Changing patterns of gap junctional intercellular communication and connexin distribution in mouse epidermis and hair follicles during embryonic development

Connexin 43: Key roles in the skin

Gap junctions are tightly packed intercellular channels that serve a common purpose of allowing the intercellular exchange of small metabolites, second messengers and electrical signals. Connexins (Cxs) are gap junction proteins. Currently, 20 and 21 members of Cxs have been characterized in mice and humans, respectively. Connexin 43 (Cx43) is the most ubiquitously expressed type of Cx in the skin. It is produced by various different types of skin cell, such as keratinocytes, fibroblasts, endothelial and basal cells, melanocytes and dermal papilla cells. At present, more evidence indicates that Cx43 has an important role in skin repair and skin tumor development, as well as in skin cell invasion and metastasis. In this review, current knowledge regarding the regulation and function of Cx43 is summarized and the therapeutic potential of regulating Cx43 activity is discussed.

Connexin 26 (GJB2) mutation in an Argentinean patient with keratitis-ichthyosis-deafness (KID) syndrome: a case report

Background

Keratitis-Ichthyosis-Deafness (KID) syndrome is a rare condition characterized by pre-lingual sensorineural deafness with skin hyperkeratinization. The primary cause of the disease is a loss-of-function mutation in the GJB2 gene. Mutations in Argentinean patients have not been described.

Case presentation

We studied a 2 year-old boy with bilateral congenital sensorineural deafness with dry skin over the entire body, hypotrichosis of the scalp, thin and light-blond hair. Analysis of the GJB2 gene nucleotide sequence revealed the substitution of guanine-148 by adenine predicted to result in an Asp50Asn amino acid substitution.

Conclusion

This is the first KID report in a patient from Argentina. This de novo mutation proved to be the cause of keratitis-ichthyosis-deafness syndrome (KID-syndrome) in the patient, and has implications in medical genetic practice.

Connexins and pannexins in the integumentary system: the skin and appendages

The integumentary system comprises the skin and its appendages, which includes hair, nails, feathers, sebaceous and eccrine glands. In this review, we focus on the expression profile of connexins and pannexins throughout the integumentary system in mammals, birds and fish. We provide a picture of the complexity of the connexin/pannexin network illustrating functional importance of these proteins in maintaining the integrity of the epidermal barrier. The differential regulation and expression of connexins and pannexins during skin renewal, together with a number of epidermal, hair and nail abnormalities associated with mutations in connexins, emphasize that the correct balance of connexin and pannexin expression is critical for maintenance of the skin and its appendages with both channel and non-channel functions playing profound roles. Changes in connexin expression during both hair and feather regeneration provide suggestions of specialized communication compartments. Finally, we discuss the potential use of zebrafish as a model for connexin skin biology, where evidence mounts that differential connexin expression is involved in skin patterning and pigmentation.

Immunohistochemical demonstration of connexins in the developing feather follicle of the chicken

Based on immunohistochemistry, the study demonstrates the varying distribution and reaction intensity of connexins (Cx26 [chicken 31sim], 30 [chicken 31], 31, 32, 43, 45) in the developing feather follicle of the chicken (White Leghorn). The different embryonal stages were identified according to the normal table of Hamburger and Hamilton (1951). The development of the feather follicle complex is closely related to skin layer development, making use of the controlling function of connexins. This was evident during feather follicle differentiation, based on communication between ectomesodermal (fibroblasts) and ectodermal cells (developing epidermis), but also by the subsequent separation of the two cell line types related to their connexin-dependent differentiation degree. With the increase in mesenchymal cell numbers during feather placode development, the multiple connexins Cx26 [chicken 31sim] and 43, supported by Cx30 [chicken 31], 31 and 32, were increasingly activating the fibroblast concentrations as related to epidermal follicle buds, the specific follicle structure, the endothelial cells of capillaries and larger blood vessels, as well as the collagen fiber production and the growing feather musculature shortly before hatching; Cx45 could not be demonstrated. In conclusion, it seems that connexin expression is not only coupled to the origin of embryonic cells, but also connected with tissue formation before the follicle system can be formed.

Gap junction remodeling in skin repair following wounding and disease

In the present review, we provide an overview of connexin expression during skin development and remodeling in wound healing, and reflect on how loss- or gain-of-function connexin mutations may change cellular phenotypes and lead to diseases of the skin. We also consider the therapeutic value of targeting connexins in wound healing.

Hair phenotype in non-syndromic deafness

The GJB2 gene is located on chromosome 13q12 and it encodes the connexin 26, a transmembrane protein involved in cell-cell attachment of almost all tissues. GJB2 mutations cause autosomal recessive (DFNB1) and sometimes dominant (DFNA3) non-syndromic sensorineural hearing loss. Moreover, it has been demonstrated that connexins are involved in regulation of growth and differentiation of epidermal tissues. Hence, mutations in GJB2 gene, which is responsible for non-syndromic deafness, may be associated with an abnormal skin and hair phenotype. We analyzed hair samples from 96 subjects: a study group of 42 patients with hearing impairments of genetic origin (38 with a non-syndromic form, 4 with a syndromic form), and a control group including 54 people, i.e. 43 patients with other, non-genetic hearing impairments and 11 healthy volunteers aged up to 10 years old. The surface structure of 49 hair samples was normal, whereas in 45 cases it was altered, with a damaged appearance. Two hair samples were considered unclassifiable: one from the patient heterozygotic for the pendrin mutation (Fig. 2C), the other from a patient from Ghana with a R134W mutation (Fig. 2D). Among the 43 altered hair samples, 31 belonged to patients with connexin mutations and the other 12 came from patients without connexin mutations.

