Keratin K6irs is specific to the inner root sheath of hair follicles in mice and humans

Disrupted citric acid metabolism inhibits hair growth

Hair follicles (HFs) play an essential role in sustaining a persistent hair growth cycle. The activities of dermal papilla cells (DPCs) and other cells inside the HFs dominate the process of hair growth. However, the detailed molecular mechanisms remain largely unknown. To investigate the role of citric acid (CA) metabolism in hair growth, we evaluated the effect of citrate synthase (CS)-CA axis on hair growth in vivo and in vitro. Mice hair growth was evaluated by morphology and histopathology analysis. The inflammation and apoptosis levels in mice, HFs, and DPCs were detected by immunohistofluorescence, qPCR, ELISA, western blot, and TUNEL assay. Cell proliferation, cell cycle, and cell apoptosis in DPCs were analyzed by real-time cell analysis and flow cytometer. We found that subcutaneous injection of CA in mice caused significant hair growth suppression, skin lesion, inflammatory response, cell apoptosis, and promotion of catagen entry, compared with the saline control, by activating p-p65 and apoptosis signaling in an NLRP3-dependent manner. In cultured human HFs, CA attenuated the hair shaft production and accelerated HF catagen entry by regulating the above-mentioned pathways. Additionally, CA hampered the proliferation rate of DPCs via inducing cell apoptosis and cell cycle arrest. Considering that citrate synthase (CS) is responsible for CA production and is a rate-limiting enzyme of the tricarboxylic acid cycle, we also investigated the role of CS in CA metabolism and hair growth. As expected, knockdown of CS reduced CA production and reversed CA-induced hair growth inhibition, anagen shrink, inflammation, and apoptosis both in HFs and DPCs. Our experiments demonstrated that CS-CA axis serves as an important mediator and might be a potential therapeutic target in hair growth.

Transcriptomic regulation of seasonal coat color change in hares

Color molts from summer brown to winter white coats have evolved in several species to maintain camouflage year-round in environments with seasonal snow. Despite the eco-evolutionary relevance of this key phenological adaptation, its molecular regulation has only recently begun to be addressed. Here, we analyze skin transcription changes during the autumn molt of the mountain hare (Lepus timidus) and integrate the results with an established model of gene regulation across the spring molt of the closely related snowshoe hare (L. americanus). We quantified differences in gene expression among three stages of molt progression-"brown" (early molt), "intermediate," and "white" (late molt). We found 632 differentially expressed genes, with a major pulse of expression early in the molt, followed by a milder one in late molt. The functional makeup of differentially expressed genes anchored the sampled molt stages to the developmental timeline of the hair growth cycle, associating anagen to early molt and the transition to catagen to late molt. The progression of color change was characterized by differential expression of genes involved in pigmentation, circadian, and behavioral regulation. We found significant overlap between differentially expressed genes across the seasonal molts of mountain and snowshoe hares, particularly at molt onset, suggesting conservatism of gene regulation across species and seasons. However, some discrepancies suggest seasonal differences in melanocyte differentiation and the integration of nutritional cues. Our established regulatory model of seasonal coat color molt provides an important mechanistic context to study the functional architecture and evolution of this crucial seasonal adaptation.

A Case of Loose Anagen Hair Syndrome in a Southeast Asian Boy

Loose anagen hair syndrome is an uncommon hair disorder, particularly in non-Caucasian children. We report the case of a 13-year-old Thai boy who presented with a single patch of hair thinning on the frontal scalp with excessive shedding, and the hairs did not grow long. Microscopic examination showed naked anagen bulbs with ruffling of the cuticle, which is compatible with loose anagen hair syndrome. To our knowledge, there is no reported case in Southeast Asian children.

Characteristics and Expression Profile of KRT71 Screened by Suppression Subtractive Hybridization cDNA Library in Curly Fleece Chinese Tan Sheep

As an important commercial trait for sheep, curly fleece has a great economic impact on production costs and efficiency in sheep industry. To identify genes that are important for curly fleece formation in mammals, a suppression subtractive hybridization analysis was performed on the shoulder skin tissues exposed to two different growth stages of Chinese Tan sheep with different phenotypes (curly fleece and noncurling fleece). BLAST analysis identified 67 differentially expressed genes, of which 31 were expressed lower and 36 were expressed higher in lambs than in adult sheep. Differential expressions of seven randomly selected genes were verified using quantitative real-time polymerase chain reaction (qRT-PCR). KRT71 gene was selected for further study due to its high correlation with the curly hair phenotype in various mammal species. Semi-qPCR showed distinctively high expression of KRT71 in skin tissues. Moreover, qPCR result showed a significantly higher expression of KRT71 in curly fleece than noncurling Tan sheep. The luciferase assay and electrophoresis mobility shift assay showed that there were transcription factor binding sites in the promoter region of KRT71 related to the differential expression of KRT71 at the two growth stages of Tan sheep. Online bioinformation tools predicted MFZ1 as a transcriptional factor that regulates the expression of KRT71. These studies on KRT71 gene revealed some mechanisms underlying the relationship between the KRT71 gene and the curly fleece phenotype of Tan sheep.

