Immunohistochemical localization of sulfhydryl oxidase correlates with disulfide crosslinking in the upper epidermis of rat skin

Vertebrate keratinization evolved into cornification mainly due to transglutaminase and sulfhydryl oxidase activities on epidermal proteins: An immunohistochemical survey

The epidermis of vertebrates forms an extended organ to protect and exchange gas, water, and organic molecules with aquatic and terrestrial environments. Herein, the processes of keratinization and cornification in aquatic and terrestrial vertebrates were compared using immunohistochemistry. Keratins with low cysteine and glycine contents form the main bulk of proteins in the anamniote epidermis, which undergoes keratinization. In contrast, specialized keratins rich in cysteine-glycine and keratin associated corneous proteins rich in cysteine, glycine, and tyrosine form the bulk of proteins of amniote soft cornification in the epidermis and hard cornification in scales, claws, beak, feathers, hairs, and horns. Transglutaminase (TGase) and sulfhydryl oxidase (SOXase) are the main enzymes involved in cornification. Their evolution was fundamental for the terrestrial adaptation of vertebrates. Immunohistochemistry results revealed that TGase and SOXase were low to absent in fish and amphibian epidermis, while they increased in the epidermis of amniotes with the evolution of the stratum corneum and skin appendages. TGase aids the formation of isopeptide bonds, while SOXase forms disulfide bonds that generate numerous cross-links between keratins and associated corneous proteins, likely increasing the mechanical resistance and durability of the amniote epidermis and its appendages. TGase is low to absent in the beta-corneous layers of sauropsids but is detected in the softer but pliable alpha-layers of sauropsids, mammalian epidermis, medulla, and inner root sheath of hairs. SOXase is present in hard and soft corneous appendages of reptiles, birds, and mammals, and determines cross-linking among corneous proteins of scales, claws, beaks, hairs, and feathers.

Immunolabeling indicates that sulfhydryl oxidase is absent in anamniote epidermis but marks the process of cornification in the skin of terrestrial vertebrates

The passage between keratinization to cornification of the epidermis and skin appendages in vertebrates requires formation of a stratum corneum rich in SS bonds among other cross-linking chemical bonds. A key enzyme, sulfhydryl oxidase (SOXase) catalyzes the oxidation of SH groups present in keratins and in corneous proteins of the epidermis into SS. Presence and distribution of SAXase has been studied by immunohistochemistry in all vertebrates, from fish to mammals. SOXase is immunohistochemically absent in all fish and amphibian species tested with the exception of a thin pre-corneous layer in the epidermis of adult anurans. SOXase is low to absent in corneous appendages such as horny teeth of lamprey or claws and horny beaks of amphibians. Conversely, SOXase is detected in the transitional (pre-corneous) and inner corneous layers of the epidermis of sauropsids and mammals. In lepidosaurian reptiles, SOXase appears in both beta- and alpha-corneous-layers, but is limited to the pre-corneous and corneous layers of the thin soft epidermises of birds and mammals, including the granular layer. SOXase is localized in pre-corneous layers and disappears in external corneous layers of amniote skin appendages such as claws, beaks of turtles and birds, and in developing feathers. This distribution further indicates that the increase activity of epidermal SOXase is/was essential, in addition to other enzymes such as epidermal transglutaminases, for the evolution of the corneous layer and of the different hard skin appendages present in terrestrial vertebrates.

[Function of Sacran as an Artificial Skin Barrier and the Development of Skincare Products]

Sacran, a large molecular-weight polysaccharide isolated from algae, is composed of 11 types of saccharides, including sulfate and carboxylic acid groups. Because of its unique structure, sacran can form a gel-like sheet in the presence of polyols such as 1,3-butanediol. In addition, those sacran gel-like sheets prevent the evaporation of water and the penetration of chemicals. The results of our previous study suggested that sacran can work as an artificial barrier against external stimuli such as air pollutants which increase the stress on humans. Topically applied sacran was localized at the surface of reconstructed human epidermal equivalents. Those results suggested that sacran inhibits excessive water evaporation from the skin and protects against environmental stimuli by forming an artificial barrier at the skin surface. Then, in a clinical study, we examined the activity of sacran in improving skin problems caused by an impaired epidermal barrier. First, we conducted a use test on a serum formulated with sacran on human volunteers who had impaired skin barrier function. The results showed that sacran provided excellent benefits to improve the maturation of corneocytes. These results suggest that sacran could play an important role in providing optimal skin conditions for keratinocytes to progress through their differentiation.

