Short term retinol treatment in vitro induces stable transdifferentiation of chick epidermal cells into mucus-secreting cells

Effects of retinoic acid and Gbx1 on feather-bud formation and epidermal transdifferentiation in chick embryonic cultured dorsal skin

BACKGROUND

Retinoic acid, an active metabolite of retinol, is known to regulate cell proliferation, differentiation, and morphogenesis during normal development of many tissues. Using chick embryonic tarsometatarsal skin, we showed previously that the expression of Gbx1, a divergent homeobox gene, is increased in the epidermis through interaction with retinol-pretreated dermal fibroblasts followed by epidermal transdifferentiation to mucous epithelium. This present study was performed to elucidate the effects of retinoic acid and Gbx1 on feather-bud formation and epidermal transdifferentiation.

RESULTS

We showed that Gbx1 was expressed in the chick embryonic dorsal epidermis as early as at placode stage (Hamburger and Hamilton stage 31) and increased in amount during feather-bud formation. Treatment with 1 μM retinoic acid for 24 hr inhibited feather-bud formation and induced the transdifferentiation of the epidermis to a mucosal epithelium with a concomitant increase in Gbx1 mRNA expression in the epithelium. Furthermore, transient transfection of the epidermis with Gbx1 cDNA by electroporation induced elongation of the feather bud, but did not result in transdifferentiation.

CONCLUSIONS

These results indicate that Gbx1 was involved in the feather-bud formation and was one of target genes of retinoic acid and that other signals in addition to Gbx1 were required for epidermal mucous transdifferentiation.

Increased retinoic acid levels through ablation of Cyp26b1 determine the processes of embryonic skin barrier formation and peridermal development

The process by which the periderm transitions to stratified epidermis with the establishment of the skin barrier is unknown. Understanding the cellular and molecular processes involved is crucial for the treatment of human pathologies, where abnormal skin development and barrier dysfunction are associated with hypothermia and perinatal dehydration. For the first time, we demonstrate that retinoic acid (RA) levels are important for periderm desquamation, embryonic skin differentiation and barrier formation. Although excess exogenous RA has been known to have teratogenic effects, little is known about the consequences of elevated endogenous retinoids in skin during embryogenesis. Absence of cytochrome P450, family 26, subfamily b, polypeptide 1 (Cyp26b1), a retinoic-acid-degrading enzyme, results in aberrant epidermal differentiation and filaggrin expression, defective cornified envelopes and skin barrier formation, in conjunction with peridermal retention. We show that these alterations are RA dependent because administration of exogenous RA in vivo and to organotypic skin cultures phenocopy Cyp26b1(-/-) skin abnormalities. Furthermore, utilizing the Flaky tail (Ft/Ft) mice, a mouse model for human ichthyosis, characterized by mutations in the filaggrin gene, we establish that proper differentiation and barrier formation is a prerequisite for periderm sloughing. These results are important in understanding pathologies associated with abnormal embryonic skin development and barrier dysfunction.

Transdifferentiation of epidermis to mucous epithelium by retinol accompanies increase in transglutaminase 2/Gh and decrease in transglutaminase 3

We showed previously that transdifferentiation of skin epidermis to mucous epithelium can be induced by treatment with 20 µM retinol for 1 d followed by culture for 4 d without retinol in chick embryonic tarsometatarsal skin. In mouse epidermal cells, 3 µM retinoic acid (an active metabolite of retinol) inhibits epidermal keratinization in consistent with an increase in transglutaminase (TG)2/Gh, while its physiological role in the skin is still unresolved. TG1, TG3 and TG5 are also found in mammalian keratinocytes and play an important role in the formation of the stratum corneum in the skin by the introduction of cross-links into proteins. The most characteristic enzyme function of TG family is calcium-dependent transamidation activity (transamidase) that introduces inter or intramolecular ε-(γ-glutamyl)lysine cross-links into the protein. TG2/Gh is a multifunctional protein and ubiquitously expressed member of transglutaminase family that has been implicated in a variety of biological processes. By in situ hybridization analysis, we showed that TG2/Gh mRNA expression started to increase throughout the skin during the culture for 1 d with retinol, while it was weak in the control skin. On the other hand, an expression of TG3 mRNA was increased in the keratinized epidermis of control skin but was decreased by retinol. In situ transamidase activity of transglutaminase was weak in retinol-pretreated skin. Therefore, it was indicated that functions other than transamidase of TG2/Gh protein might be important in retinol-induced epidermal mucous transdifferentiation.

