Flotillas of lipid rafts in transit amplifying cell-like keratinocytes

EphA2 Transmembrane Domain Is Uniquely Required for Keratinocyte Migration by Regulating Ephrin-A1 Levels

EphA2 receptor tyrosine kinase is activated by ephrin-A1 ligand, which harbors a glycosylphosphatidylinositol anchor that enhances lipid raft localization. Although EphA2 and ephrin-A1 modulate keratinocyte migration and differentiation, the ability of this cell-cell communication complex to localize to different membrane regions in keratinocytes remains unknown. Using a combination of biochemical and imaging approaches, we provide evidence that ephrin-A1 and a ligand-activated form of EphA2 partition outside of lipid raft domains in response to calcium-mediated cell-cell contact stabilization in normal human epidermal keratinocytes. EphA2 transmembrane domain swapping with a shorter and molecularly distinct transmembrane domain of EphA1 resulted in decreased localization of this receptor tyrosine kinase at cell-cell junctions and increased expression of ephrin-A1, which is a negative regulator of keratinocyte migration. Accordingly, altered EphA2 membrane distribution at cell-cell contacts limited the ability of keratinocytes to seal linear scratch wounds in vitro in an ephrin-A1-dependent manner. Collectively, these studies highlight a key role for the EphA2 transmembrane domain in receptor-ligand membrane distribution at cell-cell contacts that modulates ephrin-A1 levels to allow for efficient keratinocyte migration with relevance for cutaneous wound healing.

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.

Cholera toxin regulates a signaling pathway critical for the expansion of neural stem cell cultures from the fetal and adult rodent brains

Background

New mechanisms that regulate neural stem cell (NSC) expansion will contribute to improved assay systems and the emerging regenerative approach that targets endogenous stem cells. Expanding knowledge on the control of stem cell self renewal will also lead to new approaches for targeting the stem cell population of cancers.

Methodology/Principal Findings

Here we show that Cholera toxin regulates two recently characterized NSC markers, the Tie2 receptor and the transcription factor Hes3, and promotes the expansion of NSCs in culture. Cholera toxin increases immunoreactivity for the Tie2 receptor and rapidly induces the nuclear localization of Hes3. This is followed by powerful cultured NSC expansion and induction of proliferation both in the presence and absence of mitogen.

Conclusions/Significance

Our data suggest a new cell biological mechanism that regulates the self renewal and differentiation properties of stem cells, providing a new logic to manipulate NSCs in the context of regenerative disease and cancer.

Mutant huntingtin and glycogen synthase kinase 3-beta accumulate in neuronal lipid rafts of a presymptomatic knock-in mouse model of Huntington's disease

Patients with Huntington's disease have an expanded polyglutamine tract in huntingtin and suffer severe brain atrophy and neurodegeneration. Because membrane dysfunction can occur in Huntington's disease, we addressed whether mutant huntingtin in brain and primary neurons is present in lipid rafts, which are cholesterol-enriched membrane domains that mediate growth and survival signals. Biochemical analysis of detergent-resistant membranes from brains and primary neurons of wild-type and presymptomatic Huntington's disease knock-in mice showed that wild-type and mutant huntingtin were recovered in lipid raft-enriched detergent-resistant membranes. The association with lipid rafts was stronger for mutant huntingtin than wild-type huntingtin. Lipid rafts extracted from Huntington's disease mice had normal levels of lipid raft markers (G(alphaq), Ras, and flotillin) but significantly more glycogen synthase kinase 3-beta. Increases in glycogen synthase kinase 3-beta have been associated with apoptotic cell death. Treating Huntington's disease primary neurons with inhibitors of glycogen synthase kinase 3-beta reduced neuronal death. We speculate that accumulation of mutant huntingtin and glycogen synthase kinase 3-beta in lipid rafts of presymptomatic Huntington's disease mouse neurons contributes to neurodegeneration in Huntington's disease.