Correlations between skin hydration parameters and corneocyte-derived parameters to characterize skin conditions

BACKGROUND

Skin hydration is generally assessed using the parameters of skin surface water content (SWC) and trans-epidermal water loss (TEWL). To date, few studies have characterized skin conditions using correlations between skin hydration parameters and corneocyte parameters.

AIMS

The parameters SWC and TEWL allow the classification of skin conditions into four distinct Groups. The purpose of this study was to assess the characteristics of skin conditions classified by SWC and TEWL for correlations with parameters from corneocytes.

METHODS

A human volunteer test was conducted that measured SWC and TEWL. As corneocyte-derived parameters, the size and thick abrasion ratios, the ratio of sulfhydryl groups and disulfide bonds (SH/SS) and CP levels were analyzed.

RESULTS

Volunteers were classified by their median SWC and TEWL values into 4 Groups: Group I (high SWC/low TEWL), Group II (high SWC/high TEWL), Group III (low SWC/low TEWL), and Group IV (low SWC/high TEWL). Group IV showed a significantly smaller size of corneocytes. Groups III and IV had significantly higher thick abrasion ratios and CP levels. Group I had a significantly lower SH/SS value. The SWC/TEWL value showed a decline in order from Group I to Group IV.

CONCLUSION

Groups classified by their SWC and TEWL values showed characteristic skin conditions. We propose that the SWC and TEWL ratio is a comprehensive parameter to assess skin conditions.

Topical treatment with sacran, a sulfated polysaccharide from Aphanothece sacrum, improves corneocyte-derived parameters

Sacran, a polysaccharide isolated from Aphanothece sacrum (Suizenji-nori) alga, has unique characteristics in terms of its physiological properties and effects on the skin, and has recently become a focus of attention as a novel biomaterial. In a previous study, we reported the unique physical characteristics of sacran, which forms a gel-like film containing water in the presence of polyols. This film resists penetration by water and chemicals. We expected this unique physical characteristic to act as an artificial barrier upon the application of sacran to the skin. In the present study, we tested the efficacy of sacran application in healthy individuals who reported previous symptoms of dry or inflamed skin, to evaluate the potential benefits of sacran for skin care in patients with mild atopic dermatitis. Compared with placebo, sacran-containing serum did not significantly alter either the water content of the skin surface or transepidermal water loss. However, subjects using the serum showed improvements in corneocyte parameters including size, percentage of thick abrasion, ratio of SH to SS groups, ratio of interleukin (IL)-1 receptor antagonist to IL-1α, and carbonylated protein level. These results indicate that the sulfated polysaccharide sacran is an effective agent for improving or maintaining the skin conditions.

The X-Ray Crystal Structure of the Keratin 1-Keratin 10 Helix 2B Heterodimer Reveals Molecular Surface Properties and Biochemical Insights into Human Skin Disease

Keratins 1 (K1) and 10 (K10) are the primary keratins expressed in differentiated epidermis. Mutations in K1/K10 are associated with human skin diseases. We determined the crystal structure of the complex between the distal (2B) helices of K1 and K10 to better understand how human keratin structure correlates with function. The 3.3 Å resolution structure confirms many features inferred by previous biochemical analyses, but adds unexpected insights. It demonstrates a parallel, coiled-coil heterodimer with a predominantly hydrophobic intermolecular interface; this heterodimer formed a higher order complex with a second K1-K10-2B heterodimer via a Cys401^K10 disulfide link, although the bond angle is unanticipated. The molecular surface analysis of K1-K10-2B identified several pockets, one adjacent to the disulfide linkage and conserved in K5-K14. The solvent accessible surface area of the K1-K10 structure is 20-25% hydrophobic. The 2B region contains mixed acidic and basic patches proximally (N-terminal), whereas it is largely acidic distally (C-terminal). Mapping of conserved and nonconserved residues between K1-K10 and K5-K14 onto the structure demonstrated the majority of unique residues align along the outer helical ridge. Finally, the structure permitted a fresh analysis of the deleterious effects caused by K1/K10 missense mutations found in patients with phenotypic skin disease.

