Evidence for autoregulation of cell division and cell transit in keratinocytes grown on collagen at an air-liquid interface

Architectural characterization of organotypic cultures of H400 and primary rat keratinocytes

Organotypic epithelial structures can be cultured using primary or immortalized keratinocytes. However, there has been little detailed quantitative histological characterization of such cultures in comparison with normal mucosal architecture. The aim of this study is to identify morphological markers of tissue architecture that can be used to monitor tissue structure, maturation, and differentiation and to enable quantitative comparison of organotypic cultures (OCs) with normal oral mucosa. OCs of oral keratinocytes [immortalized H400 or primary rat keratinocytes (PRKs)] were generated using the three scaffolds of de-epidermalized dermis (DED), polyethylene terephthalate (PET), and collagen gels for up to 14 days. Cultures and normal epithelium were analyzed immunohistochemically and by using the semi-quantitative reverse transcriptase polymerase chain reaction (sq-RT-PCR) for E-cadherin, desmoglein-3, plakophilin, involucrin, cytokeratins-1, -5, -6, -10, -13, and Ki67. The epithelial thickness of OCs was measured in stained sections using image processing. Histological analysis revealed that air-liquid interface (ALI) cultures generated stratified organotypic epithelial structures by 14-days. The final thickness of these cultures as well as the degree of maturation/stratification (including stratum corneum formation) varied significantly depending on the scaffold used. For certain scaffolds, the immunohistochemical profiles obtained recapitulated those of normal oral epithelium indicating comparable in vitro differentiation and proliferation. In conclusion, quantitative microscopy approaches enabled unbiased architectural characterization of OCs. The scaffold materials used in the present study (DED, collagen type-I and PET) differentially influenced cell behavior in OCs of oral epithelia. H400 and PRK OCs on DED at the ALI demonstrated similar characteristics in terms of gene expression and protein distribution to the normal tissue architecture.

Wounding-induced synthesis of hyaluronic acid in organotypic epidermal cultures requires the release of heparin-binding egf and activation of the EGFR

Hyaluronic acid (HA), a glycosaminoglycan located between keratinocytes in the epidermis, accumulates dramatically following skin wounding. To study inductive mechanisms, a rat keratinocyte organotypic culture model that faithfully mimics HA metabolism was used. Organotypic cultures were needle-punctured 100 times, incubated for up to 24 hours, and HA analyzed by histochemical and biochemical methods. Within 15 minutes post-injury, HA levels had elevated two-fold, increasing to four-fold by 24 hours. HA elevations far from the site of injury suggested the possible involvement of a soluble HA-inductive factor. Media transfer experiments (from wounded cultures to unwounded cultures) confirmed the existence of a soluble factor. From earlier evidence, we hypothesized that an EGF-like growth factor might be responsible. This was confirmed as follows: (1) EGFR kinase inhibitor (AG1478) completely prevented wounding-induced HA accumulation. (2) Rapid tyrosine-phosphorylation of EGFR correlated well with the onset of increased HA synthesis. (3) A neutralizing antibody that recognizes heparin binding EGF-like growth factor (HB-EGF) blocked wounding-induced HA synthesis by > or =50%. (4) Western analyses showed that release of activated HB-EGF (but neither amphiregulin nor EGF) occured after wounding. In summary, rapid HA accumulation after epidermal wounding occurs through a mechanism requiring cleavage of HB-EGF and activation of EGFR signaling.

Cellular responses to disruption of the permeability barrier in a three-dimensional organotypic epidermal model

Repeated injury to the stratum corneum of mammalian skin (caused by friction, soaps, or organic solvents) elicits hyperkeratosis and epidermal thickening. Functionally, these changes serve to restore the cutaneous barrier and protect the organism. To better understand the molecular and cellular basis of this response, we have engineered an in vitro model of acetone-induced injury using organotypic epidermal cultures. Rat epidermal keratinocytes (REKs), grown on a collagen raft in the absence of any feeder fibroblasts, developed all the hallmarks of a true epidermis including a well-formed cornified layer. To induce barrier injury, REK cultures were treated with intermittent 30-s exposures to acetone then were fixed and paraffin-sectioned. After two exposures, increased proliferation (Ki67 and BrdU staining) was observed in basal and suprabasal layers. After three exposures, proliferation became confined to localized buds in the basal layer and increased terminal differentiation was observed (compact hyperkeratosis of the stratum corneum, elevated levels of K10 and filaggrin, and heightened transglutaminase activity). Thus, barrier disruption causes epidermal hyperplasia and/or enhances differentiation, depending upon the extent and duration of injury. Given that no fibroblasts are present in the model, the ability to mount a hyperplastic response to barrier injury is an inherent property of keratinocytes.