The G60S connexin43 mutant regulates hair growth and hair fiber morphology in a mouse model of human oculodentodigital dysplasia

Patients expressing mutations in the gene encoding the gap junction protein Cx43 suffer from a disease called oculodentodigital dysplasia (ODDD). Patients with ODDD are often reported to develop hair that is dry, dull, sparse, and slow growing. To evaluate the linkage between Cx43 and hair growth, structure, and follicle density we employed a mouse model of ODDD that harbors a Cx43 G60S point mutant. Regionally sparse and overall dull hair were observed in mutant mice compared with their wild-type (WT) littermates. However, histological analysis of overall hair follicle density in mutant and WT mice did not reveal any significant differences. After epilation, mutant mouse hair grew back slower, and hair growth was asynchronous. In addition, ultrastructural scanning electron microscopic imaging of hair fibers taken from mutant mice and two patients harboring the G143S mutation revealed severe cuticle weathering. Nodule formation was also observed in the proximal region of hair fibers taken from mutant mice. These results suggest that the G60S mutant mouse model mimics the hair phenotype found in at least some ODDD patients and suggests an important role for Cx43 in hair regeneration, growth, and cuticle formation.

The potency of the fs260 connexin43 mutant to impair keratinocyte differentiation is distinct from other disease-linked connexin43 mutants

Although there are currently 62 mutants of Cx43 (connexin43) that can cause ODDD (oculodentodigital dysplasia), only two mutants have also been reported to cause palmar plantar hyperkeratosis. To determine how mutants of Cx43 can lead to this skin disease, REKs (rat epidermal keratinocytes) were engineered to express an ODDD-associated Cx43 mutant always linked to skin disease (fs260), an ODDD-linked Cx43 mutant which has been reported to sometimes cause skin disease (fs230), Cx43 mutants which cause ODDD only (G21R, G138R), a mouse Cx43 mutant linked to ODDD (G60S), a non-disease-linked truncated Cx43 mutant that is trapped in the endoplasmic reticulum (Δ244*) or full-length Cx43. When grown in organotypic cultures, of all the mutants investigated, only the fs260-expressing REKs consistently developed a thinner stratum corneum and expressed lower levels of Cx43, Cx26 and loricrin in comparison with REKs overexpressing wild-type Cx43. REKs expressing the fs260 mutant also developed a larger organotypic vital layer after acetone-induced injury and exhibited characteristics of parakeratosis. Collectively, our results suggest that the increased skin disease burden exhibited in ODDD patients harbouring the fs260 mutant is probably due to multiple additive effects cause by the mutant during epidermal differentiation.

Differential expression and localization of connexins 26 and 43 in the rat gingival epithelium

We investigated the expression and localization of connexins (CX) 26 and 43 in the rat gingival epithelium. RT-PCR analysis revealed CX26 gene expression in both the upper and lower layers of the gingival epithelium and in the total epithelial layer, whereas CX43 gene expression was limited to the lower layer and the total epithelial layer. Immunoreactivity for CX43 was observed in the membranes of adjacent cells from the basal layer to the middle of the prickle cell layer, while immunoreactivity for CX26 was observed in the granular cell layer and lower part of the squamous cell layer. Merged images revealed the co-localization of CX26 and CX43 in the middle of the prickle cell layer. By immuno-electron microscopy, gap junctions appeared curved, hemi-circular, or annular within the cytoplasm, and gold particles indicating the presence of CX43 were localized at the outer edges of these cytoplasmic formations. These results suggest that CX43 is associated with the regulation of cell proliferation and that increased CX26 expression is associated with differentiation of keratinocytes. Thus, degradation of CX43 is considered to play an essential role in differentiation of the rat gingival epithelium.

A murine living skin equivalent amenable to live-cell imaging: analysis of the roles of connexins in the epidermis

Three-dimensional (3D) organotypic models are increasingly used to study the aspects of epidermal organisation and cutaneous wound-healing events. However, these are largely dependent on laborious histological analysis and immunohistochemical approaches. Despite the large resource of transgenic and knockout mice harboring mutations relevant to skin disorders, few organotypic mouse skin models are available. We have developed a versatile in vitro 3D organotypic mouse skin equivalent that reflects epidermal organisation in vivo. The system is optically transparent and ideally suited to real-time analysis using a variety of integrated in situ imaging techniques. As a paradigm for coordination of cellular events, the epidermal gap junction network was investigated and the model displayed predominant connexin 43 (Cx43) expression in basal proliferating cells and Cx26 and Cx30 expression in differentiated keratinocytes. We show that attenuation of Cx43-mediated communication by a Cx mimetic peptide enhanced wound closure rates in keratinocyte monocultures and in the living skin equivalent system, emphasising the utility of the model to systematically unravel the molecular mechanisms underlying epidermal morphogenesis, assess promising therapeutic strategies, and reduce animal experimentation. Furthermore, we visualise epidermal regeneration following injury in real time, thereby facilitating avenues to explore distinctive modes of wound re-epithelialisation in a non-invasive manner.

Connexin Levels Regulate Keratinocyte Differentiation in the Epidermis

To understand the role of connexin43 (Cx43) in epidermal differentiation, we reduced Cx43 levels by RNA-mediated interference knockdown and impaired its functional status by overexpressing loss-of-function Cx43 mutants associated with the human disease oculodentodigital dysplasia (ODDD) in rat epidermal keratinocytes. When Cx43 expression was knocked down by 50-75%, there was a coordinate 55-65% reduction in Cx26 level, gap junction-based dye coupling was reduced by 60%, and transepithelial resistance decreased. Importantly, the overall growth and differentiation of Cx43 knockdown organotypic epidermis was severely impaired as revealed by alterations in the levels of the differentiation markers loricrin and involucrin and by reductions in vital and cornified layer thicknesses. Conversely, although the expression of Cx43 mutants reduced the coupling status of rat epidermal keratinocytes by approximately 80% without altering the levels of endogenous Cx43 or Cx26, their ability to differentiate was not altered. In addition, we used a mouse model of ODDD and found that newborn mice harboring the loss-of-function Cx43(G60S) mutant had slightly reduced Cx43 levels, whereas Cx26 levels, epidermis differentiation, and barrier function remained unaltered. This properly differentiated epidermis was maintained even when Cx43 and Cx26 levels decreased by more than 70% in 3-week-old mutant mice. Our studies indicate that Cx43 and Cx26 collectively co-regulate epidermal differentiation from basal keratinocytes but play a more minimal role in the maintenance of established epidermis. Altogether, these studies provide an explanation as to why the vast majority of ODDD patients, where Cx43 function is highly compromised, do not suffer from skin disease.

