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

The moisturizing effect of Capparis spinosa fruit extract targeting filaggrin synthesis and degradation

BACKGROUND

Small molecular natural products, such as betaine, have unique moisturizing advantages. Capparis spinosa L. fruit is rich in quaternary ammonium alkaloids such as betaine and stachydrine. However, few studies investigated its efficacy and mechanism on human skin.

OBJECTIVE

Polysaccharides-free C. spinosa fruit extract (CS) was obtained to study its moisturizing effect and mechanisms focusing on filaggrin (FLG) synthesis and degradation.

METHODS

The clinical moisturizing test was carried out on human arms, calves, and faces after CS treatment for 0.5-6 h. The change in the level of FLG, caspase 14, loricrin, and transglutaminase 5 (TGM 5) was measured by immunofluorescence after CS treatment for 4 and 24 h in a reconstructed epidermis model. Also, the content of pyrrolidone carboxylic acid (PCA) in the stratum corneum was tested by high-performance liquid chromatography (HPLC) both in the epidermis model and human calves.

RESULTS

Compared with glycerin (positive control), 5% CS showed a strong skin hydration effect on arms and calves when applied for 0.5-6 h. Also, the face hydration increased at 0.5 and 4 h. In addition, 3% CS applied to the recombinant epidermis model under low humidity promoted the immunodetected levels of caspase 14 and PCA content but reduced the levels of FLG at 4 h, however, the levels of FLG, loricrin, and TGM 5 were promoted at 24 h. Meanwhile, CS treatment for 4 h in human calves increased the PCA content in the stratum corneum by 29.9%.

CONCLUSIONS

Topical application of CS on human skin showed an instant and long-lasting increase in skin hydration by regulating the FLG network. It promoted FLG degradation to form PCA at 4 h both in vivo and in vitro, increasing FLG synthesis after 24 h, potentially reforming the FLG monomer reservoir to alleviate the skin's dry condition.

RDIVpSGP motif of ASPP2 binds to 14-3-3 and enhances ASPP2/k18/14-3-3 ternary complex formulation to promote BRAF/MEK/ERK signal inhibited cell proliferation in hepatocellular carcinoma

The Apoptosis Stimulating Protein of p53 2 (ASPP2) is a heterozygous insufficient tumor suppressor; however, its molecular mechanism(s) in tumor suppression is not completely understood. ASPP2 plays an essential role in cell growth, as shown by liver hepatocellular carcinoma (LIHC) RNA-seq assay using the Cancer Genome Atlas (TCGA) and High-Throughput-PCR assay using ASPP2 knockdown cells. These observations were further confirmed by in vivo and in vitro experiments. Mechanistically, N-terminus ASPP2 interacted with Keratin 18 (k18) in vivo and in vitro. Interestingly, the RDIVpSGP motif of ASPP2 associates with 14-3-3 and promotes ASPP2/k18/14-3-3 ternary-complex formation which promotes MEK/ERK signal activation by impairing 14-3-3 and BRAF association. Additionally, ASPP2-rAd injection promotes paclitaxel-suppressed tumor growth by suppressing cell proliferation in the BALB/c nude mice model. ASPP2 and k18 were preferentially downregulated in Hepatocellular Carcinoma (HCC), which predicted poor prognosis in HCC patients. Overall, these findings suggested that ASPP2 promoted BRAF/MEK/ERK signal activation by promoting the formation of an ASPP2/k18/14-3-3 ternary complex via the RDIVpSGP motif at the N terminus. Moreover, this study provides novel insights into the molecular mechanism of tumor suppression in HCC patients.

Terminal differentiation of keratinocytes was damaged in type 2 diabetic mice

AIMS

Although skin manifestations are common in diabetic patients, its characteristics are poorly identified. This study explored the differentiation process of keratinocytes in type 2 diabetes mellitus (T2DM) in vivo.

METHODS

Back skin of T2DM model KKAy/TaJcl mice (KKAy) and C57BL/6JJcl mice (control) aged 8 and 12 weeks was used. The mRNA expression of differentiation markers of keratinocytes was measured by quantitative real-time polymerase chain reaction (qRT-PCR). The expression of each marker in situ was examined immunohistochemically.

RESULTS

KKAy mice showed hyperglycemia versus control mice. The histological findings showed increased thickness and structural impairment of epidermal tissue in KKAy mice. The qRT-PCR revealed that the expression of integrin beta 1 and keratin 14 in KKAy and control mice was identical. However, the expression of involucrin at 8 weeks, keratin 10 at 12 weeks, and filaggrin and loricrin at 8 and 12 weeks was decreased in KKAy mice. Immunohistochemical findings showed that filaggrin was markedly decreased in KKAy mice, though Ki-67 remained unchanged.

CONCLUSION

The terminal differentiation process was impaired in the diabetic skin, while keratinocyte proliferation was preserved. Damaged terminal differentiation of keratinocytes may contribute to impairment of the skin barrier function in diabetic dermatoses.

