Caveolin expression and localization in human keratinocytes suggest a role in lamellar granule biogenesis

The expanding organelle lipidomes: current knowledge and challenges

Lipids in cell membranes and subcellular compartments play essential roles in numerous cellular processes, such as energy production, cell signaling and inflammation. A specific organelle lipidome is characterized by lipid synthesis and metabolism, intracellular trafficking, and lipid homeostasis in the organelle. Over the years, considerable effort has been directed to the identification of the lipid fingerprints of cellular organelles. However, these fingerprints are not fully characterized due to the large variety and structural complexity of lipids and the great variability in the abundance of different lipid species. The process becomes even more challenging when considering that the lipidome differs in health and disease contexts. This review summarizes the information available on the lipid composition of mammalian cell organelles, particularly the lipidome of the nucleus, mitochondrion, endoplasmic reticulum, Golgi apparatus, plasma membrane and organelles in the endocytic pathway. The lipid compositions of extracellular vesicles and lamellar bodies are also described. In addition, several examples of subcellular lipidome dynamics under physiological and pathological conditions are presented. Finally, challenges in mapping organelle lipidomes are discussed.

Involvement of caveolin-1 in skin diseases

The skin is the outermost layer and largest organ in the human body. Since the skin interfaces with the environment, it has a variety of roles, including providing a protective barrier against external factors, regulating body temperature, and retaining water in the body. It is also involved in the immune system, interacting with immune cells residing in the dermis. Caveolin-1 (CAV-1) is essential for caveolae formation and has multiple functions including endocytosis, lipid homeostasis, and signal transduction. CAV-1 is known to interact with a variety of signaling molecules and receptors and may influence cell proliferation and migration. Several skin-related disorders, especially those of the inflammatory or hyperproliferative type such as skin cancers, psoriasis, fibrosis, and wound healing, are reported to be associated with aberrant CAV-1 expression. In this review, we have explored CAV-1 involvement in skin physiology and skin diseases.

Probing the effects of sweet cherry (Prunus avium L.) extract on 2D and 3D human skin models

BACKGROUND

Natural products are not only positioned in the heart of traditional medicine but also in modern medicine as many current drugs are coming from natural sources. Apart from the field of medicine and therapeutics, natural products are broadly used in other industrial fields such as nutrition, skincare products and nanotechnology.

METHODS AND RESULTS

The aim of this study was to assess the effects of sweet cherry (Prunus avium L.) fruit extract from the Greek native cultivar 'Vasiliadi', on the human 2D and 3D in vitro models in order to investigate its potential impact on skin. We focused on 2D culture of primary normal human epidermal keratinocytes (NHEK) that were treated with sweet cherry fruit extract. In the first place, we targeted fruit extract potential cytotoxicity by determining ATP intracellular levels. Furthermore, we assessed its potential skin irritability by using 3D skin model. To better understand the bioactivity of sweet cherry fruit. extract, we used qPCR to study the expression of various genes that are implicated in the skin functions. Our experiments showed that sweet cherry fruit extract is non-toxic in 2D keratinocytes culture as well as non-irritant in 3D skin model. Our results revealed that the extract mediated important pathways for the optimum epidermis function such as cell proliferation, immune and inflammatory response.

CONCLUSION

The sweet cherry fruit extracts possesses significant activity in epidermis function without any potential of cytotoxicity or skin irritability, which makes it a rather promising active agent for skincare.

In vitro protective effects of Paeonia mascula subsp. hellenica callus extract on human keratinocytes

Natural ingredients have been used to improve the state of health in humans. The genus Paeonia has been studied only limited yet it's reported to have many activities such as antioxidant and anti-inflammatory. To this context, here we focused on an endemic Paeonia species in Attica. This study aims to present the development of the Paeonia mascula subsp. hellenica callus extract and its pleiotropic bioactivity on human primary keratinocytes exploring its potential application as an active agent in skin-related products. This extract showed a high scavenging activity with high phenolic content and an interesting metabolic profile. At a molecular level, the study on the transcript accumulation of genes revealed that this extract exhibits in vitro skin-related protection properties by mediating mitochondrial energy, cell proliferation, immune and inflammatory response and positively regulates genes involved in epidermal and in stratum corneum function. Besides, the extract is proven not skin irritant on reconstructed human skin model. These findings indicate that the specific P. mascula subsp. hellenica extract possesses significant in vitro protection activity on human epidermis and provides new insights into its beneficial role in skin confirming that the advent of biotechnology contribution the past few decades.

The importance of caveolins and caveolae to dermatology: Lessons from the caves and beyond

Caveolae are flask-shaped invaginations of the cell membrane rich in cholesterol and sphingomyelin, with caveolin proteins acting as their primary structural components that allow compartmentalization and orchestration of various signalling molecules. In this review, we discuss how pleiotropic functions of caveolin-1 (Cav1) and its intricate roles in numerous cellular functions including lipid trafficking, signalling, cell migration and proliferation, as well as cellular senescence, infection and inflammation, are integral for normal development and functioning of skin and its appendages. We then examine how disruption of the homeostatic levels of Cav1 can lead to development of various cutaneous pathophysiologies including skin cancers, cutaneous fibroses, psoriasis, alopecia, age-related changes in skin and aberrant wound healing and propose how levels of Cav1 may have theragnostic value in skin physiology/pathophysiology.

