Structure and site of expression of a murine type II hair keratin

A role for the 1A and L1 rod domain segments in head domain organization and function of intermediate filaments: structural analysis of trichocyte keratin

A dynamic model is proposed to explain how the 1A and linker L1 segments of the rod domain in intermediate filament (IF) proteins affect the head domain organization and vice versa. We have shown in oxidized trichocyte IF that the head domain sequences fold back over and interact with the rod domain. This phenomenon may occur widely in reduced IF as well. Its function may be to stabilize the 1A segments into a parallel two-stranded coiled coil or something closely similar. Under differing reversible conditions, such as altered states of IF assembly, or posttranslational modifications, such as phosphorylation etc., the head domains may no longer associate with the 1A segment. This could destabilize segment 1A and cause the two alpha-helical strands to separate. Linker L1 would thus act as a hinge and allow the heads to function over a wide lateral range. This model has been explored using the amino acid sequences of the head (N-terminal) domains of Type I and Type II trichocyte keratin intermediate filament chains. This has allowed several quasi-repeats to be identified. The secondary structure corresponding to these repeats has been predicted and a model has been produced for key elements of the Type II head domain. Extant disulfide cross-link data have been used as structural constraints. A model for the head domain structure predicts that a twisted beta-sheet region may wrap around the 1A segment and this may reversibly stabilize a coiled-coil conformation for 1A. The evidence in favor of the swinging head model for IF is discussed.

Characterization of a 300 kbp region of human DNA containing the type II hair keratin gene domain

Screening of an arrayed human genomic P1 artificial chromosome DNA library by means of the polymerase chain reaction with a specific primer pair from the human type II hair keratin hHb5 yielded two P1 artificial chromosome clones covering approximately 300 kb of genomic DNA. The contig contained six type II hair keratin genes, hHb1-hHb6, and four keratin pseudogenes psihHbA-psihHbD. This hair keratin gene domain was flanked by type II epithelial keratins K6b/K6hf and K7, respectively. The keratin genes/pseudogene are 5-14 kbp in size with intergenic distances of 5-19 kbp of DNA and do not exhibit a single direction of transcription. With one exception, type II hair keratin genes are organized into nine exons and eight introns, with strictly conserved exon-intron boundaries. The functional hair keratin genes are grouped into two distinct subclusters near the extremities of the hair keratin gene domain. One subcluster encodes the highly related hair keratins hHb1, hHb3, and hHb6; The second cluster encodes the structurally less related hair keratins hHb2, hHb4, and hHb5. Reverse transcription-polymerase chain reaction shows that all hair keratin genes are expressed in the hair follicle. Pseudogene psihHbD is also transcriptionally expressed, albeit with alterations in splicing and frameshift mutations, leading to premature stop codons in the splice forms analyzed. Evolutionary tree analysis revealed a divergence of the type II hair keratin genes from the epithelial keratins, followed by their segregation into the members of the two subclusters over time. We assume that the approximately 200 kbp DNA domain contains the entire complement of human type II hair keratin genes.

Regulation of a hair follicle keratin intermediate filament gene promoter

During hair growth, cortical cells emerging from the proliferative follicle bulb rapidly undergo a differentiation program and synthesise large amounts of hair keratin proteins. To identify some of the controls that specify expression of hair genes we have defined the minimal promoter of the wool keratin intermediate filament gene K2.10. The region of this gene spanning nucleotides −350 to +53 was sufficient to direct expression of the lacZ gene to the follicle cortex of transgenic mice but deletion of nucleotides −350 to −150 led to a complete loss of promoter activity. When a four base substitution mutation was introduced into the minimal functional promoter at the binding site for lymphoid enhancer factor 1 (LEF-1), promoter activity in transgenic mice was decreased but specificity was not affected. To investigate the interaction of trans-acting factors within the minimal K2.10 promoter we performed DNase I footprinting analyses and electrophoretic mobility shift assays. In addition to LEF-1, Sp1, AP2-like and NF1-like proteins bound to the promoter. The Sp1 and AP2-like proteins bound sequences flanking the LEF-1 binding site whereas the NF1-like proteins bound closer to the transcription start site. We conclude that the LEF-1 binding site is an enhancer element of the K2.10 promoter in the hair follicle cortex and that factors other than LEF-1 regulate promoter tissue- and differentiation-specificity.

