Functional redundancy of extracellular matrix protein 1 in epidermal differentiation

BACKGROUND

Extracellular matrix protein 1 (ECM1) is a secreted protein expressed in skin. Its dermatological relevance has been highlighted by the discovery of loss-of-function mutations in ECM1 in patients with lipoid proteinosis (LiP).

OBJECTIVES

To determine the role of ECM1 in epidermal differentiation by examining gene and protein expression of epidermal differentiation markers in individuals with LiP and histological assessment of transgenic mouse skin that overexpresses Ecm1a in basal or suprabasal epidermis.

METHODS

Subconfluent, confluent and postconfluent LiP and control keratinocyte cultures were analysed by Northern and Western blotting for differences in expression of differentiation markers. Expression of these markers was analysed in skin of patients with LiP by immunohistochemistry. To study effects of Ecm1 overexpression on epidermal differentiation, transgenic mice were generated under control of either a keratin 14 or an involucrin promoter.

RESULTS

No differential expression of the different markers analysed was observed in LiP keratinocytes compared with controls. No histological differences were found in Ecm1-overexpressing mouse skin compared with wild-type.

CONCLUSIONS

Absence of ECM1 does not lead to differences in epidermal differentiation. Moreover, overexpression of Ecm1a in vivo does not exert dramatic effects on epidermal structure. Collectively, these findings suggest no role of ECM1 in epidermal differentiation.

Immunogold silver staining associated with epi-fluorescence for cucumber mosaic virus localisation on semi-thin sections of banana tissues

The immunogold-silver staining (IGSS) technique in combination with epi-fluorescence detection was used to localise cucumber mosaic virus (CMV) particles within banana infected tissues. For this purpose, tissue samples (2 mm3) were excised from CMV-infected and highly proliferating meristem cultures of Williams BSJ banana (ITC. 0570, AAA, Cavendish subgroup). These samples were immediately fixed in a 2% paraformaldehyde/0.25% glutaraldehyde mixture, dehydrated in ethanol, and finally embedded in L.R. White resin. Semi-thin sections were cut, mounted on clean treated glass slides and immunostained for CMV particles using gold-labelled secondary antibodies and silver enhancement. Sections were counterstained with basic fuchsin and examined using laser scanning confocal microscopy. Negative controls included immuno-stained samples excised from non-virus infected material as well as infected material on which primary or secondary antibodies were not applied. Images of autofluorescence (in red) and of epi-reflectance of silver-enhanced immunogold particles (in green) were recorded separately and merged, allowing the specific localisation of CMV particles at the cellular level on semi-thin sections of aldehyde-fixed banana tissues. The main advantage of this analytical approach compared to previously published protocols is that it combines a fast staining procedure, stable preparation, a high resolution, and a narrow plane of focus with the flexibility in generation, processing and analysis of images offered by laser scanning confocal microscopy. Finally, the presence of numerous CMV particles within banana meristems constitutes a clear explanation of the very low CMV elimination efficiency when using meristem-tip culture alone.

A simple reconstructed human epidermis: preparation of the culture model and utilization in in vitro studies

The preparation of a reconstructed human epidermis is described with examples of its utilization in in vitro studies. The model was obtained by culturing normal human keratinocytes at high cell density for 14 days in serum-free and high calcium (1.5 m M) medium on an inert polycarbonate filter at the air-liquid interface. These stratified cultures showed histological features similar to those observed in vivo in the epidermis: a proliferating basal layer and differentiating spinous, granular, and cornified layers. Electron microscopy illustrated lamellar bodies, junctions and keratohyalin granules. Immunofluorescent localization of epidermal markers (keratins 14 and 10, involucrin and filaggrin) revealed typical differentiation. This in vitro reconstructed tissue was used in studies of toxic effects of chemicals. The modelled tissue showed progressive cytotoxicity of a skin irritant (benzalkonium chloride) and a sensitizer (dinitrochlorobenzene) as assessed by MTT assay. Moreover, differential release of interleukin-1alpha and interleukin-8 were measured after 20 h of incubation allowing the irritant to be distinguished from the sensitizer. Permeation studies indicated efficient barrier function of the reconstructed epidermis, as well as metabolizing properties towards hormones. This model can be custom-made and is potentially useful for studies involving keratinocytes in the epidermis, in basic science, dermatology or toxicology.

