanoyl-Arg-Val-Lys-Arg-chloromethylketone (Dec-RVKR-CMK), a PC inhibitor, and LDN-193189, a BMP receptor inhibitor, suppressed squamous differentiation, promoted mucociliary differentiation, and down-regulated the BMP-2/Smad1/5/8/p38 signaling pathways. Dec-RVKR-CMK also decreased expression of PC5/6A, but not furin, another PC member, suggesting the involvement of PC5/6A in squamous differentiation of HNECs. Overexpression of PC5/6A and BMP-2 in the human nasal epithelial cell line RPMI-2650 demonstrated that PC5/6A can activate BMP-2. Under retinoic acid-sufficient culture conditions for mucociliary differentiation of HNECs, short-term expression of PC5/6A by the adenovirus system and addition of exogenous BMP-2 induced squamous differentiation. Furthermore, PC5/6A and BMP-2 were highly expressed in metaplastic squamous epithelium of human nasal polyps. Taken together, PC5/6A is involved in squamous differentiation of HNECs, possibly through up-regulation of the BMP-2/pSmad1/5/8/p38 signaling pathway, pointing to a potential therapeutic target for the prevention of chronic airway diseases that exhibit squamous metaplasia.

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.

Proprotein convertase subtilisin/kexin type 7 (PCSK7) is essential for the zebrafish development and bioavailability of transforming growth factor β1a (TGFβ1a)

Proprotein convertase subtilisin/kexin (PCSK) enzymes convert proproteins into bioactive end products. Although other PCSK enzymes are known to be essential for biological processes ranging from cholesterol metabolism to host defense, the in vivo importance of the evolutionarily ancient PCSK7 has remained enigmatic. Here, we quantified the expressions of all pcsk genes during the 1st week of fish development and in several tissues. pcsk7 expression was ubiquitous and evident already during the early development. To compare mammalian and zebrafish PCSK7, we prepared homology models, which demonstrated remarkable structural conservation. When the PCSK7 function in developing larvae was inhibited, we found that PCSK7-deficient fish have defects in various organs, including the brain, eye, and otic vesicle, and these result in mortality within 7 days postfertilization. A genome-wide analysis of PCSK7-dependent gene expression showed that, in addition to developmental processes, several immune system-related pathways are also regulated by PCSK7. Specifically, the PCSK7 contributed to the mRNA expression and proteolytic cleavage of the cytokine TGFβ1a. Consequently, tgfβ1a morphant fish displayed phenotypical similarities with pcsk7 morphants, underscoring the importance of this cytokine in the zebrafish development. Targeting PCSK activity has emerged as a strategy for treating human diseases. Our results suggest that inhibiting PCSK7 might interfere with normal vertebrate development.

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.

Regulatory role for the profilaggrin N-terminal domain in epidermal homeostasis

It is well known that profilaggrin, after its release from keratohyalin granules through dephosphorylation, becomes enzymatically processed into individual filaggrin monomers. The roles for filaggrin monomers in aggregating keratin filaments, as a component of the cornified cell envelope, and as a source of natural moisturizing factor are well established. A specific N-terminal fragment, called the PF-AB domain, becomes proteolytically released as well, but much less is known about its functional role in epidermal development. Here, the functional role of profilaggrin N-terminal (PF-N) domain was addressed by overexpressing three overlapping fragments from a lentiviral expression vector in the epidermis of living skin equivalents. The PF-N domain expression impaired the epidermal development through reducing keratinocyte proliferation and impairing differentiation. The expression of well-known differentiation markers profilaggrin, loricrin, and keratin 10 was considerably downregulated in PF-N domain overexpressing-skin equivalents. The activation of caspase 14 was also substantially affected. In contrast, total silencing of profilaggrin expression, obtained with a lentiviral miR vector, resulted in a hyperproliferative epidermis. We propose a hypothesis that profilaggrin AB domain provides a key feedback mechanism that controls epidermal homeostasis.

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

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

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.

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.

