Alpha-Keratin, Keratin-Associated Proteins and Transglutaminase 1 Are Present in the Ortho- and Parakeratinized Epithelium of the Avian Tongue

Abstract

The lingual mucosa in birds is covered with two specific types of multilayered epithelia, i.e., the para- and orthokeratinized epithelium, that differ structurally and functionally. Comprehensive information on proteins synthesized in keratinocyte during their cytodifferentiation in subsequent layers of multilayered epithelia in birds concerns only the epidermis and are missing the epithelia of the lingual mucosa. The aim of the present study was to perform an immunohistochemical (IHC) and molecular analysis (WB) of bird-specific alpha-keratin, keratin-associated proteins (KAPs), namely filaggrin and loricrin, as well as transglutaminase 1 in the para- and orthokeratinized epithelium covering the tongue in the domestic duck, goose, and turkey. The results reveal the presence of alpha-keratin and KAPs in both epithelia, which is a sign of the cornification process. In contrast to the epidermis, the main KAPs involved in the cornification process of the lingual epithelia in birds is loricrin. Stronger expression with KAPs and transglutaminase 1 in the orthokeratinized epithelium than in the parakeratinized epithelium may determine the formation of a more efficient protective mechanical barrier. The presence of alpha-keratin, KAPs, and transglutaminase 1 epitopes characteristic of epidermal cornification in both types of the lingual epithelia may prove that they are of ectodermal origin.

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

“…Likewise, transglutamination appears to protect the epithelium on the surface of the tongue and in lingual papillae. Recently, immunolabeling studies using cross-reactive antibodies have suggested expression of TGMs in suprabasal epithelial cells of avian tongue [19]. The localization of TGM enzymatic activity in our study is congruent with the pattern of immunoreactivity [19].…”

Pattern Distribution of Connexins in the Ortho- and Parakeratinized Epithelium of the Lingual Mucosa in Birds

“…Cytokeratins and other proteins of the cornification process are synthesized in the intermediate layer of the cornified epithelia of the avian tongue. Skieresz-Szewczyk et al [ 40 , 41 , 42 ] indicate the presence of avian-specific alpha- and beta-keratin, as well as KAPs (filaggrin and loricrin) and TGM-1 in the intermediate layer of both types of cornified lingual epithelia. The presence of kinase C is required to initiate the synthesis of the above proteins.…”

“…Immunohistochemical and molecular studies of the cornification process of ortho- and parakeratinized epithelia by Skieresz-Szewczyk et al [ 40 , 41 , 42 ] determined that there is a characteristic pattern of bird-specific alpha- and beta-keratins in the epithelia. Alpha-keratin is mainly accumulated in the lower layers of these epithelia, i.e., in the basal and intermediate layers, and its percentage is, on average, 38–45% [ 40 , 41 ].…”

“…Beta-keratin is present primarily in the cornified layer, whereby the orthokeratinized epithelium is characterized by a higher percentage of this keratin at 70% than the parakeratinized epithelium, in which the percentage of beta-keratin is about 61% [ 40 , 41 ]. Keratin-associated proteins (KAPs) (filaggrin and loricrin) and transglutaminase 1 (TGM-1) reveal stronger expression in the orthokeratinized epithelium than in the parakeratinized epithelium, which determines the formation of a more effective protective mechanical barrier [ 42 ]. The differences found in the expression of proteins in the cornification process are reflected in the diverse functions of these epithelia during food intake and transport into the oesophagus [ 40 , 41 , 42 ].…”

“…Keratin-associated proteins (KAPs) (filaggrin and loricrin) and transglutaminase 1 (TGM-1) reveal stronger expression in the orthokeratinized epithelium than in the parakeratinized epithelium, which determines the formation of a more effective protective mechanical barrier [ 42 ]. The differences found in the expression of proteins in the cornification process are reflected in the diverse functions of these epithelia during food intake and transport into the oesophagus [ 40 , 41 , 42 ]. Extensive studies of the epidermis and skin appendages in mammals and birds have determined that transglutaminase is a conserved protein contributing to cornification [ 43 ].…”