Analysis of interleukin-1alpha (IL-1alpha) and interleukin-8 (IL-8) expression and release in in vitro reconstructed human epidermis for the prediction of in vivo skin irritation and/or sensitization

In the present study, reconstructed human epidermis (RHE) was used as an in vitro model to discriminate 1-chloro-2,4-dinitrobenzene (DNCB), nickel sulfate (NiSO(4)), oxazolone (OXA), 2,4-dinitrofluorobenzene (DNFB) and 2,4,6-trinitrobenzenesulfonic acid (TNBS) as skin sensitizers from benzalkonium chloride (BC), benzoic acid (BA) and sodium lauryl sulfate (SLS) as skin irritants. Our criteria were (a) the differential IL-1alpha and IL-8 synthesis and release (b) cytotoxicity assessment by MTT assay. When the RHE are topically treated with the sensitizers, very low levels of extra- and intracellular IL-1alpha are observed although they induce significant cytotoxicity. In contrast, they exhibit a sharp maximum of IL-8 release. In the presence of the tested irritants, we observe the inverse cytokine release profile, although they induce dose-dependent cytotoxicity profiles similar to those observed with the sensitizers. Finally, IL-1alpha mRNA upregulation is observed after topical application of both sensitizers and irritants, but only the latter significantly increase extracellular IL-1alpha. In conclusion, our results suggest that the associated determination of IL-8, with IL-1alpha, and MTT conversion are at least necessary to discriminate and classify, in a single assay, irritant and sensitizing agents and represent a potential in vitro alternative to two classical in vivo assays.

Ultrastructural changes associated with cryopreservation of banana ( Musa spp.) highly proliferating meristems

Cryopreservation has been shown to improve the frequency of virus elimination - specifically cucumber mosaic virus and banana streak virus - from banana ( Musa spp.) plants. To understand the mode of action of cryopreservation for the eradication of viral particles, we examined the ultrastructure of meristem tips at each step of the cryopreservation process. Excised meristematic clumps produced from infected banana plants belonging to cv. Williams (AAA, Cavendish subgroup) were cryopreserved through vitrification using the PVS-2 solution. We demonstrated that the cryopreservation method used only allowed survival of small areas of cells in the meristematic dome and at the base of the primordia. Cellular and subcellular changes occurring during the cryopreservation process are discussed.

Dual role of protein kinase C on mitogen-activated protein kinase activation and human keratinocyte proliferation

Subconfluent normal human keratinocytes exhibit autonomous (autocrine growth factor driven) proliferation and express the specific markers for keratinocyte proliferation K#5 and K#14. In keratinocyte autocrine culture, the exogenously added epidermal growth factor (EGF) has no effect on cell proliferation and mitogen-activated protein kinase (MAPK) activity. PD98059 inhibits MAPK pathway and autocrine keratinocyte proliferation. Staurosporine and Gö6976 strongly inhibit autonomous keratinocyte proliferation. In contrast, Gö6983 (which does not inhibit PKC micro ) inhibits only 20% of autocrine keratinocyte proliferation. Staurosporine inhibits MAPK activity, whereas Gö6976 and Gö6983 strongly increase it. We have concluded that MAPK, PKC micro and probably PKCalpha take part in autocrine keratinocyte proliferation. The effect of Gö6976 and Gö6983 on MAPK activity could be explained by the inhibition of PKC-dependent MAPK-phosphatase expression. The effect of staurosporine could be explained by its paradoxical action (activation) on protein kinase C (PKC) in keratinocytes.

Ca(2+)/calmodulin-dependent protein kinase (CaM-kinase) inhibitor KN-62 suppresses the activity of mitogen-activated protein kinase (MAPK), c-myc activation and human keratinocyte proliferation

Autocrine growth of human epidermal keratinocytes can be maintained in subconfluent cell cultures in the absence of exogenous growth factors. We used this culture model to investigate the interactions between the mitogen-activated protein kinase (MAPK) pathway and Ca(2+)/calmodulin-dependent protein kinases (CaM-kinases) in autocrine keratinocyte proliferation. We have previously demonstrated that MAPK and protein kinase C (PKC) are both involved in keratinocyte proliferation in a complex set of interactions. Treatment of keratinocytes with PD98059, a potent inhibitor of MAPK kinase, inhibited the MAPK pathway, c-myc activation and autocrine keratinocyte proliferation. Application of the CaM-kinase inhibitor KN-62 also led to a strong inhibition of MAPK/c-myc activation and autocrine keratinocyte proliferation. Other inhibitors, such as wortmannin (selective and potent inhibitor of phosphatidylinositol 3-kinase) and AG 490 (JAK2 inhibitor) had weak effects on autocrine keratinocyte proliferation, MAPK and c-myc activation. Our results clearly demonstrate a crosstalk between CaM-kinase/MAPK pathways in transducing keratinocyte proliferation stimuli.