Ultrastructural Localization of Caspase-14 in Human Epidermis

Caspase-14 has been implicated in the formation of stratum corneum because of its specific expression and activation in terminally differentiating keratinocytes. However, its precise physiological role and its protein substrate are elusive. We studied the ultrastructural localization of caspase-14 in human epidermis to compare its distribution pattern with that of well-characterized differentiation markers. Immunogold cytochemistry confirmed that caspase-14 is nearly absent in basal and spinous layers. In the granular, layer nuclei and keratohyalin granules were labeled with increasing intensity towards the transitional layer. Particularly strong caspase-14 labeling was associated with areas known to be occupied by involucrin and loricrin, whereas F-granules, occupied by profilaggrin/filaggrin, were much less labeled. A high density of gold particles was also present at the forming cornified cell envelope, including desmosomes. In corneocytes, intense labeling was both cytoplasmic and associated with nuclear remnants and corneodesmosomes. These observations will allow focusing efforts of biochemical substrate screening on a subset of proteins localizing to distinct compartments of terminally differentiated keratinocytes.

Immunolocalization of sulfhydryl oxidase in reptilian epidermis indicates that the enzyme participates mainly to the hardening process of the beta-corneous layer

Reptilian skin is tough and scaled representing an evolutionary adaptation to the terrestrial environment. The presence of sulfhydryl oxidase during the process of hardening of the corneous layer in reptilian epidermis has been analyzed by immunocytochemistry and immunoblotting. Sulfhydryl oxidase-like immunoreactivity of proteins in the 50-65 kDa range of molecular weight is mainly observed in the transitional and pre-corneous layers of crocodilians, chelonian, and in the forming beta-layer of lepidosaurians. The ultrastructural localization of the enzyme by immunogold in lizard epidermis during renewal and resting stages shows that the labeling is mainly distributed in the cytoplasm and along the accumulating beta-packets of differentiating beta-cells while it appears very low to undetectable in differentiating alpha-cells of the lacunar, clear, mesos, and alpha-layers. The labeling however becomes absent or undetectable also in the fully mature beta-layer. The study shows that an oxidative enzyme is likely responsible of the cross-linking of the numerous cysteines present in the main proteins accumulated in corneocytes of reptilian epidermis, known as corneous beta-proteins (beta-keratins). This process of disulphide bond formation is probably largely responsible for the formation of hard beta-corneous layers in reptilian scales, a difference with alpha-corneous layers where substrate proteins of transglutaminase appear predominant.

Complementary roles of specific cysteines in keratin 14 toward the assembly, organization, and dynamics of intermediate filaments in skin keratinocytes

We recently showed that inter-keratin disulfide bonding plays an important role in the assembly, organization, and dynamics of keratin intermediate filaments in skin keratinocytes. In particular, cysteine 367 located in the central α-helical rod domain of keratin 14 is necessary for the formation of a stable perinuclear network of keratin filaments (with type II partner keratin 5) in skin keratinocytes analyzed by static and live cell imaging. Here, we show that two additional cysteine residues located in the non-helical head domain of K14, Cys-4 and Cys-40, also participate in inter-keratin disulfide bonding and tandemly play a key role complementary to that of Cys-367 in the assembly, organization, and dynamics of keratin filaments in skin keratinocytes in primary culture. Analysis of K14 variants with single or multiple substitutions of cysteine residues points to a spatial and temporal hierarchy in how Cys-4/Cys-40 and Cys-367 regulate keratin assembly in vitro and filament dynamics in live keratinocytes in culture. Our findings substantiate the importance and complexity of a novel determinant, namely inter-keratin disulfide bonding, for the regulation of several aspects of keratin filaments in surface epithelia.