Caveolin-1 as a possible target in the treatment for acne

Expression of caveolin-1 (Cav-1) is an important pathophysiological factor in acne. Cav-1 strongly interacts with such well-recognized etiopathogenic factors such as hyperseborrhea, follicular hyperkeratinization and pathogenicity of Cutibacterium acnes. Cav-1 is a strong negative regulator of transforming growth factor beta (TGF-β) expression. It acts as a critical determinant of autophagy, which is significantly induced in acne lesions through C. acnes and by absorption of fatty acids. Cav-1 also demonstrates different correlations with the development of innate immunity. We propose that normalization of Cav-1 expression can serve as a target in anti-acne therapy.

Pharmacological and Genetic Inhibition of Caveolin-1 Promotes Epithelialization and Wound Closure

Chronic wounds-including diabetic foot ulcers, venous leg ulcers, and pressure ulcers-represent a major health problem that demands an urgent solution and new therapies. Despite major burden to patients, health care professionals, and health care systems worldwide, there are no efficacious therapies approved for treatment of chronic wounds. One of the major obstacles in achieving wound closure in patients is the lack of epithelial migration. Here, we used multiple pre-clinical wound models to show that Caveolin-1 (Cav1) impedes healing and that targeting Cav1 accelerates wound closure. We found that Cav1 expression is significantly upregulated in wound edge biopsies of patients with non-healing wounds, confirming its healing-inhibitory role. Conversely, Cav1 was absent from the migrating epithelium and is downregulated in acutely healing wounds. Specifically, Cav1 interacted with membranous glucocorticoid receptor (mbGR) and epidermal growth factor receptor (EGFR) in a glucocorticoid-dependent manner to inhibit cutaneous healing. However, pharmacological disruption of caveolae by MβCD or CRISPR/Cas9-mediated Cav1 knockdown resulted in disruption of Cav1-mbGR and Cav1-EGFR complexes and promoted epithelialization and wound healing. Our data reveal a novel mechanism of inhibition of epithelialization and wound closure, providing a rationale for pharmacological targeting of Cav1 as potential therapy for patients with non-healing chronic wounds.

Caveolin-1 in skin aging - From innocent bystander to major contributor

Caveolin-1 (Cav-1) appears to be both a pathophysiological contributor and a target in different inflammatory and hyperproliferative skin conditions as well as in skin aging. Skin fibroblasts demonstrate an up-regulation of Cav-1 expression both in chronological and UV-induced aging, and such an up-regulation was observed both in vitro and in vivo. Typical alterations in aging skin involve a reduction of the dermis thickness, a significant expansion of the dermal white adipose tissue as well as modifications of the content and distribution of hyaluronan, impairment of autophagic flux, a reduction of collagen expression and an increase in tissue inflammation. All of these phenomena can be connected with changes in Cav-1 expression in the aging skin. Modified expression of Cav-1 can also significantly influence the mechanical properties of individual skin layers, thus changing the total mechanical stability of the layered composite skin/WAT, leading to typical structural modifications of the skin surface in the aging skin. Selective reduction of Cav-1 expression has the potential to exert anti-aging effects on the skin.

Marked Changes in Lamellar Granule and Trans-Golgi Network Structure Occur during Epidermal Keratinocyte Differentiation

Epidermal lamellar granules transport various lipids, proteins, and protein inhibitors from the trans-Golgi network to the extracellular space, and play an important role in skin barrier formation. We elucidated the 3-dimensional structure of lamellar granules and the trans-Golgi network in normal human skin by focused ion beam scanning electron microscopy. Reconstructed focused ion beam scanning electron microscopy 3-dimensional images revealed that the overall lamellar granule structure changed from vesicular to reticular within the second layer of the stratum granulosum. Furthermore, the trans-Golgi network was well developed within this layer and spread through the cytoplasm with branched, tubular structures that connected to lamellar granules. Our study reveals the unique overall 3-dimensional structure of lamellar granules and the trans-Golgi network within the cells of the epidermis, and provides the basis for an understanding of the skin barrier formation.

Molecular basis of the skin barrier structures revealed by electron microscopy

The barrier function of skin is indispensable for terrestrial animals. This function is mainly carried out by the epidermis, more specifically by its granular and cornified layers. The major structural components associated with this function are the intercellular lipid layer, desmosomes, corneodesmosomes, tight junctions, cornified cell envelope and keratin filaments. In this review, we discuss the current knowledge of their ultrastructure, their molecular basis and their relevance to skin disease.

The physical chemistry of the stratum corneum lipids

This article summarizes the current knowledge of the composition, self-assembly, and molecular organization of the stratum corneum (SC) lipids, reviews the evidence connecting these parameters and the barrier properties of human skin, and outlines the immediate issues in the field of SC lipid research.

Regulation of keratinocyte expression of stress proteins and antioxidants by the electrophilic nitrofatty acids 9- and 10-nitrooleic acid

