Psoriasis is a common cutaneous disease with multiple characteristics including inflammation and aberrant keratinocyte proliferation. However, the pathogenesis of psoriasis is not completely clear yet. The objective of this study is to perform an in-depth analysis of the association between SPRR and LCE in the pathogenesis of psoriasis.

The epidermal cornified cell envelope (CE) is a complex protein-lipid composite that replaces the plasma membrane of terminally differentiated keratinocytes. This lamellar structure is essential for the barrier function of the skin and has the ability to prevent the loss of water and ions and to protect from environmental hazards. The major protein of the epidermal CE is loricrin, contributing approximately 70% by mass. We have generated mice that are deficient for this protein. These mice showed a delay in the formation of the skin barrier in embryonic development. At birth, homozygous mutant mice weighed less than control littermates and showed skin abnormalities, such as congenital erythroderma with a shiny, translucent skin. Tape stripping experiments suggested that the stratum corneum stability was reduced in newborn Lor(-/-) mice compared with wild-type controls. Isolated mutant CEs were more easily fragmented by sonication in vitro, indicating a greater susceptibility to mechanical stress. Nevertheless, we did not detect impaired epidermal barrier function in these mice. Surprisingly, the skin phenotype disappeared 4-5 d after birth. At least one of the compensatory mechanisms preventing a more severe skin phenotype in newborn Lor(-/-) mice is an increase in the expression of other CE components, such as SPRRP2D and SPRRP2H, members of the family of "small proline rich proteins", and repetin, a member of the "fused gene" subgroup of the S100 gene family.

Small proline-rich proteins (SPRRs), components of cornified cell envelope precursors, have recently been found to participate in airway diseases. However, their role in allergic airway inflammatory conditions remains unknown. Here, we explored the expression of SPRR3 in house dust mite (HDM)-sensitized/challenged mice and attempted to elucidate the regulatory role of SPRR3 in allergic airway inflammation. SPRR3 was identified via bioinformatics analysis of Gene Expression Omnibus (GEO) databases and further confirmed to be upregulated in the lungs of asthmatic mice. Knockdown of SPRR3 via the intratracheal route significantly inhibited eosinophils in bronchoalveolar lavage fluid (BALF) and suppressed the expressions of type 2 cytokines (IL-4, IL-5, and IL-13) in BALF and lung tissues. Further, SPRR3 knockdown reduced the expression of IL-33 and further attenuated the activation of the PI3K/AKT/NF-κB signaling pathway in the recruitment of group 2 innate lymphoid cells (ILC2s) to inhibit allergic airway inflammation. In vitro, SPRR3 siRNA could alleviate HDM-induced inflammatory responses in BEAS-2B cells. This study reveals the regulatory role of SPRR3 in allergic airway inflammation, identifying this protein as a potential novel therapeutic target for asthma.

Excessive ultraviolet radiation (UVR) exposure of the skin is associated with adverse clinical outcomes. Although both exogenous sunscreens and endogenous tissue components (including melanins and tryptophan-derived compounds) reduce UVR penetration, the role of endogenous proteins in absorbing environmental UV wavelengths is poorly defined. Having previously demonstrated that proteins which are rich in UVR-absorbing amino acid residues are readily degraded by broadband UVB-radiation (containing UVA, UVB and UVC wavelengths) here we hypothesised that UV chromophore (Cys, Trp and Tyr) content can predict the susceptibility of structural proteins in skin and the eye to damage by physiologically relevant doses (up to 15.4 J/cm(2)) of solar UVR (95% UVA, 5% UVB). We show that: i) purified suspensions of UV-chromophore-rich fibronectin dimers, fibrillin microfibrils and β- and γ-lens crystallins undergo solar simulated radiation (SSR)-induced aggregation and/or decomposition and ii) exposure to identical doses of SSR has minimal effect on the size or ultrastructure of UV chromophore-poor tropoelastin, collagen I, collagen VI microfibrils and α-crystallin. If UV chromophore content is a factor in determining protein stability in vivo, we would expect that the tissue distribution of Cys, Trp and Tyr-rich proteins would correlate with regional UVR exposure. From bioinformatic analysis of 244 key structural proteins we identified several biochemically distinct, yet UV chromophore-rich, protein families. The majority of these putative UV-absorbing proteins (including the late cornified envelope proteins, keratin associated proteins, elastic fibre-associated components and β- and γ-crystallins) are localised and/or particularly abundant in tissues that are exposed to the highest doses of environmental UVR, specifically the stratum corneum, hair, papillary dermis and lens. We therefore propose that UV chromophore-rich proteins are localised in regions of high UVR exposure as a consequence of an evolutionary pressure to express sacrificial protein sunscreens which reduce UVR penetration and hence mitigate tissue damage.

