Abstract
The morphogenesis of feathers is a complex process that depends on a tight spatiotemporal regulation of gene expression and assembly of the protein components of mature feathers. Recent comparative genomics and gene transcription studies have indicated that genes within the epidermal differentiation complex (EDC) encode numerous structural proteins of cornifying skin cells in amniotes including birds. Here, we determined the localization of one of these proteins, termed EDMTFH (Epidermal Differentiation Protein starting with a MTF motif and rich in Histidine), which belongs to a group of EDC-encoded proteins rich in aromatic amino acid residues. We raised an antibody against an EDMTFH-specific epitope and performed immunohistochemical investigations by light microscopy and immunogold labeling by electron microscopy of chicken embryos at days 14–18 of development. EDMTFH was specifically present in the subperiderm, a transient layer of the embryonic epidermis, and in barbs and barbules of feathers. In the latter, it partially localized to bundles of so-called feather beta-keratins (corneous beta-proteins, CBPs). Cells of the embryonic periderm, the epidermis proper, and the feather sheath were immunonegative for EDMTFH. The results of this study indicate that EDMTFH may contribute to the unique mechanical properties of feathers and define EDMTFH as a common marker of the subperiderm and the feather barbules. This expression pattern of EDMTFH resembles that of epidermal differentiation cysteine-rich protein (EDCRP) and feather CBPs and is in accordance with the hypothesis that a major part of the cyclically regenerating feather follicle is topologically, developmentally and evolutionarily related to the embryonic subperiderm.
Highlights
The cornified skin barrier of amniotes and cornified skin appendages such as claws, hair, and feathers are formed by epidermal keratinocytes that differentiate by inducing the expression of specific sets of genes [1,2,3,4]
Amino acid sequence alignments showed that EDMTFH is identical to the previously reported chicken histidine-rich protein (HRP) [29, 36] with the exception of the carboxy-terminal segment (Fig 1)
The amino-terminal sequence of EDMTFH is identical to a 20-amino acid peptide previously identified by direct peptide sequencing of HRP [29] (Fig 1A, blue underline) and highly similar to the sequences of peptides reported for so-called HRP-B proteins [38] (Fig 1B)
Summary
The cornified skin barrier of amniotes and cornified skin appendages such as claws, hair, and feathers are formed by epidermal keratinocytes that differentiate by inducing the expression of specific sets of genes [1,2,3,4]. Proteins traditionally termed beta-keratins [16,17,18] but identified as Corneous Beta Proteins (CBPs) represent a major sub-cluster of EDC genes in sauropsids while they are absent in mammals [8, 19]. These proteins of a molecular mass typically in the range of 10–18 kDa possess a characteristic central region (with a most highly conserved stretch of 34 amino acid residues) that folds into an anti-parallel beta-sheet and facilitates the formation of CBP filaments of 3–4 nm thickness [20, 21]. Labeling with tritiated histidine and autoradiography suggested that histidine-containing proteins are present in the cytoplasm and in corneous bundles of barbules, the identity of these protein(s) was not determined in that study [23]
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