Nitric oxide and various by-products including nitrite contribute to tissue injury by forming novel intermediates via redox-mediated nitration reactions. Nitration of unsaturated fatty acids generates electrophilic nitrofatty acids such as 9-nitrooleic acid (9-NO) and 10-nitrooleic acid (10-NO), which are known to initiate intracellular signaling pathways. In these studies, we characterized nitrofatty acid-induced signaling and stress protein expression in mouse keratinocytes. Treatment of keratinocytes with 5-25μM 9-NO or 10-NO for 6h upregulated mRNA expression of heat shock proteins (hsp's) 27 and 70; primary antioxidants heme oxygenase-1 (HO-1) and catalase; secondary antioxidants glutathione S-transferase (GST) A1/2, GSTA3, and GSTA4; and Cox-2, a key enzyme in prostaglandin biosynthesis. The greatest responses were evident with HO-1, hsp27, and hsp70. In keratinocytes, 9-NO activated JNK and p38 MAP kinases. JNK inhibition suppressed 9-NO-induced HO-1, hsp27, and hsp70 mRNA and protein expression, whereas p38 MAP kinase inhibition suppressed HO-1. In contrast, inhibition of constitutive expression of Erk1/2 suppressed only hsp70, indicating that 9-NO modulates expression of stress proteins by distinct mechanisms. 9-NO and 10-NO also upregulated expression of caveolin-1, the major structural component of caveolae. Western blot analysis of caveolar membrane fractions isolated by sucrose density centrifugation revealed that HO-1, hsp27, and hsp70 were localized within caveolae after nitrofatty acid treatment of keratinocytes, suggesting a link between induction of stress response proteins and caveolin-1 expression. These data indicate that nitrofatty acids are effective signaling molecules in keratinocytes. Moreover, caveolae seem to be important in the localization of stress proteins in response to nitrofatty acids.

Modulation of keratinocyte expression of antioxidants by 4-hydroxynonenal, a lipid peroxidation end product

4-Hydroxynonenal (4-HNE) is a lipid peroxidation end product generated in response to oxidative stress in the skin. Keratinocytes contain an array of antioxidant enzymes which protect against oxidative stress. In these studies, we characterized 4-HNE-induced changes in antioxidant expression in mouse keratinocytes. Treatment of primary mouse keratinocytes and PAM 212 keratinocytes with 4-HNE increased mRNA expression for heme oxygenase-1 (HO-1), catalase, NADPH:quinone oxidoreductase (NQO1) and glutathione S-transferase (GST) A1-2, GSTA3 and GSTA4. In both cell types, HO-1 was the most sensitive, increasing 86-98 fold within 6h. Further characterization of the effects of 4-HNE on HO-1 demonstrated concentration- and time-dependent increases in mRNA and protein expression which were maximum after 6h with 30 μM. 4-HNE stimulated keratinocyte Erk1/2, JNK and p38 MAP kinases, as well as PI3 kinase. Inhibition of these enzymes suppressed 4-HNE-induced HO-1 mRNA and protein expression. 4-HNE also activated Nrf2 by inducing its translocation to the nucleus. 4-HNE was markedly less effective in inducing HO-1 mRNA and protein in keratinocytes from Nrf2-/- mice, when compared to wild type mice, indicating that Nrf2 also regulates 4-HNE-induced signaling. Western blot analysis of caveolar membrane fractions isolated by sucrose density centrifugation demonstrated that 4-HNE-induced HO-1 is localized in keratinocyte caveolae. Treatment of the cells with methyl-β-cyclodextrin, which disrupts caveolar structure, suppressed 4-HNE-induced HO-1. These findings indicate that 4-HNE modulates expression of antioxidant enzymes in keratinocytes, and that this can occur by different mechanisms. Changes in expression of keratinocyte antioxidants may be important in protecting the skin from oxidative stress.

Role of lipids in the formation and maintenance of the cutaneous permeability barrier

The major function of the skin is to form a barrier between the internal milieu and the hostile external environment. A permeability barrier that prevents the loss of water and electrolytes is essential for life on land. The permeability barrier is mediated primarily by lipid enriched lamellar membranes that are localized to the extracellular spaces of the stratum corneum. These lipid enriched membranes have a unique structure and contain approximately 50% ceramides, 25% cholesterol, and 15% free fatty acids with very little phospholipid. Lamellar bodies, which are formed during the differentiation of keratinocytes, play a key role in delivering the lipids from the stratum granulosum cells into the extracellular spaces of the stratum corneum. Lamellar bodies contain predominantly glucosylceramides, phospholipids, and cholesterol and following the exocytosis of lamellar lipids into the extracellular space of the stratum corneum these precursor lipids are converted by beta glucocerebrosidase and phospholipases into the ceramides and fatty acids, which comprise the lamellar membranes. The lipids required for lamellar body formation are derived from de novo synthesis by keratinocytes and from extra-cutaneous sources. The lipid synthetic pathways and the regulation of these pathways are described in this review. In addition, the pathways for the uptake of extra-cutaneous lipids into keratinocytes are discussed. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Human epidermal stem cell function is regulated by circadian oscillations

Human skin copes with harmful environmental factors that are circadian in nature, yet how circadian rhythms modulate the function of human epidermal stem cells is mostly unknown. Here we show that in human epidermal stem cells and their differentiated counterparts, core clock genes peak in a successive and phased manner, establishing distinct temporal intervals during the 24 hr day period. Each of these successive clock waves is associated with a peak in the expression of subsets of transcripts that temporally segregate the predisposition of epidermal stem cells to respond to cues that regulate their proliferation or differentiation, such as TGFβ and calcium. Accordingly, circadian arrhythmia profoundly affects stem cell function in culture and in vivo. We hypothesize that this intricate mechanism ensures homeostasis by providing epidermal stem cells with environmentally relevant temporal functional cues during the course of the day and that its perturbation may contribute to aging and carcinogenesis.

