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

Immunolabeling for filaggrin and acidic keratins in the granular layer of mammalian epidermis indicates that an acidic-basic interaction is involved in cornification

The soft epidermis of mammals derives from the accumulation of keratohyaline granules in the granular layer, before maturing into corneocytes. Main proteins accumulated in the granular layer are pro-filaggrin and filaggrin that determine keratin clumping and later moisturization of the stratum corneum that remains flexible. This soft epidermis allows the high sensitivity of mammalian skin. Presence and thickness of the stratum granulosum varies among different species of mammals and even between different body regions of the same animal, from discontinuous to multilayered. These variations are evident using antibodies for filaggrin, a large protein that share common epitopes among placentals. Here we have utilized filaggrin antibodies (8959 and 466) and an acidic keratin antibody (AK2) for labeling placental, marsupial and monotreme epidermis. AK2 labeling appears mainly to detect K24 keratin, and less likely other acidic keratins. Immunoreactivity for filaggrin is absent in platypus, discontinuous in Echidna and in the tested marsupials. In placentals, it is inconstantly or hardly detected in the thin epidermis of bat, rodents, and lagomorphs with a narrow, mono-stratified and/or discontinuous granular layer. In contrast, where the granular layer is continuous or even stratified, both filaggrin and AK2 antibodies decorate granular cells. The ultrastructural analysis using the AK2 antibody on human epidermis reveals that a weak labeling is associated with keratohyalin granules and filamentous keratins of transitional keratinocytes and corneocytes. This observation suggests that basophilic filaggrin interacts with acidic keratins like K24 and determines keratin condensation into corneocytes of the stratum corneum.

The Evolution of Transglutaminases Underlies the Origin and Loss of Cornified Skin Appendages in Vertebrates

Transglutaminases (TGMs) cross-link proteins by introducing covalent bonds between glutamine and lysine residues. These cross-links are essential for epithelial cornification which enables tetrapods to live on land. Here, we investigated which evolutionary adaptations of vertebrates were associated with specific changes in the family of TGM genes. We determined the catalog of TGMs in the main clades of vertebrates, performed a comprehensive phylogenetic analysis of TGMs, and localized the distribution of selected TGMs in tissues. Our data suggest that TGM1 is the phylogenetically oldest epithelial TGM, with orthologs being expressed in the cornified teeth of the lamprey, a basal vertebrate. Gene duplications led to the origin of TGM10 in stem vertebrates, the origin of TGM2 in jawed vertebrates, and an increasing number of epithelium-associated TGM genes in the lineage leading to terrestrial vertebrates. TGM9 is expressed in the epithelial egg tooth, and its evolutionary origin in stem amniotes coincided with the evolution of embryonic development in eggs that are surrounded by a protective shell. Conversely, viviparous mammals have lost both the epithelial egg tooth and TGM9. TGM3 and TGM6 evolved as regulators of cornification in hair follicles and underwent pseudogenization upon the evolutionary loss of hair in cetaceans. Taken together, this study reveals the gain and loss of vertebrate TGM genes in association with the evolution of cornified skin appendages and suggests an important role of TGM9 in the evolution of amniotes.

Spatio-temporal expression patterns of glycine-rich beta proteins and cysteine-rich beta proteins in setae development of Gekko japonicus

BACKGROUND

Setae on the pad lamellae of the Japanese gecko Gekko japonicus (Schlegel, 1836), a vital epidermal derivative, are primarily composed of cornified beta-proteins (CBPs) and play a pivotal role in adhesion and climbing. The amino acid composition of CBPs might be a determining factor influencing their functional properties. However, the molecular mechanisms governed by CBP genes with diverse amino acid compositions in setae development remain unexplored.