Tissue-engineered oral mucosa: a review of the scientific literature

Tissue-engineered oral mucosal equivalents have been developed for clinical applications and also for in vitro studies of biocompatibility, mucosal irritation, disease, and other basic oral biology phenomena. This paper reviews different tissue-engineering strategies used for the production of human oral mucosal equivalents, their relative advantages and drawbacks, and their applications. Techniques used for skin tissue engineering that may possibly be used for in vitro reconstruction of human oral mucosa are also discussed.

Disturbed keratinocyte differentiation in transgenic mice and organotypic keratinocyte cultures as a result of spermidine/spermine N-acetyltransferase overexpression

Overexpression of the rate-limiting enzyme in polyamine catabolism spermidine/spermine N1-acetyltransferase (SSAT) in transgenic (Tg) mouse leads to accumulation of putrescine in the skin and permanent hair loss at the age of 3 wk. The hair follicles of these mice are replaced by dermal cysts and epidermal utriculi. Increased putrescine production is also seen in hyperproliferative cutaneous disorders such as in psoriasis. These disorders are characterized by delayed onset of epidermal differentiation characterized as reduced expression of terminal differentiation markers such as cytokeratins 1/10, and filaggrin and persisting expression of basal cell cytokeratins 5/14 in the suprabasal layers. The use of these markers in immunohistological analysis of SSAT Tg skin clearly showed signs of disturbed differentiation. To exclude the possibility that changes in differentiation originated from underlying connective tissue, we introduced SSAT gene into an established rat epidermal cell line. Organotypic cultures derived from the transfected cells displayed similar changes in their differentiation pattern as keratinocytes in Tg skin. The role of accumulated putrescine in cutaneous changes of SSAT Tg mice was verified by an experiment in which putrescine level was reduced by systemic putrescine biosynthesis inhibition. The putrescine reduction was sufficient to alleviate the cutaneous changes to such an extent that distinct hair regrowth could be seen. These results suggest that the cutaneous changes of SSAT Tg animals are due to disorders of the keratinocyte differentiation. Moreover, they strengthen the view that the proper regulation of polyamine metabolism plays an important role in the keratinocyte maturation.

The DP-1 transcription factor is required for keratinocyte growth and epidermal stratification

The epidermis is a stratified epithelium constantly replenished through the ability of keratinocytes in its basal layer to proliferate and self-renew. The epidermis arises from a single-cell layer ectoderm during embryogenesis. Large proliferative capacity is central to ectodermal cell and basal keratinocyte function. DP-1, a heterodimeric partner of E2F transcription factors, is highly expressed in the ectoderm and all epidermal layers during embryogenesis. To investigate the role of DP-1 in epidermal morphogenesis, we inhibited DP-1 activity through exogenous expression of a dominant-negative mutant (dnDP-1). Expression of the dnDP-1 mutant interferes with binding of E2F/DP-1 heterodimers to DNA and inhibits DNA replication, as well as cyclin A mRNA and protein expression. Chromatin immunoprecipitation analysis demonstrated that the cyclin A promoter is predominantly bound in proliferating keratinocytes by complexes containing E2F-3 and E2F-4. Thus, the mechanisms of decreased expression of cyclin A in the presence of dnDP-1 seem to involve inactivation of DP-1 complexes containing E2F-3 and E2F-4. To assess the consequences on epidermal morphogenesis of inhibiting DP-1 activity, we expressed dnDP-1 in rat epithelial keratinocytes in organotypic culture and observed that DP-1 inhibition negatively affected stratification of these cells. Likewise, expression of dnDP-1 in embryonic ectoderm explants produced extensive disorganization of subsequently formed epidermal basal and suprabasal layers, interfering with normal epidermal formation. We conclude that DP-1 activity is required for normal epidermal morphogenesis and ectoderm-to-epidermis transition.

Hyaluronan suppresses epidermal differentiation in organotypic cultures of rat keratinocytes

Hyaluronan (hyaluronic acid, HA) is an abundant matrix component between keratinocytes of the epidermis in vivo, but its function there remains unclear. We used a lift culture model, in which rat epidermal keratinocytes (REKs) stratify at an air-liquid interface, to ask whether HA may regulate epidermal proliferation and/or differentiation. In this model, early markers of differentiation (keratin 10), and later markers (profilaggrin, keratohyalin granules, cornified layers) are faithfully expressed, both temporally and spatially. HA, measured using two different analytical techniques, accumulated to high levels only in the presence of an intact basement membrane that seals the epidermal compartment. To test whether HA has a functional role in differentiation, Streptomyces hyaluronidase (StrepH, 1 U/ml; digests >95% of HA within 4 h) was added daily to lift cultures during stratification time-course experiments over 5 days. In StrepH-treated cultures, the expression of profilaggrin and the number and size of keratohyalin granules were significantly increased relative to controls using semiquantitative histological analyses. The StrepH-related accumulation of K10 protein and profilaggrin/filaggrin were confirmed by Western analyses. Thus, it appears that the presence of intercellular HA in the epidermis acts as a brake upon intracellular events that occur during keratinocyte differentiation.

