NEWER FINDINGS ON THE ENZYMES AND PROTEINS OF HAIR FOLLICLES

Known and Unknown Features of Hair Cuticle Structure: A Brief Review

The cuticle is the outermost layer of overlapping flattened cells of hair and has been subjected to many years of study to understand its structure and how it develops in the follicle. The essential function of the cuticle with its tough inelastic protein content is to protect the inner cortex that provides the elastic properties of hair. Progress in our knowledge of hair came from studies with the electron microscope, initially transmission electron microscopy (TEM) for internal structure and later the scanning electron microscope (SEM) for cuticle surface shape and for investigating changes caused by various environmental influences such as cosmetic treatments and industrial processing of wool. Other physical techniques have been successfully applied in conjunction with proteomics. The outstanding internal features of the cuticle cells are the internal layers consisting of keratin filament proteins and the keratin-associated proteins. The stability and physical toughness of the cuticle cell is partly accounted for by the high content of disulphide crosslinking. The material between the cells that holds them tightly together, the cell membrane complex, consists of a layer of lipid on both sides of a central protein layer. The lipid contains 18-methyleicosanoic acid that is part of the hydrophobic lipid surface of hair. For the past decade there have been aspects that remained unanswered because they are difficult to study. Some of these are discussed in this brief review with suggestions for experimental approaches to shed more light.

Biology of the wool follicle: an excursion into a unique tissue interaction system waiting to be re-discovered

Wool fibres are hairs and the term 'wool' is usually restricted to describe the fine curly hairs that constitute the fleece produced by sheep. In a broader sense, it can be used to describe the fleeces produced by related species such as goat or yak. Research into the biology of wool growth and the structure of the wool fibre has been driven by the demands of the wool industry to improve both the efficiency of growing wool and the quality of the product. Well beyond this very applied perspective however, the wool follicle is a unique basic research model for the life sciences in general. These unique features include, to name just a few selected examples, accessibility for studying the molecular controls involved in branching of secondary epithelial-mesenchymal structures, the photoperiod-dependence of regenerating tissue interaction systems, the origin of fibre curliness and follicle wave pattern formation, and the effect of alterations in nutrient supply on epithelial growth and fibre structure. In this review, investigation of growth processes in the formation of the wool fibre is broadly surveyed. The relevance and potential for practical outcomes through characterization of wool follicle genes are discussed and particular features of the wool follicle contributing to our knowledge of the biology of hair growth are highlighted. The practical potential of gene discovery in wool research is the provision of molecular markers for selective breeding and for altering wool growth and wool structure by other biological pathways such as sheep transgenesis that could lead to novel wool properties. In this background, the current review attempts to revive general interest in the fascinating biology of the wool follicle which is not only of profound economic and practical importance but offers an exquisite, highly instructive research model for addressing key questions of modern biology.

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.

A unique type I keratin intermediate filament gene family is abundantly expressed in the inner root sheaths of sheep and human hair follicles

A unique type I keratin intermediate filament group, comprising three highly related proteins and expressed in the inner root sheath of hair follicles, has been identified in both sheep and human. The first members from these species are named oIRSa1 and hIRSa1 and each encodes a protein of 450 amino acids, with compositional characteristics intermediate between those of previously described hair keratin and epidermal cytokeratin type I intermediate filaments. Detection of abundant mRNA transcripts derived from the sheep and human genes by cRNA in situ hybridization only in the inner root sheath and not in the medulla concurs with the findings of earlier ultrastructural analyses that have reported intermediate filaments only in the inner root sheath. Clustering of the IRSa keratin genes is apparent in the genomes of both species. The three hIRSa genes, known to reside on human chromosome 17, are closely linked to three further type I keratin intermediate filament genes of unknown function. This new gene complex, contained almost entirely within a 156 kb BAC (hRPK.142_H_19), is likely to lie near the type I intermediate filament cytokeratin and hair keratin gene loci at 17q12-q21. A phylogenetic analysis including all known human type I intermediate filament cytokeratins, hHa keratins, hIRSa, and hIRSa-linked keratins suggests that origin of the IRSa keratin intermediate filament linkage group preceded origin of most of the epidermal cytokeratins and all hair keratins during emergence of the keratin intermediate filament genes.

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.