The p63 target HBP1 is required for skin differentiation and stratification

Genetic experiments established that p63 is crucial for the development and maintenance of pluristratified epithelia. In the RNA interference (RNAi) screening for targets of p63 in keratinocytes, we identified the transcription factor, High Mobility Group (HMG) box protein 1 (HBP1). HBP1 is an HMG-containing repressor transiently induced during differentiation of several cell lineages. We investigated the relationship between the two factors: using RNAi, overexpression, chromatin immunoprecipitations and transient transfections with reporter constructs, we established that HBP1 is directly repressed by p63. This was further confirmed in vivo by evaluating expression in p63 knockout mice and in transgenics expressing p63 in basal keratinocytes. Consistent with these findings, expression of HBP1 increases upon differentiation of primary keratinocytes and HaCaT cells in culture, and it is higher in the upper layers of human skin. Inactivation of HBP1 by RNAi prevents differentiation of keratinocytes and stratification of organotypic skin cultures. Finally, we analyzed the keratinocyte transcriptomes after HBP1 RNAi; in addition to repression of growth-promoting genes, unexpected activation of differentiation genes was uncovered, coexisting with repression of other genes involved in epithelial cornification. Our data indicate that suppression of HBP1 is part of the growth-promoting strategy of p63 in the lower layers of epidermis and that HBP1 temporally coordinates expression of genes involved in stratification, leading to the formation of the skin barrier.

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.

Subtilisin-like proprotein convertase expression, localization, and activity in the human retina and optic nerve head

PURPOSE

Subtilisin-like proprotein convertases (SPCs) are a family of calcium-dependent cleavage enzymes that act on dibasic sites of various peptide/protein substrates. The purpose of this study was to investigate the expression, localization, and activity of SPCs in the human retina and optic nerve head.

METHODS

mRNA expression of the SPC family in the human retina and optic nerve head tissues was evaluated by quantitative reverse transcription polymerase chain reaction (QRT-PCR). Double immunofluorescence staining was performed on paraffin-embedded human posterior sections to localize SPC family members. Western blot analysis was used to identify PACE4 isoform expression within the optic nerve head and retina. In addition, a fluorogenic SPC substrate-based assay was used to elucidate SPC enzyme activity within human retina and optic nerve head (ONH) tissues.

RESULTS

QPCR results indicated that PC1 and PC2 were expressed 4.1- and 5.7-fold higher in retina compared to optic nerve head, whereas PACE4 was expressed 4.1-fold higher in the ONH. PC1 and PC2 were localized primarily in neuronal cells, whereas PACE4 and PC5 were limited to the glia of the retina and optic nerve head. SPC activity in ONH lysate was significantly higher than that of retinal lysate; however, when an SPC inhibitor was added, activity in ONH decreased more than that in retina.

CONCLUSIONS

These results indicate that the SPCs are expressed in distinct patterns throughout the human retina and ONH. PC1 and PC2 were primarily expressed in neurons, whereas PACE4 appeared to be largely restricted to glia. Thus, elevated PACE4 may modulate the bioactivity of proteins secreted in the ONH and retina.

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.

Regulation of pigmentation in human epidermal melanocytes by functional high-affinity beta-melanocyte-stimulating hormone/melanocortin-4 receptor signaling

To date, the principal receptor considered to regulate human pigmentation is the melanocortin-1 receptor (MC1-R) via induction of the cAMP/protein kinase A pathway by the melanocortins alpha-MSH and ACTH. In this context, it is noteworthy that beta-MSH can also induce melanogenesis, although it has a low affinity for the MC1-R, whereas the preferred receptor for this melanocortin is the MC4-R. Because beta-MSH is present in the epidermal compartment, it was of interest to ascertain whether functioning MC4-Rs are present in human epidermal keratinocytes and melanocytes. Our results provide evidence that the MC4-R is expressed in situ and in vitro throughout the human epidermis at the mRNA and protein level using RT-PCR, Western blotting, and double immunofluorescence staining. Moreover, radioligand binding studies yielded high-affinity receptors for beta-MSH on epidermal melanocytes (3600 receptors per cell), undifferentiated keratinocytes (7200 receptors per cell), and differentiated keratinocytes (72,700 receptors per cell), indicating that MC4-R expression correlates with epidermal differentiation. Importantly, increased melanogenesis after stimulation of the beta-MSH/cAMP/microphthalmia-associated transcription factor/tyrosinase cascade proved the functionality of this signal in melanocytes, which was attenuated in the presence of the specific MC4-R antagonist HS014. In summary, our results imply an important role for the beta-MSH/MC4-R cascade in human melanocyte biology, although the function and purpose of this signal in keratinocytes needs further elucidation.