Real-time monitoring of membrane cholesterol reveals new insights into epidermal differentiation

Cholesterol is organized in distinctive liquid-ordered micro-domains within biological membranes called lipid rafts. These micro-domains direct multiple physiological functions in mammalian cells by modulating signaling processes. Recent findings suggest a role for lipid rafts in cellular processes in human keratinocytes such as early differentiation and apoptosis. However, research of lipid rafts is hindered by technological limitations in visualizing dynamic cholesterol organization in plasma membranes. This study addresses a real-time, non-invasive method for the long-term observation of cholesterol reorganization in plasma membranes. In addition, this study also addresses the dynamic process of cholesterol depletion and repletion in primary human keratinocytes. Cholesterol reorganization was measured by observed changes in cellular impedance. Disruption of lipid rafts with low concentrations of methyl-beta-cyclodextrin (MbetaCD) resulted in an increase in the proliferative capacity of keratinocytes, which was assessed using real-time proliferation curves and adenosine triphosphate (ATP)-based proliferation assays. Quantitative PCR showed a concomitant decrease in messenger RNA (mRNA) expression of the early differentiation markers keratins 1 and 10. Conversely, specific cholesterol reintegration led to a 4.5-fold increase in keratin 2 mRNA expression, a marker for late keratinocyte differentiation, whereas depletion resulted in a significant downregulation. These findings imply a strictly controlled mechanism for the regulation of membrane cholesterol composition in both early and terminal keratinocyte differentiation. The impedance-based method that this study addresses further enhances our understanding of how physiological processes in keratinocytes are controlled by membrane cholesterol.

Mannheimia haemolytica leukotoxin binds to lipid rafts in bovine lymphoblastoid cells and is internalized in a dynamin-2- and clathrin-dependent manner

Mannheimia haemolytica is the principal bacterial pathogen of the bovine respiratory disease complex. Its most important virulence factor is a leukotoxin (LKT), which is a member of the RTX family of exotoxins produced by many gram-negative bacteria. Previous studies demonstrated that LKT binds to the beta(2)-integrin LFA-1 (CD11a/CD18) on bovine leukocytes, resulting in cell death. In this study, we demonstrated that depletion of lipid rafts significantly decreases LKT-induced bovine lymphoblastoid cell (BL-3) death. After binding to BL-3 cells, some of the LKT relocated to lipid rafts in an LFA-1-independent manner. We hypothesized that after binding to LFA-1 on BL-3 cells, LKT moves to lipid rafts and clathrin-coated pits via a dynamic process that results in LKT internalization and cytotoxicity. Knocking down dynamin-2 by small interfering RNA reduced both LKT internalization and cytotoxicity. Similarly, expression of dominant negative Eps15 protein expression, which is required for clathrin coat formation, reduced LKT internalization and LKT-mediated cytotoxicity to BL-3 cells. Finally, we demonstrated that inhibiting actin polymerization reduced both LKT internalization and LKT-mediated cytotoxicity. These results suggest that both lipid rafts and clathrin-mediated mechanisms are important for LKT internalization and cytotoxicity in BL-3 cells and illustrate the complex nature of LKT internalization by the cytoskeletal network.

Ligand-independent activation of the EGFR by lipid raft disruption

Normal and immortalized keratinocytes demonstrate large aggregates of lipid rafts, detectable by membrane staining with fluorescently tagged cholera toxin (CTx). As lipid rafts are known to regulate the function of many surface receptors, we wished to investigate their impact on the EGFR in HaCaT cells. When rafts were disrupted by cholesterol sequestration with methyl-beta-cyclodextrin (MbetaCD) or filipin III, EGFR rearranged into approximately micrometer large clusters outside the CTx(bright) raft aggregates. These clusters contained high concentrations of activated, tyrosine-phosphorylated EGFR exhibiting greatly reduced mobility in the fluorescence recovery after photobleaching experiments. EGFR activation led to the stimulation of extracellular signal-regulated kinase 2, the phosphorylated form of which translocated to the nucleus and stimulated growth of the MbetaCD-treated cells. Experiments with the specific antagonistic antibody proved that the activation of EGFR by lipid raft disruption occurred without the participation of the ligand. We hypothesize that cholesterol depletion leads to the release of EGFR from the damaged rafts into the small confined areas of the membrane, where the receptor molecules are likely to be spontaneously activated owing to a very high density and/or separation from the inhibitory factors remaining in the surrounding portions of the membrane.

Adherence and blocking of Candida albicans to cultured vaginal epithelial cells: treatments to decrease adherence

BACKGROUND

Pathogenesis of mucosal microorganisms depends on adherence to the tissues they colonize and infect. For Candida albicans, cell surface hydrophobicity may play a significant role in tissue binding ability.

METHODS

A continuous cell line of vaginal epithelial cells (VEC) was grown in keratinocyte serum-free medium (KSFM) with supplements and harvested by trypsinization. VEC were combined with yeast cells to evaluate adherence and inhibition of adherence. In this experimental setup, yeast stained with fluorescein isothiocyanate were allowed to attach to VEC and the resulting fluorescent VEC were detected by flow cytometry.

