Comparative genomics suggests loss of keratin K24 in three evolutionary lineages of mammals

Keratins are the main cytoskeletal proteins of epithelial cells and changes in the expression of keratins have contributed to the evolutionary adaptation of epithelia to different environments. Keratin K24 was proposed to be a differentiation marker of epidermal keratinocytes but the significance of K24 expression in the epidermis versus other tissues has remained elusive. Here, we show by RT-PCR, western blot, and immunofluorescence analyses that K24 is highly expressed in the epithelium of the cornea whereas its expression levels are significantly lower in other stratified epithelia including in the epidermis. To investigate the evolutionary history of K24, we screened the genome sequences of vertebrates for orthologs of the human KRT24 gene. The results of this comparative genomics study suggested that KRT24 originated in a common ancestor of amniotes and that it was lost independently in three clades of mammals, i.e. camels, cetaceans, and a subclade of pinnipeds comprising eared seals and the walrus. Together, the results of this study identify K24 as component of the cytoskeleton in the human corneal epithelium and reveal previously unknown differences of keratin gene content among mammalian species.

Differential Evolution of the Epidermal Keratin Cytoskeleton in Terrestrial and Aquatic Mammals

Keratins are the main intermediate filament proteins of epithelial cells. In keratinocytes of the mammalian epidermis they form a cytoskeleton that resists mechanical stress and thereby are essential for the function of the skin as a barrier against the environment. Here, we performed a comparative genomics study of epidermal keratin genes in terrestrial and fully aquatic mammals to determine adaptations of the epidermal keratin cytoskeleton to different environments. We show that keratins K5 and K14 of the innermost (basal), proliferation-competent layer of the epidermis are conserved in all mammals investigated. In contrast, K1 and K10, which form the main part of the cytoskeleton in the outer (suprabasal) layers of the epidermis of terrestrial mammals, have been lost in whales and dolphins (cetaceans) and in the manatee. Whereas in terrestrial mammalian epidermis K6 and K17 are expressed only upon stress-induced epidermal thickening, high levels of K6 and K17 are consistently present in dolphin skin, indicating constitutive expression and substitution of K1 and K10. K2 and K9, which are expressed in a body site-restricted manner in human and mouse suprabasal epidermis, have been lost not only in cetaceans and manatee but also in some terrestrial mammals. The evolution of alternative splicing of K10 and differentiation-dependent upregulation of K23 have increased the complexity of keratin expression in the epidermis of terrestrial mammals. Taken together, these results reveal evolutionary diversification of the epidermal cytoskeleton in mammals and suggest a complete replacement of the quantitatively predominant epidermal proteins of terrestrial mammals by originally stress-inducible keratins in cetaceans.

Mitotic Diversity in Homeostatic Human Interfollicular Epidermis

Despite decades of skin research, regulation of proliferation and homeostasis in human epidermis is still insufficiently understood. To address the role of mitoses in tissue regulation, we utilized human long-term skin equivalents and systematically assessed mitoses during early epidermal development and long-term epidermal regeneration. We now demonstrate four different orientations: (1) horizontal, i.e., parallel to the basement membrane (BM) and suggestive of symmetric divisions; (2) oblique with an angle of 45°-70°; or (3) perpendicular, suggestive of asymmetric division. In addition, we demonstrate a fourth substantial fraction of suprabasal mitoses, many of which are committed to differentiation (Keratin K10-positive). As verified also for normal human skin, this spatial mitotic organization is part of the regulatory program of human epidermal tissue homeostasis. As a potential marker for asymmetric division, we investigated for Numb and found that it was evenly spread in almost all undifferentiated keratinocytes, but indeed asymmetrically distributed in some mitoses and particularly frequent under differentiation-repressing low-calcium conditions. Numb deletion (stable knockdown by CRISPR/Cas9), however, did not affect proliferation, neither in a three-day follow up study by life cell imaging nor during a 14-day culture period, suggesting that Numb is not essential for the general control of keratinocyte division.

Keratins K2 and K10 are essential for the epidermal integrity of plantar skin

BACKGROUND

K1 and K2 are the main type II keratins in the suprabasal epidermis where each of them heterodimerizes with the type I keratin K10 to form intermediate filaments. In regions of the ears, tail, and soles of the mouse, only K2 is co-expressed with K10, suggesting that these keratins suffice to form a mechanically resilient cytoskeleton.

OBJECTIVE

To determine the effects of the suppression of both main keratins, K2 and K10, in the suprabasal plantar epidermis of the mouse.

METHODS

Krt2(-/-) Krt10(-/-) mice were generated by crossing Krt2(-/-) and Krt10(-/-) mice. Epidermal morphology of soles of hind-paws was examined macroscopically and histologically. Immunofluorescence analysis and quantitative PCR analysis were performed to analyze the expression of keratins in sole skin of wildtype and Krt2(-/-) Krt10(-/-) mice. Highly abundant proteins of the sole stratum corneum were determined by electrophoretic and chromatographic separation and subsequent mass spectrometry.

RESULTS

K2 and K10 are the most prominent suprabasal keratins in normal mouse soles with the exception of the footpads where K1, K9 and K10 predominate. Mice lacking both K2 and K10 were viable and developed epidermal acanthosis and hyperkeratosis in inter-footpad epidermis of the soles. The expression of keratins K1, K9 and K16 was massively increased at the RNA and protein levels in the soles of Krt2(-/-) Krt10(-/-) mice.

CONCLUSIONS

This study demonstrates that the loss of the main cytoskeletal components of plantar epidermis, i.e. K2 and K10, can be only partly compensated by the upregulation of other keratins. The thickening of the epidermis in the soles of Krt2(-/-) Krt10(-/-) mice may serve as a model for pathomechanistic aspects of palmoplantar keratoderma.

Localisation of keratin K78 in the basal layer and first suprabasal layers of stratified epithelia completes expression catalogue of type II keratins and provides new insights into sequential keratin expression

Among the 26 human type II keratins, K78 is the only one that has not yet been explored with regard to its expression characteristics. Here, we show that, at both the transcriptional and translational levels, K78 is strongly expressed in the basal and parabasal cell layers with decreasing intensity in the lower suprabasal cells of keratinising and non-keratinising squamous epithelia and keratinocyte cultures. The same pattern has been detected at the transcriptional level in the corresponding mouse epithelia. Murine K78 protein, which contains an extraordinary large extension of its tail domain, which is unique among all known keratins, is not detectable by the antibody used. Concomitant studies in human epithelia have confirmed K78 co-expression with the classical basal keratins K5 and K14. Similarly, K78 co-expression with the differentiation-related type I keratins K10 (epidermis) and K13 (non-keratinising epithelia) occurs in the parabasal cell layer, whereas that of the corresponding type II keratins K1 (epidermis) and K4 (non-keratinising epithelia) unequivocally starts subsequent to the respective type I keratins. Our data concerning K78 expression modify the classical concept of keratin pair K5/K14 representing the basal compartment and keratin pairs K1/K10 or K4/K13 defining the differentiating compartment of stratified epithelia. Moreover, the K78 expression pattern and the decoupled K1/K10 and K4/K13 expression define the existence of a hitherto unperceived early differentiation stage in the parabasal layer characterized by K78/K10 or K78/K13 expression.