Proteomic analysis of differentially expressed skin proteins in iRhom2(Uncv) mice

A mouse homozygous for the spontaneous mutation uncovered (Uncv) has a hairless phenotype. A 309-bp non-frameshift deletion mutation in the N-terminal cytoplasmic domain of iRhom2 was identified in Uncv mice (iRhom2^Uncv) using target region sequencing. The detailed molecular basis for how the iRhom2 mutation causes the hairless phenotype observed in the homozygous iRhom2^Uncv mouse remains unknown. To identify differentially expressed proteins in the skin of wild-type and homozygous iRhom2^Uncv littermates at postnatal day 5, proteomic approaches, including two-dimensional gel electrophoresis and mass spectrometry were used. Twelve proteins were differentially expressed in the skin in a comparison between wild-type and homozygous iRhom2^Uncv mice. A selection of the proteomic results were tested and verified using qRT-PCR, western blot and immunohistochemistry. These data indicate that differentially expressed proteins, especially KRT73, MEMO1 and Coro-1, might participate in the mechanism by which iRhom2 regulates the development of murine skin. [BMB Reports 2015; 48(1): 19-24]

Defining the complex epithelia that comprise the canine claw with molecular markers of differentiation

Canine claws are complex epithelial structures resembling the mammalian hair fibre, and human nail plate, in terms of tissue-specific differentiation. They are composed of several distinct epithelial cell lineages undergoing either hard or soft keratinization. The claw plate has three distinct regions: stratum externum, stratum medium (SM) and stratum internum and the underside and tip are cushioned by a soft keratinizing epithelium, the sole. We have examined keratin expression in the canine claw and associated epithelia. Digits from German shepherd dogs were decalcified, processed and sectioned by sledge microtome. Sections were stained with haematoxylin and eosin or treated with specific antibodies to various keratins (immunohistochemistry). Proteins were extracted from claw components and analysed by SDS-PAGE and Western blotting. The keratinized canine claw plate expressed hair-specific keratins (type I, K25-K38 and type II, K71-K86) but only the inner region of the SM contained K6- and K16-positive tubules, soft epithelia running through the hard keratinized claw plate. The soft keratinaceous sole epithelium expressed keratins K5, K6, K14, K16 and K17 and contained cells with abundant envelopes. The canine claw had two slippage zones, the inner claw bed, between the claw plate and ungula process, which expressed K17 and the region between the inner and outer claw sheath, equivalent to the hair follicle companion layer, which expressed K6, K77, K16 and K17. In conclusion, several different cell types have been defined in the canine claw presenting a complex mechanism of cellular differentiation.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Disruption of epidermal specific gene expression and delayed skin development in AP-2 gamma mutant mice

Summary Sentence: Conditional ablation of AP-2 gamma results in a delay in skin development and abnormal expression of p63, K14, K1, filaggrin, repetin and secreted Ly6/Plaur domain containing 1, key genes required for epidermal development and differentiation. The development of the epidermis, a stratified squamous epithelium, is dependent on the regulated differentiation of keratinocytes. Differentiation begins with the initiation of stratification, a process tightly controlled through proper gene expression. AP-2 gamma is expressed in skin and previous research suggested a pathway where p63 gene induction results in increased expression of AP-2 gamma, which in turn is responsible for induction of K14. This study uses a conditional gene ablation model to further explore the role of AP-2 gamma in skin development. Mice deficient for AP-2 gamma exhibited delayed expression of p63, K14, and K1, key genes required for development and differentiation of the epidermis. In addition, microarray analysis of E16.5 skin revealed delayed expression of additional late epidermal differentiation genes: filaggrin, repetin and secreted Ly6/Plaur domain containing 1, in mutant mice. The genetic delay in skin development was further confirmed by a functional delay in the formation of an epidermal barrier. These results document an important role for AP-2 gamma in skin development, and reveal the existence of regulatory factors that can compensate for AP-2 gamma in its absence.