RESULTS

Based on RNA-seq analyses, this study confirmed that all G. japonicus CBPs (GjCBPs) are involved in setae formation. Cysteine-rich CBPs encoding genes (ge-cprp-17 to ge-cprp-26) and glycine-rich CBPs encoding genes (ge-gprp-17 to ge-gprp-22) were haphazardly selected, with quantitative real-time PCR revealing their expression patterns in embryonic pad lamellae and dorsal epidermis. It is inferred that glycine-rich CBPs are integral to the formation of both dorsal scales and lamellar setae, cysteine-rich CBPs are primarily associated with setae development. Additionally, fluorescence in situ hybridization revealed spatiotemporal differences in the expression of a glycine-rich CBP encoding gene (ge-gprp-19) and a cysteine-rich CBP encoding gene (ge-cprp-17) during dorsal scales and/or lamellar development.

CONCLUSIONS

All 66 CBPs are involved in the formation of setae. Glycine-rich CBPs hold a significant role in the development of dorsal scales and lamellar setae, whereas most cysteine-rich CBPs appear to be essential components of G. japonicus setae. Even GjCBPs with similar amino acid compositions may play diverse functions. The clear spatio-temporal expression differences between the glycine-rich and cysteine-rich CBP encoding genes during epidermal scale and/or setae formation were observed. Embryonic developmental stages 39 to 42 emerged as crucial phases for setae development. These findings lay the groundwork for deeper investigation into the function of GjCBPs in the development of G. japonicus setae.

A comprehensive exploration of diverse skin cell types in the limb of the desert tortoise (Testudo graeca) through light, transmission, scanning electron microscopy, and immunofluorescence techniques

The Greek tortoise, inhabiting harsh desert environments, provides a compelling case for investigating skin adaptations to extreme conditions. We have utilized light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and immunofluorescence analysis to describe the structure of the arid-adapted limb skin in the Greek tortoise. Our aim was to identify the cell types that reflect the skin adaptation of this tortoise to arid conditions. Utilizing seven antibodies, we localized and elucidated the functions of various skin cells, shedding light on how the tortoise adapts to adverse environmental conditions. Our findings unveiled numerous scales on the limbs, varying in size and color, acting as protective armor against abrasions, bites, and other potential threats in their rocky habitats. The epidermis comprises four layers: stratum basalis, stratum spinosum, peri-corneous layer, and stratum corneum. Cytokeratin 14 (CK14) was explicitly detected in the basal layer of the epidermis, suggesting a role in maintaining epidermal integrity and cellular function. Langerhans cells were observed between epidermal cells filled with ribosomes and Birbeck granules. Numerous dendritic-shaped Langerhans cells revealed through E-Cadherin signify strong immunity in tortoises' skin. Melanophores were identified using the Melan-A antibody, labeling the cytoplasm, and the SOX10 antibody, labeling the nucleus, providing comprehensive insights into melanophores morphology and distribution. Two types of melanophores were found: dendritic below the stratum basalis of the epidermis and clustered oval melanophores in the deep dermal layer. Varied melanophores distribution resulted in a spotted skin pattern, potentially offering adaptive camouflage and protection against environmental challenges. Numerous myofibroblasts were discerned through alpha-smooth actin (α-SMA) expression, indicating that the Greek tortoise's skin possesses a robust tissue repair and remodeling capacity. B-cell lymphocytes detected via CD20 immunostaining exhibited sporadic distribution in the dermis, concentrating in lymphoid aggregates and around vessels, implying potential roles in local immune responses and inflammation modulation. Employing Tom20 to identify skin cells with abundant mitochondria revealed a notable presence in melanophores and the basal layer of the epidermis, suggesting high metabolic activity in these cell types and potentially influencing cellular functions. These findings contribute to our comprehension of tortoise skin anatomy and physiology, offering insights into the remarkable adaptations of this species finely tuned to their specific environmental habitats.

