Genetic and biochemical aspects of keratin synthesis by hair follicles

In this review article the data about synthesis and gene regulation of keratin by hair follicles have been summarized. It has been shown that both differentiation of hair follicle matrix cells and normal growth of hair require the coordinated activities of the genes encoding structural proteins. The keratin genes are clustered in families and are usually 5-10 kb in the genome. The separate clusters of two keratin IF gene families and five KAP gene families have been discovered and some of them have been mapped. The close relation between these clusters suggests that the "global" regulatory domains might govern their expression.     

The organization of the keratin I and II gene clusters in placental mammals and marsupials show a striking similarity

The genomic database for a marsupial, the opossum Monodelphis domestica, is highly advanced. This allowed a complete analysis of the keratin I and keratin II gene cluster with some 30 genes in each cluster as well as a comparison with the human keratin clusters. Human and marsupial keratin gene clusters have an astonishingly similar organization. As placental mammals and marsupials are sister groups a corresponding organization is also expected for the archetype mammal. Since hair is a mammalian acquisition the following features of the cluster refer to its origin. In both clusters hair keratin genes arose at an interior position. While we do not know from which epithelial keratin genes the first hair keratins type-I and -II genes evolved, subsequent gene duplications gave rise to a subdomain of the clusters with many neighboring hair keratin genes. A second subdomain accounts in both clusters for 4 neighboring genes encoding the keratins of the inner root sheath (irs) keratins. Finally the hair keratin gene subdomain in the type-I gene cluster is interrupted after the second gene by a region encoding numerous genes for the high/ultrahigh sulfur hair keratin-associated proteins (KAPs). We also propose a tentative synteny relation of opossum and human genes based on maximal sequence conservation of the encoded keratins. The keratin gene clusters of the opossum seem to lack pseudogenes and display a slightly increased number of genes. Opossum keratin genes are usually longer than their human counterparts and also show longer intergenic distances.     

Structure and expression of genes for a class of cysteine-rich proteins of the cuticle layers of differentiating wool and hair follicles

The major histological components of the hair follicle are the hair cortex and cuticle. The hair cuticle cells encase and protect the cortex and undergo a different developmental program to that of the cortex. We report the molecular characterization of a set of evolutionarily conserved hair genes which are transcribed in the hair cuticle late in follicle development. Two genes were isolated and characterized, one expressed in the human follicle and one in the sheep follicle. Each gene encodes a small protein of 16 kD, containing greater than 50 cysteine residues, ranging from 31 to 36 mol% cysteine. Their high cysteine content and in vitro expression data identify them as ultra-high-sulfur (UHS) keratin proteins. The predicted proteins are composed almost entirely of cysteine-rich and glycine-rich repeats. Genomic blots reveal that the UHS keratin proteins are encoded by related multigene families in both the human and sheep genomes. Tissue in situ hybridization demonstrates that the expression of both genes is localized to the hair fiber cuticle and occurs at a late stage in fiber morphogenesis.     

Terrestrial vertebrates have two keratin gene clusters; striking differences in teleost fish

Keratins I and II form the largest subgroups of mammalian intermediate filament (IF) proteins and account as obligatory heteropolymers for the keratin filaments of epithelia. All human type I genes except for the K18 gene are clustered on chromosome 17q21, while all type II genes form a cluster on chromosome 12q13, that ends with the type I gene K18. Highly related keratin gene clusters are found in rat and mouse. Since fish seem to lack a keratin II cluster we screened the recently established draft genomes of a bird (chicken) and an amphibian (Xenopus). The results show that keratin I and II gene clusters are a feature of all terrestrial vertebrates. Because hair with its multiple hair keratins and inner root sheath keratins is a mammalian acquisition, the keratin gene clusters of chicken and Xenopus tropicalis have only about half the number of genes found in mammals. Within the type I clusters all genes have the same orientation. In type II clusters there is a rare gene of opposite orientation. Finally we show that the genes for keratins 8 and 18, which are the first expression pair in embryology, are not only adjacent in mammals, but also in Xenopus and three different fish. Thus neighboring K8 and K18 genes seem a feature shared by all vertebrates. In contrast to the two well defined keratin gene clusters of terrestrial vertebrates, three teleost fish show an excess of type I over type II genes, the lack of a keratin type II gene cluster and a striking dispersal of type I genes, that are probably the result of the teleost-specific whole genome duplication followed by a massive gene loss. This raises the question whether keratin gene clusters extend beyond the ancestral bony vertebrate to cartilage fish and lamprey. We also analyzed the complement of non-keratin IF genes of the chicken. Surprisingly, an additional nuclear lamin gene, previously overlooked by cDNA cloning, is documented on chromosome 10. The two splice variants closely resemble the lamin LIII a + b of amphibia and fish. This lamin gene is lost on the mammalian lineage.     