THE INCORPORATION AND FATE OF H3-TYROSINE IN THE HAIR CORTEX OF RATS OBSERVED BY RADIOAUTOGRAPHY

The incorporation of H³-tyrosine into the protein of the cells in the cortex of rat hair has been investigated by radioautography. In growing hairs, radioactivity is found in the matrix, the upper bulb, and the whole of the keratogenous zone up to the fully keratinized part of the shaft, 10 and 30 minutes after an injection of labelled tyrosine. This is unequivocal evidence of protein synthesis at these sites. There is a very precise relationship between the end of protein synthesis and the hardening of the cortical cells at the top of the keratogenous zone. The way in which the silver grains of the radioautographs are clustered indicates that at 30 minutes after the injection the isotope is distributed more evenly in the matrix and upper bulb than in the top of the keratogenous zone. Possibly this reflects a difference, at these sites, in the cell components engaged in protein synthesis, or in the proteins being synthesized. The fully keratinized and hardened part of the hair was not radioactive at 10 and 30 minutes after the injection of H³-tyrosine. The rate at which the radioactivity moves into this region shows that the hair of rats grows 0.9 mm/24 hours. Comparison of the degree of radioactivity along the growing hair in the 30-minute, 12-hour, and 36-hour materials shows conclusively that protein accumulates in the cortical cells during their keratinization. An injection of a labelled amino acid does not behave as an ideal pulse dose; consequently, the grain density over the hair cortex at 36 hours is 100 per cent larger than would be expected if an ideal pulse dose situation existed.

A STUDY OF THE ULTRASTRUCTURAL LOCALIZATION OF HAIR KERATIN SYNTHESIS UTILIZING ELECTRON MICROSCOPIC AUTORADIOGRAPHY IN A MAGNETIC FIELD

The sites of the incorporation of labeled cystine into keratinizing structures were studied in electron microscopic autoradiographs. The tracer used was cystine labeled with S(35) emitting long-range ionizing particles. During exposure for 1 to 2 months, according to our method of electron microscopic autoradiography, emulsion-coated specimens were exposed to a static magnetic field which appeared to result in a marked increase in the number of reacted silver grains. In young Swiss mice receiving intraperitoneal injections at 1, 3, and 6 hours before biopsy, conventional autoradiography demonstrated that S(35)-cystine was intensely localized in the keratogenous zone of anagen hair follicles, and that the radioactivity there increased in intensity progressively with time while the radioactivity in the hair bulb always remained very low. Our observations with electron microscopic autoradiography in a magnetic field appeared to indicate that at 3 and 6 hours after injection the S(35)-cystine was directly and specifically incorporated into tonofibrils in the hair cortex and into amorphous keratin granules of the hair cuticle layer, possibly without any particular concentration of this substance in the other cellular components. There seemed to be an appreciable concentration of cystine in tonofibrils of the cuticle of the inner root sheath. However, trichohyalin granules in the hair medulla and inner root sheath failed to show any evidence of cystine concentration. The improved sensitivity of the electron microscopic autoradiography with S(35)-cystine appeared to be partly due to the application of a static magnetic field. However, the reason for this could not be explained theoretically.

Trichohyalin: purification from porcine tongue epithelium and characterization of the native protein

Trichohyalin, a protein of Mr between 190 and 220 kDa in different species, was first demonstrated in large granules of the inner root sheath and medulla of hair follicles and may provide a matrix for keratin filaments. We have purified trichohyalin in milligram quantities from a citric acid-insoluble fraction derived from pig tongue epithelium. Trichohyalin was extracted under conditions of low ionic strength from the citric acid-insoluble fraction, separated by gel-filtration chromatography in buffer containing 1 M NaBr, and concentrated by ion-exchange chromatography in buffer containing 4 M urea. The purified material, which is soluble in buffers containing 1 M NaBr, was considered to be trichohyalin because of its characteristic molecular weight and amino acid composition and its localization to hair follicle inner root sheath and medulla by indirect immunofluorescence using antibodies against the purified protein. Immunofluorescence showed that trichohyalin is a major protein of filiform papillae of the tongue. Unlike trichohyalin from other animals examined, the porcine protein is a doublet on SDS polyacrylamide gels of 195 and 210 kDa; both bands are recognized by different antibodies, their two-dimensional peptide maps are nearly identical, and they have nearly identical isoelectric points of about 6.6. Trichohyalin has a Stokes radius of 124 A on gel filtration and a Svedberg constant of 6, consistent with an extended structure. The protein probably associates reversibly in solution, and the native protein we have isolated may be dimeric, because crosslinking of the iodinated purified protein with disuccinimidyl suberate demonstrated the presence of a dimer, which could be dissociated in the presence of high concentrations of urea. Rotary shadowing electron microscopy of the native protein showed a filamentous structure averaging 85 nm in length with a single globular-appearing end-domain. The purification of native trichohyalin provides a basis for future functional studies.