Skin tumors with matrical differentiation: lessons from hair keratins, beta-catenin and PHLDA-1 expression

BACKGROUND

Pilomatricomas are tumors that emulate the differentiation of matrix cells of the hair follicle, showing cortical differentiation, with sequential expression of K35 and K31 keratins. Beta-catenin gene is frequently mutated in pilomatricoma, leading to beta-catenin nuclear accumulation, and to downstream expression of LEF1. Skin matrical tumors other than pilomatricoma are very rare, and comprise purely matrical tumors and focally matrical tumors. We aimed at studying cortical differentiation, beta-catenin pathway and expression of the follicular stem-cell marker PHLDA1 in a series of matrical tumors other than pilomatricoma.

METHODS

In 36 prospectively collected tumors, K31, K35, CK17, LEF1, HOXC13, beta-catenin and PHLDA1 expressions were evaluated. Five pilomatricomas were used as controls.

RESULTS

In 18 purely matrical tumors (11 matrical carcinomas, 4 melanocytic matricomas, 3 matricomas) and 18 focally matrical tumors (11 basal cell carcinomas, 3 trichoepithelioma/trichoblastomas, 4 others), sequential K35, HOXC13 and K31 expressions were found, indicating cortical differentiation. Germinative matrix cells were always CK17-, and showed nuclear beta-catenin accumulation, with LEF1 and PHLDA1 expressions.

CONCLUSIONS

Nuclear beta-catenin and LEF1 expression was highly conserved in matrical tumors, and suggested a common tumorigenesis driven by Wnt pathway activation. PHLDA1 was consistently expressed in matrical tumors and in areas of matrical differentiation.

Activation of Nrf2 in keratinocytes causes chloracne (MADISH)-like skin disease in mice

The transcription factor Nrf2 is a key regulator of the cellular stress response, and pharmacological Nrf2 activation is a promising strategy for skin protection and cancer prevention. We show here that prolonged Nrf2 activation in keratinocytes causes sebaceous gland enlargement and seborrhea in mice due to upregulation of the growth factor epigen, which we identified as a novel Nrf2 target. This was accompanied by thickening and hyperkeratosis of hair follicle infundibula. These abnormalities caused dilatation of infundibula, hair loss, and cyst development upon aging. Upregulation of epigen, secretory leukocyte peptidase inhibitor (Slpi), and small proline-rich protein 2d (Sprr2d) in hair follicles was identified as the likely cause of infundibular acanthosis, hyperkeratosis, and cyst formation. These alterations were highly reminiscent to the phenotype of chloracne/“metabolizing acquired dioxin-induced skin hamartomas” (MADISH) patients. Indeed, SLPI, SPRR2, and epigen were strongly expressed in cysts of MADISH patients and upregulated by dioxin in human keratinocytes in an NRF2-dependent manner. These results identify novel Nrf2 activities in the pilosebaceous unit and point to a role of NRF2 in MADISH pathogenesis.

New facets of keratin K77: interspecies variations of expression and different intracellular location in embryonic and adult skin of humans and mice

The differential expression of keratins is central to the formation of various epithelia and their appendages. Structurally, the type II keratin K77 is closely related to K1, the prototypical type II keratin of the suprabasal epidermis. Here, we perform a developmental study on K77 expression in human and murine skin. In both species, K77 is expressed in the suprabasal fetal epidermis. While K77 appears after K1 in the human epidermis, the opposite is true for the murine tissue. This species-specific pattern of expression is also found in conventional and organotypic cultures of human and murine keratinocytes. Ultrastructure investigation shows that, in contrast to K77 intermediate filaments of mice, those of the human ortholog are not attached to desmosomes. After birth, K77 disappears without deleterious consequences from human epidermis while it is maintained in the adult mouse epidermis, where its presence has so far gone unnoticed. After targeted Krt1 gene deletion in mice, K77 is normally expressed but fails to functionally replace K1. Besides the epidermis, both human and mouse K77 are present in luminal duct cells of eccrine sweat glands. The demonstration of a K77 ortholog in platypus but not in non-mammalian vertebrates identifies K77 as an evolutionarily ancient component of the mammalian integument that has evolved different patterns of intracellular distribution and adult tissue expression in primates.

Nonsense mutations in AAGAB cause punctate palmoplantar keratoderma type Buschke-Fischer-Brauer

Punctate palmoplantar keratodermas (PPKPs) are rare autosomal-dominant inherited skin diseases that are characterized by multiple hyperkeratotic plaques distributed on the palms and soles. To date, two different loci in chromosomal regions 15q22-15q24 and 8q24.13-8q24.21 have been reported. Pathogenic mutations, however, have yet to be identified. In order to elucidate the genetic cause of PPKP type Buschke-Fischer-Brauer (PPKP1), we performed exome sequencing in five affected individuals from three families, and we identified in chromosomal region 15q22.33-q23 two heterozygous nonsense mutations-c.370C>T (p.Arg124(∗)) and c.481C>T (p.Arg161(∗))-in AAGAB in all affected individuals. Using immunoblot analysis, we showed that both mutations result in premature termination of translation and truncated protein products. Analyses of mRNA of affected individuals revealed that the disease allele is either not detectable or only detectable at low levels. To assess the consequences of the mutations in skin, we performed immunofluorescence analyses. Notably, the amount of granular staining in the keratinocytes of affected individuals was lower in the cytoplasm but higher around the nucleus than it was in the keratinocytes of control individuals. AAGAB encodes the alpha-and gamma-adaptin-binding protein p34 and might play a role in membrane traffic as a chaperone. The identification of mutations, along with the results from additional studies, defines the genetic basis of PPKP1 and provides evidence that AAGAB plays an important role in skin integrity.

A decisive function of transforming growth factor-β/Smad signaling in tissue morphogenesis and differentiation of human HaCaT keratinocytes

By interfering with the TGFβ/Smad pathway in the human HaCaT keratinocytes, this study provides novel insights into the role of Smad signaling for regular tissue homeostasis and demonstrates its crucial role in terminal epidermal differentiation and in the decision between alternative epithelial differentiation programs.