Sonic hedgehog signaling is essential for hair development

BACKGROUND

The skin is responsible for forming a variety of epidermal structures that differ amongst vertebrates. In each case the specific structure (for example scale, feather or hair) arises from an epidermal placode as a result of epithelial-mesenchymal interactions with the underlying dermal mesenchyme. Expression of members of the Wnt, Hedgehog and bone morphogenetic protein families (Wnt10b, Sonic hedgehog (Shh) and Bmp2/Bmp4, respectively) in the epidermis correlates with the initiation of hair follicle formation. Further, their expression continues into either the epidermally derived hair matrix which forms the hair itself, or the dermal papilla which is responsible for induction of the hair matrix. To address the role of Shh in the hair follicle, we have examined Shh null mutant mice.

RESULTS

We found that follicle development in the Shh mutant embryo arrested after the initial epidermal-dermal interactions that lead to the formation of a dermal papilla anlage and ingrowth of the epidermis. Wnt10b, Bmp2 and Bmp4 continued to be expressed at this time, however. When grafted to nude mice (which lack T cells), Shh mutant skin gave rise to large abnormal follicles containing a small dermal papilla. Although these follicles showed high rates of proliferation and some differentiation of hair matrix cells into hair-shaft-like material, no hair was formed.

CONCLUSIONS

Shh signaling is not required for initiating hair follicle development. Shh signaling is essential, however, for controlling ingrowth and morphogenesis of the hair follicle.

Hard alpha-keratin intermediate filament chains: substructure of the N- and C-terminal domains and the predicted structure and function of the C-terminal domains of type I and type II chains

The quantity of sequence data now available for both Type I and Type II hard alpha-keratin IF proteins makes it possible to analyze their N- and C-terminal domains and ascertain features of likely structural and/or functional importance. The N-terminal domains of both chain types can be divided into acidic (NA) and basic (NB) subdomains, where NA is 29 and 34 residues long, respectively, for Type I and II chains and is located immediately adjacent to the end of the rod domain. NB constitutes the remainder of the N-terminal domain and is about 27 and 70 residues long for the two chain types, respectively. The glycine residue contents, however, are high in NA(I) and NB(II), but low in NA(II) and NB(I). Subdomain NB(II) contains four consecutive nonapeptide quasirepeats of the form GGGFGYRSX. The C-terminal domain of Type I chains, termed C(I), is characterized by a PCX motif repeated 10 times, 7 of them contiguously. From an analysis of the conformation of like peptides from crystal structures it has been shown that this region will probably adopt a polyproline II left-handed helical structure with three residues per turn. In contrast, the C-terminal domain of Type II hard alpha-keratin chains (known as C(II)) contains a periodic distribution of hydrophobicities that, together with other predictive techniques, allow its conformation (a twisted four-stranded antiparallel beta-sheet) to be predicted with some degree of confidence. In addition, it is possible to suggest two partners with which this domain will interact. The first is with segment L12 in the rod domain and the second is with another C(II) domain in an antiparallel neighboring molecule. The latter possibility appears most likely. In either case the aggregation would likely serve to stabilize the molecular assembly through the interaction of two beta-sheets via their apolar faces and, in so doing, would position a number of cysteine residues in external positions that would allow them to form a number of covalent disulfide bonds with other molecules.

Hoxc13 mutant mice lack external hair

Hox genes are usually expressed temporally and spatially in a colinear manner with respect to their positions in the Hox complex. Consistent with the expected pattern for a paralogous group 13 member, early embryonic Hoxc13 expression is found in the nails and tail. Hoxc13 is also expressed in vibrissae, in the filiform papillae of the tongue, and in hair follicles throughout the body; a pattern that apparently violates spatial colinearity. Mice carrying mutant alleles of Hoxc13 have been generated by gene targeting. Homozygotes have defects in every region in which gene expression is seen. The most striking defect is brittle hair resulting in alopecia (hairless mice). One explanation for this novel role is that Hoxc13 has been recruited for a function common to hair, nail, and filiform papilla development.