Structure and biochemistry of mammalian hard keratin

In this review, the structure and biological formation of hard alpha-keratin are drawn together. The hard keratins comprising wool, hairs, quills, hooves, horns, nails and baleen contain partly alpha-helical polypeptides which show homology with epidermal polypeptides only in the helical regions. These polypeptides (about 32 chains) are organized into intermediate filaments (IFs) of 7.5 nm diameter which are embedded in variable amounts of a matrix of non-helical cystine-rich proteins and glycine-tyrosine-rich proteins. The total number of proteins may exceed 100. In addition keratins contain a variety of lipid components. Wool and hair are produced in follicles in a multistep procedure. In the lower levels of the follicle, IFs without associated matrix are found. Subsequently matrix proteins are laid down between the IFs and further synthesis takes place concurrently. Finally the proteins are insolubilized by the oxidative formation of disulphide bonds. Keratinized fibres shows considerable complexity and diversity in the structural arrangement of IFs and matrix within cortical cells. Typically the IFs show hexagonal packing or give a whorl-like appearance in cross-section.

The isolation and characterization of alpha-keratin microfibrils

A method of isolating alpha-keratin microfibrils which avoids the degradation previously associated with the use of chemical, physical or enzymic procedures has been developed. Electron microscope studies of the isolation procedure establish that the microfibrils originate from the presumptive cortical cells. A purification procedure, monitored by electron microscopy, has enabled microfibrils to be isolated on a scale sufficient for chemical characterization. The amino acid composition of the microfibrils is very similar to that of low-sulphur protein fractions extracted from a range of hard mammalian keratins and thus provides direct experimental evidence for the assumption that the low-sulphur proteins comprise the microfibril in alpha-keratin.

Characterization of the proteins of guinea-pig hair and hair-follicle tissue

This work forms a part of a study of the mechanism and control of protein synthesis in the hair follicle and concerns the characterization of the proteins of hair-follicle tissue and for comparative reasons those of the hair itself. 1. Five different groups of reduced carboxymethylated proteins were delineated from both tissues; these were: group 1A proteins, which appeared to be aggregates of the group 2 proteins; group 1B proteins, soluble at pH4.4, which were thought to originate from the medulla and inner-rootsheath layers; group 2 proteins, which were defined as the main low-sulphur keratin proteins insoluble at pH4.4; group 3 proteins, the precise origin of which is not known; and the group 4 proteins, which were defined as the main high-sulphur keratin proteins soluble at pH4.4. 2. With the single exception of the group 1B proteins, the types and properties of all hair and hair-follicle proteins were identical as far as could be determined by use of such criteria as multiplicity of components, molecular charge, molecular weight and amino acid composition. 3. Two significant quantitative differences were noted: in follicle extracts there were more group 2 proteins but less group 3 and group 4 proteins than in hair extracts; and secondly, in the follicle group 4 proteins, there were more proteins of lowest molecular weight and S-carboxymethylcysteine content, but fewer proteins of the highest molecular weight and S-carboxymethylcysteine conent than in the hair group 4 proteins. 4. These quantitative differences are discussed in terms of the mechanism of synthesis of the keratin proteins. 5. Follicle group 1B proteins are postulated to have arisen from the trichohyalin droplets of the developing medulla and inner-root-sheath layers of the follicle and may be precursors of the proteins of the mature medulla and inner root sheath.

Transglutaminase from hair follicle of guinea pig (crosslinking-fibrin-glutamyllysine-isoenzymes-purified enzyme)

Two transglutaminases are found in homogenates of the inner root sheaths of guinea pig hair-follicles. One is indistinguishable from the well-characterized liver transglutaminase [J. Biol. Chem., 246, 1093 (1971)]. The other, which is present in far greater quantity, has not been detected in other organs or tissues. Gel filtration and polyacrylamide gel electrophoresis studies indicate that the native hair-follicle enzyme, of molecular weight 54,000, is composed of two subunits of identical molecular weight. Specificity studies suggest that the intermolecular cross-linking of fibrin and fibrinogen that is catalyzed by this enzyme is a result of the formation of epsilon(gamma-glutamyl)lysine bonds. The probable participation of hair-follicle transglutaminase in the formation of these cross-links in the proteins of hair is discussed.