Unveiling the expansion of keratin genes in lungfishes: a possible link to terrestrial adaptation

Keratins are intermediate filament proteins that are important for epidermal strength and protection from desiccation. Keratin genes are highly duplicated and have diversified by forming two major clusters in the genomes of terrestrial vertebrates. The keratin genes of lungfishes, the closest fish to tetrapods, have not been studied at the genomic level, despite the importance of lungfishes in terrestrial adaptation. Here, we identified keratin genes in the genomes of two lungfish species and performed syntenic and phylogenetic analyses. Additionally, we identified keratin genes from two gobies and two mudskippers, inhabiting underwater and terrestrial environments. We found that in lungfishes, keratin genes were duplicated and diversified within two major clusters, similar to but independent of terrestrial vertebrates. By contrast, keratin genes were not notably duplicated in mudskippers. The results indicate that keratin gene duplication occurred repeatedly in lineages close to tetrapods, but not in teleost fish, even in species adapted to terrestrial environments.

General aspects on skin development in vertebrates with emphasis on sauropsids epidermis

General cellular aspects of skin development in vertebrates are presented with emphasis on the epidermis of sauropsids. Anamniote skin develops into a multilayered mucogenic and soft keratinized epidermis made of Intermediate Filament Keratins (IFKs) that is reinforced in most fish and few anurans by dermal bony and fibrous scales. In amniotes, the developing epidermis in contact with the amniotic fluid initially transits through a mucogenic phase recalling that of their anamniotes progenitors. A new gene cluster termed EDC (Epidermal Differentiation Complex) evolved in amniotes contributing to the origin of the stratum corneum. The EDC contains numerous genes coding for over 100 types of corneous proteins (CPs). In sauropsids 2-8 layers of embryonic epidermis accumulate soft keratins (IFKs) but do not form a compact corneous layer. The embryonic epidermis of reptiles and birds produces small amount of other, poorly known proteins in addition to IFKs and mucins. In the following development, a resistant corneous layer is formed underneath the embryonic epidermis that is shed before hatching. The definitive corneous epidermis of sauropsids is mainly composed of CBPs (Corneous beta proteins, formerly indicated as beta-keratins) derived from the EDC. CBPs belong to a gene sub-family of CPs unique for sauropsids, contain an inner amino acid region formed by beta-sheets, are rich in cysteine and glycine, and make most of the protein composition of scales, claws, beaks and feathers. In mammalian epidermis CPs missing the beta-sheet region are instead produced, and include loricrin, involucrin, filaggrin and various cornulins. Small amount of CPs accumulate in the 2-3 layers of mammalian embryonic epidermis and their appendages, that is replaced with the definitive corneous layers before birth. Differently from sauropsids, mammals utilize KAPs (keratin associated proteins) rich in cysteine and glycine for making the hard corneous material of hairs, claws, hooves, horns, and occasionally also scales.

Immunolocalization of Some Epidermal Proteins and Glycoproteins in the Growing Skin of the Australian Lungfish (Neoceratodus forsteri)

Here we report the immunolocalization of mucin, nestin, elastin and three glycoproteins involved in tissue mineralization in small and large juveniles of Neoceratodus forsteri. Both small and larger juvenile epidermis are mucogenic and contain a diffuse immunolabeling for nestin. Sparse PCNA-labeled cells, indicating proliferation, are found in basal and suprabasal epidermal layers. No scales are formed in small juveniles but are present in a 5 cm long juvenile and in larger juveniles. Elastin and a mineralizing matrix are localized underneath the basement membrane of the tail epidermis where lepidotriches are forming. The latter appears as "circular bodies" in cross sections and are made of elongated cells surrounding a central amorphous area containing collagen and elastin-like proteins that undergo calcification as evidenced using the von Kossa staining. However, the first calcification sites are the coniform teeth of the small juveniles of 2-3 cm in length. In the superficial dermis of juveniles (16-26 cm in length) where scales are formed, the spinulated outer bony layer (squamulin) of the elasmoid scales contains osteonectin, alkaline phosphatase, osteopontin, and calcium deposits that are instead absent in the underlying layer of elasmodin. In particular, these glycoproteins are localized along the scale margin in juveniles where scales grow, as indicated by the presence of PCNA-labeled cells (proliferating). These observations suggest a continuous deposition of new bone during the growth of the scales, possibly under the action of these mineralizing glycoproteins, like in the endoskeleton of terrestrial vertebrates.