Cav1 suppresses tumor growth and metastasis in a murine model of cutaneous SCC through modulation of MAPK/AP-1 activation

Caveolin-1 (Cav1) is a scaffolding protein that serves to regulate the activity of several signaling molecules. Its loss has been implicated in the pathogenesis of several types of cancer, but its role in the development and progression of cutaneous squamous cell carcinoma (cSCC) remains largely unexplored. Herein, we use the keratinocyte cell line PAM212, a murine model of cSCC, to determine the function of Cav1 in skin tumor biology. We first show that Cav1 overexpression decreases cell and tumor growth, whereas Cav1 knockdown increases these attributes in PAM212 cells. In addition, Cav1 knockdown increases the invasive ability and incidence of spontaneous lymph node metastasis. Finally, we demonstrate that Cav1 knockdown increases extracellular signaling-related kinase 1/2 mitogen-activated protein kinase/activator protein-1 pathway activation. We attribute the growth and invasive advantage conferred by Cav1 knockdown to increased expression of activator protein-1 transcriptional targets, including cyclin D1 and keratin 18, which show inverse expression in PAM212 based on the expression level of Cav1. In summary, we demonstrate that loss of Cav1 affects several characteristics associated with aggressive human skin tumors and that this protein may be an important modulator of tumor growth and invasion in cSCC.

Immediate and long-term effects of polysaccharides-based formulations on human skin

A new trend in cosmetic formulations is the use of biotechnological raw materials as the polysaccharides from Klebsiella pneumoniae, which are supposed to enhance cell renewal, improve skin hydration and micro-relief. Botanical extracts of Myrtus communis leaves contain different sugars, which may provide the same benefits. Thus, the objective of this study was to evaluate through objective and subjective analysis the immediate and long-term effects of cosmetic formulations containing polysaccharides biotechnologically-originated and / or the ones contained in Myrtus communis extracts. Three polysaccharide-based and placebo formulations were applied on the forearm skin of 40 volunteers. Skin hydration, transepidermal water loss (TEWL), viscoelasticity and skin micro-relief measurements were made before and 2 hours after a single application and after 15 and 30 day-periods of daily applications. Answers to a questionnaire about perceptions of formulation cosmetic features constituted the subjective analysis. All polysaccharide-based formulations enhanced skin hydration. Formulations with isolated or combined active substances improved skin barrier function as compared to placebo, in the short and long term studies. Formulations containing Myrtus communis extracts had the highest acceptance. Results suggest that daily use of formulations containing these substances is important for protection of the skin barrier function.

Regulation of Hsp27 and Hsp70 expression in human and mouse skin construct models by caveolae following exposure to the model sulfur mustard vesicant, 2-chloroethyl ethyl sulfide

Dermal exposure to the vesicant sulfur mustard causes marked inflammation and tissue damage. Basal keratinocytes appear to be a major target of sulfur mustard. In the present studies, mechanisms mediating skin toxicity were examined using a mouse skin construct model and a full-thickness human skin equivalent (EpiDerm-FT™). In both systems, administration of the model sulfur mustard vesicant, 2-chloroethyl ethyl sulfide (CEES, 100-1000μM) at the air surface induced mRNA and protein expression of heat shock proteins 27 and 70 (Hsp27 and Hsp70). CEES treatment also resulted in increased expression of caveolin-1, the major structural component of caveolae. Immunohistochemistry revealed that Hsp27, Hsp70 and caveolin-1 were localized in basal and suprabasal layers of the epidermis. Caveolin-1 was also detected in fibroblasts in the dermal component of the full thickness human skin equivalent. Western blot analysis of caveolar membrane fractions isolated by sucrose density centrifugation demonstrated that Hsp27 and Hsp70 were localized in caveolae. Treatment of mouse keratinocytes with filipin III or methyl-β-cyclodextrin, which disrupt caveolar structure, markedly suppressed CEES-induced Hsp27 and Hsp70 mRNA and protein expression. CEES treatment is known to activate JNK and p38 MAP kinases; in mouse keratinocytes, inhibition of these enzymes suppressed CEES-induced expression of Hsp27 and Hsp70. These data suggest that MAP kinases regulate Hsp 27 and Hsp70; moreover, caveolae-mediated regulation of heat shock protein expression may be important in the pathophysiology of vesicant-induced skin toxicity.

Transcriptional profiling after lipid raft disruption in keratinocytes identifies critical mediators of atopic dermatitis pathways

Lipid rafts are cholesterol-rich cell signaling platforms, and their physiological role can be explored by cholesterol depletion. To characterize transcriptional changes ongoing after lipid raft disruption in epidermal keratinocytes, a cell type that synthesizes its cholesterol in situ, we performed whole-genome expression profiling. Microarray results show that over 3,000 genes are differentially regulated. In particular, IL-8, urokinase-like plasminogen activator receptor, and metalloproteinases are highly upregulated after cholesterol extraction. Quantitative reverse transcriptase PCR validation and protein release measurements demonstrate the physiological relevance of microarray data. Major enriched terms and functions, determined by Ingenuity Pathways Analysis, identify cholesterol biosynthesis as a major function, illustrating the specificity of keratinocyte response toward cholesterol depletion. Moreover, the inflammatory skin disorder atopic dermatitis (AD) is identified as the disease most closely associated with the profile of lipid raft-disrupted keratinocytes. This finding is confirmed in skin of AD patients, in whom transcript levels of major lipid raft target genes are similarly regulated in lesional atopic skin, compared with non-lesional and normal skin. Thus, lipid raft disruption evokes typical features of AD, thereby suggesting that lipid raft organization and signaling could be perturbed in atopic keratinocytes.