Characterization of connexin30.3-deficient mice suggests a possible role of connexin30.3 in olfaction

We have generated connexin30.3-deficient mice in which the coding region of the connexin30.3 gene was replaced by the lacZ reporter gene. The expression pattern of this connexin was characterized using beta-galactosidase staining and immunoblot analyses. In skin, beta-galactosidase/connexin30.3 protein was expressed in the spinous and granulous layers of the epidermis. Specific beta-galactosidase/connexin30.3 expression was also detected in the thin ascending limb of Henle's loop in the kidney. In addition, we found beta-galactosidase/connexin30.3 in progenitor cells of the olfactory epithelium and in a subpopulation of cells in the apical layer of the vomeronasal organ. Connexin30.3-deficient mice were fertile and displayed no abnormalities in the skin or in the chemosensory systems. Furthermore, they showed normal auditory thresholds as measured by brain stem evoked potentials. These mice did, however, exhibit reduced behavioural responses to a vanilla scent.

Differentiation of Organotypic Epidermis in the Presence of Skin Disease-Linked Dominant-Negative Cx26 Mutants and Knockdown Cx26

In this study, we chose a differentiation-competent rat epidermal keratinocyte (REK) cell line to examine the role of Cx26 and disease-linked Cx26 mutants in organotypic epidermal differentiation. First, we generated stable REK cell lines expressing three skin disease-linked mutants (G59A, D66H and R75W). Second, we used an RNAi approach to knock down the expression of Cx26 in REKs. Interestingly, the three-dimensional (3D) architecture of the organotypic epidermis altered the intracellular spatial distribution of the mutants in comparison to 2D cultured REKs, highlighting the importance of using organotypic cultures. Unexpectedly, the presence of disease-linked mutants or the overexpression of wild-type Cx26 had little effect on the differentiation of the organotypic epidermis as determined by the architecture of the epidermis, expression of molecular markers indicative of epidermis differentiation (keratin 10, keratin 14, involucrin, loricrin) and stratification/cornification of the epidermis. Likewise, organotypic epidermis continued to differentiate normally upon Cx26 knockdown. While Cx26 has been reported to be upregulated during wound healing, no reduction in wound closure was observed in 2D REK cultures that expressed loss-of-function, dominant Cx26 mutants. In conclusion, we demonstrate that gain or loss of Cx26 function does not disrupt organotypic epidermal differentiation and offer insights into why patients harboring Cx26 mutations do not frequently present with more severe disease that encompasses thin skin.

Functional expression of connexin30 and connexin31 in the polarized human airway epithelium

Gap junctions are documented in the human airway epithelium but the functional expression and molecular identity of their protein constituents (connexins, Cx) in the polarized epithelium is not known. To address this question, we documented the expression of a family of epithelial Cx (Cx26, Cx30, Cx30.3, Cx31, Cx31.1, Cx32, Cx37, Cx40, and Cx43) in primary human airway epithelial cells (AEC) grown on porous supports. Under submerged conditions, AEC formed a monolayer of airway cells whereas the air-liquid interface induced within 30-60 days AEC differentiation into a polarized epithelium for up to 6-9 months. Maturation of AEC was associated with the down-regulation of Cx26 and Cx43. The well-differentiated airway epithelium exhibited gap junctional communication between ciliated and between ciliated and basal cells. Interestingly, Cx30 was mostly present between ciliated cells whereas Cx31 was found between basal cells. These results are supportive of the establishment of signal-selective gap junctions with maturation of AEC, likely contributing to support airway epithelium function. These results lay the ground for studying the role of Cx-mediated cell-cell communication during repair following AEC injury and exploring Cx-targeted interventions to modulate the healing process.

Connexin 26 regulates epidermal barrier and wound remodeling and promotes psoriasiform response

Inflammatory skin disorders result in significant epidermal changes, including keratinocyte hyperproliferation, incomplete differentiation, and impaired barrier. Here we test whether, conversely, an impaired epidermal barrier can promote an inflammatory response. Mice lacking the transcription factor Kruppel-like factor 4 (Klf4) have a severe defect in epidermal barrier acquisition. Transcription profiling of Klf4(-/-) newborn skin revealed similar changes in gene expression to involved psoriatic plaques, including a significant upregulation of the gap junction protein connexin 26 (Cx26). Ectopic expression of Cx26 from the epidermis-specific involucrin (INV) promoter (INV-Cx26) demonstrated that downregulation of Cx26 is required for barrier acquisition during development. In juvenile and adult mice, persistent Cx26 expression kept wounded epidermis in a hyperproliferative state, blocked the transition to remodeling, and led to an infiltration of immune cells. Mechanistically, ectopic expression of Cx26 in keratinocytes resulted in increased ATP release, which delayed epidermal barrier recovery and promoted an inflammatory response in resident immune cells. These results provide a molecular link between barrier acquisition in utero and epidermal remodeling after wounding. More generally, these studies suggest that the most effective treatments for inflammatory skin disorders might concomitantly suppress the immune response and enhance epidermal differentiation to restore the barrier.