Heparan sulfate-independent cell binding and infection with furin-precleaved papillomavirus capsids

Papillomavirus infection normally involves virion binding to cell surface heparan sulfate proteoglycans (HSPGs). However, we found that human papillomavirus type 16 pseudovirions efficiently bound and infected cells lacking HSPGs if their L2 capsid protein was precleaved by furin, a cellular protease required for infection. The inability of pseudovirions to efficiently bind and infect cultured primary keratinocytes was also overcome by furin precleavage, suggesting that the defect involves altered HSPG modification. We conclude that the primary function of HSPG binding is to enable cell surface furin cleavage of L2 and that binding to a distinct cell surface receptor(s) is a subsequent step of papillomavirus infection.

Matrix metalloproteinase-21 expression is associated with keratinocyte differentiation and upregulated by retinoic acid in HaCaT cells

In the skin, expression of several matrix metalloproteinases (MMPs) occurs in response to tissue injury, tumorigenesis, angiogenesis, apoptosis, and inflammation. The recently cloned MMP-21 has been implicated in skin development and various epithelial cancers. In this study, we found that it is also expressed by differentiated keratinocytes (KCs) in various benign skin disorders, in which it was not associated with KC apoptosis or proliferation, and in organotypic cultures. Furthermore, MMP-21 was induced in keratinocytes in association with increased calcium and presence of the differentiation marker filaggrin. In stably transfected A431 and HEK293 cell lines, MMP-21 increased invasion of cells but did not associate with increased apoptosis, proliferation, or epithelial-to-mesenchymal transition. Of various agents tested in HaCaT cell cultures, only retinoic acid (10(-6) M) and staurosporine (2.5 x 10(-8) M) upregulated MMP-21 mRNA and protein expression, whereas tumor promoters, hormones, or dexamethasone were without effect. Our results suggest that MMP-21 may be an important protease in the terminal differentiation of keratinocytes.

The Ca2+-binding capacity of epidermal furin is disrupted by H2O2-mediated oxidation in vitiligo

The Ca(2+)-dependent precursor convertase furin is abundantly expressed in epidermal keratinocytes and melanocytes. In this context, it is noteworthy that proopiomelanocortin (POMC) cleavage is also processed by furin, leading to ACTH, beta-lipotropin, and beta-endorphin. All prohormone convertases including furin are regulated by Ca(2+). Because numerous epidermal peptides and enzymes are affected by H(2)O(2)-mediated oxidation, including the POMC-derived peptides alpha-MSH and beta-endorphin as shown in the epidermis of patients with vitiligo, we here asked the question of whether furin could also be a possible target for this oxidation mechanism by using immunofluorescence, RT-PCR, Western blotting, Ca(2+)-binding studies, and computer modeling. Our results demonstrate significantly decreased in situ immunoreactivity of furin in the epidermis of patients with progressive vitiligo (n = 10), suggesting H(2)O(2)-mediated oxidation. This was confirmed by (45)Ca(2+)-binding studies with human recombinant furin identifying the loss of one Ca(2+)-binding site from the enzyme after oxidation with H(2)O(2). Computer simulation supported alteration of one of the two Ca(2+)-binding sites on furin. Taken together, our results implicate that the Ca(2+)-dependent proteolytic activity of this convertase is targeted by H(2)O(2), which in turn could contribute to the reduced epidermal expression of the POMC-derived peptides alpha-MSH and beta-endorphin as documented earlier in patients with vitiligo.

Potentiation of ATP- and bradykinin-induced [Ca2+]c responses by PTHrP peptides in the HaCaT cell line