Disulfide bond formation in the cytoplasm

SIGNIFICANCE

Disulfide bond formation is critical for biogenesis of many proteins. While most studies in this field are aimed at elucidating the mechanisms in the endoplasmic reticulum, intermembrane space of mitochondria, and prokaryotic periplasm, structural disulfide bond formation also occurs in other compartments including the cytoplasm. Such disulfide bond formation is essential for biogenesis of some viruses, correct epidermis biosynthesis, thermal adaptation of some extremophiles, and efficient recombinant protein production.

RECENT ADVANCES

The majority of work in this new field has been reported in the past decade. Within the past few years very significant new data have emerged on the catalytic and noncatalytic mechanisms for disulfide bond formation in the cytoplasm. This includes the crystal structure of a key component of viral oxidative protein folding, identification of a missing component in cytoplasmic disulfide bond formation in hyperthermophiles, and introduction of de novo dithiol oxidants in engineered oxidative folding pathways.

CRITICAL ISSUES AND FUTURE DIRECTIONS

While a broad picture of cytoplasmic disulfide bond formation has emerged many critical questions remain unanswered. The individual components in the natural systems are largely known, but the molecular mechanisms by which these processes occur are largely deduced from the mechanisms of analogous components in other compartments. This prevents full understanding and manipulation of these systems, including the potential for novel anti-viral drugs based on the unique features of their sulfhydryl oxidases and the generation of more efficient cell factories for the large-scale production of therapeutic and industrial proteins.

Structural basis for heteromeric assembly and perinuclear organization of keratin filaments

There is as yet no high-resolution data regarding the structure and organization of keratin intermediate filaments, which are obligate heteropolymers providing vital mechanical support in epithelia. We report the crystal structure of interacting 2B regions from the central coiled-coil domains of keratins 5 and 14 (K5 and K14), expressed in progenitor keratinocytes of epidermis. The interface of the K5-K14 coiled-coil heterodimer has asymmetric salt bridges, hydrogen bonds and hydrophobic contacts, and its surface exhibits a notable charge polarization. A trans-dimer homotypic disulfide bond involving Cys367 in K14's stutter region occurs in the crystal and in skin keratinocytes, where it is concentrated in a keratin filament cage enveloping the nucleus. We show that K14-Cys367 impacts nuclear shape in cultured keratinocytes and that mouse epidermal keratinocytes lacking K14 show aberrations in nuclear structure, highlighting a new function for keratin filaments.

Gain of Function in an ERV/ALR Sulfhydryl Oxidase by Molecular Engineering of the Shuttle Disulfide

The ERV/ALR sulfhydryl oxidase domain is a versatile module adapted for catalysis of disulfide bond formation in various organelles and biological settings. Its four-helix bundle structure juxtaposes a Cys-X-X-Cys dithiol/disulfide motif with a bound flavin adenine dinucleotide (FAD) cofactor, enabling transfer of electrons from thiol substrates to non-thiol electron acceptors. ERV/ALR family members contain an additional di-cysteine motif outside the four-helix-bundle core. Although the location and context of this "shuttle" disulfide differs among family members, it is proposed to perform the same basic function of mediating electron transfer from substrate to the enzyme active site. We have determined by X-ray crystallography the structure of AtErv1, an ERV/ALR enzyme that contains a Cys-X4-Cys shuttle disulfide and oxidizes thioredoxin in vitro, and compared it to ScErv2, which has a Cys-X-Cys shuttle and does not oxidize thioredoxin at an appreciable rate. The AtErv1 shuttle disulfide is in a region of the structure that is disordered and thus apparently mobile and exposed. This feature may facilitate access of protein substrates to the shuttle disulfide. To test whether the shuttle disulfide region is modular and can confer on other enzymes oxidase activity toward new substrates, we generated chimeric enzyme variants combining shuttle disulfide and core elements from AtErv1 and ScErv2 and monitored oxidation of thioredoxin by the chimeras. We found that the AtErv1 shuttle disulfide region could indeed confer thioredoxin oxidase activity on the ScErv2 core. Remarkably, various chimeras containing the ScErv2 Cys-X-Cys shuttle disulfide were found to function efficiently as well. Since neither the ScErv2 core nor the Cys-X-Cys motif is therefore incapable of participating in oxidation of thioredoxin, we conclude that wild-type ScErv2 has evolved to repress activity on substrates of this type, perhaps in favor of a different, as yet unknown, substrate.