GJB2 mutations and degree of hearing loss: a multicenter study

Hearing impairment (HI) affects 1 in 650 newborns, which makes it the most common congenital sensory impairment. Despite extraordinary genetic heterogeneity, mutations in one gene, GJB2, which encodes the connexin 26 protein and is involved in inner ear homeostasis, are found in up to 50% of patients with autosomal recessive nonsyndromic hearing loss. Because of the high frequency of GJB2 mutations, mutation analysis of this gene is widely available as a diagnostic test. In this study, we assessed the association between genotype and degree of hearing loss in persons with HI and biallelic GJB2 mutations. We performed cross-sectional analyses of GJB2 genotype and audiometric data from 1,531 persons, from 16 different countries, with autosomal recessive, mild-to-profound nonsyndromic HI. The median age of all participants was 8 years; 90% of persons were within the age range of 0-26 years. Of the 83 different mutations identified, 47 were classified as nontruncating, and 36 as truncating. A total of 153 different genotypes were found, of which 56 were homozygous truncating (T/T), 30 were homozygous nontruncating (NT/NT), and 67 were compound heterozygous truncating/nontruncating (T/NT). The degree of HI associated with biallelic truncating mutations was significantly more severe than the HI associated with biallelic nontruncating mutations (P<.0001). The HI of 48 different genotypes was less severe than that of 35delG homozygotes. Several common mutations (M34T, V37I, and L90P) were associated with mild-to-moderate HI (median 25-40 dB). Two genotypes--35delG/R143W (median 105 dB) and 35delG/dela(GJB6-D13S1830) (median 108 dB)--had significantly more-severe HI than that of 35delG homozygotes.

Expression and function of connexins in the epidermis, analyzed with transgenic mouse mutants

Eight different connexins are expressed in mouse epidermis with overlapping expression patterns in different epidermal layers. Analyses of mice with deficiency or modifications of distinct connexins yielded insights into the large variety of connexins in the epidermis. Connexin43 (Cx43) deficiency in mouse epidermis resulted in a significant acceleration of wound closure. Truncation by 125 amino acid residues of the Cx43 C-terminal region led to an altered epidermal expression pattern of Cx43 and defective development of the epidermal water barrier in transgenic mice, although the truncated Cx43 protein could still form open gap junctional channels in transfected HeLa cells. Thus, the phenotypic abnormalities observed in mice with truncated Cx43 protein (Cx43K258Stop) are more likely due to defective regulation of this protein rather than the closed Cx43 channel. Our studies of connexin-deficient mice revealed an extensive redundancy of connexins expressed in mouse epidermis. Epidermal connexins seem to form two functional groups in which deficiency of one connexin isoform can be compensated by other connexin isoforms of the same group.

Liver cell lines for the study of hepatocyte functions and immunological response

BACKGROUND

Liver cell lines closely resembling primary hepatocyte are essential for research on hepatitis viruses and hepatocyte function. Currently used cell lines are derived from hepatic tumours and have altered gene expression.

AIMS

The generation and characterisation of novel human hepatocyte lines (HHLs) derived from healthy human liver, retaining the primary hepatocyte phenotype.

RESULTS

Primary hepatocytes were immortalised with Moloney's mouse leukaemia virus expressing E6 and E7 proteins of human papillomavirus, and cultures propagated long-term. All HHLs contained markers of hepatocyte and biliary phenotype (cytokeratins 7, 8, 18 and 19), Cytochrome P450 and albumin. The HHLs did not express high levels of p53 or alpha-fetoprotein. When grown in a collagen sandwich culture, or at the air-liquid interface, HHLs were maintained as monolayer whereas Huh-7 and HepG2 formed thick layers. All HHLs showed increased capacity to bind recombinant hepatitis C virus-like particles in comparison with Huh-7 and HepG2. We also demonstrate that HHLs contained active gap junctions, and that the cells respond to stimulation with IFN-alpha by upregulation of major histocompatibility complex (MHC)-I and -II.

CONCLUSIONS

These HHLs retain primary hepatocyte phenotype and should be useful for investigating mechanisms of entry and replication of hepatotropic viruses, and should also be valuable in the study of hepatocyte biology and pathology.

Novel Connexin 43 (GJA1) mutation causes oculo-dento-digital dysplasia with curly hair

Oculo-dento-digital dysplasia (ODDD) [OMIM 164200] is a rare autosomal dominant pleiotropic disorder comprising ocular, craniofacial, and digital anomalies, caused by mutations in the gap junction alpha-1 gene (GJA1 or Connexin 43 (CX43)) [Paznekas et al., 2003]. In a Danish family affected over five generations, we found a novel mutation, 286G --> A, resulting in Val96Met. We provide an easy method for mutation detection by use of the restriction enzyme Nde1 and discuss possible pathogenetic mechanisms, arguing that loss of function cannot be excluded. This is the second article reporting ODDD mutations.

Directing stem cells into the keratinocyte lineage in vitro

A major area of research in regenerative medicine is the potential application of stem cells in skin grafting and tissue engineering. This would require well defined and efficient protocols for directing the commitment and differentiation of stem cells into the keratinocyte lineage, together with their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying skin tissue biology, as well as facilitate the genetic manipulation of stem cells for therapeutic applications. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for skin-related biomaterials and drugs could also utilize protocols developed for the commitment and differentiation of stem cells into the keratinocyte lineage. Hence, this review critically examines the various strategies that could be employed to direct the commitment and differentiation of stem cells into the keratinocyte lineage in vitro.

The effects of a mutant connexin 26 on epidermal differentiation

To elucidate the mode of action of dominant mutant connexins in causing inherited skin diseases, transgenic mice were produced that express the true Vohwinkel syndrome-associated mutant Cx26 (D66H), from a keratin 10 promoter, specifically in the suprabasal epidermal keratinocytes. Following birth, the transgenic mice developed keratoderma similar to that of human carriers of Cx26 (D66H). Expression of the transgene resulted in a loss of Cx26 and Cx30 at intercellular junctions of epidermal keratinocytes and accumulation of these connexins in the cytoplasm. Injection of primary mouse keratinocytes with Lucifer Yellow showed no difference in terms of dye spreading between transgenic and non transgenic keratinocytes in vitro. Expression of the mutant Cx26 (D66H) did not interfere with the formation of the epidermal water barrier during late embryonic development. Attempts to produce transgenic mice expressing the wild type form of Cx26 from the K10 promoter failed to produce viable animals although transgenic embryos were recovered at days 9 and 12 of gestation, suggesting that the transgene might be embryonic lethal.