Mutations in the helix termination motif of mouse type I IRS keratin genes impair the assembly of keratin intermediate filament

Two classical mouse hair coat mutations, Rex (Re) and Rex wavy coat (Re(wc)), are linked to the type I inner root sheath (IRS) keratin genes of chromosome 11. An N-ethyl-N-nitrosourea-induced mutation, M100573, also maps close to the type I IRS keratin genes. In this study, we demonstrate that Re and M100573 mice bear mutations in the type I IRS gene Krt25; Re(wc) mice bear an additional mutation in the type I IRS gene Krt27. These three mutations are located in the helix termination motif of the 2B alpha-helical rod domain of a type I IRS keratin protein. Immunohistological analysis revealed abnormal foam-like immunoreactivity with an antibody raised to type II IRS keratin K71 in the IRS of Re/+ mice. These results suggest that the helix termination motif is essential for the proper assembly of types I and II IRS keratin protein complexes and the formation of keratin intermediate filaments.

Epidermal hyperplasia and oral carcinoma in mice overexpressing the transcription factor ATF3 in basal epithelial cells

ATF3 is a highly conserved eukaryotic transcription factor that is ubiquitously upregulated transcriptionally during cellular responses to a variety of stresses, in particular DNA damage. However, the role of ATF3 in the DNA damage response is unclear. Transgenic mice that overexpress human ATF3 in basal epithelial cells under the control of the bovine keratin 5 (K5) promoter were constructed and characterized for epidermal alterations. Strong, nuclear expression of the exogenous ATF3 protein was seen in basal cells of the epidermis, hair follicles, and oral mucosa. Hyperplastic changes in the K5-expressing, outer root sheath (ORS) cells of the hair follicle were observed in young mice, resulting in multiple layers of ORS cells in the mature follicle and large aberrantly shaped follicles. Mild hyperplasia of the interfollicular epidermis was also noted, increasing with age. However, no epidermal tumors were identified in BK5.ATF3 mice observed for 16 mo. At 16 mo of age, most transgenic mice exhibited multi-focal areas of hyperplasia and dysplasia in the oral mucosa, with cellular atypia and underlying acute inflammatory changes. Neoplastic lesions were also seen in the oral cavity of BK5.ATF3 mice, including oral squamous cell carcinoma (60% incidence) and basal cell tumors with follicular differentiation (70% incidence), but not in non-transgenic FVB/N littermates. Heterogeneous nuclear expression (or stabilization) of p53 protein was seen in some oral dysplasias, with a patchy distribution primarily in the least differentiated layers of the lesions. This represents the first indication that ATF3 may have oncogenic properties in epithelial cells.

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).

Keratin expression provides novel insight into the morphogenesis and function of the companion layer in hair follicles

Hair follicles cycle between stages of growth (anagen) and metabolic quiescence (telogen) throughout life. In mature follicles, transition from telogen back into anagen involves the activation, proliferation, and differentiation of epithelial stem cells located in the bulge, a specialization of the outer root sheath. Recent studies identified keratin 6a (K6a) transcripts as enriched in bulge epithelial stem cells in mouse skin. We used messenger RNA probes, antibodies, a LacZ reporter mouse model, and whole-mount staining assays to investigate the regulation of mK6a during mouse postnatal hair cycling, and compare it to mK75, a companion layer (Cl) marker. We find that mK75 regulation parallels that of inner root sheath (IRS) markers, with expression onset at anagen IIIa above the new hair bulb and subsequent spreading towards the bulge. Although also occurring in the Cl, mK6a expression begins at anagen IIIb in differentiating cells located proximal to the bulge, and subsequently spreads towards the hair bulb. mK6a and mK75 thus exhibit temporally distinct, and spatially opposed, expression patterns in the Cl during postnatal anagen. These findings provide novel insight into the morphogenesis and properties of the Cl, and raise the distinct possibility that it is an integral part of the IRS compartment.

Morphologic and molecular characterization of two novel Krt71 (Krt2-6g) mutations: Krt71rco12 and Krt71rco13

We have analyzed two novel mouse mutant strains, Rco12 and Rco13, displaying a wavy pelage and curly vibrissae that have been identified in an ENU screen for dominant mutations affecting the pelage. The mutations were mapped to mouse Chromosome 15 and identified as missense point mutations in the first exon of the Krt71 (formerly called Krt2-6g) gene causing alterations of amino acid residue 143 from alanine to glycine (Rco12) and residue 146 from isoleucine to phenylalanine. The morphologic analyses demonstrated that both mutations cause identical phenotypes leading to the formation of filamentous aggregates in Henle's and Huxley's layers of the inner root sheath (IRS) of the hair follicle that leads to the bending of the hair shaft. Both novel mutations are located in the immediate vicinity of previously identified mutations in murine Krt71 that cause similar phenotypes and alter the helix initiation motif of the keratin. The characterization of these mutants demonstrates the importance of this Krt71 domain for the formation of linear IRS intermediate filaments.