Cytochemical, biochemical and molecular aspects of the process of keratinization in the epidermis of reptilian scales

The characteristics of scaled skin of reptiles is one of their main features that distinguish them from the other amniotes, birds and mammals. The different scale patterns observed in extant reptiles result from a long evolutive history that allowed each species to adapt to its specific environment. The present review deals with comparative aspects of epidermal keratinization in reptiles, chelonians (turtles and tortoises), lepidosaurian (lizards, snakes, sphenodontids), archosaurians (crocodilians). Initially the morphology and cytology of reptilian scales is outlined to show the diversity in the epidermis among different groups. The structural proteins (alpha-keratins and associated proteins), and enzymes utilized to form the corneous layer of the epidermis are presented. Aside cytokeratins (alpha-keratins), used for making the cytoskeleton, reptilian alpha-keratinocytes produce interkeratin (matrix) and corneous cell envelope proteins. Keratin bundles and degraded cell organelles constitute most of the corneous material of alpha-keratinocytes. Matrix, histidine-rich and sulfur-rich proteins are produced in the soft epidermis and accumulated in the cornified cell envelope. Main emphasis is given to the composition and to the evolution of the hard keratins (beta-keratins). Beta-keratins constitute the hard corneous material of scales. These small proteins are synthesized in beta-keratinocytes and are accumulated into small packets that rapidly merge into a compact corneous material and form densely cornified layers. Beta-keratins are smaller proteins (8-20 kDa) in comparison to alpha-keratins (40-70 kDa), and this size may determine their dense packing in corneocytes. Both glycine-sulfur-rich and glycine-proline-rich proteins have been so far sequenced in the corneous material of scales in few reptilian species. The latter keratins possess C- and N-amino terminal amino acid regions with sequence homology with those of mammalian hard keratins. Also, reptilian beta-keratins possess a central core with homology with avian scale/feather keratins. Multiple genes code for these proteins and their discovery and sequentiation is presently an active field of research. These initial findings however suggest that ancient reptiles already possessed some common genes that have later diversified to produce the specific keratin-associated proteins in their descendants: extant reptiles, birds and mammals. The evolution of these small proteins in lepidosaurians, chelonians and archosaurians represent the next step to understand the evolution of cornification in reptiles and derived amniotes (birds and mammals).

Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

This review presents comparative aspects of epidermal keratinization in vertebrates, with emphasis on the evolution of the stratum corneum in land vertebrates. The epidermis of fish does not contain proteins connected with interkeratin matrix and corneous cell envelope formation. Mucus-like material glues loose keratin filaments. In amphibians a cell corneous envelope forms but matrix proteins, aside from mucus/glycoproteins, are scarce or absent. In reptiles, birds, and mammals specific proteins associated with keratin become relevant for the production of a resistant corneous layer. In reptiles some matrix, histidine-rich and sulfur-rich corneous cell envelope proteins are produced in the soft epidermis. In avian soft epidermis low levels of matrix and cornified proteins are present while lipids become abundant. In mammalian keratinocytes, interkeratin proteins, cornified cell envelope proteins, and transglutaminase are present. Topographically localized areas of dermal-epidermal interactions in amniote skin determine the formation of skin derivatives such as scales, feathers, and hairs. New types of keratin and associated proteins are produced in these derivatives. In reptiles and birds beta-keratins form the hard corneous material of scales, claws, beaks, and feathers. In mammals, small sulfur-rich and glycine-tyrosine-rich proteins form the corneous material of hairs, horns, hooves, and claws. Molecular studies on reptilian beta-keratins show they are glycine-rich proteins. They have C- and N-terminal amino acid regions homologous to those of mammalian proteins and a central core with homology to avian scale/feather keratins. These findings suggest that ancient reptiles already possessed some common genes that later diversified to produce some keratin-associated protein in extant reptiles and birds, and others in mammals. The evolution of these small proteins represents the more recent variation of the process of cornification in vertebrates.