Major protein components of the mammalian skin barrier are encoded by genes clustered in the Epidermal Differentiation Complex (EDC). The skin of cetaceans, i.e. whales, porpoises and dolphins, differs histologically from that of terrestrial mammals. However, the genetic regulation of their epidermal barrier is only incompletely known. Here, we investigated the EDC of cetaceans by comparative genomics. We found that important epidermal cornification proteins, such as loricrin and involucrin are conserved and subtypes of small proline-rich proteins (SPRRs) are even expanded in numbers in cetaceans. By contrast, keratinocyte proline rich protein (KPRP), skin-specific protein 32 (XP32) and late-cornified envelope (LCE) genes with the notable exception of LCE7A have been lost in cetaceans. Genes encoding proline rich 9 (PRR9) and late cornified envelope like proline rich 1 (LELP1) have degenerated in subgroups of cetaceans. These data suggest that the evolution of an aquatic lifestyle was accompanied by amplification of SPRR genes and loss of specific other epidermal differentiation genes in the phylogenetic lineage leading to cetaceans.

Epidermal keratinocyte differentiation on the body surface is a carefully choreographed process that leads to assembly of a barrier that is essential for life. Perturbation of keratinocyte differentiation leads to disease. Activator protein 1 (AP1) transcription factors are key controllers of this process. We have shown that inhibiting AP1 transcription factor activity in the suprabasal murine epidermis, by expression of dominant-negative c-jun (TAM67), produces a phenotype type that resembles human keratoderma. However, little is understood regarding the structural and molecular changes that drive this phenotype. In the present study we show that TAM67-positive epidermis displays altered cornified envelope, filaggrin-type keratohyalin granule, keratin filament, desmosome formation and lamellar body secretion leading to reduced barrier integrity. To understand the molecular changes underlying this process, we performed proteomic and RNA array analysis. Proteomic study of the corneocyte cross-linked proteome reveals a reduction in incorporation of cutaneous keratins, filaggrin, filaggrin2, late cornified envelope precursor proteins, hair keratins and hair keratin-associated proteins. This is coupled with increased incorporation of desmosome linker, small proline-rich, S100, transglutaminase and inflammation-associated proteins. Incorporation of most cutaneous keratins (Krt1, Krt5 and Krt10) is reduced, but incorporation of hyperproliferation-associated epidermal keratins (Krt6a, Krt6b and Krt16) is increased. RNA array analysis reveals reduced expression of mRNA encoding differentiation-associated cutaneous keratins, hair keratins and associated proteins, late cornified envelope precursors and filaggrin-related proteins; and increased expression of mRNA encoding small proline-rich proteins, protease inhibitors (serpins), S100 proteins, defensins and hyperproliferation-associated keratins. These findings suggest that AP1 factor inactivation in the suprabasal epidermal layers reduces expression of AP1 factor-responsive genes expressed in late differentiation and is associated with a compensatory increase in expression of early differentiation genes.

Small proline rich protein (SPRR) 2A is one of 14 SPRR genes that encodes for a skin cross-linking protein, which confers structural integrity to the cornified keratinocyte cell envelope. New evidence, however, shows that SPRR2A is also a critical stress and wound repair modulator: it enables a variety of barrier epithelia to transiently acquire mesenchymal characteristics (EMT) and simultaneously quench reactive oxygen species during wound repair responses. p53 is also widely recognized as the node in cellular stress responses that inhibits EMT and triggers cell-cycle arrest, apoptosis, and cellular senescence. Since some p53-directed processes would seem to impede wound repair of barrier epithelia, we hypothesized that SPRR2A up regulation might counteract these effects and enable/promote wound repair under stressful environmental conditions.