RESULTS

VEC were cultured and examined daily after plating and showed morphology similar to basal epithelial cells. Culture media supplemented with estradiol showed increased VEC proliferation initially (first 24 h) but cell morphology was not altered. Fluorescinated Candida cells bound effectively to the cultured VEC. Using fresh cells exposed to various preparations of K-Y, we showed that all formulations of the product reduced Candida binding to VEC by 25% to 50%. While VEC were generally harvested for use in experiments when they were near confluent growth, we allowed some cultures to grow beyond that point and discovered that cells allowed to become overgrown or stressed appeared to bind yeast cells more effectively.

CONCLUSION

Flow cytometry is a useful method for evaluating binding of stained yeast cells to cultured VEC and has demonstrated that commercially available products have the ability to interfere with the process of yeast adherence to epithelial cells.

Effect of JP-8 jet fuel exposure on protein expression in human keratinocyte cells in culture

Dermal exposure to jet fuel is a significant occupational hazard. Previous studies have investigated its absorption and disposition in skin, and the systemic biochemical and immunotoxicological sequelae to exposure. Despite studies of JP-8 jet fuel components in murine, porcine or human keratinocyte cell cultures, proteomic analysis of JP-8 exposure has not been investigated. This study was conducted to examine the effect of JP-8 administration on the human epidermal keratinocyte (HEK) proteome. Using a two-dimensional electrophoretic approach combined with mass spectrometric-based protein identification, we analyzed protein expression in HEK exposed to 0.1% JP-8 in culture medium for 24 h. JP-8 exposure resulted in significant expression differences (p<0.02) in 35 of the 929 proteins matched and analyzed. Approximately, a third of these alterations were increased in protein expression, two-thirds declined with JP-8 exposure. Peptide mass fingerprint identification of effected proteins revealed a variety of functional implications. In general, altered proteins involved endocytotic/exocytotic mechanisms and their cytoskeletal components, cell stress, and those involved in vesicular function.

Disruption of lipid rafts causes apoptotic cell death in HaCaT keratinocytes

Lipid rafts are cholesterol-enriched microdomains in plasma membranes. The functional activity of many membrane proteins, including death and growth factor receptors, depends on their insertion in lipid rafts. We have previously demonstrated the presence of lipid rafts in keratinocytes and shown that lipid rafts are involved in the control of keratinocyte proliferation and metabolic activity. In this work, we investigated the effect of lipid-raft disruption on HaCaT keratinocyte survival. Lipid rafts could be disrupted or rearranged with cholesterol-targeting detergents: methyl-beta-cyclodextrin and filipin III. Moreover, cholesterol oxidation by a specific oxidase or blocking of cholesterol synthesis by mevastatin had a similar effect on lipid rafts. All cholesterol-modifying substances caused cell death in a concentration-dependent manner. More detailed studies on the effects of cyclodextrin revealed apoptotic cell death at concentrations >or=0.5% (w/v). The molecular mechanism of apoptosis precipitated by raft disruption remains unknown but does not seem to be dependent of either membrane permeabilization or cell-cycle arrest imposed by cholesterol-modifying compounds.

New advances on the functions of epidermal growth factor receptor and ceramides in skin cell differentiation, disorders and cancers

Recent advances in understanding of the biological functions of the epidermal growth factor and epidermal growth factor receptor (EGF-EGFR) system and ceramide production for the maintenance of skin integrity and barrier function are reported. In particular, the opposite roles of EGFR and ceramide cascades in epithelial keratinocyte proliferation, migration and terminal differentiation are described. Moreover, the functions of ceramides in the epidermal permeability barrier are reviewed. The alterations in EGFR signaling and ceramide metabolism, which might be involved in the etiopathogenesis of diverse skin disorders and cancers, are described. New progress in understanding of skin organization, which might provide the basis for the design of new transcutaneous drug delivery techniques as well as for the development of new therapies of skin disorders and cancers, are reported.