In the epidermis, local and systemic factors including extracellular nucleotides and parathyroid hormone-related protein (PTHrP) regulate keratinocyte proliferation and differentiation. Extracellular nucleotides increase proliferation via activation of P2 receptors and induction of calcium transients, while endoproteases cleave PTHrP, resulting in fragments with different cellular functions. We investigated the effects of adenosine 5'-triphosphate (ATP) alone and in combination with synthetic PTHrP peptides on calcium transients in HaCaT cells. ATP induced calcium transients, while PTHrP peptides did not. C-terminal and mid-molecule PTHrP peptides (1-100 pM) potentiated ATP-induced calcium transients independently of calcium influx. 3-Isobutyl-1-methylxanthine potentiated ATP-induced calcium transients, suggesting that a cyclic monophosphate is responsible. Cyclic AMP is not involved, but cyclic GMP is a likely candidate since the protein kinase G inhibitor, KT5823, inhibited potentiation. Co-stimulation with ATP and either PTHrP (43-52) or PTHrP (70-77) increased proliferation, suggesting that this is important in the regulation of cell turnover and wound healing and may be a mechanism for hyperproliferation in skin disorders such as psoriasis. Finally, PTHrP fragments potentiated bradykinin-induced calcium transients, suggesting a role in inflammation in the skin. Since PTHrP is found in many normal and malignant cells, potentiation is likely to have a wider role in modulating signal transduction events.

Epidermal terminal differentiation depends on B lymphocyte-induced maturation protein-1

The cornified layer is a compacted lattice of lipid-embedded corneocytes that provides an organism's barrier to the external environment. Cornification is the final differentiative step for epidermal keratinocytes and involves dramatic cell condensation before death. Using conditional gene deletion in mice, we identified the transcriptional repressor Blimp-1 (B lymphocyte-induced maturation protein-1) as an important regulator of keratinocyte transition from the granular to the cornified layer. More than 250 genes are misregulated in conditional knockout epidermis, including those encoding transcription factors, signal transduction components, proteinases, and enzymes involved in lipid metabolism. Steady-state mRNA and ChIP analyses of a subset of these genes provide evidence that nfat5, fos, prdm1, and dusp16 are novel direct targets of Blimp-1. Identifying nfat5 as a target of Blimp-1 repression indicates that cornification involves suppression of normal osmotic regulation in granular cells. Consistently, conditional knockout mice have delayed barrier formation as embryos, enlarged granular layer cells and corneocytes, and a morphologically abnormal cornified layer. These studies provide insight into cornification, identifying transcriptional regulatory circuitry and indicating the importance of blocking osmotic homeostasis.

LEKTI fragments specifically inhibit KLK5, KLK7, and KLK14 and control desquamation through a pH-dependent interaction

LEKTI is a 15-domain serine proteinase inhibitor whose defective expression underlies the severe autosomal recessive ichthyosiform skin disease, Netherton syndrome. Here, we show that LEKTI is produced as a precursor rapidly cleaved by furin, generating a variety of single or multidomain LEKTI fragments secreted in cultured keratinocytes and in the epidermis. The identity of these biological fragments (D1, D5, D6, D8-D11, and D9-D15) was inferred from biochemical analysis, using a panel of LEKTI antibodies. The functional inhibitory capacity of each fragment was tested on a panel of serine proteases. All LEKTI fragments, except D1, showed specific and differential inhibition of human kallikreins 5, 7, and 14. The strongest inhibition was observed with D8-D11, toward KLK5. Kinetics analysis revealed that this interaction is rapid and irreversible, reflecting an extremely tight binding complex. We demonstrated that pH variations govern this interaction, leading to the release of active KLK5 from the complex at acidic pH. These results identify KLK5, a key actor of the desquamation process, as the major target of LEKTI. They disclose a new mechanism of skin homeostasis by which the epidermal pH gradient allows precisely regulated KLK5 activity and corneodesmosomal cleavage in the most superficial layers of the stratum corneum.

Efficient Production of Bioactive Insulin from Human Epidermal Keratinocytes and Tissue-Engineered Skin Substitutes: Implications for Treatment of Diabetes

Despite many years of research, daily insulin injections remain the gold standard for diabetes treatment. Gene therapy may provide an alternative strategy by imparting the ability to secrete insulin from an ectopic site. The epidermis is a self-renewing tissue that is easily accessible and can provide large numbers of autologous cells to generate insulin-secreting skin substitutes. Here we used a recombinant retrovirus to modify human epidermal keratinocytes with a gene encoding for human proinsulin containing the furin recognition sequences at the A-C and B-C junctions. Keratinocytes were able to process proinsulin and secrete active insulin that promoted glucose uptake. Primary epidermal cells produced higher amounts of insulin than cell lines, suggesting that insulin secretion may depend on the physiological state of the producer cells. Modified cells maintained the ability to stratify into 3-dimensional skin equivalents that expressed insulin at the basal and suprabasal layers. Modifications at the furin recognition sites did not improve proinsulin processing, but a single amino acid substitution in the proinsulin B chain enhanced C-peptide secretion from cultured cells and bioengineered skin substitutes 10- and 28-fold, respectively. These results suggest that gene-modified bioengineered skin may provide an alternative means of insulin delivery for treatment of diabetes.