Comprehensive analysis of keratin gene clusters in humans and rodents

Here, we present the comparative analysis of the two keratin (K) gene clusters in the genomes of man, mouse and rat. Overall, there is a remarkable but not perfect synteny among the clusters of the three mammalian species. The human type I keratin gene cluster consists of 27 genes and 4 pseudogenes, all in the same orientation. It is interrupted by a domain of multiple genes encoding keratin-associated proteins (KAPs). Cytokeratin, hair and inner root sheath keratin genes are grouped together in small subclusters, indicating that evolution occurred by duplication events. At the end of the rodent type I gene cluster, a novel gene related to K14 and K17 was identified, which is converted to a pseudogene in humans. The human type II cluster consists of 27 genes and 5 pseudogenes, most of which are arranged in the same orientation. Of the 26 type II murine keratin genes now known, the expression of two new genes was identified by RT-PCR. Kb20, the first gene in the cluster, was detected in lung tissue. Kb39, a new ortholog of K1, is expressed in certain stratified epithelia. It represents a candidate gene for those hyperkeratotic skin syndromes in which no K1 mutations were identified so far. Most remarkably, the human K3 gene which causes Meesmann's corneal dystrophy when mutated, lacks a counterpart in the mouse genome. While the human genome has 138 pseudogenes related to K8 and K18, the mouse and rat genomes contain only 4 and 6 such pseudogenes. Our results also provide the basis for a unified keratin nomenclature and for future functional studies.     

Concerted gene duplications in the two keratin gene families

Summary Evolutionary trees were derived from the keratin protein sequences using the Phylogeny Analysis Using Parsimony (PAUP) set of programs. Three major unexpected conclusions were derived from the analysis: The smallest keratin protein subunit, K#19 (Moll et al. 1982), is not the most primitive one, but has evolved to fulfill a highly specialized function, presumably to redress the unbalanced synthesis of keratin subunits. Second, the ancestors of keratins expressed in the early embryonic stages, K#8 and K#18, were the first to diverge from the ancestors of all the other keratins. The branches leading to these two keratins are relatively short, indicating a comparatively strong selection against changes in the sequences of these two proteins. Third, the two keratin families show extraodinary parallelism in their patterns of gene duplications. In both families the genes expressed in embryos diverged first, later bursts of gene duplications created the subfamilies expressed in various differentiated cells, and relatively recent gene duplications gave rise to the hair keratin genes and separated the basal cell-specific keratin from those expressed under hyperproliferative conditions. The parallelism of gene duplications in the two keratin gene families implies a mechanism in which duplications in one family influence duplication events in the other family.     

Hair follicle differentiation and regulation

Ten years ago, Hardy (1992) wrote a timely review on the major features of hair follicle development and hair growth which she referred to as a secret life. Many of these secrets are now being revealed. The information discussed in this brief review comprises the structure of the hair and hair follicle, the continuing characterisation of the genes for keratin and keratin associated proteins, the determination of the location of their expression in the different cell layers of the hair follicle, molecular signals which control keratin gene expression and post-translational events in the terminal stages of hair formation.     

