Cornification in reptilian epidermis occurs through the deposition of keratin‐associated beta‐proteins (beta‐keratins) onto a scaffold of intermediate filament keratins

Abstract

The isolation of genes for alpha-keratins and keratin-associated beta-proteins (formerly beta-keratins) has allowed the production of epitope-specific antibodies for localizing these proteins during the process of cornification epidermis of reptilian sauropsids. The antibodies are directed toward proteins in the alpha-keratin range (40-70 kDa) or beta-protein range (10-30 kDa) of most reptilian sauropsids. The ultrastructural immunogold study shows the localization of acidic alpha-proteins in suprabasal and precorneous epidermal layers in lizard, snake, tuatara, crocodile, and turtle while keratin-associated beta-proteins are localized in precorneous and corneous layers. This late activation of the synthesis of keratin-associated beta-proteins is typical for keratin-associated and corneous proteins in mammalian epidermis (involucrin, filaggrin, loricrin) or hair (tyrosine-rich or sulfur-rich proteins). In turtles and crocodilians epidermis, keratin-associated beta-proteins are synthesized in upper spinosus and precorneous layers and accumulate in the corneous layer. The complex stratification of lepidosaurian epidermis derives from the deposition of specific glycine-rich versus cysteine-glycine-rich keratin-associated beta-proteins in cells sequentially produced from the basal layer and not from the alternation of beta- with alpha-keratins. The process gives rise to Oberhäutchen, beta-, mesos-, and alpha-layers during the shedding cycle of lizards and snakes. Differently from fish, amphibian, and mammalian keratin-associated proteins (KAPs) of the epidermis, the keratin-associated beta-proteins of sauropsids are capable to form filaments of 3-4 nm which give rise to an X-ray beta-pattern as a consequence of the presence of a beta-pleated central region of high homology, which seems to be absent in KAPs of the other vertebrates.

Formation of adherens and communicating junctions coordinate the differentiation of the shedding‐layer and beta‐epidermal generation in regenerating lizard epidermis

“…The lack of nuclear labeling for beta‐catenin during this stage suggests that alpha keratinization is the basic (“default”) program of differentiation present in lizard keratinocytes, confirming previous views (Flaxman, ; Maderson et al, ). As shown in Figure A, it appears that when beta‐cells are produced in the central region of the elongating pegs this constitutive program is changed and newly synthesized proteins are deposited onto the alpha‐keratin scaffold present in beta‐cells (Alibardi, ). This is evidenced by the nuclear labeling for beta‐catenin that becomes temporary visible, especially in basal cells of the elongating caudal side of the pegs at Stages 2–3 of the shedding cycle, destined to produce new keratinocytes for the formation of the Oberhäutchen and beta‐layer.…”

Vitamin A administration in lizards during tail regeneration determines epithelial mucogenesis and delays muscle and cartilage differentiation

“…In the regenerating epidermis, retinol and/or its derived metabolites have shown to inhibit the process of keratinization (accumulation of IF‐keratins) and the following cornification (addition of corneous and keratin‐associated proteins to the keratin framework; see Alibardi, ). This interference was previously noted in the avian and mammalian epidermis and keratinocytes cultivated in vitro (Barnett & Szabo, ; Chopra and Flaxman, ).…”

“…Other sections of tissues embedded in Bioacryl were immunostained for the detection of CBPs (formerly indicated as β‐keratins) using a rabbit antibody (precore box, see Alibardi, ). Briefly, the sections were preincubated for 20 min in 0.1 M Tris buffer solution at pH 7.4, with the addition of 5% of bovine serum albumin for blocking nonspecific antigenic sites.…”

Immunolocalization of large corneous beta-proteins in the green anole lizard (Anolis carolinensis) suggests that they form filaments that associate to the smaller beta-proteins in the beta-layer of the epidermis

“…A further indication that true intermediate filament keratins (alpha‐keratins) mix and interact with beta‐proteins during epidermal maturation derives from the detection of immunolabeled bands in 1D Western blots or immunolabeled spots in 2D Western blots within the range of alpha‐keratins, above 40 kDa (Alibardi and Toni, ; Rizzo et al, ; Toni et al, ). Also double‐labeling studies using immunogold at the ultrastructural level demonstrated the coexpression and mixing of these two types of proteins (Alibardi, ).…”

“…These proteins have been provisionally subdivided into 4 subfamilies: High Glycine (HgG, 17 members), High Glycine Cysteine (HGC, 16 members), High Cysteine (HC, 2 members), and Low Glycine and Cysteine (LwGC, 2 members). Systematic immunohistochemical studies have mapped the distribution of representative members of the above 37 CBPs in the skin of A. carolinensis (Alibardi et al, ; Alibardi, ). The presence of so many small CBPs suggests that these proteins mix in different proportion in scales localized in different body areas to impart them their specific mechanical or barrier performances.…”