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.

Mature pig oligodendrocytes rapidly process human recombinant pro-nerve growth factor and do not undergo cell death

The neurotrophin family with its first member, nerve growth factor (NGF), binds two classes of receptors, more specifically to Trk receptors and to a shared p75NTR receptor. It has been shown that proNGF rather than NGF is predominant in the mature central nervous system. A recent finding indicated that a furin-resistant proNGF preferentially binds to p75NTR, initiating a pro-apoptotic cascade even in the presence of TrkA. In this context, rodent oligodendrocytes were reported to undergo cell death when exposed to proNGF. We have investigated the effect of a non-mutated 32 kDa human recombinant proNGF (rhproNGF) on cultured pig oligodendrocytes which express TrkA, p75NTR and sortilin. Pig oligodendrocytes respond to rhproNGF (50 ng/mL) with an enhanced regeneration of their processes as already observed for NGF. Activity of mitogen-activated protein kinase (MAPK), which plays an important role in oligodendroglial process formation, was increased even when rhproNGF processing was inhibited by the furin inhibitor Decanoyl-RVKR-CMK. Similarly, a cleavage-resistant proNGF (R-1G) activated MAPK and promoted oligodendroglial process regeneration. High concentrations of rhproNGF (300 ng/mL) did not induce cell death. Sodium dodecyl sulfate - polyacrylamide gel electrophoresis and Western blotting revealed that oligodendrocytes process rhproNGF to NGF. NGF was detected in Western blots of oligodendroglial lysates already 10 min after rhproNGF exposure, followed by a release of NGF into the culture medium. Indirect evidence indicates that rhproNGF processing occurs via an endocytotic route.

Human mast cells in the neurohormonal network: expression of POMC, detection of precursor proteases, and evidence for IgE-dependent secretion of alpha-MSH

Human mast cells have been shown to release histamine in response to the neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH), but it is unknown whether these cells express proopiomelanocortin (POMC) or POMC-derived peptides. We therefore examined highly purified human skin mast cells and a leukemic mast cell line-1 (HMC-1) for their ability to express POMC and members of the prohormone convertase (PC) family known to process POMC. Furthermore, we investigated whether these cells store and secrete alpha-MSH. Reverse transcriptase-PCR (RT-PCR) analysis revealed that both skin mast cells and HMC-1 cells express POMC mRNA and protein. Expression of the POMC gene at the RNA level in HMC-1 cells could be confirmed by Northern blotting. Transcripts for both PC1 and furin convertase were detectable in skin-derived mast cells and HMC-1 cells, as shown by RT-PCR. In contrast, PC2 transcripts were detected only in skin mast cells, whereas transcripts for paired basic amino acid converting enzyme 4 (PACE4) were present only in HMC-1 cells. Radioimmunoassays performed on cell lysates and cell culture supernatants from human skin-derived mast cells disclosed immunoreactive amounts of alpha-MSH in both fractions. Stimulation with an anti-IgE antibody significantly reduced intracellular alpha-MSH and increased extracellular levels, indicating IgE-mediated secretion of this neuropeptide. Our findings show that human mast cells are active players in the cutaneous POMC system. Mast cell-derived alpha-MSH may contribute to cutaneous hyperpigmentation as seen in patients with urticaria pigmentosa. Moreover, IgE-dependent release of alpha-MSH suggests an immunomodulatory role of this neurohormone during inflammatory and allergic reactions of the skin.

Cleavage of the papillomavirus minor capsid protein, L2, at a furin consensus site is necessary for infection

Papillomaviruses (PV) comprise a large family of nonenveloped DNA viruses that include the oncogenic PV types that are the causative agents of human cervical cancer. As is true of many animal DNA viruses, PV are taken into the cell by endocytosis and must escape from the endosomal compartment to the cytoplasm to initiate infection. Here we show that this step depends on the site-specific enzymatic cleavage of the PV minor virion protein L2 at a consensus furin recognition site. Cleavage by furin, a cell-encoded proprotein convertase, is known to be required for endosome escape by many bacterial toxins. However, to our knowledge, furin has not been previously implicated in the viral entry process. This step is potentially a target for PV inhibition.