Immunohistochemical evidence of caveolin-1 expression in the human fetal and neonatal striated muscle and absence in the adult's

BACKGROUND

Caveolin-1 (Cav-1) is a 22-kd protein, which exerts essential roles in the regulation of cell proliferation and in transmembrane transport processes. It is mainly expressed in adipocytes, smooth muscle, fibroblasts, and endothelial cells. Its expression in striated muscle fibers is controversial. Indeed, most authors have attributed Cav-1 detection in striated muscle to endothelial cells, adipocytes, and fibroblasts secretion. Nonetheless, recent in vitro studies have shown that Cav-1 is expressed in L6 myoblasts and maintained during the differentiation process. In view of this, and, because only one study has heretofore explored Cav-1 expression in human striated muscle, the aim of the present study was to evaluate and to compare Cav-1 immunohistochemical expression in the human striated muscles of fetus, newborn, and adult.

DESIGN

Samples of skeletal muscles of different sites and of myocardium were taken at autopsy from 13 fetuses and 4 newborns and submitted to the immunohistochemical analysis for Cav-1 together with 10 samples of adult skeletal muscle.

RESULTS

Myocardial fibers displayed a weak immunoreaction in all samples, from both the newborns and the fetuses, independently of the week of gestation. Conversely, skeletal muscle fibers were only labeled in specimens from fetuses at late gestation and from the newborns, whereas no immunoreaction was evidenced in muscles taken from fetuses at mid-gestation and in the adult samples.

CONCLUSIONS

This novel and unexpected pattern of Cav-1 expression in human skeletal muscle suggests a role for Cav-1 in terminal differentiation processes, which need to be clarified by further studies.

Caveolin-1 and -2 interact with connexin43 and regulate gap junctional intercellular communication in keratinocytes

Connexin43 (Cx43) has been reported to interact with caveolin (Cav)-1, but the role of this association and whether other members of the caveolin family bind Cx43 had yet to be established. In this study, we show that Cx43 coimmunoprecipitates and colocalizes with Cav-1 and Cav-2 in rat epidermal keratinocytes. The colocalization of Cx43 with Cav-1 was confirmed in keratinocytes from human epidermis in vivo. Our mutation and Far Western analyses revealed that the C-terminal tail of Cx43 is required for its association with Cavs and that the Cx43/Cav-1 interaction is direct. Our results indicate that newly synthesized Cx43 interacts with Cavs in the Golgi apparatus and that the Cx43/Cavs complex also exists at the plasma membrane in lipid rafts. Using overexpression and small interfering RNA approaches, we demonstrated that caveolins regulate gap junctional intercellular communication (GJIC) and that the presence of Cx43 in lipid raft domains may contribute to the mechanism modulating GJIC. Our results suggest that the Cx43/Cavs association occurs during exocytic transport, and they clearly indicate that caveolin regulates GJIC.

Rab11 Is Associated with Epidermal Lamellar Granules

Epidermal lamellar granules (LGs) are specialized organelles that transport and secrete various molecules, including lipids, proteases, protease inhibitors, and structural proteins, thereby providing a protective barrier against the environment. Abnormalities in LG-related molecules result in severe skin diseases, but their transport mechanisms are poorly understood. We studied the distribution of Rab11, a common GTPase in recycling endosomes, in normal human epidermis. Confocal laser scanning microscopy detected Rab11 immunoreactivity in differentiated epidermal keratinocytes. Staining was strong at the apical side of each cell, a pattern commonly seen in LG-associated molecules. Around the nuclei, Rab11 was colocalized with TGN46, a trans-Golgi network marker. Rab11 was also colocalized with known LG-molecules, namely lymphoepithelial Kazal-type-related inhibitor, corneodesmosine, cathepsin D, and glucosylceramides. Immunoelectron microscopy revealed that Rab11 was widely distributed along TGN and tubular-vesicular structures containing different LG molecules. The present results suggest that Rab11 plays a role in the intracellular trafficking of various types of LG-molecule from the TGN to the cell surface.

Skin structure and cornification proteins in the soft-shelled turtle Trionyx spiniferus

In contrast to most chelonians, the fully aquatic soft-shelled turtles have a smooth, unscaled, and pliable shell. The skin of the shell, tail, limbs, and neck of juveniles of Trionyx spiniferus has been studied by ultrastructural, immunocytochemical, and immunoblotting methods. The epidermis of the carapace and plastron has a thick corneous layer composed of alpha-corneocytes surrounded by a cornified cell envelope. The softer epidermis is similar to that of the shell but the epidermis and corneous layer are much thinner. Pre-corneous cells in both soft and shell epidermis are rich in vesicles produced in the Golgi apparatus and smooth endoplasmic vesicles, and contain numerous dense-core mucus-like and vesicular (lamellar) bodies. Secreted material is present among corneocytes where it probably forms an extensive intercellular lipid-mucus waterproof barrier. The dermis is very thick and composed of several layers of collagen bundles that form a plywood-patterned dermis. This dermis constitutes a strong mechanical barrier that compensates for the low content in beta-keratin, and lack of cornified scutes and dermal bones. The growth of the shell mainly occurs along the lateral margins. Immunocytochemistry reveals the presence of some beta-keratin in soft and shell epidermis, and this is confirmed by immunoblotting where bands at 18 and 32-35 kDa are present. Other proteins of the cornified cell envelope (loricrin and sciellin) or associated to lipid trafficking (caveolin-1) are also detected by immunoblotting. Loricrin positive bands at 24 and 57 kDa are present while bands cross-reactive for sciellin are seen at 45 and 53 kDa. Caveolin-1 positive bands are seen at 21-22 kDa. The presence of these proteins indicates that the epidermis is both coriaceous and waterproof. These results suggest that the shell of Trionyx is tough enough to be as mechanically efficient as the hard shell of the other turtles. At the same time, a soft shell is lighter, its shape is more easily controlled by muscles, and it allows a more controlled closure and retraction of limbs and neck inside the shell. Thus, the shell makes the animal more streamlined for swimming.