Human EGF receptor (HER) family and heregulin members are differentially expressed in epidermal keratinocytes and modulate differentiation

Human EGF receptor (HER), also designated HER1, is an activatable tyrosine kinase receptor. HER1 activation regulates cell growth and differentiation in epidermal keratinocytes. Expression of other HER family members was investigated in human keratinocytes cultured under autocrine conditions. HER2 and HER3 are expressed and upregulated by confluence, concurrent with induction of epidermal differentiation. HER4 is not expressed by keratinocytes. Maximum expression of the cognate ligand, heregulin, is observed in subconfluent keratinocytes, and the expression of both heregulin alpha and beta isoforms is downregulated with confluence. Recombinant heregulin isoforms have no effect on colony formation and keratinocyte proliferation, but heregulin beta activates tyrosine phosphorylation of HER2 and HER3, with no effect on HER1, in confluent differentiating keratinocyte cultures. Also, heregulin beta increases HER2/HER3 heterodimerization under those conditions. Treatment of confluent cultures by heregulin beta correlates with cell signaling and inhibition of epidermal differentiation. Together, HER2, HER3, and heregulin constitute a potential autocrine-paracrine system involved in epidermal homeostasis and repair, as well as in hyperproliferative pathologies.

Differentiation-dependent alternative splicing and expression of the extracellular matrix protein 1 gene in human keratinocytes

The human extracellular matrix protein 1 (Ecm1) gene is located at chromosome band 1q21 close to the epidermal differentiation complex and is transcribed in two discrete mRNAs: a full length Ecm1a and a shorter, alternatively spliced, Ecm1b transcript, the expression of which is restricted to tonsils and skin. The chromosomal localization and the Ecm1b expression in skin prompted us to investigate the role of Ecm1 in keratinocyte differentiation. In this study, we provide evidence for the existence of a relationship between keratinocyte differentiation and expression of the Ecm1b transcript. Cultures of subconfluent undifferentiated normal human keratinocytes express only Ecm1a. Upon reaching confluence, the cells start to differentiate, as measured by keratin K10 mRNA expression. Concomitantly Ecm1b mRNA expression is induced, although expression of Ecm1a mRNA remains unchanged. In addition, treatment of undifferentiated normal human keratinocyte cells with 12-O-tetradecanoyl-phorbol-13-acetate strongly induces the expression of Ecm1b mRNA. Expression of Ecm1b can also be induced by coculturing normal human keratinocytes with lethally irradiated feeder cells and by a diffusible factor secreted by stromal cells. In adult human skin, Ecm1a mRNA is expressed throughout the epidermis with the strongest expression in the basal and first suprabasal cell layers, whereas expression of Ecm1b mRNA is predominantly found in spinous and granular cell layers. Immunohistochemically, Ecm1a expression is almost completely restricted to the basal cell layer, whereas Ecm1b is detected in the suprabasal layers. These results are strongly suggestive of a role for Ecm1b in terminal keratinocyte differentiation, which is also supported by the localization of the Ecm1 gene at 1q21. Refinement of its genomic localization, however, placed Ecm1 centromeric of the epidermal differentiation complex.

The tetraspanin CD9 associates with the integrin alpha6beta4 in cultured human epidermal keratinocytes and is involved in cell motility

Integrins are involved in several ways in keratinocyte physiology, including cell motility. CD9 is a member of the tetraspanin protein family which is found in association with other transmembrane proteins like the integrins. CD9 is expressed in the epidermal tissue, but this expression is not regulated by differentiation. The present work focuses on association of CD9 with the integrin alpha6beta4 in keratinocytes. In vivo, CD9 does not co-localize with alpha6beta4, and is not internalized with the integrin upon basal detachment with dispase. In vitro, CD9 is found partly in co-localization with alpha6beta4 and is internalized with the integrin after keratinocyte detachment with dispase. Using blocking antibodies in a phagokinetic tracks assay, we show that CD9, and to a lesser extent alpha6beta4, but not the tetraspanin CD82, promote motility of subconfluent keratinocytes on collagen I. Our observations also suggest that CD9 is involved in the formation of lamellipodia. We also report that the phorbol ester TPA has no effect on CD9 expression and association with alpha6beta4, but increases keratinocyte motility, possibly through modulation of integrin subunits expression, or through upregulation of collagenase-1 expression. Together, these results confirm that CD9 associates with alpha6beta4 in cultured keratinocytes, possibly in order to regulate the function of the integrin, and that CD9 is involved in keratinocyte motility on collagen. The data suggest that regulation of adhesion characteristics by CD9 in keratinocytes may play a role in epidermal repair.