Distribution of caspase-14 in epidermis and hair follicles is evolutionarily conserved among mammals

Caspase-14, a member of the caspase family of cysteine proteases, is almost exclusively expressed in the epidermis. Studies on human and mouse cells and tissues have implicated caspase-14 in terminal differentiation of epidermal keratinocytes and in the formation of the stratum corneum. Here we investigated evolutionary aspects of the role of caspase-14 by analyzing its distribution in the epidermis and hair follicles of representative species of placental mammals, marsupials, and monotremes. Immunocytochemical staining showed that caspase-14 is consistently expressed in the granular and corneous layer of the epidermis of all mammalian species investigated. Ultrastructural analysis using gold-labeled anticaspase-14 antibodies revealed that caspase-14 is associated preferentially with keratin bundles and amorphous material of keratohyalin granules, but is also present in nuclei of transitional cells of the granular layer and in corneocytes. In hair follicles, caspase-14 was diffusely present in cornifying cells of the outer root sheath, in the companion layer, and, most abundantly, in the inner root sheath of all mammalian species here analyzed. In Henle and Huxley layers of the inner root sheath, labeling was seen in nuclei and, more diffusely, among trichohyalin granules of cornifying cells. In summary, the tissue expression pattern and the intracellular localization of caspase-14 are highly conserved among diverse mammalian species, suggesting that this enzyme is involved in a molecular process that appeared early in the evolution of mammalian skin. The association of caspase-14 with keratohyalin and trichohyalin granules may indicate a specific role of caspase-14 in the maturation of these keratinocyte-specific structures.

Cell-specific localization of the sulphydryl oxidase QSOX in rat peripheral tissues

Rat quiescin/sulphydryl oxidase (rQSOX) introduces disulphide bridges into peptides and proteins with the reduction of molecular oxygen to hydrogen peroxide. Its occurrence has been previously highlighted in a wide range of organs by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analyses, methods that have provided information concerning its expression in whole organs but that do not reveal the cell types expressing this enzyme. In this report, in addition to RT-PCR and Western blot experiments, the cell-specific localization of rQSOX has been investigated in a wide range of male and female adult rat tissues by using in situ hybridization and immunohistochemistry. Labelling was detected in most organs and systems including the immune, endocrine and reproductive systems, the respiratory, digestive and urinary tracts and the skin. No labelling was observed in the heart, blood vessel endothelium, liver or smooth and skeletal muscles. rQSOX expression was mainly localized in epithelial cells specialized in secretion, strengthening the hypothesis that QSOX enzymes play an important role in the mechanism of secretion, notably in the folding of secreted proteins. The intracellular patterns of immunolabelling indicate that the protein usually follows the secretory pathway, which is in accordance with its secreted nature and its presumed involvement in the elaboration of the extracellular matrix. In seminiferous tubules, where a high level of expression was noticed, QSOX might play an important physiological role in sperm function and serve as a marker for the diagnosis of male infertility.

Immuno-Cross reactivity of transglutaminase and cornification marker proteins in the epidermis of vertebrates suggests common processes of soft cornification across species