Cholesterol Depletion Upregulates Involucrin Expression in Epidermal Keratinocytes Through Activation of p38

Cholesterol has been recently suggested to regulate the early steps of keratinocyte differentiation through lipid rafts. In many cell types, depletion of cholesterol activates signaling proteins like epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 2 (HER2), or extracellular signal-regulated kinase (ERK) known to affect cell differentiation. In this study, we explored the effects of cholesterol depletion on the phenotype of cultured keratinocytes, using a treatment with methyl-beta-cyclodextrin (MbetaCD) to extract cholesterol and a treatment with lovastatin to inhibit cholesterol neosynthesis. Analysis of the expression of differentiation marker genes in early differentiating confluent cultures reveals that cholesterol depletion induces downregulation of keratin 14 (K14) and keratin 10 (K10) and upregulation of involucrin. MbetaCD treatment induces phosphorylation of EGFR, HER2, and ERK, but not HER3. Inhibition of EGFR with PD153035 impairs the MbetaCD-induced phosphorylation of EGFR, HER2, and ERK, but does not impair the alteration of K14, K10, or involucrin gene expression, indicating that other signaling proteins regulate this phenomenon. p38 has been suggested to regulate the expression of involucrin during keratinocyte differentiation. We found that MbetaCD treatment induces a prolonged phosphorylation of p38 in general and p38alpha in particular. An inhibition of p38 with PD169316 impairs the upregulation of involucrin mRNAs by a treatment with MbetaCD, but not by a p38delta-activating TPA treatment, which might suggest that cholesterol depletion alters involucrin gene expression through activation of p38alpha/beta.

Depletion of membrane cholesterol causes ligand-independent activation of Fas and apoptosis

Fas is a member of the tumour necrosis factor receptor superfamily. Fas-mediated apoptosis is an essential mechanism protecting against skin cancer. Activation of Fas by specific ligand or agonistic antibodies leads to the formation of a membrane associated death-inducing signalling complex comprising aggregates of Fas, the Fas-associated death domain protein (FADD), and caspase-8. It has recently been suggested that activity of Fas is not only regulated by its cognate ligand but also by the association of this receptor with cholesterol-enriched lipid domains in the plasma membrane (lipid rafts). We report here that disruption of lipid rafts by cholesterol-depleting compounds (methyl-beta-cyclodextrin, filipin III, cholesterol oxidase, and mevastatin) leads to a spontaneous clustering of Fas in the non-raft compartment of the plasma membrane, formation of Fas-FADD complexes, activation of caspase-8, and apoptosis. We propose that in some cell types exclusion of Fas from lipid rafts leads to the spontaneous, ligand-independent activation of this death receptor, a mechanism that can potentially be utilized in anticancer therapy.

Lipid raft-enriched stem cell-like keratinocytes in the epidermis, hair follicles and sinus tracts in hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a disease, that causes considerable morbidity in patients. A histological hallmark of the disorder is the formation of sinus tracts in the dermis and the subcutis. Biologically, they represent a poorly understood phenomenon involving the infiltrative growth of proliferating non-malignant keratinocytes. Lipid domains in plasma membranes (lipid rafts) play a role in the function of growth factors and are suspected of having a pathogenic role in cell migration and invasive growth. Using HS as a model, the presence of lipid rafts was studied using cholera toxin conjugated with FITC (CTx-FITC) and antibeta1 integrin (CD29)-CyChrome conjugate fluorescence staining of unfixed and acetone-fixed cryostat sections of lesional and paralesional skin samples. The double-labeled skin samples were observed in the confocal laser-scanning fluorescence microscope. Samples were obtained from five patients with HS. The lesional epidermis of HS contained three populations of keratinocytes: CD29(bright)CTx(dim), CD29(dim)CTx(bright) and a third hitherto unseen population containing double-positive CD29(bright)CTx(bright) cells. The CD29(bright)CTx(dim) population resembles the earlier described epidermal stem-like cells, while the CD29(dim)CTx(bright) basal keratinocytes overlap with the transit-amplifying cell pool. The new population of double-positive CD29(bright)CTx(bright) cells was localized on the slopes of the papillas, focally in the suprabasal epidermal layers, in some hair follicles and in the majority of sinus tracts. Such double-positive cells have not previously been encountered by us in normal epidermis and hair follicles. Using HS as a model, it is suggested that the keratinocytes involved in sinus tract formation are CD29(bright)CTx(bright) cells. Owing to the physical proximity of the cells, it is hypothesised that the described CD29(bright)CTx(bright) cells result from an increased expression of CD29 on the CTx(bright) cells. It is likely that the double-positive CD29(bright)CTx(bright) cells emerge due to the influence of local inflammatory cytokines. Sinus tract formation may represent an aberrant epidermal repair response executed by the activated CD29(bright)CTx(bright) keratinocytes capable of non-malignant infiltrative growth in the dermis and subcutis.