Keratinocyte growth factor stimulates migration and hyaluronan synthesis in the epidermis by activation of keratinocyte hyaluronan synthases 2 and 3

Keratinocyte growth factor (KGF) activates keratinocyte migration and stimulates wound healing. Hyaluronan, an extracellular matrix glycosaminoglycan that accumulates in wounded epidermis, is known to promote cell migration, suggesting that increased synthesis of hyaluronan might be associated with the KGF response in keratinocytes. Treatment of monolayer cultures of rat epidermal keratinocytes led to an elongated and lifted cell shape, increased filopodial protrusions, enhanced cell migration, accumulation of intermediate size hyaluronan in the culture medium and within keratinocytes, and a rapid increase of hyaluronan synthase 2 (Has2) mRNA, suggesting a direct influence on this gene. In stratified, organotypic cultures of the same cell line, both Has2 and Has3 with the hyaluronan receptor CD44 were up-regulated and hyaluronan accumulated in the epidermis, the spinous cell layer in particular. At the same time the expression of the early differentiation marker keratin 10 was inhibited, whereas filaggrin expression and epidermal permeability were less affected. The data indicate that Has2 and Has3 belong to the targets of KGF in keratinocytes, and support the idea that enhanced hyaluronan synthesis acts an effector for the migratory response of keratinocytes in wound healing, whereas it may delay keratinocyte terminal differentiation.

EGF upregulates, whereas TGF-beta downregulates, the hyaluronan synthases Has2 and Has3 in organotypic keratinocyte cultures: correlations with epidermal proliferation and differentiation

Hyaluronan, a major extracellular matrix molecule in the vital cell layers of skin epidermis, has been suggested to support proliferation and migration of keratinocytes, during challenges like wounding and inflammation. An organotypic keratinocyte culture originated from continuous rat epidermal keratinocyte cell line was subjected to the proliferative and antiproliferative growth factors epidermal growth factor and transforming growth factor beta, respectively, to study their influence on hyaluronan synthesis and epidermal morphology. Epidermal growth factor induced a 4-fold increase of epidermal hyaluronan concentration. This was associated with upregulation of the hyaluronan synthases Has2 and Has3, and the hyaluronan receptor CD44. 5-Bromo-2'-deoxyuridine labeling, basal cell height, and the thickness of vital epidermis were increased, reflecting the hyperplastic effects of epidermal growth factor. The expression of keratin 10 and the maturation of filaggrin were inhibited, and epidermal permeability barrier became less efficient, indicating compromised terminal differentiation by epidermal growth factor. In contrast, transforming growth factor beta reduced the content of hyaluronan and the mRNA of Has2 and Has3. At the same time, transforming growth factor beta suppressed keratinocyte proliferation and epidermal thickness, but retained intact differentiation. The results suggest that epidermal hyaluronan synthesis, controlled by epidermal growth factor and transforming growth factor beta through changes in the expression of Has2 and Has3, correlates with epidermal proliferation, thickness, and differentiation.

Hyaluronan enters keratinocytes by a novel endocytic route for catabolism

Hyaluronan synthesized in the epidermis has an exceptionally short half-life, indicative of its catabolism by epidermal keratinocytes. An intracellular pool of endogenously synthesized hyaluronan, from 1 to 20 fg/cell, inversely related to cell density, was observed in cultured rat epidermal keratinocytes. More than 80% of the intracellular hyaluronan was small (<90 kDa). Approximately 25% of newly synthesized hyaluronan was endocytosed by the keratinocytes and had a half-life of 2-3 h. A biotinylated aggrecan G(1) domain/link protein probe demonstrated hyaluronan in small vesicles of approximately 100 nm diameter close to the plasma membrane, and in large vesicles and multivesicular bodies up to 1300 nm diameter around the nucleus. Hyaluronan did not co-localize with markers of lysosomes. However, inhibition of lysosomal acidification with NH(4)Cl or chloroquine, or treating the cells with the hyaluronidase inhibitor apigenin increased intracellular hyaluronan staining, suggesting that it resided in prelysosomal endosomes. Competitive displacement of hyaluronan from surface receptors using hyaluronan decasaccharides, resulted in a rapid disappearance of this endosomal hyaluronan (t(12) approximately 5 min), indicating its transitory nature. The ultrastructure of the hyaluronan-containing vesicles, co-localization with marker proteins for different vesicle types, and application of specific uptake inhibitors demonstrated that the formation of hyaluronan-containing vesicles did not involve clathrin-coated pits or caveolae. Treatment of rat epidermal keratinocytes with the OX50 monoclonal antibody against the hyaluronan receptor CD44 increased endosomal hyaluronan. However, no CD44-hyaluronan co-localization was observed intracellularly unless endosomal trafficking was retarded by monensin, or cultivation at 20 degrees C, suggesting CD44 recycling. Rat epidermal keratinocytes thus internalize a large proportion of their newly synthesized hyaluronan into non-clathrin-coated endosomes in a receptor mediated way, and rapidly transport it to slower degradation in the endosomal/lysosomal system.