Corsican Helichrysum italicum essential oil (HIEO) is characterized by high concentrations of neryl acetate, and we previously demonstrated that Corsican HIEO increases the expression of genes that are part of the differentiation complex (involucrin, small proline rich proteins, late cornified envelope, S100 protein family). The biological activities of HIEO and neryl acetate (NA) were compared to identify how NA contributes to HIEO activity on human skin. NA, as a part component of HIEO, was tested on skin explant models for 24 hours and 5 days in comparison with HIEO. We analyzed the biological regulations in the skin explant by transcriptomic analysis, skin barrier protein immunofluorescence, lipid staining and ceramide analysis by liquid chromatography-mass spectrometry. Transcriptomic analysis revealed that 41.5% of HIEO-modulated genes were also regulated by NA and a selected panel of genes were confirmed by qquantitative reverse transcription PCR analysis. Those genes are involved in epidermal differentiation, skin barrier formation and ceramide synthesis. Involucrin (IVL), involved in formation of the cornified envelope (CE), was upregulated at both gene and protein levels after 24 hours and 5 days respectively. After 5 days of treatment, total lipids and ceramides were also increased. Our results demonstrate that NA mediates a large part of Corsican HIEO activity on skin barrier formation.

The skin provides an efficient permeability barrier and protects from microbial invasion and oxidative stress. Here, we show that these essential functions are linked through the Nrf2 transcription factor. To test the hypothesis that activation of Nrf2 provides skin protection under stress conditions, we determined the consequences of pharmacological or genetic activation of Nrf2 in keratinocytes. Surprisingly, mice with enhanced Nrf2 activity in keratinocytes developed epidermal thickening, hyperkeratosis and inflammation resembling lamellar ichthyosis. This resulted from upregulation of the cornified envelope proteins small proline-rich proteins (Sprr) 2d and 2h and of secretory leukocyte peptidase inhibitor (Slpi), which we identified as novel Nrf2 targets in keratinocytes. Since Sprrs are potent scavengers of reactive oxygen species and since Slpi has antimicrobial activities, their upregulation contributes to Nrf2's protective function. However, it also caused corneocyte fragility and impaired desquamation, followed by alterations in the epidermal lipid barrier, inflammation and overexpression of mitogens that induced keratinocyte hyperproliferation. These results identify an unexpected role of Nrf2 in epidermal barrier function, which needs to be considered for pharmacological use of Nrf2 activators.

The function of the skin as a barrier against the environment depends on the differentiation of epidermal keratinocytes into highly resilient corneocytes that form the outermost skin layer. Many genes encoding structural components of corneocytes are clustered in the epidermal differentiation complex (EDC), which has been described in placental and marsupial mammals as well as non-mammalian tetrapods. Here, we analyzed the genomes of the platypus (Ornithorhynchus anatinus) and the echidna (Tachyglossus aculeatus) to determine the gene composition of the EDC in the basal clade of mammals, the monotremes. We report that mammal-specific subfamilies of EDC genes encoding small proline-rich proteins (SPRRs) and late cornified envelope proteins as well as single-copy EDC genes such as involucrin are conserved in monotremes, suggesting that they have originated in stem mammals. Monotremes have at least one gene homologous to the group of filaggrin (FLG), FLG2 and hornerin (HRNR) in placental mammals, but no clear one-to-one pairwise ortholog of either FLG, FLG2 or HRNR. Caspase-14, a keratinocyte differentiation-associated protease implicated in the processing of filaggrin, is encoded by at least 3 gene copies in the echidna. Our results reveal evolutionarily conserved and clade-specific features of the genetic regulation of epidermal differentiation in monotremes.

Rationale: Cdc42 is a Rho GTPase that regulates diverse cellular functions. Here, we used genetic techniques to investigate the role of Cdc42 in epidermal development and epidermal barrier formation. Methods: Keratinocyte-restricted Cdc42 knockout mice were generated with the Cre-LoxP system under the keratin 14 (K14) promoter. The skin and other tissues were collected from mutant and wild-type mice, and their cellular, molecular, morphological, and physiological features were analyzed. Results: Loss of Cdc42 in the epidermis in vivo resulted in neonatal lethality and impairment of epidermal barrier formation. Cdc42 deficiency led to the loss of epidermal stem cells. The absence of Cdc42 led to increased thickening of the epidermis, which was associated with increased proliferation and reduced apoptosis of keratinocytes. In addition, Cdc42 deficiency damaged tight junctions, adherens junctions and desmosomes. RNA sequencing results showed that the most significantly altered genes were enriched by the terms of "keratinization" and "cornified envelope" (CE). Among the differentially expressed genes in the CE term, several members of the small proline-rich protein (SPRR) family were upregulated. Further study revealed that there may be a Cdc42-SPRR pathway, which may correlate with epidermal barrier function. Conclusions: Our study indicates that Cdc42 is essential for epidermal development and epidermal barrier formation. Defects in Cdc42-SPRR signaling may be associated with skin barrier dysfunction and a variety of skin diseases.