 The mechanism by which transforming growth factor-β (TGFβ) regulates differentiation in human epidermal keratinocytes is still poorly understood. To assess the role of Smad signaling, we engineered human HaCaT keratinocytes either expressing small interfering RNA against Smads2, 3, and 4 or overexpressing Smad7 and verified impaired Smad signaling as decreased Smad phosphorylation, aberrant nuclear translocation, and altered target gene expression. Besides abrogation of TGFβ-dependent growth inhibition in conventional cultures, epidermal morphogenesis and differentiation in organotypic cultures were disturbed, resulting in altered tissue homeostasis with suprabasal proliferation and hyperplasia upon TGFβ treatment. Neutralizing antibodies against TGFβ, similar to blocking the actions of EGF-receptor or keratinocyte growth factor, caused significant growth reduction of Smad7-overexpressing cells, thereby demonstrating that epithelial hyperplasia was attributed to TGFβ-induced “dermis”-derived growth promoting factors. Furthermore impaired Smad signaling not only blocked the epidermal differentiation process or caused epidermal-to-mesenchymal transition but induced a switch to a complex alternative differentiation program, best characterized as mucous/intestinal-type epithelial differentiation. As the same alternative phenotype evolved from both modes of Smad-pathway interference, and reduction of Smad7-overexpression caused reversion to epidermal differentiation, our data suggest that functional TGFβ/Smad signaling, besides regulating epidermal tissue homeostasis, is not only essential for terminal epidermal differentiation but crucial in programming different epithelial differentiation routes.

Against the rules: human keratin K80: two functional alternative splice variants, K80 and K80.1, with special cellular localization in a wide range of epithelia

Of the 54 human keratins, five members have, at present, only been characterized at the gene level. In this study we have investigated the expression patterns of keratin K80, whose gene is located at the centromeric end of the type II keratin gene domain. K80 possesses a number of highly unusual properties. Structurally, it is distinctly closer to type II hair keratins than to type II epithelial keratins. Nonetheless, it is found in virtually all types of epithelia (stratified keratinizing/non-keratinizing, hard-keratinizing, as well as non-stratified tissues, and cell cultures thereof). This conspicuously broad expression range implies an unprecedented in vivo promiscuity of K80, which involves more than 20 different type I partners for intermediate filament (IF) formation. Throughout, K80 expression is related to advanced tissue or cell differentiation. However, instead of being part of the cytoplasmic IF network, K80 containing IFs are located at the cell margins close to the desmosomal plaques, where they are tightly interlaced with the cytoplasmic IF bundles abutting there. In contrast, in cells entering terminal differentiation, K80 adopts the "conventional" cytoplasmic distribution. In evolutionary terms, K80 is one of the oldest keratins, demonstrable down to fish. In addition, KRT80 mRNA is subject to alternative splicing. Besides K80, we describe a smaller but fully functional splice variant K80.1, which arose only during mammalian evolution. Remarkably, unlike the widely expressed K80, the expression of K80.1 is restricted to soft and hard keratinizing epithelial structures of the hair follicle and the filiform tongue papilla.

N-WASP is a novel regulator of hair-follicle cycling that controls antiproliferative TGF{beta} pathways

N-WASP is a cytoplasmic molecule mediating Arp2/3 nucleated actin polymerization. Mice with a keratinocyte-specific deletion of the gene encoding N-WASP showed normal interfollicular epidermis, but delayed hair-follicle morphogenesis and abnormal hair-follicle cycling, associated with cyclic alopecia and prolonged catagen and telogen phases. The delayed anagen onset correlated with an increased expression of the cell-cycle inhibitor p21CIP, and increased activity of the TGFbeta pathway, a known inducer of p21CIP expression. Primary N-WASP-null keratinocytes showed reduced growth compared with control cells and enhanced expression of the gene encoding the cell-cycle inhibitor p15INK4B, a TGFbeta target gene. Inhibition of TGFbeta signaling blocked overexpression of p15INK4B and restored proliferation of N-WASP-deficient keratinocytes in vitro. However, induction of N-WASP gene deletion in vitro did not result in obvious changes in TGFbeta signaling or growth of keratinocytes, indicating that the in vivo environment is required for the phenotype development. These data identify the actin nucleation regulator N-WASP as a novel element of hair-cycle control that modulates the antiproliferative and pro-apoptotic TGFbeta pathway in keratinocytes in vivo and in vitro.

The keratins of the human beard hair medulla: the riddle in the middle

We have investigated the expression of 52 of the 54 keratins in beard hair medulla. We found that not only 12 hair keratins but, unexpectedly, also 12 epithelial keratins are potentially expressed in medulla cells. The latter comprise keratins also present in outer- and inner-root sheaths and in the companion layer. Keratins K5, K14, K17, K25, K27, K28, and K75 define a "pre-medulla," composed of cells apposed to the upper dermal papilla. Besides K6, K16, K7, K19, and K80, all pre-medullary epithelial keratins continue to be expressed in the medulla proper, along with the 12 hair keratins. Besides this unique feature of cellular keratin co-expression, the keratin pattern itself is highly variable in individual medulla cells. Remarkably, both epithelial and hair keratins behave highly promiscuously with regard to heterodimer- and IF formation, which also includes keratin chain interactions in IF bundles. We also identified cortex cells within the medullary column. These exhibit all the properties of genuine cortex cells, including a particular type of keratin heterogeneity of their compact IF bundles. In both keratin expression profile and keratin number, medulla cells are distinct from all other cells of the hair follicle or from any other epithelium.

Prolactin--a novel neuroendocrine regulator of human keratin expression in situ

The controls of human keratin expression in situ remain to be fully elucidated. Here, we have investigated the effects of the neurohormone prolactin (PRL) on keratin expression in a physiologically and clinically relevant test system: organ-cultured normal human hair follicles (HFs). Not only do HFs express a wide range of keratins, but they are also a source and target of PRL. Microarray analysis revealed that PRL differentially regulated a defined subset of keratins and keratin-associated proteins. Quantitative immunohistomorphometry and quantitative PCR confirmed that PRL up-regulated expression of keratins K5 and K14 and the epithelial stem cell-associated keratins K15 and K19 in organ-cultured HFs and/or isolated HF keratinocytes. PRL also up-regulated K15 promoter activity and K15 protein expression in situ, whereas it inhibited K6 and K31 expression. These regulatory effects were reversed by a pure competitive PRL receptor antagonist. Antagonist alone also modulated keratin expression, suggesting that "tonic stimulation" by endogenous PRL is required for normal expression levels of selected keratins. Therefore, our study identifies PRL as a major, clinically relevant, novel neuroendocrine regulator of both human keratin expression and human epithelial stem cell biology in situ.

Identification of a novel family of laminin N-terminal alternate splice isoforms: structural and functional characterization

The laminins are a family of heterotrimeric basement membrane proteins that play roles in cellular adhesion, migration, and tissue morphogenesis. Through in silico analysis of the laminin-encoding genes, we identified a novel family of alternate splice isoforms derived from the 5'-end of the LAMA3 and LAMA5 genes. These isoforms resemble the netrins in that they contain a laminin N-terminal domain followed by a short stretch of laminin-type epidermal growth factor-like repeats. We suggest the terms LaNt (laminin N terminus) alpha3 and LaNt alpha5, for the predicted protein products of these mRNAs. RT-PCR confirmed the presence of these transcripts at the mRNA level. Moreover, they exhibit differential, tissue-specific, expression profiles. To confirm the existence of LaNt alpha3 protein, we generated an antibody to a unique domain within the putative polypeptide. This antibody recognizes a protein at the predicted molecular mass of 64 kDa by immunoblotting. Furthermore, immunofluorescence analyses revealed a basement membrane staining in epithelial tissue for LaNt alpha3 and LaNt alpha3 localized along the substratum-associated surface of cultured keratinocytes. We have also tested the functionality LaNt alpha3 through RNAi-mediated knockdown. Keratinocytes exhibiting specific knockdown of LaNt alpha3 displayed impaired adhesion, stress resistance, and reduced ability to close scratch wounds in vitro.