Subcellular distribution of α2-adrenoceptor subtypes in the rodent kidney

Renal α2-adrenoceptors have been reported to play a role in the regulation of urinary output, renin secretion, and water and sodium excretion in the kidneys. However, the distribution of α2-adrenoceptor subtypes in the kidneys remains unclear. In this study, we aimed to investigate the localization of α2-adrenoceptor subtypes in rat kidneys using 8-week-old Sprague-Dawley rats. Immunofluorescence imaging revealed that both α2A- and α2B-adrenoceptors were expressed in the basolateral, but not apical, membrane of the epithelial cells of the proximal tubules. We also found that α2A- and α2B-adrenoceptors were not expressed in the glomeruli, collecting ducts, or the descending limb of the loop of Henle and vasa recta. In contrast, α2C-adrenoceptors were found to be localized in the glomeruli and lumen of the cortical and medullary collecting ducts. These results suggest that noradrenaline may act on the basement membrane of the proximal tubules through α2A- and α2B-adrenoceptors. Moreover, noradrenaline may be involved in the regulation of glomerular filtration and proteinuria through the induction of morphological changes in mesangial cells and podocytes via α2C-adrenoceptors. In the collecting ducts, urinary noradrenaline may regulate morphological changes of the microvilli through α2C-adrenoceptors. Our findings provide an immunohistochemical basis for understanding the cellular targets of α2-adrenergic regulation in the kidneys. This may be used to devise therapeutic strategies targeting α2-adrenoceptors.

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.

Long-Term and Clinically Relevant Full-Thickness Human Skin Equivalent for Psoriasis

Psoriasis is an incurable, immune-mediated inflammatory disease characterized by the hyperproliferation and abnormal differentiation of keratinocytes. To study in depth the pathogenesis of this disease and possible therapy options suitable, pre-clinical models are required. Three-dimensional skin equivalents are a potential alternative to simplistic monolayer cultures and immunologically different animal models. However, current skin equivalents lack long-term stability, which jeopardizes the possibility to simulate the complex disease-specific phenotype followed by long-term therapeutic treatment. To overcome this limitation, the cell coating technique was used to fabricate full-thickness human skin equivalents (HSEs). This rapid and scaffold-free fabrication method relies on coating cell membranes with nanofilms using layer-by-layer assembly, thereby allowing extended cultivation of HSEs up to 49 days. The advantage in time is exploited to develop a model that not only forms a disease phenotype but can also be used to monitor the effects of topical or systemic treatment. To generate a psoriatic phenotype, the HSEs were stimulated with recombinant human interleukin 17A (rhIL-17A). This was followed by systemic treatment of the HSEs with the anti-IL-17A antibody secukinumab in the presence of rhIL-17A. Microarray and RT-PCR analysis demonstrated that HSEs treated with rhIL-17A showed downregulation of differentiation markers and upregulation of chemokines and cytokines, while treatment with anti-IL-17A antibody reverted these gene regulations. Gene ontology analysis revealed the proinflammatory and chemotactic effects of rhIL-17A on the established HSEs. These data demonstrated, at the molecular level, the effects of anti-IL-17A antibody on rhIL-17A-induced gene regulations. This shows the physiological relevance of the developed HSE and opens venues for its use as an alternative to ex vivo skin explants and animal testing.

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.

Filaggrin and filaggrin 2 processing are linked together through skin aspartic acid protease activation

Skin aspartic acid protease (SASPase) is believed to be a key enzyme involved in filaggrin processing during epidermal terminal differentiation. Since little is known about the regulation of SASPase function, the aim of this study was to identify involved protein partners in the process. Yeast two hybrid analyses using SASPase as bait against a human reconstructed skin library identified that the N-terminal domain of filaggrin 2 binds to the N-terminal fragment of SASPase. This interaction was confirmed in reciprocal yeast two hybrid screens and by Surface Plasmon Resonance analyses. Immunohistochemical studies in human skin, using specific antibodies to SASPase and the N-terminal domain of filaggrin 2, showed that the two proteins partially co-localized to the stratum granulosum. In vitro enzymatic assays showed that the N-terminal domain of filaggrin 2 enhanced the autoactivation of SASPase to its 14 kDa active form. Taken together, the data suggest that the N-terminal domain of filaggrin 2 regulates the activation of SASPase that may be a key event upstream of filaggrin processing to natural moisturizing factors in the human epidermis.