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.

Functional analysis of keratin components in the mouse hair follicle inner root sheath

BACKGROUND

Recently, a family of novel type I keratins of the inner root sheath of the hair follicle were discovered, increasing the number of keratins known to be expressed in the hair follicle. The mouse database shows three keratins that are possible orthologues of these inner root sheath keratins. The sequences of these keratins include rather unusual changes to a highly conserved motif at the end of the alpha-helical rod domain of the proteins, thought to be important in filament assembly.

OBJECTIVES

To investigate whether these keratins are expressed in the inner root sheath and to determine whether they assemble normally.

METHODS

To investigate this, polyclonal antibodies were raised for immunolocalization of the keratins and their cDNAs were cloned for transfection into cultured cells.

RESULTS

At least two of these keratins were expressed in the inner root sheath but the timing of expression of the different keratins was variable. Transfection of the relevant cDNAs into cells in culture indicated that these keratins were capable of integrating into existing keratin networks without disruption, but that de novo filament assembly with the type II inner root sheath keratin, mK6irs, was poor.

CONCLUSIONS

These results provide further evidence of the complexity of keratin expression in the three concentric layers of the inner root sheath.

Alopecia in a novel mouse model RCO3 is caused by mK6irs1 deficiency

Reduced coat 3 (Rco3) is a new spontaneous autosomal recessive mutation with defects in hair structure and progressive alopecia. Here we describe chromosomal mapping and molecular identification of the Rco3 mutation. The murine Rco3 locus maps to a 2-Mb interval on chromosome 15 encompassing the keratin type II gene cluster. Recently, mK6irs1 was described as a type II keratin expressed in Henle's and Huxley's layer of the murine inner root sheath. Genomic sequencing revealed a 10-bp deletion in exon 1 of mK6irs1 resulting in a frameshift after 58 amino acid residues and, therefore, the absence of 422 carboxy-terminal amino acid residues containing the complete alpha-helical rod domain. Henle's and Huxley's layers show no immunoreactivity with mK6irs1-specific antibodies and the absence of intermediate filament formation in electron microscopic images. These results indicate that the expression of functional mK6irs1 is indispensable for intermediate filament formation in the inner root sheath and highlights the importance of the keratinization of the inner root sheath in the normal formation of the hair shaft.

A Small Deletion Hotspot in the Type II Keratin Gene mK6irs1/Krt2-6g on Mouse Chromosome 15, a Candidate for Causing the Wavy Hair of the Caracul (Ca) Mutation

A new mutation has arisen in a colony of mice transgenic for human α-galactosidase. The mutation is independent of the transgenic insertion, autosomal dominant, and morphologically very similar to the classical wavy coat mutation, caracul (Ca), on chromosome 15. Therefore, we designated this locus the caracul Rinshoken (CaRin). Applying a positional cloning approach, we identified the mK6irs1/Krt2-6g gene as a strong candidate for CaRin because among five Ca alleles examined mutations always occurred in the highly conserved positions of the α-helical rod domain (1A and 2B subdomain) of this putative gene product. The most striking finding is that four independently discovered alleles, the three preexistent alleles CaJ, Ca9J, Ca10J, and our allele CaRin, all share one identical amino acid deletion (N 140 del) and the fifth, CamedJ, has an amino acid substitution (A 431 D). These findings indicate that a mutation hotspot exists in the Ca locus. Additionally, we describe a Ca mutant allele induced by ENU mutagenesis, which also possesses an amino acid substitution (L 424 W) in the mK6irs1/Krt2-6g gene. The identification of the Ca candidate gene enables us to further define the nature of the genetic pathway required for hair formation and provides an important new candidate that may be implicated in human hair and skin diseases.