Detection of functionally active melanocortin receptors and evidence for an immunoregulatory activity of alpha-melanocyte-stimulating hormone in human dermal papilla cells

Proopiomelanocortin (POMC)-derived peptides and their receptors have been identified in many peripheral organs including the skin in which they exert a diversity of biological actions. We investigated the expression and potential role of the POMC system in human dermal papilla cells (DPCs), a specialized cutaneous mesenchymal cell type regulating hair follicle activity. In culture, these cells expressed POMC and displayed immunoreactivity for ACTH, alphaMSH, and beta-endorphin. Among the prohormone convertases (PCs) tested, only PC2, its chaperone 7B2, and furin convertase but not PC1 and paired basic amino acid cleaving enzyme 4 gene were detected. Human DPCs in vitro expressed both the melanocortin-1 receptor (MC-1R) and MC-4R, and immunoreactivity for these receptors was also present in cells of the human dermal papilla in situ. In contrast to the dermal papilla of agouti mice, agouti signaling protein, a natural and highly selective MC-1R and MC-4R antagonist, was undetectable in human DPCs. The MC-Rs detected in human DPCs were functionally active because alphaMSH increased intracellular cAMP and calcium. Preincubation of the cells with a synthetic peptide corresponding to the C-terminal domain of agouti signaling protein abrogated cAMP induction by alphaMSH. Furthermore, alphaMSH was capable of antagonizing the expression of intercellular adhesion molecule-1 induced by the proinflammatory cytokine interferon-gamma. Our data suggest a regulatory function of alphaMSH within the dermal papilla whose disruption may lead to deregulation of immune and inflammatory responses of the hair follicle, thereby possibly contributing to the development of inflammatory forms of alopecia.

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.

Inhibitors of proprotein convertases

The discovery of mammalian subtilases, proprotein convertases (PCs) or subtilisin-like proprotein convertases (SPCs), in 1990 was a result of sustained efforts in searching for enzyme/s responsible for maturation of inactive protein precursors. Since then, seven PCs have so far been discovered that cleave at the carboxy-terminal of a basic amino acid characterized by the consensus sequence Arg/Lys/His-X-X/Lys/Arg-Arg downward arrow, where X denotes any amino acid other than Cys. Two additional PC subtypes--called subtilisin kexin isozyme 1 (SKI-1) or site 1 protease (S1P) and neural apoptosis regulated convertase 1 (NARC-1), also known as PCSK9--that cleave at the carboxy terminus of nonbasic amino acids were discovered later. Numerous studies revealed various important functional roles of PCs in health and diseases such as tumorigenesis, diabetes, viral infections, bacterial pathogenesis, atherosclerosis, and neurodegenarative diseases such as Alzheimer's. Owing to these findings, PCs became a promising frontier for treatment of diverse pathologies. Thus modulation of PC activity with designed inhibitors is an attractive proposition not only for intervention of diseases, but also for biochemical characterization of these enzymes. Various physiological and bioengineered proteins as well as small molecules such as peptide, peptidomimetic, and nonpeptide compounds as inhibitors of PCs have been described in the literature. Among the strategies used for design of PC inhibitors, the most successful is the one based on bioengineered serpin proteins, of which the best example is alpha1-PDX, the double mutant variant of alpha1-antitrypsin (from A(355)IPM(358) to R(355)IPR(358)). Others include small peptide inhibitors with C-terminal carboxyl function modified with a potent neucleophile or those containing pseudo or isosteric peptide bond at the scissile site of a suitable peptide substrate. Among nonpeptide PC inhibitors, the number is very limited. So far, these include 20-carbon atoms containing alicyclic diterpenes of andrographolide family and heterocyclic compounds that are ligands of Zn2+ and Cu2+ ions. Overall, these molecules display only a modest enzyme inhibition; however, they may serve as important lead structures for further development of more potent and specific nonpeptide PC inhibitors as potential therapeutic agents. Many PC inhibitors display their functional properties in proliferation, fertilization, tumorigenesis, obesity, embryogenesis, or diabetes via their inhibitory action on PC activities.