In differentiating mammalian keratinocytes proteins are linked to the plasma membrane by epidermal transglutaminases through N-epsilon-(gamma-glutamyl)-lysine isopeptide bonds to form the cornified cell envelope. The presence of transglutaminases and their protein substrates in the epidermis of nonmammalian vertebrates is not known. The present study analyses the presence and localization of the above proteins in the epidermis using immuno-cross reactivity across different classes of amniotes. After immunoblotting, some protein bands appear labelled for loricrin, sciellin, and transglutaminase in most species. These proteins are scarce to absent in the epidermis of aquatic species (goldfish and newt) where a stratum corneum is absent or very thin. The molecular weight of transglutaminase immunoreactive bands generally varies between 40 to 62 kDa, with the most represented bands at 52-57 kDa in most species. The more intense loricrin- and sciellin-immunoreactive bands are seen at 50-55-62 kDa, but are weak or absent in aquatic vertebrates. Loricrine-like immunoreactivity is present in the epidermis where alpha-(soft)-keratinization occurs. Isopeptide bonds are mainly associated to bands in the range of 50-62 kDa. In vertebrates where hard-keratin is expressed (the beta-keratin corneous layer of sauropsids and in feathers) or in hair cortex of mammals, no loricrin-like, transglutaminase-, and isopeptide-bond-immunoreactivities are seen. Immunoblotting however shows loricrin-, sciellin-, and trasnsglutaminase-positive bands in the corneous layers containing beta-keratin. Histologically, the epidermis of most amniotes shows variable transglutaminase immunoreactivity, but isopeptide-bond and sciellin immunoreactivities are weak or undetactable in most species. The limitations of immunohistochemical methods are discussed and compared with results from immunoblotting. In reptilian epidermis transglutaminase is mainly localized in 0.15-0.3 microm dense granules or diffuse in transitional alpha-keratogenic cells. In beta-keratogenic cells few small dense granules show a weak immunolabeling. Transglutaminase is present in nuclei of terminal differentiating alpha- and beta-keratinocytes, as in those of mature inner and outer root sheath. The present study suggests that keratinization based on loricrin, sciellin and transglutaminase was probably present in the stratum corneoum of basic amniotes in the Carboniferous. These proteins were mainly maintained in alpha-keratogenic layers of amniotes but decreased in beta-keratogenic layers of sauropsids (reptiles and birds). The study suggests that similar proteins for the formation of the cornified cell envelope are present in alpha-keratinocytes across vertebrates but not in beta-keratinocytes.

Comparative aspects of the inner root sheath in adult and developing hairs of mammals in relation to the evolution of hairs

The inner root sheath (IRS) allows the exit of hairs through the epidermal surface. The fine structure of monotreme and marsupial IRS and trichohyalin is not known. Using electron microscopy and immunocytochemistry, the localization of trichohyalin and transglutaminase have been studied in monotreme and marsupial hairs, and compared with trichohyalin localization in placental hairs. Trichohyalin in all mammalian species studied here is recognized by a polyclonal antibody against sheep trichohyalin. This generalized immunoreactivity suggests that common epitopes are present in trichohyalin across mammals. In differentiating IRS cells, trichohyalin granules of variable dimensions are composed of an immunolabelled amorphous matrix associated with a network of 10-12-nm-thick keratin filaments. Transglutaminase labelling is present among keratin bundles and trichohyalin granules, and in condensed nuclei of terminally differentiating cells of the inner root sheath. The IRS in monotreme hairs is multistratified but lacks a distinguishable Henle layer. Cornification of IRS determines the sculpturing of the fibre cuticle and later shedding from the follicle for the exit of the hair fibre on the epidermal surface. It is hypothesized that the stratification of IRS in Henle, Huxley and IRS cuticle layers is derived from a simpler organization, like that present in the IRS of monotremes. The IRS is regarded as a localized shedding/sloughing layer needed for the exit of hairs without injury to the epidermis. The formation of the IRS during the evolution of mammalian epidermis allowed the physiological exit of hairs produced inside the skin. The peculiar morphogenesis of hairs in possible primitive skins, such as those of the monotremes (mammals with some reptilian characteristics) or the tails of some rodents (a scaled skin), may elucidate the evolution of hairs. In monotreme and rodent tail skin, the dermal papilla remains localized on the proximal side of the hair peg and forms a hair placode with bilateral symmetry. The papilla is progressively surrounded by the down-growing hair peg until a dermal papilla with radial symmetry is formed. It is speculated that the progressive reduction of the extended dermal papilla of reptilian scales into small and deep papillae of therapsid reptiles produced hairs in mammals.