Introduction to the Development of Skin in Vertebrates

The integument of vertebrates is a complex and large organ positioned at the interface with the aquatic or terrestrial environment, and is derived from the embryonic ectoderm (epidermis) and mesoderm (dermis and hypodermis) [...].

The Story of the Finest Armor: Developmental Aspects of Reptile Skin

The reptile skin is a barrier against water loss and pathogens and an armor for mechanical damages. The integument of reptiles consists of two main layers: the epidermis and the dermis. The epidermis, the hard cover of the body which has an armor-like role, varies among extant reptiles in terms of structural aspects such as thickness, hardness or the kinds of appendages it constitutes. The reptile epithelial cells of the epidermis (keratinocytes) are composed of two main proteins: intermediate filament keratins (IFKs) and corneous beta proteins (CBPs). The outer horny layer of the epidermis, stratum corneum, is constituted of keratinocytes by means of terminal differentiation or cornification which is a result of the protein interactions where CBPs associate with and coat the initial scaffold of IFKs. Reptiles were able to colonize the terrestrial environment due to the changes in these epidermal structures, which led to various cornified epidermal appendages such as scales and scutes, a beak, claws or setae. Developmental and structural aspects of the epidermal CBPs as well as their shared chromosomal locus (EDC) indicate an ancestral origin that gave rise to the finest armor of reptilians.

Transglutaminase Activity Is Conserved in Stratified Epithelia and Skin Appendages of Mammals and Birds

The cross-linking of structural proteins is critical for establishing the mechanical stability of the epithelial compartments of the skin and skin appendages. The introduction of isopeptide bonds between glutamine and lysine residues depends on catalysis by transglutaminases and represents the main protein cross-linking mechanism besides the formation of disulfide bonds. Here, we used a fluorescent labeling protocol to localize the activity of transglutaminases on thin sections of the integument and its appendages in mammals and birds. In human tissues, transglutaminase activity was detected in the granular layer of the epidermis, suprabasal layers of the gingival epithelium, the duct of sweat glands, hair follicles and the nail matrix. In the skin appendages of chickens, transglutaminase activity was present in the claw matrix, the feather follicle sheath, the feather sheath and in differentiating keratinocytes of feather barb ridges. During chicken embryogenesis, active transglutaminase was found in the cornifying epidermis, the periderm and the subperiderm. Transglutaminase activity was also detected in the filiform papillae on the tongue of mice and in conical papillae on the tongue of chickens. In summary, our study reveals that transglutaminase activities are widely distributed in integumentary structures and suggests that transglutamination contributes to the cornification of hard skin appendages such as nails and feathers.

Identification of Chicken Transglutaminase 1 and In Situ Localization of Transglutaminase Activity in Avian Skin and Esophagus

Transglutaminase 1 (TGM1) is a membrane-anchored enzyme that cross-links proteins during terminal differentiation of epidermal and esophageal keratinocytes in mammals. The current genome assembly of the chicken, which is a major model for avian skin biology, does not include an annotated region corresponding to TGM1. To close this gap of knowledge about the genetic control of avian cornification, we analyzed RNA-sequencing reads from organotypic chicken skin and identified TGM1 mRNA. By RT-PCR, we demonstrated that TGM1 is expressed in the skin and esophagus of chickens. The cysteine-rich sequence motif required for palmitoylation and membrane anchorage is conserved in the chicken TGM1 protein, and differentiated chicken keratinocytes display membrane-associated transglutaminase activity. Expression of TGM1 and prominent transglutaminase activity in the esophageal epithelium was also demonstrated in the zebra finch. Altogether, the results of this study indicate that TGM1 is conserved among birds and suggest that chicken keratinocytes may be a useful model for the study of TGM1 in non-mammalian cornification.