Expression of lysosome-associated membrane protein 1 (Lamp-1) and galectins in human keratinocytes is regulated by differentiation

Lysosomes and their components are suspected to be involved in epidermal differentiation. In this study, lysosomal enzyme activities, expression of the lysosome-associated membrane protein 1 (Lamp-1) and expression of the epidermal galectins-1, -3 and -7 were investigated in human keratinocytes cultured at different cell densities (subconfluence, confluence and postconfluence) in order to induce differentiation. Detected by Western blot and immunofluorescence, Lamp-1 expression is transiently upregulated at culture confluence, but reduced at postconfluence. Northern blot analyses performed on subconfluent, confluent and post-confluent cultures of keratinocytes show that Lamp-1 mRNA expression is also upregulated at culture confluence, but downregulated at postconfluence. Measurements of lysosomal enzyme activities indicate a transient upregulation at culture confluence, whereas cathepsins B, C and L are particularly downregulated at postconfluence. Cell density and differentiation of epidermal cells also differentially regulates galectin expression in autocrine cultures. As the expression of galectin-1 mRNA is high in subconfluent cells, it is assumed to be associated with their proliferative state. On the other hand, as the mRNA levels for galectins-3 and -7 are notably upregulated at culture confluence (galectin-7) or at postconfluence (galectin-3), their expression is thought to be related to the differentiated state of keratinocytes. However, we collected evidence by confocal microscopy that galectin-3 and Lamp-1 do not colocalize in vitro in keratinocytes. Altogether, our results suggest that the upregulated Lamp-1 expression at confluence could be involved in keratinocyte differentiation, but apparently not through interaction with galectin-3.

An update of the defensive barrier function of skin

Skin, as the outermost organ in the human body, continuously confronts the external environment and serves as a primary defense system. The protective functions of skin include UV-protection, anti-oxidant and antimicrobial functions. In addition to these protections, skin also acts as a sensory organ and the primary regulator of body temperature. Within these important functions, the epidermal permeability barrier, which controls the transcutaneous movement of water and other electrolytes, is probably the most important. This permeability barrier resides in the stratum corneum, a resilient layer composed of corneocytes and stratum corneum intercellular lipids. Since the first realization of the structural and biochemical diversities involved in the stratum corneum, a tremendous amount of work has been performed to elucidate its roles and functions in the skin, and in humans in general. The perturbation of the epidermal permeability barrier, previously speculated to be just a symptom involved in skin diseases, is currently considered to be a primary pathophysiologic factor for many skin diseases. In addition, much of the evidence provides support for the idea that various protective functions in the skin are closely related or even co-regulated. In this review, the recent achievements of skin researchers focusing on the functions of the epidermal permeability barrier and their importance in skin disease, such as atopic dermatitis and psoriasis, are introduced.

Cytochemical, biochemical and molecular aspects of the process of keratinization in the epidermis of reptilian scales

The characteristics of scaled skin of reptiles is one of their main features that distinguish them from the other amniotes, birds and mammals. The different scale patterns observed in extant reptiles result from a long evolutive history that allowed each species to adapt to its specific environment. The present review deals with comparative aspects of epidermal keratinization in reptiles, chelonians (turtles and tortoises), lepidosaurian (lizards, snakes, sphenodontids), archosaurians (crocodilians). Initially the morphology and cytology of reptilian scales is outlined to show the diversity in the epidermis among different groups. The structural proteins (alpha-keratins and associated proteins), and enzymes utilized to form the corneous layer of the epidermis are presented. Aside cytokeratins (alpha-keratins), used for making the cytoskeleton, reptilian alpha-keratinocytes produce interkeratin (matrix) and corneous cell envelope proteins. Keratin bundles and degraded cell organelles constitute most of the corneous material of alpha-keratinocytes. Matrix, histidine-rich and sulfur-rich proteins are produced in the soft epidermis and accumulated in the cornified cell envelope. Main emphasis is given to the composition and to the evolution of the hard keratins (beta-keratins). Beta-keratins constitute the hard corneous material of scales. These small proteins are synthesized in beta-keratinocytes and are accumulated into small packets that rapidly merge into a compact corneous material and form densely cornified layers. Beta-keratins are smaller proteins (8-20 kDa) in comparison to alpha-keratins (40-70 kDa), and this size may determine their dense packing in corneocytes. Both glycine-sulfur-rich and glycine-proline-rich proteins have been so far sequenced in the corneous material of scales in few reptilian species. The latter keratins possess C- and N-amino terminal amino acid regions with sequence homology with those of mammalian hard keratins. Also, reptilian beta-keratins possess a central core with homology with avian scale/feather keratins. Multiple genes code for these proteins and their discovery and sequentiation is presently an active field of research. These initial findings however suggest that ancient reptiles already possessed some common genes that have later diversified to produce the specific keratin-associated proteins in their descendants: extant reptiles, birds and mammals. The evolution of these small proteins in lepidosaurians, chelonians and archosaurians represent the next step to understand the evolution of cornification in reptiles and derived amniotes (birds and mammals).