CONNEXINS AND CELL SIGNALING IN DEVELOPMENT AND DISEASE

Gap junctions contain hydrophilic membrane channels that allow direct communication between neighboring cells through the diffusion of ions, metabolites, and small cell signaling molecules. They are made up of a hexameric array of polypeptides encoded by the connexin multi-gene family. Cell-cell communication mediated by connexins is crucial to various cellular functions, including the regulation of cell growth, differentiation, and development. Mutations in connexin genes have been linked to a variety of human diseases, including cardiovascular anomalies, peripheral neuropathy, deafness, skin disorders, and cataracts. In addition to their coupling function, recent studies suggest that connexin proteins may also mediate signaling. This could involve interactions with other protein partners that may play a role not only in connexin assembly, trafficking, gating and turnover, but also in the coordinate regulation of cell-cell communication with cell adhesion and cell motility. The integration of these cell functions is likely to be important in the role of gap junctions in development and disease.

An expression atlas of connexin genes in the mouse

Connexin genes are involved in several human diseases such as hearing and dermatological and peripheral nerve disorders. Connexins are protein units of gap junctions and form homotypic, heterotypic, or heteromeric complexes known as connexons. Data on the expression patterns of members of this family are partial and fragmentary. We therefore cloned all the identifiable murine homologs of human CONNEXIN genes and analyzed their expression patterns in embryonic and neonatal mouse tissues. We found that connexins are preferentially expressed in tissues derived from ectoderm and/or endoderm. Our data provide a comprehensive and detailed atlas of expression of connexin genes and in some cases suggest possible interactions of proteins that are coexpressed in the same tissue. Knowledge of temporal and spatial distribution of connexins also allows the identification of candidate genes for human diseases and provides important insight into mechanisms that lead to human disorders due to mutations in CONNEXIN genes.

Defective epidermal barrier in neonatal mice lacking the C-terminal region of connexin43

More than 97% of mice in which the C-terminal region of connexin43 (Cx43) was removed (designated as Cx43K258stop) die shortly after birth due to a defect of the epidermal barrier. The abnormal expression of Cx43K258stop protein in the uppermost layers of the epidermis seems to perturb terminal differentiation of keratinocytes. In contrast to Cx43-deficient mice, neonatal Cx43K258stop hearts show no lethal obstruction of the right ventricular outflow tract, but signs of dilatation. Electrocardiographies of neonatal hearts reveal repolarization abnormalities in 20% of homozygous Cx43K258stop animals. The very rare adult Cx43K258stop mice show a compensation of the epidermal barrier defect but persisting impairment of cardiac function in echocardiography. Female Cx43K258stop mice are infertile due to impaired folliculogenesis. Our results indicate that the C-terminally truncated Cx43K258stop mice lack essential functions of Cx43, although the truncated Cx43 protein can form open gap junctional channels.

Functional Domain Mapping and Selective Trans-dominant Effects Exhibited by Cx26 Disease-causing Mutations

Mutations in Cx26 are a major cause of autosomal dominant and recessive forms of sensorineural deafness. Some mutations in Cx26 are associated not only with deafness but also with skin disease. We examined the subcellular localization and function of two green fluorescent protein (GFP)-tagged Cx26 point mutants that exhibit both phenotypes, G59A-GFP and D66H-GFP. D66H-GFP was retained within the brefeldin A-insensitive trans-Golgi network, whereas a population of G59A-GFP was transported to the cell surface. Neither G59A nor D66H formed gap junctions that were permeable to small fluorescent dyes, suggesting they are loss-of-function mutations. When co-expressed with wild-type Cx26, both G59A and D66H exerted dominant-negative effects on Cx26 function. G59A also exerted a trans-dominant negative effect on co-expressed wild type Cx32 and Cx43, whereas D66H exerted a trans-dominant negative effect on Cx43 but not Cx32. We propose that the severity of the skin disease is dependent on the specific nature of the Cx26 mutation and the trans-dominant selectivity of the Cx26 mutants on co-expressed connexins. Additional systematic mutations at residue D66, in which the overall charge of this motif was altered, suggested that the first extracellular loop is critical for Cx26 transport to the cell surface as well as function of the resulting gap junction channels.

Gap junction development in the human fetal hair follicle and bulge region

BACKGROUND

Gap junctions, composed of connexin (Cx) subunits, are channels that allow intercellular communication between adjacent cells and are thought to play a key role in the regulation of cell proliferation and differentiation. The Cx expression pattern and formation of gap junctions in human fetal hair follicles has yet to be clarified, including the prominent follicular bulge region that is believed to be a site rich in stem cells.

OBJECTIVES

To study the expression of two major Cxs, Cx26 and Cx43, in developing hair follicles in skin samples from a series of human fetuses of estimated gestational age (EGA) 88-163 days, and to determine quantitatively the presence of gap junctions.

METHODS

We used immunofluorescence labelling to investigate the sequential expression pattern of Cx26 and Cx43 in developing human hair follicles. Gap junction formation was observed by electron microscopy and the numbers of gap junctions were analysed quantitatively. Results Both Cx26 and Cx43 expression were observed at 88 days' EGA in the inner part of the hair peg. At 135 days' EGA, Cx26 was expressed in the outer root sheath (ORS) and the inner root sheath (IRS), while Cx43 was expressed chiefly in the IRS, hair matrix and sebaceous glands. At 163 days' EGA, Cx26 expression was most intense in the outermost layer of the ORS, in contrast to Cx43 expression which was in the inner part of the ORS. In the bulge region, only Cx43 was expressed in a subset of cells in the bulge. Ultrastructurally, gap junctions were observed at 102 days' EGA in the hair peg, and the number of gap junctions increased as the hair follicle matured. Gap junctions were also observed between the bulge cells in considerable numbers.