Claudin-7 regulates EpCAM-mediated functions in tumor progression

EpCAM has been described as a therapeutically relevant tumor marker. We noted an interaction between EpCAM and the tight junction protein claudin-7 and here explored the nature of this interaction and its effect on EpCAM-mediated functions. The interaction between EpCAM and claudin-7 was defined in HEK293 cells transfected with rat claudin-7 and EpCAM cDNA. Deletions of the epidermal growth factor-like and the thyroglobin repeat domains of EpCAM or the cytoplasmic domain of EpCAM or claudin-7 did not prevent the EpCAM-claudin-7 association. A chimeric EpCAM molecule with an exchange of the cytoplasmic and transmembrane domains and an EpCAM molecule with point mutations in an AxxxG motif in the transmembrane region do not associate with claudin-7. HEK cells and the rat pancreatic tumor line BSp73AS, transfected with (mutated) EpCAM and claudin-7 cDNA, revealed that the association of both molecules severely alters the functional activity of EpCAM. Claudin-7-associated EpCAM is recruited into tetraspanin-enriched membrane microdomains (TEM). The TEM-located claudin-7-EpCAM complex supports proliferation accompanied by sustained extracellular signal-regulated kinase-1/2 phosphorylation, up-regulation of antiapoptotic proteins, and drug resistance, but not EpCAM-mediated cell-cell adhesion. Enhanced motility may be supported by colocalization of claudin-7 with actin bundles, which is only seen in EpCAM-claudin-7-expressing cells. The EpCAM-claudin-7 complex strongly promotes tumorigenicity, accelerates tumor growth, and supports ascites production and thymic metastasis formation. High expression of the tumor marker EpCAM is frequently associated with poor prognosis, which could well rely on the EpCAM-claudin-7 association that prohibits EpCAM-mediated cell-cell adhesion but promotes migration, proliferation, apoptosis resistance, and tumorigenicity.

Human female hair follicles are a direct, nonclassical target for thyroid-stimulating hormone

Pituitary thyroid-stimulating hormone (TSH) regulates thyroid hormone synthesis via receptors (TSH-R) expressed on thyroid epithelial cells. As the hair follicle (HF) is uniquely hormone-sensitive and, hypothyroidism with its associated, increased TSH serum levels clinically can lead to hair loss, we asked whether human HFs are a direct target for TSH. Here, we report that normal human scalp skin and microdissected human HFs express TSH-R mRNA. TSH-R-like immunoreactivity is limited to the mesenchymal skin compartments in situ. TSH may alter HF mesenchymal functions, as it upregulates alpha-smooth muscle actin expression in HF fibroblasts. TSH-R stimulation by its natural ligand in organ culture changes the expression of several genes of human scalp HFs (for example keratin K5), upregulates the transcription of classical TSH target genes and enhances cAMP production. Although the functional role of TSH in human HF biology awaits further dissection, these findings document that intracutaneous TSH-Rs are fully functional in situ and that HFs of female individuals are direct targets for nonclassical, extrathyroidal TSH bioregulation. This suggests that organ-cultured scalp HFs provide an instructive and physiologically relevant human model for exploring nonclassical functions of TSH, in and beyond the skin.

New concepts on the histogenesis of eccrine neoplasia from keratin expression in the normal eccrine gland, syringoma and poroma

BACKGROUND

Peripheral and luminal layers of eccrine sweat gland ducts are self-renewing structures. Proliferation is restricted to the lowermost luminal layer, but randomly scattered in the peripheral layer. Each layer exhibits differential expression of keratins K5/K14 and K6/K16. Keratin K1 occurs only in peripheral cells and the novel keratin K77 is specific for luminal cells.

OBJECTIVES

To investigate the expression of luminal (K77), peripheral (K1) and further discriminatory keratins in two eccrine sweat gland tumours: syringoma, thought to show differentiation towards luminal cells of intraepidermal sweat ducts and eccrine poroma, considered to arise from poroid cells, i.e. peripheral duct cells; and keratinocytes of the lower acrosyringium/sweat duct ridge differentiating towards cells of intradermal/intraepidermal duct segments.

METHODS

Paraffin-embedded sections were examined by immunohistochemistry using several keratin, smooth muscle actin and Ki-67 antibodies.

RESULTS

We confirmed the ductal nature of syringomas. Despite drastic morphological alterations in both layers, their keratin patterns remained almost undisturbed compared with normal ducts. In eccrine poroma epidermal keratins K5/K14 were ubiquitously expressed in all poroid cells. Cell islands deviating morphologically from poroid cells contained epidermal keratins K1/K10. K77 expression was limited to luminal cells of intact duct structures within the tumours.

CONCLUSIONS

Syringomas are benign tumours of luminal cells of the lowermost intraglandular sweat duct. Poroid precursor cells of poromas do not comprise peripheral duct cells nor do poromas differentiate towards peripheral or luminal duct cells. Instead, poroid cells consist only of keratinocytes of the lowermost acrosyringium and the sweat duct ridge and poromas tend to differentiate towards the cells of the upper acrosyringium.

The Ki67+ proliferation index correlates with increased cellular retinol-binding protein-1 and the coordinated loss of plakophilin-1 and desmoplakin during progression of cervical squamous lesions

AIMS

To investigate the modulation of cellular retinol-binding protein (CRBP)-1 and the desmosomal plaque proteins plakophilin (PKP)-1 and desmoplakin (DP) in correlation with the Ki67+ proliferation index (PI) during the progression of cervical squamous intraepithelial lesions (SIL) to squamous cell carcinoma (SCC).

METHODS

Using in situ imaging by brightfield and confocal laser scanning microscopy, the expression of CRBP-1 protein and transcripts, PKP-1, DP and the Ki67 PI were analysed in 38 low-grade (L) SIL, 56 high-grade (H) SIL, 49 SCC, 30 control cervices and 10 human papillomavirus-positive condylomatous lesions.

RESULTS

CRBP-1+ cells increased from 11.4% in the normal cervix to 80.3% in LSILs, 92.3% in HSILs and slightly decreased to 78.3% in invasive SCCs (P = 0.0001) in close association with the Ki67 PI (r =0.41; P < 0.0001). PKP-1+ and DP+ cells were correlated (0.32; P < 0.0001) and decreased from normal (81% versus 92.3%) to LSIL (53.1% versus 85.3%), to HSIL (46.4% versus 67.5%) and SCC (35.1% versus 35.9%). The Ki67+ PI was inversely correlated with DP (-0.20, P = 0.0014) and PKP-1 (-0.19, P = 0.015). Condylomata retained low CRBP-1 and high expression of PKP-1 and DP.