Molecular control of epithelial-mesenchymal interactions during hair follicle cycling

Epithelial-mesenchymal interactions play pivotal roles in the morphogenesis of many organs and various types of appendages. During hair follicle development, extensive interactions between two embryologically different hair follicle compartments (epidermal keratinocytes and dermal papilla fibroblasts) lead to the formation of the hair shaft-producing mini-organ that shows cyclic activity during postnatal life with periods of active growth, involution and resting. During the hair cycle, the epithelium and the mesenchyme are regulated by a distinct set of molecular signals that are unique for every distinct phase of the hair cycle. In telogen hair follicles, epithelial-mesenchymal interactions are characterized by a predominance of inhibitory signals that retain the hair follicle in a quiescent state. During anagen, a large variety of growth stimulatory pathways are activated in the epithelium and in the mesenchyme, the coordination of which are essential for proper hair fiber formation. During catagen, the termination of anagen-specific signaling interactions between the epithelium and the mesenchyme leads to apoptosis in the hair follicle epithelium, while activation of selected signaling pathways promotes the transition of the dermal papilla into a quiescent state. The signaling exchange between the follicular epithelium and the mesenchyme is modulated by proteoglycans, such as versican, which may significantly enhance or reduce the biological activities of secreted growth stimulators. However, additional research will be required to bridge the gap between our current understanding of mechanisms underlying epithelial-mesenchymal interactions in hair follicles and the potential clinical application of growth modulators involved in those interactions. Further progress in this area of research will hopefully lead to the development of new drugs for the treatment of hair growth disorders.

K6irs1, K6irs2, K6irs3, and K6irs4 represent the inner-root-sheath-specific type II epithelial keratins of the human hair follicle

In this study we report on the cloning of two novel human type II keratin cDNAs, K6irs3 and K6irs4, which were specifically expressed in the inner root sheath of the hair follicle. Together with the genes of two previously described type II inner root sheath keratins, K6irs1 and K6irs2, the K6irs3 and K6irs4 genes were subclustered in the type II keratin/hair keratin gene domain on chromosome 12q13. Evolutionary tree analysis using all known type II epithelial and hair keratins revealed that the K6irs1-4 formed a branch separate from the other epithelial and hair keratins. RNA in situ hybridization and indirect immunofluorescence studies of human hair follicles, which also included the K6irs2 keratin, demonstrated that both K6irs2 and K6irs3 were specifically expressed in the inner root sheath cuticle, but showed a different onset of expression in this compartment. Whereas the K6irs3 expression began in the lowermost bulb region, that of K6irs2 was delayed up to the height of the apex of the dermal papilla. In contrast, the K6irs4 keratin was specifically expressed in the Huxley layer. Moreover, K6irs4 was ideally suited to further investigate the occurrence of Flügelzellen, i.e., Huxley cells, characterized by horizontal cell extensions that pass through the Henle layer, abut upon the companion layer, and form desmosomal connections with the surrounding cells. Previously, we detected Flügelzellen only in the region along the differentiated Henle layer. Using the Huxley-cell-specific K6irs4 antiserum, we now demonstrate this cell type to be clearly apposed to the entire Henle layer. We provide evidence that Flügelzellen penetrate the Henle layer actively and may play a role in conferring plasticity and resilience to the otherwise rigid upper Henle layer.

Cloning of human, murine, and marsupial keratin 7 and a survey of K7 expression in the mouse

Keratins are cytoplasmic intermediate filament proteins expressed by epithelial cells. Keratin 7 (K7) is expressed in a wide range of epithelial structures in humans. We have cloned and fully sequenced the human and mouse K7 genes and mRNAs, and the K7 mRNA from the marsupial Potorous tridactylis, from which the widely used simple epithelial cell lines PtK1 and PtK2 are derived. Percentage identity plots comparing the mouse and human genomic sequences revealed a number of conserved CpG islands associated with the K7 gene. There was considerable conservation of introns between the two species, which may indicate the presence of intronic regulatory elements. Only the most proximal 500bp of the promoter was conserved, although an additional conserved sequence island was found 2-3kb upstream. Protein sequence comparisons between the three species allowed identification of conserved regions of the keratin variable domains that may be candidates for protein-protein interactions and/or regulatory modification. From the mouse sequence, we generated a polyclonal rabbit antibody specific for murine K7. This antibody was used to perform a survey of K7 expression in the mouse. The expression pattern was similar to the reported human distribution, with substantial expression observed in lung, bladder, mesothelium, hair follicle, and ductal structures. We also noted previously unreported expression of K7 in the gastrointestinal tract and filiform papillae of the tongue and specific K7 expression in a range of "hard" epithelial tissues. The distribution of K7 in mouse and availability of genomic sequence from the 129/Sv mouse strain will allow the generation and analysis of transgenic mice expressing mutant forms of K7 and to predict the phenotype of human genetic disorders caused by mutations in this keratin.