DHCR24 gene knockout mice demonstrate lethal dermopathy with differentiation and maturation defects in the epidermis

Desmosterolosis is an autosomal recessive disorder due to mutations in the 3beta-hydroxysterol-Delta24 reductase (DHCR24) gene that encodes an enzyme catalyzing the conversion of desmosterol to cholesterol. To date, only two patients have been reported with severe developmental defects including craniofacial abnormalities and limb malformations. We employed mice with targeted disruption of DHCR24 to understand the pathophysiology of desmosterolosis. All DHCR24-/- mice died within a few hours after birth. Their skin was wrinkleless and less pliant, leading to restricted movement and inability to suck (empty stomach). DHCR24 gene was expressed abundantly in the epidermis of control but not of DHCR24-/- mice. Accordingly, cholesterol was not detected whereas desmosterol was abundant in the epidermis of DHCR24-/- mice. Skin histology revealed thickened epidermis with few and smaller keratohyaline granules. Aberrant expression of keratins such as keratins 6 and 14 suggested hyperproliferative hyperkeratosis with undifferentiated keratinocytes throughout the epidermis. Altered expression of filaggrin, loricrin, and involcrin were also observed in the epidermis of DHCR24-/-. These findings suggested impaired skin barrier function. Indeed, increased trans-epidermal water loss and permeability of Lucifer yellow were observed in DHCR24-/- mice. DHCR24 thus plays crucial role for skin development and its proper function.

Caveolins and flotillin-2 are present in the blood stages of Plasmodium vivax

Blood stages of Plasmodium vivax induce the development of caveolae and caveola-vesicle complexes (CVC) in the membrane of their host erythrocyte. Caveolae are found in almost all types of cells and are involved in endogenous processes as calcium and cholesterol homeostasis, cell signalling, transporting, ligand internalization and transcytosis of serum components. Major structural components of caveolae are the proteins caveolins and flotillins. The functional role of caveolae in the P. vivax-infected erythrocyte is not properly understood. As these organelles have been shown to contain malaria antigens, it has been suggested that they are involved in the transport and release of specific parasite antigens from the infected erythrocyte and in the uptake of plasma proteins. Using specific antibodies to classical caveolae proteins and an immunolocalization approach, we found caveolin-2, caveolin-3, and flotillin-2 in the vesicle profiles and some CVC of P. vivax-infected erythrocytes. Caveolin-1-3 were not found in uninfected erythrocytes. This is the first report of identification and localization of caveolins in the CVC present in erythrocytes infected with P. vivax, thereby providing evidence of the role of this particular organelle in the protein-trafficking pathway that connect parasite-encoded proteins with the erythrocyte cytoplasm and the cell surface throughout the asexual blood cycle of vivax malaria parasite.

CD98, a novel marker of transient amplifying human keratinocytes

Identification of plasma membrane markers of basal keratinocytes is essential for sorting basal cells and, subsequently, adult epidermal stem cells. In this study, we isolated caveolin-1-enriched microdomains from human HaCaT keratinocytes and identified proteins representing potential cell surface markers of the epidermis by a proteomic approach. The purification of this caveolae domain allowed us to characterize 53 proteins of which 26% were transmembrane and 32% associated-membrane proteins. One of them, CD98, was found to be co-localized with beta1 integrin at the plasma membrane of the basal keratinocytes of healthy human epidermis. We then isolated CD98-positive keratinocytes from fresh skin biopsies. Using clonogenic assays, we demonstrate that CD98 may be considered as a marker of transient amplifying human keratinocytes.

Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

This review presents comparative aspects of epidermal keratinization in vertebrates, with emphasis on the evolution of the stratum corneum in land vertebrates. The epidermis of fish does not contain proteins connected with interkeratin matrix and corneous cell envelope formation. Mucus-like material glues loose keratin filaments. In amphibians a cell corneous envelope forms but matrix proteins, aside from mucus/glycoproteins, are scarce or absent. In reptiles, birds, and mammals specific proteins associated with keratin become relevant for the production of a resistant corneous layer. In reptiles some matrix, histidine-rich and sulfur-rich corneous cell envelope proteins are produced in the soft epidermis. In avian soft epidermis low levels of matrix and cornified proteins are present while lipids become abundant. In mammalian keratinocytes, interkeratin proteins, cornified cell envelope proteins, and transglutaminase are present. Topographically localized areas of dermal-epidermal interactions in amniote skin determine the formation of skin derivatives such as scales, feathers, and hairs. New types of keratin and associated proteins are produced in these derivatives. In reptiles and birds beta-keratins form the hard corneous material of scales, claws, beaks, and feathers. In mammals, small sulfur-rich and glycine-tyrosine-rich proteins form the corneous material of hairs, horns, hooves, and claws. Molecular studies on reptilian beta-keratins show they are glycine-rich proteins. They have C- and N-terminal amino acid regions homologous to those of mammalian proteins and a central core with homology to avian scale/feather keratins. These findings suggest that ancient reptiles already possessed some common genes that later diversified to produce some keratin-associated protein in extant reptiles and birds, and others in mammals. The evolution of these small proteins represents the more recent variation of the process of cornification in vertebrates.