Increase in expression of the homeobox gene, GBX1, in retinol-induced epidermal mucous metaplasia

Using a degenerate RT-PCR-based screening method, we isolated the homeobox gene, Gbx1, from the shank skin of 13-day-old chick embryos. By in situ hybridization analysis we showed that the Gbx1 was expressed in the epidermis of the skin and the mucous epithelium of the intestine, and that among many homeobox genes isolated, expression of the Gbx1 strongly increased in the epidermis when the skin was cultured with 20 microM retinol, which induces epidermal mucous metaplasia. The Gbx1 expression in the epidermis was increased by interaction with the retinol-pretreated dermal fibroblasts, resulting in mucous metaplasia. These results suggest that the Gbx1 regulates the differentiation and transdifferentiation of the epithelium and controls the morphology of the epithelium. We isolated the chick Gbx1 cDNA clones. The amino acid sequences in homeodomain and its downstream encoded by human and chick Gbx1 cDNA were almost the same, but those upstream of the homeodomain were rather different.

Inhibition by tunicamycin of mucin synthesis, not morphological changes, in epidermis during retinol-induced mucous metaplasia of chick embryonic cultured skin

BACKGROUND

Our previous studies have shown that epidermal mucous metaplasia of chick embryonic skin can be induced by culture in medium containing 20 microM retinol for only 8 hr and then in a chemically defined medium without retinol for 2 days and that retinol primarily affects the dermal cells, which then transform the epithelial cells into mucus-secreting cells.

METHODS

Tarsometatarsal skin of 13-day-old chick embryo was cultured with 20 microM retinol for 1 day and then without the vitamin but with 0.1 microgram/ml tunicamycin for 5 days. Effect of tunicamycin on epidermal mucous metaplasia was studied biochemically and morphologically.

RESULTS

Tunicamycin, which prevents the formation of N-glycans and inhibits maturation or morphological organization of various epithelial cells, irreversibly inhibited the synthesis of sulfated glycoproteins (O-glycans, mucin) in the epidermis only when applied to retinol-pretreated skin. Microvilli on the surface of the cells were well developed, but mucous granules surrounded by a limiting membrane were not observed in the upper cell layer of the epidermis, and many vesicles without electron-dense materials (mucin) and dilated rough endoplasmic reticulum were seen in the intermediate cell layers of the epidermis. When recombinants of 13-day-old normal epidermis and cultured dermis, which had been treated with retinol for 24 hr and with only tunicamycin for 2 days, were cultured without the antibiotic for 5 days, epidermal mucous metaplasia was induced.

CONCLUSION

These results suggest that tunicamycin did not prevent morphological changes induced by retinol but inhibited mucin synthesis by a direct action on the epidermis of retinol-pretreated skin. Because in some cell-line mucin precursors contain high mannose N-linked oligosaccharides side chains, tunicamycin may have inhibited mucin synthesis. Interaction between epidermal basal cells and retinol-pretreated dermal fibroblasts is prerequisite for epidermal mucous metaplasia. Thus, the present study suggests that N-linked protein glycosylation is not required for this interaction.

Changes in expression of two endogenous beta-galactoside-binding isolectins in the dermis of chick embryonic skin during development in ovo and in vitro

In order to elucidate the roles of metal-independent animal lectins, we systematically investigated changes in expression of 2 kinds of beta-galactoside-binding isolectins (MW 14 and 16 kDa) in the dermis of chick embryonic tarsometatarsal skin during the course of development. These lectins were immunohistochemically located at different stages of development both in ovo and in vitro by light and electron microscopy. Light-microscopic observation showed that while positive staining for the 14-kDa lectin was weak at days 8 and 10 it became intense after day 13. In contrast, staining for the 16-kDa lectin was intense at days 8, 10, and 13, but it became weak after day 17 when keratinization of the epidermis was completed. Immuno-electron-microscopic observation revealed that both the 14 and 16-kDa lectins were located on the basement membrane, in the extracellular matrix, and in both the cytoplasm and the nucleus of dermal fibroblasts. Distribution of the 2 isolectins was also examined in cultured skin explants in vitro. The results were almost the same as those obtained in ovo when the skin explant was keratinized in the presence of hydrocortisone. However, in the skin explant where keratinization was prevented and mucous metaplasia was induced by the addition of vitamin A, the distribution of the 14-kDa lectin in the epidermis was significantly affected. These results indicate that (1) the expression of the 2 isolectins is differently regulated in both the dermis and epidermis, (2) the 16-kDa lectin is involved in the early stage of the formation of the dermis and the basement membrane and is replaced by the 14-kDa lectin as keratinization of the epidermis occurs, and (3) the expression of the 2 isolectins in the dermis is not significantly affected by the induction of mucous metaplasia, in contrast to their drastic changes in the epidermis.