Gene expression profiles of TNF-alpha, TACE, furin, IL-1beta and matrilysin in UVA- and UVB-irradiated HaCat cells

BACKGROUND/PURPOSE

It is known that solar ultraviolet (UV) irradiation exerts multiple effects on mammalian skin tissues, one of which is the induction of local and systemic immunosuppression as well as inflammation. Tumor necrosis factor-alpha (TNF-alpha) and other cytokines are suggested to play a role in these responses. Quantitative real-time polymerase chain reaction (TaqMan RTPCR) was used to elucidate the effect of UVA and UVB irradiation on the expression of genes coding for TNF-alpha, IL-1beta, IL-10, FasL, matrilysin, TACE and furin in HaCaT cells over a 48 h period (IL-1beta, interleukin-1beta; FasL, Fas ligand).

METHODS

Cultured HaCaT cells were either sham irradiated (control) or exposed to UVA (2000 and 8000 J/m2) or UVB (200 and 2000 J/m2) radiation. RNA was extracted from cells at 0, 4, 8, 12, 16, 24, 48 h post-irradiation and reverse transcribed to generate cDNA for subsequent real-time PCR amplification.

RESULTS

Significant increases in the mRNA levels for all genes tested were detected in both UVA- and UVB-irradiated HaCaT cells compared with control (sham-irradiated) cells. TNF-alpha mRNA levels were immediately up-regulated (0 h) after irradiation, with maximal induction at 8 h post 2000 J/m2 UVA and 200 J/m2 UVB irradiation, at 4 h post 8000 J UVA irradiation and at 48 h post 2000 J/m2 UVB irradiation. No correlation was observed between TNF-alpha, TACE and furin mRNA induction in the different irradiated cohorts.

CONCLUSION

Results suggest that time-distinct gene induction of TNF-alpha, furin, IL-1beta and matrilysin may be involved in UV-induced cellular responses, but not for TACE. In general, mRNA induction was dose dependent at some time points post-irradiation, but not throughout the whole time course tested. Our results show that quantitative real-time PCR is a useful tool in the analysis of quantitative changes of mRNA levels in cultured HaCaT cells after UV exposure.

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.

Delayed epidermal permeability barrier formation and hair follicle aberrations in Inv-Cldn6 mice

Homozygous mice overexpressing Claudin-6 (Cldn6) exhibit a perturbation in the epidermal differentiation program leading to a defective epidermal permeability barrier (EPB) and dehydration induced death ensuing within 48 h of birth [Turksen, K., Troy, T.C., 2002. Permeability barrier dysfunction in transgenic mice overexpressing claudin 6. Development 129, 1775-1784]. Their heterozygous counterparts are also born with an incomplete EPB; however, barrier formation continues after birth and normal hydration levels are achieved by postnatal day 12 allowing survival into adulthood. Heterozygous Inv-Cldn6 mice exhibit a distinct coat phenotype and histological analysis shows mild epidermal hyperkeratosis. Expression of K5 and K14 is aberrant, extending beyond the basal layer into the suprabasal layer where they are not co-localized suggesting that their expression is uncoupled. There is also atypical K17 and patchy K15 expression in the basal layer with no K6 expression in the interfollicular epidermis; together with marked changes in late differentiation markers (e.g. profilaggrin/filaggrin, loricrin, transglutaminase 3) indicating that the normal epidermal differentiation program is modified. The expression compartment of various Cldns is also perturbed although overall protein levels remained comparable. Most notably induction of Cldn5 and Cldn8 was observed in the Inv-Cldn6 epidermis. Heterozygous Inv-Cldn6 animals also exhibit subtle alterations in the differentiation program of the hair follicle including a shorter anagen phase, and altered hair type distribution and length compared to the wild type; the approximately 20% increase in zig-zag hair fibers at the expense of guard hairs and the approximately 30% shorter guard hairs contribute to coat abnormalities in the heterozygous mice. In addition, the transgenic hair follicles exhibit a decreased expression of K15 as well as some hair-specific keratins and express Cldn5 and Cldn18, which are not detectable in the wild type. These data indicate that Cldn6 plays a role in the differentiation processes of the epidermis and hair follicle and supports the notion of a link between Cldn regulation and EPB assembly/maintenance as well as the hair cycle.