Epidermal growth factor activates hyaluronan synthase 2 in epidermal keratinocytes and increases pericellular and intracellular hyaluronan

Hyaluronan is an abundant and rapidly turned over matrix molecule between the vital cell layers of the epidermis. In this study, epidermal growth factor (EGF) induced a coat of hyaluronan and a 3-5-fold increase in its rate of synthesis in a rat epidermal keratinocyte cell line that has retained its ability for differentiation. EGF also increased hyaluronan in perinuclear vesicles, suggesting concurrent enhancement in its endocytosis. Cell-associated hyaluronan was most abundant in elongated cells that were stimulated to migrate by EGF, as determined in vitro in a wound healing assay. Large fluctuations in the pool size of UDP-N-acetylglucosamine, the metabolic precursor of hyaluronan, correlated with medium glucose concentrations but not with EGF. Reverse transcriptase-polymerase chain reaction (RT-PCR) showed no increase in hyaluronan synthases 1 and 3 (Has1 and Has3), whereas Has2 mRNA increased 2-3-fold in less than 2 h following the introduction of EGF, as estimated by quantitative RT-PCR with a truncated Has2 mRNA internal standard. The average level of Has2 mRNA increased from approximately 6 copies/cell in cultures before change of fresh medium, up to approximately 54 copies/cell after 6 h in EGF-containing medium. A control medium with 10% serum caused a maximum level of approximately 21 copies/cell at 6 h. The change in the Has2 mRNA levels and the stimulation of hyaluronan synthesis followed a similar temporal pattern, reaching a maximum level at 6 h and declining toward 24 h, a finding in line with a predominantly Has2-dependent hyaluronan synthesis and its transcriptional regulation.

Herpes simplex virus type 1 infection has two separate modes of spread in three-dimensional keratinocyte culture

This study describes the outcome of herpes simplex virus type 1 (HSV-1) infection in an organotypic raft culture of spontaneously immortalized HaCat keratinocytes and human fibroblasts, as related to the virus load and epithelial stratification and differentiation. In this model, a confluent monolayer of HaCat keratinocytes was formed 60 h after seeding. Inoculation of HSV-1 before induction of differentiation by lifting of the culture to the air-liquid interface always resulted in a productive infection, but the virus yield was highest when the inoculation took place 72 h after seeding. Even at 0.1 p.f.u. per culture, the HaCat cultures became HSV positive. Infection of the full-thickness epithelium at 5 p.f.u. per culture resulted in a productive infection of the whole epithelium. The HaCat cells were about 10 times more sensitive to HSV-1 infection than the Vero cells in which the virus stocks were titrated. The raft cultures infected 30 min after lifting were negative by HSV-1 culture, and no HSV-1 antigen was detected by immunocytochemistry. PCR showed the presence of HSV-1 DNA and in situ hybridization showed reactivity with a latency-associated RNA probe, indicating the presence of a non-productive infection. Two different patterns of virus spread in epithelia were found: (i) lateral spread through the superficial layers of the epithelium and (ii) a demarcated infection throughout the whole thickness of the epithelium at the margins of the culture.

Hyaluronan bound to CD44 on keratinocytes is displaced by hyaluronan decasaccharides and not hexasaccharides

Abundant hyaluronan is present between epidermal keratinocytes. However, virtually nothing is known regarding its organization in the limited extracellular space between these cells. We have used metabolic labeling with [3H]glucosamine and [35S]sulfate and a hyaluronan-specific biotinylated probe to study the metabolism of hyaluronan and its localization in monolayer cultures of a rat epidermal keratinocyte cell line. Hyaluronan (approximately 20 fg/cell) was present on the apical and lateral surfaces of the cells in two nearly equal pools, either in patches (approximately 160/cell) or diffusely spread. The hyaluronan in the patches is bound to CD44 as indicated by co-localization with an antibody to CD44, and by displacement with hyaluronan decasaccharides as well as with an antibody that blocks hyaluronan binding to CD44. The inability of hyaluronan oligomers shorter than 10 monosaccharides to displace hyaluronan suggests that CD44 dimerization or cooperative interactions are required for tight binding. The diffuse hyaluronan pool is likely bound to hyaluronan synthase during its biosynthesis.