Live imaging of alterations in cellular morphology and organelles during cornification using an epidermal equivalent model

The stratum corneum plays a crucial role in epidermal barrier function. Various changes occur in granular cells at the uppermost stratum granulosum during cornification. To understand the temporal details of this process, we visualized the cell shape and organelles of cornifying keratinocytes in a living human epidermal equivalent model. Three-dimensional time-lapse imaging with a two-photon microscope revealed that the granular cells did not simply flatten but first temporarily expanded in thickness just before flattening during cornification. Moreover, before expansion, intracellular vesicles abruptly stopped moving, and mitochondria were depolarized. When mitochondrial morphology and quantity were assessed, granular cells with fewer, mostly punctate mitochondria tended to transition to corneocytes. Several minutes after flattening, DNA leakage from the nucleus was visualized. We also observed extension of the cell-flattening time induced by the suppression of filaggrin expression. Overall, we successfully visualized the time-course of cornification, which describes temporal relationships between alterations in the transition from granular cells to corneocytes.

Proteomic Analysis of Loricrin Knockout Mouse Epidermis

The crosslinked envelope of the mammalian epidermal corneocyte serves as a scaffold for assembly of the lipid barrier of the epidermis. Thus, deficient envelope crosslinking by keratinocyte transglutaminase (TGM1) is a major cause of the human autosomal recessive congenital ichthyoses characterized by barrier defects. Expectations that loss of some envelope protein components would also confer an ichthyosis phenotype have been difficult to demonstrate. To help rationalize this observation, the protein profile of epidermis from loricrin knockout mice has been compared to that of wild type. Despite the mild phenotype of the knockout, some 40 proteins were incorporated into envelope material to significantly different extents compared to those of wild type. Nearly half were also incorporated to similarly altered extents into the disulfide bonded keratin network of the corneocyte. The results suggest that loss of loricrin alters their incorporation into envelopes as a consequence of protein-protein interactions during cell maturation. Mass spectrometric protein profiling revealed that keratin 1, keratin 10, and loricrin are prominent envelope components and that dozens of other proteins are also components. This finding helps rationalize the potential formation of functional envelopes, despite loss of a single component, due to the availability of many alternative transglutaminase substrates.

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.

In a three-dimensional reconstructed human epidermis filaggrin-2 is essential for proper cornification

Atopic dermatitis is a chronic inflammatory skin disease with defects in the epidermal barrier. In a cohort of African-American children, a FLG2 nonsense mutation has been associated with the disease. In the epidermis of European patients, the expression of filaggrin-2, the filaggrin-related protein encoded by FLG2, is decreased. To describe the function of filaggrin-2 and evaluate the impact of its deficiency, its expression was downregulated using lentivirus-mediated shRNA interference in a three-dimensional reconstructed human epidermis (RHE) model. This resulted in parakeratosis and a compact stratum corneum, presence of abnormal vesicles inside the corneocytes, increased pH and reduced amounts of free amino acids at the RHE surface, leading to increased sensitivity to UVB radiations. The expression of differentiation markers was slightly modified. However, we observed reduced proteolytic processing of corneodesmosin, hornerin and filaggrin in parallel with reduced amounts of caspase-14 and bleomycin hydrolase. Our data demonstrated that filaggrin-2 is important for a proper cornification and a functional stratum corneum. Its downregulation in atopic patients may be involved in the disease-associated epidermis impairment.

Knock-down of filaggrin does not affect lipid organization and composition in stratum corneum of reconstructed human skin equivalents

Human skin mainly functions as an effective barrier against unwanted environmental influences. The barrier function strongly relies on the outermost layer of the skin, the stratum corneum (SC), which is composed of corneocytes embedded in an extracellular lipid matrix. The importance of a proper barrier function is shown in various skin disorders such as atopic dermatitis (AD), a complex human skin disorder strongly associated with filaggrin (FLG) null mutations, but their role in barrier function is yet unclear. To study the role of FLG in SC barrier properties in terms of SC lipid organization and lipid composition, we generated an N/TERT-based 3D-skin equivalent (NSE) after knock-down of FLG with shRNA. In these NSEs, we examined epidermal morphogenesis by evaluating the expression of differentiation markers keratin 10, FLG, loricrin and the proliferation marker ki67. Furthermore, the SC was extensively analysed for lipid organization, lipid composition and SC permeability. Our results demonstrate that FLG knock-down (FLG-KD) did not affect epidermal morphogenesis, SC lipid organization, lipid composition and SC permeability for a lipophilic compound in NSEs. Therefore, our findings indicate that FLG-KD alone does not necessarily affect the functionality of a proper barrier function.

The role of filaggrin in the skin barrier and disease development

Filaggrin is a structural protein that is fundamental in the development and maintenance of the skin barrier. The function of filaggrin and its involvement in various cutaneous and extracutaneous disorders has been the subject of considerable research in recent years. Mutations in FLG, the gene that encodes filaggrin, have been shown to cause ichthyosis vulgaris, increase the risk of atopic dermatitis and other atopic diseases, and exacerbate certain conditions. The present article reviews the current knowledge on the role of filaggrin in the skin barrier, FLG mutations, and the consequences of filaggrin deficiency.

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.