Novel aspects in cutaneous biology of acetylcholine synthesis and acetylcholine receptors

Extraneuronal acetylcholine (ACh) has been demonstrated to influence a plethora of cutaneous cell functions in an autocrine, paracrine and endocrine fashion. Through the differentiation-specific expression of its different nicotinic (nACh-R) and muscarinic (mACh-R) receptors, ACh acts upon keratinocyte proliferation and migration, terminal differentiation and barrier formation, sweat and sebum secretion as well as microcirculation and angiogenesis. Only very recently it has been recognized that acetylcholinesterase, but not cholineacetyltransferase, activity is regulated by hydrogen peroxide. Considering that the outer layer of the human skin can be a target for UV-generated H2O2 in the millimolar range, this mechanism needs to be taken into account for the regulation of ACh homeostasis in skin biology. Consequently, ACh can accumulate, as shown, for example, in the depigmentation process in vitiligo. There is a highly regulated distribution of ACh-R in human epidermis and adnexal structures, supporting previously observed effects of cholinergic compounds on keratinocyte biology. Most significantly, the regulated expression of ACh-R in sebaceous glands advocates a role for ACh in sebum production and as a promoter of sebocyte differentiation, thus offering an explanation for skin diseases associated with altered sebum production after chronic nicotine exposure. So far, ACh-induced sweat production has been thought to be under the exclusive control of mACh-R. However, recently, the presence of both different nACh-R and mACh-R in myoepithelial and acinar cells of eccrine sweat glands has been documented, indicating a more complex regulation of sweat production and expulsion.

A Proteomic Characterization of the Plasma Membrane of Human Epidermis by High-Throughput Mass Spectrometry

Membrane proteins are responsible for many critical cellular functions and identifying cell surface proteins on different keratinocyte populations by proteomic approaches would improve our understanding of their biological function. The ability to characterize membrane proteins, however, has lagged behind that of soluble proteins both in terms of throughput and protein coverage. In this study, a membrane proteomic investigation of keratinocytes using a two-dimensional liquid chromatography (LC) tandem-mass spectrometry (MS/MS) approach that relies on a buffered methanol-based solubilization, and tryptic digestion of purified plasma membrane is described. A highly enriched plasma membrane fraction was prepared from newborn foreskins using sucrose gradient centrifugation, followed by a single-tube solubilization and tryptic digestion of membrane proteins. This digestate was fractionated by strong cation-exchange chromatography and analyzed using microcapillary reversed-phase LC-MS/MS. In a set of 1306 identified proteins, 866 had a gene ontology (GO) annotation for cellular component, and 496 of these annotated proteins (57.3%) were assigned as known integral membrane proteins or membrane-associated proteins. Included in the identification of a large number of aqueous insoluble integral membrane proteins were many known intercellular adhesion proteins and gap junction proteins. Furthermore, 121 proteins from cholesterol-rich plasma membrane domains (caveolar and lipid rafts) were identified.

Epidermal lamellar granules transport different cargoes as distinct aggregates

Lamellar granules (LG) of the epidermis appear as discrete round or oblong shaped granules in classical transmission electron micrographs, but a recent cryo-transmission electron microscopy study has claimed that LG are in fact branched tubular structures. LG contain various cargoes including lipids, hydrolytic enzymes, and several other proteins. It is not known whether there are any differences in the timing of expression among them and whether they are sorted into the granules individually or collectively. In order to address these questions, we studied the expression of glucosylceramides (GlcCer), cathepsin D (CatD), corneodesmosin (Cdsn), kallikrein (KLK)7, and KLK8 in normal human epidermis using confocal laser scanning microscopy and immunoelectron microscopy. The results were consistent with the model that LG are parts of a branched tubular structure. In this structure, all the components were shown to be distributed as separate aggregates. In the trans-Golgi network (TGN), bulbous protrusions containing GlcCer, Cdsn, KLK7 and KLK8, and small CatD-positive vesicles were observed. The molecules were shown to be delivered to the apical region of granular keratinocytes. This study provides strong evidence for the sequential synthesis and independent trafficking of various LG cargoes, including for the first time CatD and KLK8, from TGN.

Aquaporin 3 colocates with phospholipase d2 in caveolin-rich membrane microdomains and is downregulated upon keratinocyte differentiation

Aquaporin 3 is a channel that transports both water and glycerol. Aquaporin 3-deficient mice exhibit skin defects, including decreased glycerol content and impairment of water holding capacity, barrier recovery, and wound healing. Whether aquaporin 3 and its glycerol transporting capacity are involved in regulating keratinocyte function, we have previously shown that phospholipase D2 can metabolize phospholipids in the presence of glycerol to yield phosphatidylglycerol. We hypothesized that aquaporin 3 is involved in the regulation of keratinocyte function by a mechanism involving the interaction between aquaporin 3 and phospholipase D. Using sucrose gradient centrifugation, immunoprecipitation analysis, and confocal microscopy, we found that aquaporin 3 and phospholipase D2 colocalized in caveolin-rich membrane microdomains. In addition, aquaporin 3 expression was downregulated at the transcriptional level and glycerol uptake was reduced upon primary mouse keratinocytes to differentiation in response to an elevated extracellular calcium concentration or 1,25-dihydroxyvitamin D3. Our results suggest that aquaporin 3 and phospholipase D2 form a signaling module in lipid rafts, where aquaporin 3 transports glycerol to phospholipase D2 for the synthesis of phosphatidylglycerol. Phosphatidylglycerol, as a bioactive lipid, could potentially mediate the effects of the aquaporin 3-phospholipase D2 signaling module, with aquaporin 3 as a modulatory unit, in the regulation of keratinocyte function.