Sequences and differential expression of three novel human type-II hair keratins

As part of a program designed to characterize human hair keratin genes and their expression, we present the cDNA sequences and deduced amino acid sequences of three type-II hair keratins hHb3, hHb5, and hHb6, which by virtue of their amino acid homologies are the orthologs of the previously described sheep wool keratins, K2.10, K2.12, and K.211 [29]. Amino acid sequences comparisons of these keratins, including the previously characterized human K2.9 ortholog hHb1, show extreme conservation not only in the alpha-helices but also in the aminoterminal and proximal carboxyterminal domains. They also demonstrate higher sequence relationships between hHb1, hHb3, and hHb6 as compared to hHb5, which exhibits chain-specific sequences in both the head and tail domains. In situ hybridization studies using specific 3'-probes for the four type-II hair keratins reveal sequential patterns of gene expression in human anagen follicles. Remarkably the onset of hHb5 mRNA synthesis occurs immediately above a small population of matrix cells at the base of the hair bulb and the trichocytes lining the dermal papilla. hHb5 mRNA synthesis extends upward through the matrix and ends in the lower part of the cortex of the hair shaft. In contrast, both hHb1 and hHb3 mRNA synthesis begins simultaneously in the cortex 10-15 cell layers above the apex of the dermal papilla, thus partially overlapping that of hHb5 but continuing to a point well beyond hHb5 in the upper cortex. Synthesis of hHb6 mRNA starts slightly higher than either hHb1 or hHb3 mRNA and proceeds much farther up into the keratogenous zone of the hair shaft. Our study demonstrates that the differentiation of human hair in terms of hair keratin expression begins much earlier than previously assumed, i.e. in lower matrix cells of the hair bulb. This early phase of hair differentiation is followed by a late cortical phase of terminal differentiation which comprises at least three type-II hair keratins in the zone of elongation and the keratogenous zone of the hair shaft.

The role of keratin proteins and their genes in the growth, structure and properties of hair

The importance of wool in the textile industry has inspired extensive research into its structure since the 1960s. Over the past several years, however, the hair follicle has increased in significance as a system for studying developmental events and the process of terminal differentiation. The present chapter seeks to integrate the expanding literature and present a broad picture of what we know of the structure and formation of hair at the cellular and molecular level. We describe in detail the hair keratin proteins and their genes, their structure, function and regulation in the hair follicle, and also the major proteins and genes of the inner and outer root sheaths. We discuss hair follicle development with an emphasis on the factors involved and describe some hair genetic diseases and transgenic and gene knockout models because, in some cases, they stimulate natural mutations that are advancing our understanding of cellular interactions in the formation of hair.

Characterization of a gene encoding a cysteine-rich keratin associated protein synthesized late in rabbit hair follicle differentiation

Many different cysteine-rich proteins are synthesized during hair follicle differentiation, forming part of the interstitial matrix between bundles of intermediate filaments. We have isolated a rabbit gene (rKAP4L), a member of a multigene family that encodes a small cysteine-rich hair keratin associated protein. This is the first complete gene sequence for this family. The rKAP4L gene is expressed in the cortex of rabbit pelage hair follicles at a late stage of hair follicle differentiation, well after the synthesis of the other major hair proteins, the intermediate filament and glycine/tyrosine-rich keratin associated proteins, has commenced. The protein contains 36 mol % cysteine, with a molecular size of 13593 daltons, and its sequence appears to be based on a pentapeptide repeat. It is predicted to adopt a folded conformation characterized by beta-turns interspersed with short stretches of beta-sheet or random coil.