CONCLUSIONS

The gain of CRBP-1 and the loss of desmosomal proteins occur early in cervical carcinogenesis.

A complex of EpCAM, claudin-7, CD44 variant isoforms, and tetraspanins promotes colorectal cancer progression

High expression of EpCAM and the tetraspanin CO-029 has been associated with colorectal cancer progression. However, opposing results have been reported on CD44 variant isoform v6 (CD44v6) expression. We recently noted in rat gastrointestinal tumors that EpCAM, claudin-7, CO-029, and CD44v6 were frequently coexpressed and could form a complex. This finding suggested the possibly that the complex, rather than the individual molecules, could support tumor progression. The expression of EpCAM, claudin-7, CO-029, and CD44v6 expression was evaluated in colorectal cancer (n = 104), liver metastasis (n = 66), and tumor-free colon and liver tissue. Coexpression and complex formation of the molecules was correlated with clinical variables and apoptosis resistance. EpCAM, claudin-7, CO-029, and CD44v6 expression was up-regulated in colon cancer and liver metastasis. Expression of the four molecules did not correlate with tumor staging and grading. However, coexpression inversely correlated with disease-free survival. Coexpression was accompanied by complex formation and recruitment into tetraspanin-enriched membrane microdomains (TEM). Claudin-7 contributes to complex formation inasmuch as in the absence of claudin-7, EpCAM hardly associates with CO-029 and CD44v6 and is not recruited into TEMs. Notably, colorectal cancer lines that expressed the EpCAM/claudin-7/CO-029/CD44v6 complex displayed a higher degree of apoptosis resistance than lines devoid of any one of the four molecules. Expression of EpCAM, claudin-7, CO-029, and CD44v6 by themselves cannot be considered as prognostic markers in colorectal cancer. However, claudin-7-associated EpCAM is recruited into TEM and forms a complex with CO-029 and CD44v6 that facilitates metastasis formation.

Integrin-linked kinase is required for epidermal and hair follicle morphogenesis

Integrin-linked kinase (ILK) links integrins to the actin cytoskeleton and is believed to phosphorylate several target proteins. We report that a keratinocyte-restricted deletion of the ILK gene leads to epidermal defects and hair loss. ILK-deficient epidermal keratinocytes exhibited a pronounced integrin-mediated adhesion defect leading to epidermal detachment and blister formation, disruption of the epidermal–dermal basement membrane, and the translocation of proliferating, integrin-expressing keratinocytes to suprabasal epidermal cell layers.

The mutant hair follicles were capable of producing hair shaft and inner root sheath cells and contained stem cells and generated proliferating progenitor cells, which were impaired in their downward migration and hence accumulated in the outer root sheath and failed to replenish the hair matrix. In vitro studies with primary ILK-deficient keratinocytes attributed the migration defect to a reduced migration velocity and an impaired stabilization of the leading-edge lamellipodia, which compromised directional and persistent migration. We conclude that ILK plays important roles for epidermis and hair follicle morphogenesis by modulating integrin-mediated adhesion, actin reorganization, and plasma membrane dynamics in keratinocytes.

Hair follicle-specific keratins and their diseases

The human keratin family comprises 54 members, 28 type I and 26 type II. Out of the 28 type I keratins, 17 are epithelial and 11 are hair keratins. Similarly, the 26 type II members comprise 20 epithelial and 6 hair keratins. As, however, 9 out of the 37 epithelial keratins are specifically expressed in the hair follicle, the total number of hair follicle-specific keratins (26) almost equals that of those expressed in the various forms of epithelia (28). Up to now, more than half of the latter have been found to be involved in inherited diseases, with mutated type I and type II members being roughly equally causal. In contrast, out of the 26 hair follicle-specific keratins only 5 have, at present, been associated with inherited hair disorders, while one keratin merely acts as a risk factor. In addition, all hair follicle-specific keratins involved in pathologies are type II keratins. Here we provide a detailed description of the respective hair diseases which are either due to mutations in hair keratins (monilethrix, ectodermal dysplasia of hair and nail type) or hair follicle-specific epithelial keratins (two mouse models, RCO3 and Ca(Rin) as well as pseudofolliculitis barbae).

Characterization of Human KAP24.1, A Cuticular Hair Keratin-Associated Protein with Unusual Amino-Acid Composition and Repeat Structure

In a search for genes overexpressed in human sexual hairs, several partial complementary DNA (cDNA) sequences were isolated. Screening of a human scalp cDNA library with one fragment led to the isolation of a full-length cDNA clone, which showed identity to another known sequence, termed KAP24-1 (AB09693). Bioinformatic analysis revealed that the gene for this cDNA consisted of one exon and was located ca. 86 kb away from the chromosome 21q22.1 keratin-associated protein (KAP) gene domain. RT-PCR analysis of a variety of organs showed that KAP24.1 was only present in human scalp. The KAP24.1 protein consisted of 254 amino acids, exhibited a high content of serine, proline, and tyrosine, but low cysteine content and possessed several carboxyterminal tyrosine-containing tandem decameric repeat structures. Evolutionary tree analysis showed no association to other KAP family members. In situ hybridization and indirect immunofluorescence microscopy studies using an antibody derived from KAP24.1 demonstrated specific expression in the middle/upper hair cuticle. The structure of the KRTAP24, its proximity to the chromosome 21q22.1 KAP gene domain, the presence of repeat motifs in the protein and its localization in the hair cuticle points to KAP24.1 being a novel human KAP family member.

Integrity and barrier function of the epidermis critically depend on glucosylceramide synthesis

Ceramides are vital components of the water barrier in mammalian skin. Epidermis-specific, a major ceramide portion contains omega-hydroxy very long chain fatty acids (C30-C36). These omega-hydroxy ceramides (Cers) are found in the extracellular lamellae of the stratum corneum either as linoleic acyl esters or protein bound. Glucosylceramide is the major glycosphingolipid of the epidermis. Synthesized from ceramide and UDP-glucose, it is thought to be itself an intracellular precursor and carrier for extracellular omega-hydroxy ceramides. To investigate whether GlcCer is an obligatory intermediate in ceramide metabolism to maintain epidermal barrier function, a mouse with an epidermis-specific glucosylceramide synthase (Ugcg) deficiency has been generated. Four days after birth animals devoid of GlcCer synthesis in keratinocytes showed a pronounced desquamation of the stratum corneum and extreme transepidermal water loss leading to death. The stratum corneum appeared as a thick unstructured mass. Lamellar bodies of the stratum granulosum did not display the usual ordered inner structure and were often irregularly arranged. Although the total amount of epidermal protein-bound ceramides remained unchanged, epidermal-free omega-hydroxy ceramides increased 4-fold and omega-hydroxy sphingomyelins, almost not detectable in wild type epidermis, emerged in quantities comparable with lost GlcCer. We conclude that the transient formation of GlcCer is vital for a regular arrangement of lipids and proteins in lamellar bodies and for the maintenance of the epidermal barrier.