Fine structure of marsupial hairs, with emphasis on trichohyalin and the structure of the inner root sheath

The fine structure and cornification of marsupial hairs are unknown. The distribution of keratins, trichohyalin, and transglutaminase in marsupial hairs was studied here for the first time by electron microscopy and immunocytochemistry. The localization of acidic and basic keratins in marsupial hairs is similar to that of hairs in placental mammals, and the keratins are mainly localized in the outer root sheath and surrounding epidermis. Marsupial trichohyalin in both medulla and inner root sheath (IRS) cross-reacts with a trichohyalin antibody that recognizes trichohyalin across placental species, indicating a common epitope(s) among mammalian trichohyalin. Roundish to irregular trichohyalin granules are composed of a network of immunolabeled 10-15-nm-thick coarse filaments within an amorphous matrix in which a weak labeling for transglutaminases is present. This suggests that the enzyme, and its substrate trichohyalin, are associated in mature granules. Transglutaminase labeling mainly occurs in condensing chromatin of mature cells of the outer and inner root sheaths, suggesting formation of the nuclear envelope connected with terminal differentiation of these cells. In mature Huxley or Henle layers the filaments lose the immunolabeling for trichohyalin when they are reoriented into parallel rows linked by short bridges, thus suggesting that the filaments with their reactive epitopes are chemically modified during cornification, as seen in the IRS of hairs of placental mammals. The Huxley layer probably acts as a cushion, absorbing the tensions connected with the distalward movement of the growing hair fiber. Variations in stratification of the Huxley layer are probably related to the diameter of the hair shaft. The cytoplasmic and junctional connections between cells of the Huxley layer and the companion layer and the outer root sheath enhance the grip of the IRS and hair fiber within the follicle. The role of cells of the IRS in sculpturing the fiber cuticle and in the mechanism of shedding that allows the exit of hair on the epidermal surface in mammals are discussed.

Sulfhydryl oxidase (SOx) from mouse epidermis: molecular cloning, nucleotide sequence, and expression of recombinant protein in the cultured cells

Skin sulfhydryl oxidase (SOx) is an enzyme that catalyzes disulfide (S-S) cross-linking through the oxidation of sulfhydryl compounds in the skin. In this study, using the enzyme purified from rat seminal vesicle, we obtained peptide sequences for SOx by mass spectrometry. We then searched for SOx nucleotides corresponding highly to the rat peptide sequences by assembling murine-expressed sequence tags (ESTs) from the GeneBank database. The assembled mouse SOx cDNA has an open reading frame of 1704-bp nucleotides, translating into a size of 568 amino acids. The calculated molecular mass of the mouse SOx protein is 65 kDa. This mouse sequence can be amplified from total RNAs of various mouse tissue samples by reverse transcription polymerase chain reaction, especially highly amplified from those of the seminal vesicles and epidermis. The cDNA fragment was subsequently cloned into the mammalian expression vector (pTARGET-MSSOx), allowing us to express mouse recombinant SOx protein in cultured cells. When pTARGET-MSSOx was transfected, Western blot analysis using anti-SOx antiserum could detect a 65 kDa-band of recombinant SOx in both samples from the whole cell extract and the medium after the harvest of the HEK cells. In immunohistochemical analysis, the Pt-K2 cells, following the introduction of pTARGET-MSSOx, seemed to generate a SOx protein reactive to anti-SOx antiserum in the cells. Moreover, the indirect staining of the S-S bonds using N-(7-dimethylamino-4-methyl coumarinyl) maleimide (DACM), following the addition of N-ethylmaleimide and dithiothreitol, showed that the formation of S-S bridges almost matched the localization of SOx expression in the Pt-K2 cells after the transfection. In essence, we cloned skin SOx cDNA and characterized it as one of the S-S cross-linking enzymes. The SOx clone from mouse epidermis seems to be useful for investigating the potential function of the enzyme in the epidermis, especially for understanding the physiological role of SOx in the differentiation of keratinocytes.