CONCLUSIONS

The changing expression patterns of Cx26 and Cx43 and the increasing gap junction numbers suggest a close association of Cx expression and gap junction formation with hair follicle morphogenesis. In addition, the present ultrastructural observations demonstrate that considerable numbers of the bulge cells, a putative site rich in hair follicle stem cells, form gap junctions during human hair follicle development.

Altered connexin expression and wound healing in the epidermis of connexin-deficient mice

To analyze the effect of connexin loss on the repair of wounded tail skin, we have studied the following transgenic mouse mutants: connexin30-/-, connexin31-/- and connexin43Cre-ER(T)/fl (for inducible deletion of the connexin43 coding region). Connexin43 and connexin31 are expressed in the basal and spinous layers of wild-type epidermis, whereas connexin31 and small amounts of connexin30, as well as connexin26 proteins, were found in the granulous layer. Connexin43 was downregulated in connexin31-deficient mice, whereas mice with reduced connexin43 exhibited an upregulation of connexin30. During wound healing, connexin30 and connexin26 proteins were upregulated in all epidermal layers, whereas connexin43 and connexin31 protein expression were downregulated. In connexin31-/- mice, reduced levels of connexin30 protein were observed on days 1 and 2 after wounding. The closure of epidermal wounds in mice with decreased amounts of connexin43 protein occurred one day earlier. Under these conditions the expression profiles of connexin30 and connexin31 were also temporarily shifted by one day. Furthermore, dye transfer between keratinocytes in skin sections from connexin43-deficient mice was decreased by 40%. These results suggest that downregulation of connexin43 appears to be a prerequisite for the coordinated proliferation and mobilization of keratinocytes during wound healing.

Communication network in the follicular papilla and connective tissue sheath through gap junctions in human hair follicles

Epithelial-mesenchymal interactions play a crucial role in the induction of life-long cyclic transformations of hair follicles. Many studies have already demonstrated several candidates for the soluble factors secreted from the mesenchymal components of the hair follicle, i.e. the follicular papilla (FP) and connective tissue sheath (CTS), which may be responsible for hair cycling. In this paper, we focused on cell-cell contact between FP cells (FPCs), between CTS cells (CTSCs), and between FPCs and CTSCs that may allow these mesenchymal components to function as a syncytium during hair cycling. Electron microscopic examination of the FP and the CTS obtained from human scalp revealed a tri-lamellar structure of the plasma membranes, which is a characteristic of gap junctions at the cell-cell contacting area. The immunohistochemical study with anticonnexin 43 Ab using a confocal laser scanning microscope demonstrated numerous spotted positive signals scattered throughout the FP. In the CTS, spotted positive signals were arranged linearly along the basement membrane of the hair follicle. In particular, these positive spots were aggregated in the transitional region between the FP and the CTS. By Western blot analysis of total protein extracts from the cultured FPCs and neonatal human dermal fibroblasts using anticonnexin 43 antibody, a positive band corresponding to connexin 43 was detected at 43 kDa on both the FPC lane and fibroblast lane. These findings suggest that the FP and the CTS form a communicating network through gap junctions, which may play a role in controlling the dynamic structural changes of hair follicles during hair cycling.

Changes in gap junction distribution and connexin expression pattern during human fetal skin development

Gap junctions are intercellular channels composed of connexin subunits that mediate cell-cell communication. The functions of gap junctions are believed to be associated with cell proliferation and differentiation and to be important in maintaining tissue homeostasis. We therefore investigated the expression of connexins (Cx)26 and 43, the two major connexins in human epidermis, and examined the formation of gap junctions during human fetal epidermal development. By immunofluorescence, Cx26 expression was observed between 49 and 96 days' estimated gestational age (EGA) but was not present from 108 days' EGA onwards. Conversely, Cx43 expression was observed from 88 days' EGA onwards. Using electron microscopy, the typical structure of gap junctions was observed from 120 days' EGA. The number of gap junctions increased over time and they were more common in the upper layers, within the periderm and intermediate keratinocyte layers rather than the basal layer. Immunoelectron microscopy revealed Cx43 labeling on the gap junction structures after 105 days' EGA. Formation of gap junctions increased as skin developed, suggesting that gap junctions may play an important role in fetal skin development. Furthermore, the changing patterns of connexin expression suggest that Cx26 is important for early fetal epidermal development.

Polyamines regulate gap junction communication in connexin 43-expressing cells

The control of cell-cell communication through gap junctions is thought to be crucial in normal tissue function and during various stages of tumorigenesis. However, few natural regulators of gap junctions have been found. We show here that increasing the activity of ornithine decarboxylase, or adding polyamines to the outside of cells, increases the level of gap junction communication between various epithelial cells. Conversely, reduction of ornithine decarboxylase activity decreases the level of gap junction communication. This regulation is dependent upon the expression of connexin 43 (Cx43 or Cxalpha1), which is a major connexin expressed in many different cell types, and involves an increase in Cx43 and its cellular re-distribution.

trans-dominant inhibition of connexin-43 by mutant connexin-26: implications for dominant connexin disorders affecting epidermal differentiation