High-cell-density phorbol ester and retinoic acid upregulate involucrin and downregulate suprabasal keratin 10 in autocrine cultures of human epidermal keratinocytes

Autocrine growth of human epidermal keratinocytes is initiated in subconfluent cell cultures in the absence of exogenous growth factors, at low calcium concentration of the medium and with sufficient cell density. Culture confluence inhibits keratinocyte proliferation and upregulates expression of early, keratin 10 (K10), and late, involucrin, markers of differentiation. In this report, the phenotype of autocrine keratinocytes was studied at high cell density (postconfluence), specifically after treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA), or all-trans retinoic acid (RA). At postconfluence, K10 is decreased but not involucrin. TPA upregulates involucrin expression, but not K10 in subconfluent keratinocytes. Treatment of confluent keratinocytes with RA downregulates K10, but upregulates involucrin. This in vitro culture model, unlike others, simulates for the first time the in vivo effects of RA, a member of the retinoid family which potently modulates keratinocyte differentiation and the expression of selected gene products. It thus can be developed to further examine epidermal differentiation.

Human epidermal keratinocytes upregulate expression of the prolactin receptor after the onset of terminal differentiation, but do not respond to prolactin

Growing and differentiating keratinocytes maintain the epidermal barrier. This is partly controlled by growth factors and hormones. Prolactin (PRL) is named after its hormonal role in mammals during lactation, but is found in all vertebrates where PRL exerts various effects. In serum-free keratinocyte cultures, PRL was thought to be the factor responsible for the proliferative effect of bovine pituitary extract. Here, we evaluated PRL as a clonogenic factor for keratinocytes and found no mitogenic activity. Studying the expression of the PRL receptor by keratinocytes, we found the receptor upregulated only after culture confluence, in differentiating keratinocytes, but we were unable to detect any cellular response to PRL. The hormone does not alter the gene expression of either early (suprabasal keratin) or late (involucrin) differentiation markers by keratinocytes. Accordingly, no activation of the transcription factor Stat5 by PRL can be detected in keratinocytes, Stat5 being nevertheless detected by Western blot.

In Vitro models of epidermal differentiation

The differentiation program followed by the epidermal keratinocyte in skin is intended to continuously produce and maintain a cornified layer made of fully keratinized cells. This outer layer of the body provides a certain protection against external pathogens and chemical or physical agents, together with a barrier that prevents loss of body fluids. Considerable knowledge of epidermal differentiation and understanding of its regulation has progressively emerged from the availability of keratinocyte cultures, and from the consecutive possibility of unlimited in vitro experimentation. This short review briefly presents the main current in vitro models of epidermal differentiation and emphasises their advantages of pitfalls when studying particular steps of the differentiation program or analyzing their regulation.

Cell density and culture factors regulate keratinocyte commitment to differentiation and expression of suprabasal K1/K10 keratins

Irreversible growth arrest and commitment to differentiation are among the earliest events in the program of cellular terminal differentiation. The transition from highly proliferative human keratinocytes in subconfluent culture to stationary cells in confluent culture was studied in a serum-free culture system to identify conditions that regulate the initiation of terminal differentiation in keratinocytes. We observed that culture confluence strongly induced commitment to terminal differentiation, as demonstrated by a dramatic loss of keratinocyte clonogenicity. Commitment was accompanied by the rapid induction of early differentiation markers, represented by expression of suprabasal keratin 1 (K1) and 10 (K10) genes. Induction of differentiation was independent of low (0.1 mM) or high (1.5 mM) calcium concentration in the medium. Epidermal growth factor suppressed expression of K1 and K10 mRNA in cultures induced to differentiate. Suspension of keratinocytes in methylcellulose medium failed to induce in subconfluent cultures, or enhance in confluent cultures, the expression of K1 and K10 genes. Subconfluent cells cultured in medium containing high calcium and no exogenous growth factor induced expression of K1 and K10 transcripts, but commitment and loss of proliferative potential were not observed. Confluent cell density primarily controlled keratinocyte commitment to terminal differentiation and differentiated keratin gene expression. However, suprabasal K1 and K10 gene expression also was regulated by medium calcium and exogenous growth-factor concentrations in subconfluent cultures that promoted cell-cell association. Epidermal growth factor inhibited the expression of suprabasal keratins but not the commitment to terminal differentiation mediated by cell confluence. Control of keratinocyte commitment and expression of selected differentiation genes are mediated by cell confluence and, at subconfluence, by specific culture factors.