Rac1 is crucial for hair follicle integrity but is not essential for maintenance of the epidermis

Rac1 is a small GTPase that regulates the actin cytoskeleton but also other cellular processes. To investigate the function of Rac1 in skin, we generated mice with a keratinocyte-restricted deletion of the rac1 gene. Rac1-deficient mice lost nearly all of their hair within a few weeks after birth. The nonpermanent part of mutant hair follicles developed constrictions; lost expression of hair follicle-specific keratins, E-cadherin, and alpha6 integrin; and was eventually removed by macrophages. The permanent part of hair follicles and the sebaceous glands were maintained, but no regrowth of full-length hair follicles was observed. In the skin of mutant mice, epidermal keratinocytes showed normal differentiation, proliferation, cell-cell contacts, and basement membrane deposition, demonstrating no obvious defects of Rac1-deficient epidermis in vivo. In vitro, Rac1-null keratinocytes displayed a strong spreading defect and slightly impaired adhesion. These data show that Rac1 plays an important role in sustaining the integrity of the lower part of hair follicles but not in maintenance of the epidermis.

K25 (K25irs1), K26 (K25irs2), K27 (K25irs3), and K28 (K25irs4) represent the type I inner root sheath keratins of the human hair follicle

The recent elucidation of the human type I keratin gene domain allowed the completion of the so far only partially characterized subcluster of type I keratin genes, KRT25-KRT28 (formerly KRT25A-KRT25D), representing the counterparts of the type II inner root sheath (IRS) keratin genes, KRT71-KRT74 (encoding proteins K71-K74, formerly K6irs1-K6irs4). Here, we describe the expression patterns of the type I IRS keratin proteins K25-K28 (formerly K25irs1-K25irs4) and their mRNAs. We found that K25 (K25irs1), K27 (K25irs3), and K28 (K25irs4) occur in the Henle layer, the Huxley layer, and in the IRS cuticle. Their expression extends from the bulb region up to the points of terminal differentiation of the three layers. In contrast, K26 (K25irs2) is restricted to the upper IRS cuticle. Apart from the three IRS layers, K25 (K25irs1), K27 (K25irs3), and K28 (K25irs4) are also present in the hair medulla. Based on previous, although controversial claims of the occurrence in the IRS of various "classical" epithelial keratins, we undertook a systematic study using antibodies against the presently described human epithelial and hair keratins and show that the type I keratins K25-K28 (K25irs1-K25irs4) and the type II keratins K71-K74 (K6irs1-K6irs4) represent the IRS keratins of the human hair follicle.

New consensus nomenclature for mammalian keratins

Keratins are intermediate filament–forming proteins that provide mechanical support and fulfill a variety of additional functions in epithelial cells. In 1982, a nomenclature was devised to name the keratin proteins that were known at that point. The systematic sequencing of the human genome in recent years uncovered the existence of several novel keratin genes and their encoded proteins. Their naming could not be adequately handled in the context of the original system. We propose a new consensus nomenclature for keratin genes and proteins that relies upon and extends the 1982 system and adheres to the guidelines issued by the Human and Mouse Genome Nomenclature Committees. This revised nomenclature accommodates functional genes and pseudogenes, and although designed specifically for the full complement of human keratins, it offers the flexibility needed to incorporate additional keratins from other mammalian species.

Desmoglein 4 is expressed in highly differentiated keratinocytes and trichocytes in human epidermis and hair follicle

Desmosomes are critical for the tissue integrity of stratified epithelia and their appendages. Desmogleins (DSGs) and desmocollins (DSCs) are transmembrane desmosomal cadherins that interact extracellularly to link neighboring epithelial cells. We recently identified a new member of the DSG family, designated desmoglein 4, whose mutations cause hypotrichosis in human, mouse and rat. In this study, we analyzed in detail the expression domains of human desmoglein 4 protein (DSG4) in human skin relative to differentiation markers and other DSGs. Our results show that DSG4 protein is expressed in the more highly differentiated layers of the epidermis. This expression pattern in vivo is recapitulated in highly differentiated HaCaT human keratinocytes and normal human keratinocytes in vitro. In the human hair follicle, DSG4 is expressed specifically in the hair shaft cortex, the lower hair cuticle, and the upper inner root sheath (IRS) cuticle. Using a green fluorescent protein-tagged version of mouse or rat desmoglein 4 protein (Dsg4) and immuno-electron microscopy, we demonstrate that Dsg4 localizes to desmosomes both in vitro and in vivo. The highly specific expression pattern of DSG4 in the human hair follicle, combined with the phenotype of rodent models and human patients with desmoglein 4 mutations, underscores the importance of this adhesion molecule in the integrity of the hair shaft.

Cdc42 controls progenitor cell differentiation and beta-catenin turnover in skin

Differentiation of skin stem cells into hair follicles (HFs) requires the inhibition of beta-catenin degradation, which is controlled by a complex containing axin and the protein kinase GSK3beta. Using conditional gene targeting in mice, we show now that the small GTPase Cdc42 is crucial for differentiation of skin progenitor cells into HF lineage and that it regulates the turnover of beta-catenin. In the absence of Cdc42, degradation of beta-catenin was increased corresponding to a decreased phosphorylation of GSK3beta at Ser 9 and an increased phosphorylation of axin, which is known to be required for binding of beta-catenin to the degradation machinery. Cdc42-mediated regulation of beta-catenin turnover was completely dependent on PKCzeta, which associated with Cdc42, Par6, and Par3. These data suggest that Cdc42 regulation of beta-catenin turnover is important for terminal differentiation of HF progenitor cells in vivo.

Expression patterns of hair and epithelial keratins and transcription factors HOXC13, LEF1, and beta-catenin in a malignant pilomatricoma: a histological and immunohistochemical study

BACKGROUND

We have previously shown that benign pilomatricomas not only maintain the sequential expression of the hair matrix and precortex keratins hHa5 and hHa1 of normal hair follicles in their transitional cell compartment, but also preserve the association of hHa5 expression with that of its regulatory homeoprotein HOXC13 in the lower transitional cell compartment. In contrast, hHa1 expression in the upper transitional cell compartment is uncoupled from the nuclear co-expression of the LEF1/beta-catenin complex seen in normal hair follicles (Cribier et al., J Invest Dermatol 2004; 122: 1078).

METHODS

Formalin-fixed paraffin sections of the tumor were examined using a panel of mono- and polyclonal hair and epithelial keratin antibodies as well as antibodies against HOXC13, LEF1, and beta-catenin.