Dominant mutations of GJB2-encoding connexin-26 (Cx26) have pleiotropic effects, causing either hearing impairment (HI) alone or in association with palmoplantar keratoderma (PPK/HI). We examined a British family with the latter phenotype and identified a new dominant GJB2 mutation predicted to eliminate the amino acid residue E42 (ΔE42) in Cx26. To dissect the pathomechanisms that result in diverse phenotypes of dominant GJB2 mutations, we studied the effect of three Cx26 mutants (ΔE42, D66H and R75W) identified in individuals with PPK/HI, and another (W44C) present in individuals with non-syndromic HI on gap junctional intercellular communication. We expressed mutant Cx26 alone and together with the epidermal connexins Cx26, Cx37 and Cx43 in paired Xenopus oocytes, and measured the intercellular coupling by dual voltage clamping. Homotypic expression of each connexin as well as co-expression of wild-type (wt) Cx26/wtCx43 and wtCx26/wtCx37 yielded variable, yet robust, levels of channel activity. However, all four Cx26 mutants were functionally impaired and failed to induce intercellular coupling. When co-expressed with wtCx26, all four mutants suppressed the wtCx26 channel activity consistent with a dominant inhibitory effect. However, only those Cx26 mutants associated with a skin phenotype also significantly (P&lt;0.05) inhibited intercellular conductance of co-expressed wtCx43, indicating a direct interaction of mutant Cx26 units with wtCx43. These results demonstrate, for the first time, a trans-dominant negative effect of Cx26 mutants in vitro. Furthermore, they support a novel concept suggesting that the principal mechanism for manifestation of dominant GJB2 mutations in the skin is their dominant interference with the function of wtCx43. This assumption is further corroborated by our finding that Cx26 and Cx43 focally colocalize at gap junctional plaques in affected skin tissue of two carriers of ΔE42.

Controls of hair follicle cycling

Nearly 50 years ago, Chase published a review of hair cycling in which he detailed hair growth in the mouse and integrated hair biology with the biology of his day. In this review we have used Chase as our model and tried to put the adult hair follicle growth cycle in perspective. We have tried to sketch the adult hair follicle cycle, as we know it today and what needs to be known. Above all, we hope that this work will serve as an introduction to basic biologists who are looking for a defined biological system that illustrates many of the challenges of modern biology: cell differentiation, epithelial-mesenchymal interactions, stem cell biology, pattern formation, apoptosis, cell and organ growth cycles, and pigmentation. The most important theme in studying the cycling hair follicle is that the follicle is a regenerating system. By traversing the phases of the cycle (growth, regression, resting, shedding, then growth again), the follicle demonstrates the unusual ability to completely regenerate itself. The basis for this regeneration rests in the unique follicular epithelial and mesenchymal components and their interactions. Recently, some of the molecular signals making up these interactions have been defined. They involve gene families also found in other regenerating systems such as fibroblast growth factor, transforming growth factor-beta, Wnt pathway, Sonic hedgehog, neurotrophins, and homeobox. For the immediate future, our challenge is to define the molecular basis for hair follicle growth control, to regenerate a mature hair follicle in vitro from defined populations, and to offer real solutions to our patients' problems.

Differential expression of connexins during stratification of human keratinocytes

To assess whether gap junctions and connexins change during keratinocyte differentiation, we have studied epidermal equivalents obtained in organotypic cultures of keratinocytes from the outer root sheath of human hair follicles. These reconstituted tissues exhibit a number of differentiation and proliferation markers of human epidermis, including gap junctions, connexins, and K6 and Ki67 proteins. Immunostaining and northern blots showed that gap junctions of the epidermal equivalents were made of Cx26 and Cx43. Cx26 was expressed in all keratinocyte layers, throughout the development of the epidermal equivalents. In contrast, Cx43 was initially observed only in the basal layer of keratinocytes and became detectable in the stratum spinosum and granulosum only after the epidermal equivalents had thickened. The levels of Cx26 and its transcript markedly increased as a function of stratification of the epidermal equivalents, whereas those of Cx43 remained almost constant. Microinjection of Lucifer Yellow into individual keratinocytes showed that gap junctions were similarly permeable at all stages of development of the epidermal equivalents. The data show that epidermal equivalents (i) feature a pattern of connexins typical of an actively renewing human interfollicular epidermis, and (ii) provide a model that reproduces the tridimensional organization of intact epidermis and that is amenable for experimentally testing the function of junctional communication between human keratinocytes.

Induction of gap junctional intercellular communication, connexin43 expression, and subsequent differentiation in human fetal neuronal cells by stimulation of the cyclic AMP pathway

Expression of gap junction proteins and cell-cell communication was studied in the human neural-glial cell line, SVG, as a first step in defining whether the SVG cells could be used as a model system to study the role of gap junctions in neuronal precursor cells. SVG cells were found to express connexin43 protein that co-migrated with WB-F344 rat liver connexin43 and that reacted with connexin43-specific antibodies on Western blots. However, fluorescence recovery after photobleaching analysis of 5,6-carboxyfluorescein-loaded cells failed to show significant dye coupling. Agents that stimulate the adenylyl cyclase/cAMP pathway were used to induce gap junctional intercellular communication in the SVG cultures. A 24-48 h treatment of SVG cells with 5 microM forskolin or 5 microM forskolin + 200 microM 3-isobutyl-1-methylxanthine increased the percentage of dye-coupled cells from 5-65%, using the fluorescent recovery after photobleaching method. The increase in dye coupling induced by forskolin or forskolin + 3-isobutyl-1-methylxanthine was inhibited by octanol, which is known to block gap junction-mediated cell communication. Western blot analysis of total protein extracts revealed the appearance of a higher molecular weight connexin43 protein band after treatment of SVG cells with forskolin or forskolin + 3-isobutyl-1-methylxanthine, that was not observed in vehicle-treated controls. Alkaline phosphatase treatment of total protein extracts from forskolin or forskolin + 3-isobutyl-1-methylxanthine-treated cells reduced the higher molecular weight band to approximately 41,000 the same as observed in the control extracts. The alkaline phosphatase treatment demonstrates that the higher molecular weight band was due to a phosphorylation event stimulated by forskolin or the forskolin + 3-isobutyl-1-methylxanthine combination. In addition, treatment of the SVG cells with the forskolin or forskolin + 3-isobutyl-l-methylxanthine stimulated outgrowth of neurite-like processes from the cell body which immunostained positive for the connexin43 protein as well as protein markers for neurons and oligodendrocytes. We hypothesize that the SVG cells may represent a neuronal progenitor cell population that has the ability to differentiate when exposed to the appropriate signals.

