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

Abstract

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.

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

“…The cross-linking of epidermal proteins depends on disulfide bonds between cysteine residues and isopeptide bonds between glutamine and lysine residues [1,13]. The latter bond is formed under the control of transglutaminases (TGMs) [14].…”

Keratinization and Cornification are not equivalent processes but keratinization in fish and amphibians evolved into cornification in terrestrial vertebrates

“…The stratified epidermis of anamniotes (fish and amphibians) is mucogenic (Figure 1A) and forms an external soft keratinized layer where transglutaminase activity, evidenced by the formation of isopeptide bonds in the external layers, is absent or weakly detectable by immunohistochemistry only in more terrestrial‐adapted anurans, frogs and toads (Figure 1B) 3–5 . While during keratinization epidermal cells accumulate variable amounts of IFKs, instead corneous cells (corneocytes) also accumulate other types of proteins indicated as CPs and KAPs that often overcame the initially deposited scaffold of keratins, determining cell death and cornification 14 …”

“…Other studies were done after extraction and electrophoretic separation, autoradiography and Western blot analyses of proteins from the epidermis of numerous vertebrates. [4][5][6][7][8][9] Bioinformatics coupled to gene expression analysis derived from experimental studies have supported the general interpretation of the process of terminal differentiation and of alpha-vs beta-cornification in the skin of all vertebrates. 6,[10][11][12][13] cells accumulate variable amounts of IFKs, instead corneous cells (corneocytes) also accumulate other types of proteins indicated as CPs and KAPs that often overcame the initially deposited scaffold of keratins, determining cell death and cornification.…”

“…This confusing issue should be definitely clarified since cornification is not only different from keratinization but is derived from keratinization during vertebrate evolution 3,4 . The present summarizing review derives from comparative morphological, immunohistochemical, proteomic and genomic studies carried out over the skin and skin appendages of numerous species of vertebrates, from fish to mammals, including humans 3–5 . Here, I briefly discuss the present interpretation of the process of terminal differentiation of keratinocytes in vertebrates referring to more extensive data, studies and explanations in some cited references 3–5 .…”

“…The stratified epidermis of anamniotes (fish and amphibians) is mucogenic (Figure1A) and forms an external soft keratinized layer where transglutaminase activity, evidenced by the formation of isopeptide bonds in the external layers, is absent or weakly detectable by immunohistochemistry only in more terrestrial-adapted anurans, frogs and toads (Figure1B) [3][4][5]. While during keratinization epidermal…”