Basal detachment of the epidermis using dispase: tissue spatial organization and fate of integrin alpha 6 beta 4 and hemidesmosomes

Dispase has been utilized to produce basal detachment of the epidermis of human skin biopsies and to study the consequences induced afterwards during incubations of the detached tissue. Spatial reorganization of the epidermis is observed under these conditions and is characterized by disappearance of the typical basal keratinocyte layer. Immunofluorescent labelings reveal upward migration of several cells exhibiting the basal phenotype between suprabasal differentiating keratinocytes and demonstrate progressive intracellular expression of hemidesmosomal components: the integrin alpha 6 beta 4 and two plaque components, the 230-kDa bullous pemphigoid antigen and HD1, a 500-kDa protein. Using electron microscopy and immunogold techniques, we demonstrate that the hemidesmosome-containing basal membrane domains enter the cell cytoplasm after detachment of the epidermal tissue. Partial recycling of internalized hemidesmosomal components is also suggested. Our findings illustrate the processing of released hemidesmosomes in detached basal keratinocytes and suggest some heterogeneity between basal cells migrating towards a suprabasal position and those remaining in the basal layer. These results suggest that the dispase-detached epidermis is a self-remodeling tissue in which basal keratinocytes' and tissue's polarities observed in the anchored epidermis are progressively changing.

Specific internalization of basal membrane domains containing the integrin alpha 6 beta 4 in dispase-detached cultured human keratinocytes

The integrin alpha 6 beta 4 is polarized towards the basal side of basal keratinocytes and helps anchor these cells to the basement membrane components. We have found that cultured human epidermal keratinocytes, when detached from their culture substratum, as for grafting, using the enzyme dispase, rapidly internalize the basal membrane domains containing the integrin alpha 6 beta 4, while integrins of the very late antigen subtype remain on the cell surface. Detachment and incubation at 4 degrees C prevent this internalization, as well as the contraction of the detached sheet area. Subsequent incubation at 37 degrees C initializes this contraction and allows the basal integrin alpha 6 beta 4 to be internalized. We took advantage of this blockage to label upon detachment using immunogold techniques, the alpha 6 subunit present on the basal cell surface; then we studied its internalization with the electron microscope. This internalization pathway differs from classical receptor-mediated endocytosis, and intermediate filaments might possibly play a role in this process. Interestingly, 1 h after their internalization from the basal membrane, a third of the gold particles labeling the alpha 6 subunit was found between lateral membranes of basal cells, strongly suggesting that the integrin alpha 6 beta 4 can be partly recycled to the cell surface in these conditions.

Basal cell adhesion to a culture substratum controls the polarized spatial organization of human epidermal keratinocytes into proliferating basal and terminally differentiating suprabasal populations

The contribution of adhesion to an extracellular matrix in the polarized spatial organization of keratinocytes was studied in dispase-detached cultures stored as floating sheets. Proliferating and terminally differentiating cell populations were, therefore, localized on tissue sections by their DNA-synthesizing ability and involucrin immunostaining, respectively. A progressive reorganization was induced from superposed proliferating and differentiating layers into clusters exhibiting differentiating cells on the outside. Measurements of proliferation and terminal differentiation in detached cultures revealed the progressive disappearance of proliferating cells, followed by an increase in involucrin-positive cells. Attempts to block the spatial reorganization by the addition of components of the extracellular matrix remained unsuccessful. These results suggest that basal anchorage is responsible for the superposition of proliferating and differentiating cells in keratinocyte cultures. They afford new arguments for the induction of terminal differentiation in non-adhesive keratinocytes which exhibit a concomitant modification of cell shape.