RESULTS

Morphologically, the malignant pilomatricoma investigated here clearly deviated from the described major tumor type by a large number of differently sized parakeratotic squamoid whorls emerging within the mass of basaloid cells and surrounded by cells remembering transitional cells, but only rarely containing shadow cells and signs of calcification. We show that hHa5/HOXC13 co-expression was maintained in transitional cell areas, in which hHa1 expression was much stronger than in benign pilomatricomas, but again uncoupled from concomitant nuclear LEF1/beta-catenin expression. Surprisingly, however, and in clear contrast to benign pilomatricomas, these transitional cells co-expressed the epithelial keratins K5, K14, and K17, with the latter being as strongly expressed as hHa1, both also staining the entire inner mass of the parakeratotic whorls.

CONCLUSIONS

Although the malignant pilomatricoma investigated here was distinctive in that it contained a multitude of parakeratinizing whorls and no signs of calcification, it shared both hHa5/HOXC13 co-expression and disrupted hHa1/beta-catenin-LEF1 expression in its transitional cell compartment around the whorls with benign pilomatricomas. However, in clear contrast to the latter, transitional cells of the malignant tumor also strongly expressed the epithelial keratins K5, K14, and K17. We speculate that the observed dominance of the epithelial differentiation pathway over the competing conventional shadow cell differentiation pathway may prevent massive calcification of the tumor.

Human hair keratin-associated proteins (KAPs)

Elucidation of the genes encoding structural proteins of the human hair follicle has advanced rapidly during the last decade, complementing nearly three previous decades of research on this subject in other species. Primary among these advances was both the characterization of human hair keratins, as well as the hair keratin associated proteins (KAPs). This review describes the currently known human KAP families, their genomic organization, and their characteristics of expression. Furthermore, this report delves into further aspects, such as polymorphic variations in human KAP genes, the role that KAP proteins might play in hereditary hair diseases, as well as their modulation in several different transgenic mouse models displaying hair abnormalities.

Impaired expression of acyl-CoA synthetase 5 in sporadic colorectal adenocarcinomas

Several pathways of fatty acid metabolism have been shown to be associated with the pathogenesis of colorectal cancer. Fatty acid acyl-CoA thioesters are formed from free fatty acids and coenzyme A by the activity of acyl-CoA synthetases (ACSs). Whilst an increase in ACS4 expression has been associated with colorectal carcinogenesis, little is known about possible pathogenetic functions of other ACS isoforms, such as ACS5, in tumourigenesis. In the present study, gene expression, protein synthesis, and enzymatic activity of ACS5 in sporadic colorectal adenocarcinomas, adenomas, and established cell lines were analysed using RT-PCR, western blot analysis, immunofluorescence, and an enzymatic assay. Enhanced expression of ACS5 mRNA and protein as well as enzymatic activity was found in adenomas and in 11 (73%; group 1) of 15 colorectal adenocarcinomas investigated, while a decrease of ACS5 was seen in four tumours (27%; group 2). However, basal ACS5 enzymatic activity was increased as a percentage of the total activity of ACSs in both groups, arguing for an absolute (group 1) or relative (group 2) increase in ACS5 enzymatic activity in all adenocarcinomas investigated. These findings are reflected by in vitro analysis of three established colorectal adenocarcinoma cell lines, in which activity of ACS5 occurred. The results suggest the involvement of ACS5 in the early genesis of colorectal cancer, most likely by modification of the transport and pool formation of long-chain acyl-CoA thioesters, as recently demonstrated for other isoforms of the ACS family.

Disturbed Epidermal Structure in Mice with Temporally Controlled Fatp4 Deficiency

So far, little is known about the physiological role of fatty acid transport protein 4 (Fatp4, Slc27a4). Mice with a targeted disruption of the Fatp4 gene display features of a human neonatally lethal restrictive dermopathy with a hyperproliferative hyperkeratosis, a disturbed epidermal barrier, a flat dermal-epidermal junction, a reduced number of pilo-sebaceous structures, and a compact dermis, demonstrating that Fatp4 is necessary for the formation of the epidermal barrier. Because Fatp4 is widely expressed, it is unclear whether intrinsic Fatp4 deficiency in the epidermis alone can cause changes in the epidermal structure or whether the abnormalities observed are secondary to the loss of Fatp4 in other organs. To evaluate the functional role of Fatp4 in the skin, we generated a mouse line with Fatp4 deficiency inducible in the epidermis. Mice with epidermal keratinocyte-specific Fatp4 deficiency developed a hyperproliferative hyperkeratosis with a disturbed epidermal barrier. These changes resemble the histological abnormalities in the epidermis of newborn mice with total Fatp4 deficiency. We conclude that Fatp4 in epidermal keratinocytes is essential for the maintenance of a normal epidermal structure.

The cell-cell adhesion molecule EpCAM interacts directly with the tight junction protein claudin-7

We recently described that in the metastasizing rat pancreatic carcinoma line BSp73ASML the cell-cell adhesion molecule EpCAM, CD44 variant isoforms and the tetraspanins D6.1A and CD9 form a complex that is located in glycolipid-enriched membrane microdomains. This complex contains, in addition, an undefined 20 kDa protein. As such complex formation influenced cell-cell adhesion and apoptosis resistance, it became of interest to identify the 20 kDa polypeptide. This 20 kDa protein, which co-precipitated with EpCAM in BSp73ASML lysates, was identified as the tight junction protein claudin-7. Correspondingly, an association between EpCAM and claudin-7 was noted in rat and human tumors and in non-transformed tissues of the gastrointestinal tract. Co-localization of the two molecules was most pronounced at basolateral membranes, but was also observed in tight junctions. Evidence for direct protein-protein interactions between EpCAM and claudin-7 was obtained by co-immunoprecipitation after treatment of tumor cells with a membrane-permeable chemical cross-linker. The complex, which is located in glycolipid-enriched membrane microdomains, is not disrupted by partial cholesterol depletion, but claudin-7 phosphorylation is restricted to the localization in glycolipid-enriched membrane microdomains. This is the first report on an association between EpCAM and claudins in both non-transformed tissues and metastasizing tumor cell lines.

Characterization of a novel human type II epithelial keratin K1b, specifically expressed in eccrine sweat glands

In this study, we show that a novel human type II epithelial keratin, K1b, is exclusively expressed in luminal duct cells of eccrine sweat glands. Taking this luminal K1b expression as a reference, we have used antibodies against a plethora of epithelial keratins to systematically investigate their expression in the secretory globule and the two-layered sweat duct, which was divided into the intraglandular, intradermal, and intraepidermal (acrosyringium) segments, the latter being further subdivided into the sweat duct ridge and upper intraepidermal duct. We show that (i) each of the eccrine sweat gland tissue compartments expresses their own keratin patterns, (ii) the peripheral and luminal duct layers exhibit a sequential keratin expression, with both representing self-renewing cell layers, (iii) the intradermal duct and the sweat duct ridge display hitherto unknown length variations, and (iv) out of all cell layers, the luminal cell layer is the most robust layer and expresses the highest number of keratins, these being concentrated at the apical side of the cells to form the cuticle. We provide evidence that the cellular and intercellular properties of the peripheral and the luminal layers reflect adaptations to different functions.