Chromosomal localization of acidic and basic keratin genes of the domestic dog

Our laboratories are interested in characterizing genes involved in the myriad of heritable diseases affecting the domestic dog, Canis lupus familiaris, and in development of detailed genetic and physical maps of the canine genome. Included in these efforts is examination of conservation of the genetic organization, structure, and function of gene families involved in diseases of the canine skin, skeleton, and eye. To that end, study of the highly conserved keratin gene family was undertaken. Keratins belong to the superfamily of intermediate filaments and are the major structural proteins of the epidermis, hair, and nail. The keratins are highly conserved throughout vertebrate evolution both at the DNA and amino acid sequence levels. Mutations in genes encoding epithelial keratins are known to cause various diseases in humans, and similar histopathological presentations have been reported in the dog. The keratins are divided into two groups, type I (acidic) and type II (basic). In the human, the genes encoding the acidic and basic keratins are clustered on Chrs 17 and 12, respectively. The same genetic arrangement is seen in the mouse with the acidic and basic keratin gene clusters found on Chrs 11 and 15, respectively. Reported here are the chromosomal localization of acidic and basic canine keratin genes as well as supportive sequence data. Fluorescence in situ hybridization (FISH) experiments with clones isolated from a canine genomic library suggest that the acidic keratin gene cluster resides on CFA9 and the basic keratin gene cluster is located on CFA27. Received: 25 September 1998 / Accepted: 1 December 1998     

Identification of two novel clusters of ultrahigh-sulfur keratin-associated protein genes on human chromosome 11

We analyzed two novel clusters of keratin-associated protein (KAP) genes on human chromosome 11 (11p15.5 and 11q13.5) in which we identified two known human KRTAP5 genes, KerA (=KRN1) and KerB, and nine novel KRTAP5 family genes. RT-PCR analysis of these KAP genes showed preferential expression in human hair root, suggesting these gene products are required for hair formation. Based on the deduced amino acid sequences, all these KAP proteins were classified into an ultrahigh-sulfur (UHS) type KAP with high cysteine content (> 30 mol%). These KAPs also showed high glycine and serine contents (average 24.30 and 21.13 mol%, respectively), distinguishing from other UHS/HS KAP families located on human chromosomes 17 and 21. Dot-matrix analysis revealed a significant similarity between these two KAP gene clusters. We postulated a mechanism by which these two KAP gene clusters are generated via genomic duplication of a primordial gene cluster followed by genetic modification during evolution.     

山羊不同组织来源EST片段的生物信息学分析

利用生物信息学方法,对GeneBank中截至2006年9月收录的全部来源于山羊基因的共计637条表达序列标签（EST）进行了综合及分类分析。结果显示,在来源于绒山羊含毛囊皮肤的392条EST序列中有48条为编码角蛋白或角蛋白关联蛋白的基因;在乳腺来源的245条EST中则无此类基因,而是如免疫球蛋白基因、维生素转运蛋白基因、MHC等诸多种类基因,以免疫和酶类居多。两类不同组织来源的FST相应基因相似之处最显著的是编码核糖体蛋白的基因,其中含毛囊皮肤组织的EST中有15．1％,乳腺组织为21．6％,并且有两种不同组织来源的EST组成的26条基序（contigs）中,编码核糖体蛋白的有17条,高达65．4％。     

Hoxc13在毛囊发育中的作用

Hoxc13属于Hox(Homobox)基因家族Abd-B类成员之一, 与毛囊形成和毛发生长密切相关。毛发结构蛋白KP(角蛋白)和KAP(角蛋白关联蛋白)的表达都受Hoxc13的严格调控, Hoxc13表达水平会直接影响毛发的特性, 对维持毛囊的正常形态也至关重要。文章就Hoxc13的表达水平对毛囊发育和毛发生长的影响及Hoxc13与相关基因的调控进行了综述。     

Cyclic hair-loss and regrowth in transgenic mice overexpressing an intermediate filament gene.

We have produced transgenic mice containing up to 250 copies of a sheep wool intermediate filament keratin gene to study the effect of its expression on hair structure and development. Several transgenic lines expressed the gene and in the one containing 250 transgenes, a pattern of hair-loss and regrowth was stably established. Successive waves of hair growth follow periods of denuding like the natural progression of hairs in the mouse hair cycle. By in situ hybridization we have shown that the sheep transgenes are expressed at the correct stage in mouse hair development and at a high level. The transgenic hairs contain not only an elevated level of intermediate filament keratin protein but also a decreased level of the filament-associated proteins. This imbalance disrupts the normal ordered array of these proteins in the cells of the hair cortex and leads to weakened fibres which are prematurely lost.