Signal transduction pathways controlling the switch between keratinocyte growth and differentiation

Self-renewing epithelia are characterized by a high turnover rate and a fine balance between growth and differentiation. Such a balance is influenced by many exogenous factors, including gradients of diffusible molecules, cell/substrate adhesion contacts, and direct cell-cell communication. The inter-connection between these various extracellular signals and underlying intracellular pathways is clearly of great interest. Primary keratinocytes of either human or murine origin provide an ideal experimental system to elucidate early signaling events involved in the control of epithelial differentiation. Relative to established cell lines, use of a primary system eliminates the possibility of alterations in critical regulatory events which may occur during prolonged propagation in culture. Primary keratinocytes are easily grown in large numbers, and their differentiation can be induced under well-defined culture conditions. The ensuing rapid and homogeneous response is amenable to careful biochemical analysis. Gene transfer technology (transient transfections, adenoviral and retroviral vectors), together with the use of keratinocytes derived from gene knockout and transgenic mice, makes it possible to assess the specific contribution of individual genes to the control of the differentiation process. This review focuses on the significant progress that has been made over the last few years in our understanding of the specific signals that trigger keratinocyte differentiation, the underlying signaling pathways, and how they impinge on specific transcription and cell-cycle control mechanisms associated with the onset of keratinocyte differentiation. Recent developments and future directions in this important area of research will be highlighted.

What controls hair follicle cycling?

Despite more than a hundred years of professional hair research, and substantial recent progress in unravelling the molecular controls of hair follicle morphogenesis, the chronobiological control system that cyclically drives the hair follicle through dramatic remodelling processes between phases of growth (anagen), regression (catagen), and relative resting (telogen) have remained disappointingly obscure. In view of the vast literature that has become available over the past decades on numerous genetic, biochemical, cellular and pharmacological aspects of hair growth follicle control under physiological and pathological conditions, it is astounding how comparatively few researchers in the field have published theoretical concepts that explore how hair follicle cycling might be controlled. Since this question is at the very heart of basic and clinically applied hair biology, it deserves a much more systematic and serious public exploration, which the following contributions are designed to stimulate.

Mitogen-activated protein kinase and phosphorylation of connexin43 are not sufficient for the disruption of gap junctional communication by platelet-derived growth factor and tetradecanoylphorbol acetate

Disruption of gap junctional communication (GJC) by various compounds, including growth factors and tumor promoters, is believed to be modulated by the phosphorylation of a gap junctional protein, connexin43 (Cx43). We have previously demonstrated a platelet-derived growth factor (PDGF)-induced blockade of GJC and phosphorylation of Cx43 in T51B rat liver epithelial cells expressing wild-type PDGF receptor beta (PDGFr beta). Both of these actions of PDGF required participation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). Similar requirements of MAPK were suggested in the modulation of GJC by other agents, including epidermal growth factor (EGF) and lysophosphatidic acid (LPA). Since many of these agents activate additional protein kinases, our present study examined whether activation of MAPK was sufficient for Cx43 phosphorylation and GJC blockade. By utilizing a variety of MAPK activators, we now show that activation of MAPK is not always associated with either Cx43 phosphorylation or disruption of GJC, which suggests a requirement for additional factors. Furthermore, pretreatment with hydrogen peroxide (H2O2), a potent MAPK activator but inefficient GJC/Cx43 modulator, abrogated PDGF- or TPA-induced disruption of GJC. While a 5 min H2O2 pretreatment abolished both PDGF- and TPA-induced Cx43 phosphorylation and GJC blockade, a simultaneous H2O2 treatment interfered only with GJC closure but not with the phosphorylation of Cx43 induced by PDGF and TPA. This finding indicates that, in addition to the Cx43 phosphorylation step, inhibition of GJC requires interaction with other components. H2O2-mediated abrogation of PDGF/TPA signaling can be neutralized by the antioxidant N-acetylcysteine (NAC) or by the tyrosine kinase inhibitor genistein. Taken together, our results suggest that disruption of GJC is not solely mediated by either activated MAPK or Cx43 phosphorylation but requires the participation of additional kinases and regulatory components. This complex mode of regulation is perhaps essential for the proposed functional role of GJC.

Upregulation of connexin 26 is a feature of keratinocyte differentiation in hyperproliferative epidermis, vaginal epithelium, and buccal epithelium

In epidermis, it has been suggested, intercellular communication through gap junctions is important in coordinating cell behavior. The connexins, may facilitate selective assembly or permeability of gap junctions, influencing the distribution of metabolites between cells. Using immunohistochemistry, we have compared the distribution of connexins 26 and 43 with that of proliferating cells (Ki67 labeling) in normal epidermis, hyperplastic epidermis (tape-stripped epidermis, psoriatic lesions, and viral warts), and vaginal and buccal epithelia. Connexin 43 was abundant in spinous layers of all epidermal specimens and in vaginal and buccal epithelia. Connexin 26 was absent from the interfollicular and interductal epidermis of normal hair-bearing skin, and nonlesional psoriatic epidermis but present at very low levels in plantar epidermis. Connexin 26 was prominent in lesional psoriatic epidermis and viral warts and in vaginal and buccal epithelia. In three independent experiments connexin 26 appeared in a patchy intercellular distribution in the basal epidermis within 24 h of tape stripping, proceeding to more extensive distribution in basal and suprabasal layers by 48 h. The increase in connexin 26 preceded that in cell proliferation. In vaginal epithelium, buccal epithelium, and viral warts connexin 26 was restricted mainly to suprabasal, nonproliferating cells. In psoriatic lesional epidermis connexin 26 was also located mainly in suprabasal, nonproliferating cells. Connexin 26 was present in a patchy distribution in the basal layer of psoriatic lesional epidermis, but double labeling for connexin 26 and Ki67 showed that many connexin 26 positive basal cells were nonproliferative, suggesting that connexin 26 may be related to differentiation rather than to proliferation. These observations would be consistent with a role for connexin 26 containing gap junctions during both early and later stages of keratinocyte differentiation in hyperplastic epidermis and in vaginal and buccal epithelia.