A novel human type I hair keratin gene: evidence for two keratin hHa3 isoforms

We present the nucleotide and amino acid sequence for a novel human type I hair keratin, which could be identified through its high sequence homology and strict carboxyterminal length identity as a human ortholog of the murine hair keratin mHa3. Our hHa3 sequence differs, however, from that of a previously described hHa3 hair keratin (published only as an amino acid sequence; [13]) in 24 amino acid position, 8 of which occur in the middle of the carboxyterminal domain. PCR of genomic DNA from 25 normal human subjects using a primer pair derived from sequence segments located in the 3'-region of our hHa3 clone that encode conserved amino acid sequences in both keratins, resulted in the amplification of two distinct products of 0.38 kbp and 1.0 kbp. DNA sequence analysis of the cloned PCR products allowed identification of the 0.38 kb sequence as that originating from Yu et al. [13] and the 1.0 kb sequence as that being derived from our data. The difference in fragment length was due to unique intron 6 sequences, indicating that these two keratin species are encoded by genes of their own. Moreover, extensive Southern blot analyses with DNA from 25 unrelated individuals of different races using a 3'-noncoding sequence from our keratin and the intron 6 sequence of the keratin of Yu et al. [13], as hybridization probes showed that both keratin genes are present as single copy sequences occurring ubiquitously and without gross alterations in the human genome. Collectively, these data demonstrate that the human type I hair keratin described in this paper represents an isoform of the previously described hHa3 keratin.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence and expression of human hair keratin genes

Normal hair growth and differentiation requires co-ordinate expression of many hair specific structural protein genes. It has been established that one of the 4 major groups of hair structural proteins, low-sulphur hair keratins, belongs to the intermediate filament (IF) multigene family. Hair keratin IF proteins differ from those of other epithelia as they contain cysteine-rich terminal domains allowing more extensive disulphide bonding to the high-sulphur hair matrix proteins. Until recently, little information concerning the primary sequence of hair keratins was available but cloning of some mouse hair and sheep wool keratins has now been reported. Using these sequences, we have polymerase chain reaction (PCR) amplified genomic fragments of human hair-specific keratin IF genes and isolated cosmid clones containing full length genes. We have sequenced part of these genes and studied their expression in human hair follicles. Hair specific keratin fragments were amplified from placental gDNA by PCR primed with synthetic oligonucleotides. Fragments were cloned and sequenced after ligation into pGEM-3Z and labelled riboprobes were generated for in situ hybridization on human skin sections. A human cosmid library was screened with PCR fragments and clones encoding human hair keratin genes were characterised by southern hybridization and sequencing. The type I human hair-specific keratin clones obtained (HaKA1-b2, 386 bp; hHaKA1-XH1, 1202 bp) encoded 2B helix, C-terminal and 3'nc regions and were 65% homologous to mouse sequences. The type II hair keratin clone (hHaKB2-1, 829 bp) also encoded 2B helix and C-terminal regions and was 95% homologous to mouse. In situ hybridization on human skin sections showed a specific reaction with precortical cells of the hair follicle. One human cosmid clone, isolated with the hHaKB2-1 probe, contained two type II hair keratin genes about 7 kb apart, each of which had 9 exons spanning approximately 6 kb. The coding sequences were homologous to mouse cDNA (77-88%). These human hair-specific keratin clones are useful molecular tools for studies of hair differentiation.

The large type II 70-kDa keratin of mouse epidermis is the ortholog of human keratin K2e

The basic keratin pattern of mammalian epidermis consists of the basal keratin pair K5/K14 and the differentiation-specific keratin pair K1/K10. Distinct skin sites of the adult mouse, i.e., ear, sole of the foot, and interscale regions of tail skin, express an additional, type II 70-kilodalton (kDa) keratin without a defined new type I partner in suprabasal epidermal cells. Until now, the question whether this large keratin is specific for the mouse (or related small rodents) or whether orthologous keratins exist in other species has not yet been answered. In the present study, we have determined the full-length amino acid sequence of the 70-kDa keratin. The keratin comprises 707 amino acid residues and has a calculated molecular weight of 70,976.70 Da. From the structural point of view, the 70-kDa keratin is remarkable in that more than half of both the V1 and V2 subdomains of its non alpha-helical head and tail portions consist of different glycine-rich peptide motifs that are configured consecutively at least two times and as much as seven times in tandem. By means of sequence comparisons and phylogenetic investigations, we show that the 70-kDa keratin represents the murine ortholog of the human 65-kDa keratin K2e, whose nature as a genuine keratin has recently been demonstrated. The unusually large size difference of 5 kDa between MK2e and HK2e is due mainly to a different duplication rate of the glycine-rich peptide motifs in the respective V subdomains of the orthologous keratins. We discuss the properties of these highly specialized keratins, which in both species define locally restricted epidermal keratin phenotypes, and compare them with other orthologous keratins that belong to the basic epidermal keratin pattern.

