Gap junctional communication of primary human keratinocytes: characterization by dual voltage clamp and dye transfer

Epidermal stem cells do not communicate through gap junctions

Although enrichment of putative epidermal stem cells has been achieved, a need for additional markers that can enable isolation of live keratinocytes is crucial for characterization of these cells. Earlier work has shown that connexin proteins are absent from basal cells in the limbal epithelium, a region of the corneal epithelium enriched in corneal stem cells. Accordingly, we investigated whether connexin 43, a gap junction protein present in the basal layer of normal human epidermis, can serve as a negative marker for keratinocyte stem cells. In humans, cells with immunohistochemically undetectable levels of connexin 43 are found in the epidermal basal layer of neonatal foreskin and in the follicular bulge region. About 10% of the basal keratinocytes are connexin 43 negative, as determined by flow cytometry. These cells are uniformly small and low in granularity. Restricted gap junction communication was confirmed by the failure of low molecular weight dyes to transfer between cells. Experiments carried out in mouse epidermis demonstrated that most of the slowly cycling cells, detected as label-retaining cells, do not express connexin 43. Thus, presumptive keratinocyte stem cells can be identified and separated based on connexin 43 expression.

In vivo modulation of connexins 43 and 26 of human epidermis by topical retinoic acid treatment

After 14 weeks of topical application of 0.1% all-trans-retinoic acid to the napes of volunteers, we observed a 2.5-fold increase in the thickness of epidermis, owing to an increase (p < 0.001) in the number and size of keratinocytes and the induction of keratin 6. These changes in the differentiation of epidermal keratinocytes were paralleled by an increase in the amount of Cx43, a connexin that is normally expressed in human epidermis, and by the massive induction of Cx26, which is barely detectable in normal interfollicular epidermis, as judged at both the transcript (Northern blotting) and the protein level (immunolabeling). In contrast, retinoic acid treatment did not alter the morphology and connexin pattern of hair follicles or of sebaceous and sweat glands, and did not induce the expression of other connexins (C32, Cx37, Cx40) in either skin adnexae or epidermis. These observations suggest that the expression of two distinct connexins by interfollicular keratinocytes is related to selective changes in the differentiation program of epidermis that are induced by retinoic acid.

Regeneration of rat corneal epithelium is delayed by the inhibitory epidermal pentapeptide (EPP)

The effect of the inhibitory epidermal pentapeptide (EPP) on regeneration of rat corneal epithelium was studied over a 24-h period after removal of the central part of the corneal epithelium by means of n-heptanol and scraping. Both unphosphorylated and phosphorylated EPP inhibited the mitotic rate and the formation of new cells to the same extent. Thus, the mitosis inhibitor that originally was isolated from mouse epidermis, acts even on the ectodermally derived corneal epithelial cells.

Dye-coupling in three-dimensional histoculture of rat lingual frenulum

A three-dimensional histoculture of wet stratified squamous epithelium of rat lingual frenulum was cultured on a liquid-air interface. The tissue retained its morphology for many days in culture. During this period the vast majority of the epithelial cells remained viable and exhibited dye (lucifer yellow) coupling in all living epithelial strata. Dye coupling was determined using two methods: the conventional intracellular injection method, and a new method--"cut-loading." In the cut-loading method, an incision is made in the epithelium in the presence of dye, and intracellular diffusion of dye throughout the epithelium was measured using confocal microscopy. The basolateral surface of the lingual frenulum also acted as a substrate for neuroblastoma cells to grow without exogenously added trophic factors. These neuroblastoma cells grow neurites that establish contacts with epithelial cells. This preparation can serve as a model for investigating interactions among epithelial cells and between nerves and epithelial cells.

Topography of mammalian connexins in human skin

We have explored the expression of gap junction proteins in normal human skin by immunostaining cryostat sections (indirect immunofluorescence) or lyophilized epidermis (Western blotting) with antibodies against four mammalian connexins Cx26, Cx32, Cx40, Cx43; and by hybridizing total epidermal RNA (Northern blotting) with cRNA probes for Cx26, Cx32, and Cx43. We found that epidermal keratinocytes express Cx43 but not Cx26, Cx32, or Cx40. This expression was minimal in the basal layer, much higher in the spinous layer, reduced in the granular layer, and absent in the stratum corneum. Immunostaining for Cx43 was also observed in sebaceous glands, hairs, and eccrine sweat ducts. The two latter epidermal adnexae were also markedly labeled by antibodies against Cx26, a gap junction protein that was undetectable by immunofluorescence in interfollicular keratinocytes. Immunoblots of polyacrylamide gel electrophoresis-separated epidermal proteins and hybridization of epidermal RNA confirmed the presence of Cx43 in epidermis. These observations indicate that 1) Cx43 and Cx26 are components of human keratinocyte gap junctions; 2) these two proteins are differentially expressed in the interfollicular epidermis and the skin adnexae; 3) in interfollicular epidermis, Cx43 is a predominant gap junction protein, mostly expressed by the differentiating spinous cells; 4) Cx43 distribution is in accordance with the extensive dye coupling previously observed in this epidermal compartment.

Expression of gap junction proteins connexin 26 and 43 is modulated during differentiation of keratinocytes in newborn mouse epidermis

We examined the expression of the gap junction proteins connexin 26 (Cx26), 32 (Cx32), and 43 (Cx43) in keratinocytes of newborn mouse epidermis to elucidate which connexins are expressed in keratinocytes in intact skin of newborn mice, and whether the expression of connexins is modulated during terminal differentiation of keratinocytes. Immunofluorescent staining using antibodies against Cx26, Cx32, and Cx43 combined with type-specific anti-keratin immunohistochemistry showed that Cx26 was expressed in keratinocytes in the granular layer and in the upper part of the squamous layer, whereas Cx43 was localized in keratinocytes in the basal layer and in the lower part of the squamous layer. No specific staining of Cx32 was found in mouse epidermis. Double staining of Cx26 and Cx43 revealed that some keratinocytes in the squamous layer expressed both connexins, but that in most cases localization of the two kinds of connexins was different, i.e., Cx26 was localized on the upper surface, whereas Cx43 was present on the lower surface of the plasma membrane of keratinocytes. Northern and Western blot analyses confirmed that Cx26 and Cx43, but not Cx32, were expressed at mRNA and protein levels in newborn mouse skin. These results suggest that the modulation of connexin expression from Cx43 to Cx26 takes place during terminal differentiation of keratinocytes in mouse epidermis.