The human type I keratin gene family: characterization of new hair follicle specific members and evaluation of the chromosome 17q21.2 gene domain

In general concurrence with recent studies, bioinformatic analysis of the chromosome 17q21.2 DNA sequence found in the EBI/Genebank database shows the presence of 27 type I keratin genes and five keratin pseudogenes present on 8 contiguous Bacterial Artificial Chromosome (BAC) sequences. This constitutes the 970 kb type I keratin gene domain. Inserted into this domain is a 350 kb region harboring 32 previously characterized keratin-associated protein genes. Of the 27 keratin genes found in this region, six have not been characterized in detail. This study reports the isolation of cDNA sequences for these keratin genes, termed K25irs1-K25irs4, Ka35, and Ka36, as well as cDNA sequences for the previously reported hair keratins hHa3-I, hHa7, and hHa8. RT-PCR analysis of 14 epithelial tissues using primers for the six novel keratins, as well as for keratins 23 and 24, shows that the six novel keratins appear to be hair follicle associated. Previous expression data, coupled with evolutionary analysis studies point to K25irs1-K25irs4 probably being inner root sheath specific keratins. Ka35 and Ka36 are, based on their exon-intron structure and expression characteristics, hair keratins. In contrast, K23 and K24 appear to be epithelial keratins associated with simple/glandular or stratified, non-cornified epithelia, respectively. A literature analysis coupled with the data presented here confirms that all of the 27 keratin genes found on this domain have been characterized at the transcriptional level. Together with K18, a type I keratin gene found on the type II keratin domain, this seems to be the entire complement of functional type I keratins in humans.

Characterization of new members of the human type II keratin gene family and a general evaluation of the keratin gene domain on chromosome 12q13.13

The recent completion of a reference sequence of the human genome now allows a complete characterization of the type II keratin gene domain on chromosome 12q13.13. This, domain, approximately 780 kb in size, is present on nine bacterial artificial chromosome clones sequenced by the Human Genome Sequencing Project. The type II keratin domain contains 27 keratin genes and eight pseudogenes. Twenty-three of these genes and four pseudogenes have been previously reported. This study describes, in addition to the genomic sequencing of the K2p gene and the bioinformatic identification of four keratin pseudogenes, the characterization of cDNA corresponding to three previously undescribed keratin genes K1b, K6l, and Kb20, as well as cDNA sequences for the previously described keratin genes hHb2, hHb4, and K3. Northern analysis of the new keratins K1b, K6l, K5b, and Kb20 using mRNA of major organs as well as of specific epithelial subtypes shows singular expression of these keratins in skin, hair follicles and, for K5b and Kb20, in tongue, respectively. In addition, the obvious discrepancies between the current reference sequence of the human genome and the previously described gene/cDNA sequences for K6c, K6d, K6e, K6f, K6h are investigated, leading to the conclusion that K6c, K6d as well as K6e, K6f are probably polymorphic variants of K6a and K6h, respectively. All 26 human type II keratins found on this domain as well as K18, dtype 1 Keratin, are identified at the genomic and transcriptional level. This appears to be the total complement of functional type II keratins in humans.

Keratins of the Human Hair Follicle

Substantial progress has been made regarding the elucidation of differentiation processes of the human hair follicle. This review first describes the genomic organization of the human hair keratin gene family and the complex expression characteristics of hair keratins in the hair-forming compartment. Sections describe the role and fate of hair keratins in the diseased hair follicle, particularly hereditary disorders and hair follicle-derived tumors. Also included is a report on the actual state of knowledge concerning the regulation of hair keratin expression. In the second part of this review, essentially the same principles are applied to outline more recent and, thus, occasionally fewer data on specialized epithelial keratins expressed in various tissue constituents of the external sheaths and the companion layer of the follicle. A closing outlook highlights issues that need to be explored further to deepen our insight into the biology and genetics of the hair follicle.

Expression of hair keratins in the adult nail unit: an immunohistochemical analysis of the onychogenesis in the proximal nail fold, matrix and nail bed

BACKGROUND

Recently, the expression profiles of the members of the complex hair keratin family have been determined in the human anagen hair follicle. In contrast, the details of hair keratin expression in the human nail unit are poorly known.

OBJECTIVES

In order to fill this gap, we have performed an immunohistochemical study of the adult human nail unit by means of specific antibodies against nine hair keratins of both types (hHa2, hHb2, hHa5, hHb5, hHa1, hHb1, hHb6, hHa4 and hHa8) as well as three epithelial keratins (K5, K17 and K10).

METHODS

Formalin-fixed paraffin sections of adult nails were examined using monoclonal and polyclonal keratin antibodies, respectively. Longitudinal as well as transverse sections were investigated.

RESULTS

Our study revealed two types of epithelial tissue compartments in the nail unit. The first comprised the eponychium and hyponychium and the nail bed, which expressed only epithelial keratins. While keratins K5, K17 (basal) and K10 (suprabasal) were found in the orthokeratinizing eponychium and hyponychium, throughout, the nail bed epithelium expressed only K5 and K17. The second type comprised the apical and ventral matrix which exhibited a mixed pattern of epithelial and hair keratin expression. Thus, K5 and K17 were expressed in the entire multilayered basal cell compartment of the apical and ventral matrix; however, in the latter, K5 and K17 also occurred in the lowermost layers of the overlying keratogenous zone. The hair matrix keratin hHb5, but not its type II partner hHa5, was seen in the entire keratogenous zone of the apical and ventral matrix, but was also located in the uppermost cell layers of the basal compartment of the ventral matrix, where it overlapped with K5 and K17. Similar to their sequential expression in the hair follicle cortex, hair keratins hHa1, hHb1, hHb6 and hHa4 were consecutively expressed in the keratogenous zone of both the ventral and, albeit less distinctly, apical matrix, with hHa1 initiating in the lowermost cell layers. The expression of hHa8 in only single cortex cells of the hair follicle was also preserved in cells of the keratogenous zone. In the region of the so-called dorsal matrix, we observed two histologically and histochemically distinct types of epithelia: (i) a dominant type, histologically similar to the eponychium and an associated K5, K17 and K10 keratin pattern which clearly extended into the apical matrix, and (ii) a minor type, histologically resembling the postulated dorsal matrix without a granular layer and a cuticle, and exhibiting extended K5 expression as well as hair keratin expression in superficial cells.

CONCLUSIONS

The coexpression of hHb5 with K5 and K17 in the uppermost cell layers of the basal compartment and the lowermost layers of the keratogenous zone of the ventral matrix prompts us to designate this region the prekeratogenous zone of the ventral matrix. The two alternating types of histology and keratin expression in the dorsal matrix identify this region as a transitional zone between the eponychium and the apical matrix. Finally, our data clearly show that the ventral matrix is the main source of the nail plate. In addition, the mixed scenario of hair and epithelial keratins, including demonstrable amounts of K10, in superficial cells of the apical matrix, lends support to the notion that the dorsal portion of the nail is generated by the apical matrix.