Immunohistochemical detection of sulfhydryl oxidase in chick skin appendages and feathers suggests that the enzyme contributes to maturation of the corneous material

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.

Immunolocalization of epidermal differentiation complex proteins reveals distinct molecular compositions of cells that control structure and mechanical properties of avian skin appendages

The epidermal differentiation complex (EDC) is a cluster of genes that encode structural proteins of skin derivatives with variable mechanical performances, from the scales of reptiles and birds to the hard claws and beaks, and to the flexible but resistant corneous material of feathers. Corneous proteins with or without extended beta-regions are produced from avian genomes, and include the largely prevalent corneous beta proteins (CβPs, formerly indicated as beta-keratins), and minor contribution from histidine-rich proteins, trichohyalin-like proteins (scaffoldin), loricrin, and other proteins rich in cysteine or other types of amino acids. The light-microscopic and ultrastructural immunolocalization of major and minor EDC-proteins in avian skin (feather CβPs, EDKM, EDWM, EDMTFH, EDDM, and scaffoldin) suggests that each specific appendage consists of a particular mix of these proteins in addition to the main proteins containing a peculiar beta-region of 34 amino acids, indicated as feather/scale/claw/beak CβPs (fCβPs, sCβPs, cCβPs, bCβPs). This indicates that numerous proteins of the EDC are added to the variable meshwork of intermediate filament keratins to produce avian epidermis with different mechanical and functional properties. Although the specific roles for these proteins are not known they likely make an important contribution to the final material properties of the different skin appendages of birds. The highest number of sauropsid CβPs is found in birds, suggesting a relation to the evolution of feathers, and additional epidermal differentiation proteins have contributed to the evolutionary adaptations of avian skin.

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.