Characterization of a hair (wool) keratin intermediate filament gene domain

In epithelial differentiation keratin intermediate filament genes are expressed in multifarious tissue-specific and stage-specific patterns. Pairs of type I and type II intermediate filament genes, belonging to multigene families, are coordinately regulated, and 4-5 genes of each type are expressed in the hair follicle. Accumulating chromosomal mapping data points to a major locus for each intermediate filament multigene family on separate chromosomes. In this report we describe the isolation of a sheep hair keratin cosmid by chromosome walking that overlaps two previously described cosmids and establishes a continuous 100-kb segment of cloned DNA containing three hair and three hair-like type II intermediate filament keratin genes. A new hair keratin type II intermediate filament gene, KRT2.11, is located in the middle of the cluster, and partial sequence data reveal a striking conservation of its predicted N-terminal region with other sheep hair keratin type II intermediate filament proteins. Expression analyses demonstrate the presence of a 2.4-kb KRT2.11 transcript in wool follicle RNA and show that expression occurs in the follicle cortical keratinocytes above the dermal papilla. The three hair genes are clustered within about 40 kb and flanked by hair-like genes that are not expressed in the hair follicle, thereby demarcating a hair keratin gene domain.

Complete sequence of a hair-like intermediate filament type II keratin gene

The Intermediate Filament (IF) superfamily comprises several multigene families, of which the two keratin families are the largest. The keratin IF genes are expressed in epithelial tissues in differentiation-specific patterns and recently we reported the sequence and expression of a hair IF type II keratin gene (KRT2.9). Two related genes were present in the cosmid containing KRT2.9 and we have now sequenced one of them and found that it encodes a hair-like IF type II protein (KRT2.13). However, KRT2.13 is not expressed in the hair follicle. Interestingly there is significant sequence homology between introns 1, 5 and 6 of KRT2.13 and KRT2.9 to suggest gene conversion of these regions or possibly conservation of functional sequences.

Structural features and sites of expression of a new murine 65 kD and 48 kD hair-related keratin pair, associated with a special type of parakeratotic epithelial differentiation

In the course of studies on local keratin phenotypes in the epidermis of the adult mouse, we have identified a new 65 kD and 48 kD keratin pair. In mouse skin, this keratin pair is only expressed in suprabasal cells of adult mouse tail scale epidermis which is characterized by the complete absence of a granular layer and the formation of a remarkably compact stratum corneum. A second site in which the 65 kD and 48 kD keratin pair is suprabasally expressed and whose morphology corresponds to that of tail scale epidermis is found in the posterior unit of the complex filiform papillae of mouse tongue. The causal relationship of the expression of the 65 kD and 48 kD keratins with this particular type of a non-pathological epithelial parakeratosis is emphasized by the suppression of the mRNA synthesis of the two keratins during retinoic acid mediated orthokeratotic conversion of tail scale epidermis. Apart from tail scale epidermis and the posterior unit of the filiform papillae, the 65 kD and 48 kD keratin pair is, however, also coexpressed with "hard" alpha keratins in suprabulbar cells of hair follicles and in suprabasal cells of the central core unit of the lingual filiform papillae. The non alpha-helical domains of the two new keratins are rich in cysteine and proline residues and lack the typical subdomains into which epithelial keratins of both types can be divided. This structural resemblance of the 65 kD and 48 kD keratins to "hard" alpha keratins is supported by comparative flexibility predictions for their non alpha-helical domains. Phylogenetic investigations then show that the 65 kD and 48 kD keratin pair has evolved together with hair keratins, but has diverged from these during evolution to constitute an independent branch of a pair of hair-related keratins. In view of this exceptional position of the 65 kD and 48 kD keratins within the keratin multigene family, their expression has apparently been adopted by rare anatomical sites in which an orthokeratinized stratum corneum would be too soft and a hard keratinized structure would be too rigid to meet the functional requirement of the respective epithelia.