Review: mapping epidermal beta-protein distribution in the lizard Anolis carolinensis shows a specific localization for the formation of scales, pads, and claws

Abstract

The epidermis of lizards is made of multiple alpha- and beta-layers with different characteristics comprising alpha-keratins and corneous beta-proteins (formerly beta-keratins). Three main modifications of body scales are present in the lizard Anolis carolinensis: gular scales, adhesive pad lamellae, and claws. The 40 corneous beta-proteins present in this specie comprise glycine-rich and glycine-cysteine-rich subfamilies, while the 41 alpha-keratins comprise cysteine-poor and cysteine-rich subfamilies, the latter showing homology to hair keratins. Other genes for corneous proteins are present in the epidermal differentiation complex, the locus where corneous protein genes are located. The review summarizes the main sites of immunolocalization of beta-proteins in different scales and their derivatives producing a unique map of body distribution for these structural proteins. Small glycine-rich beta-proteins participate in the formation of the mechanically resistant beta-layer of most scales. Small glycine-cysteine beta-proteins have a more varied localization in different scales and are also present in the pliable alpha-layer. In claws, cysteine-rich alpha-keratins prevail over cysteine-poor alpha-keratins and mix to glycine-cysteine-rich beta-proteins. The larger beta-proteins with a molecular mass similar to that of alpha-keratins participate in the formation of the fibrous meshwork present in differentiating beta-cells and likely interact with alpha-keratins. The diverse localization of alpha-keratins, beta-proteins, and other proteins of the epidermal differentiation complex gives rise to variably pliable, elastic, or hard corneous layers in different body scales. The corneous layers formed in the softer or harder scales, in the elastic pad lamellae, or in the resistant claws possess peculiar properties depending on the ratio of specific corneous proteins.

Sauropsids Cornification is Based on Corneous Beta‐Proteins, a Special Type of Keratin‐Associated Corneous Proteins of the Epidermis

“…According to the traditional interpretation of the process of keratinization, alpha‐keratins are present in the epidermis and skin derivatives of all vertebrates, while beta‐keratins are exclusively present in sauropsids, including feathers, and they contain internal regions conformed as beta sheets. Using specific antibodies against these beta‐proteins on sections of scales, claws, beaks, or feathers, only their corneous or precorneous layers appear significantly immunolabel indicating they are differentiation‐specific proteins (Sawyer et al., ; Alibardi et al., ; Alibardi, ; Figs. and ).…”

“…). However, recent studies have indicated that also the alpha layers of lepidosaurians contain specific types of CβPs as well as the shell of the soft‐shelled turtle (Dalla Valle et al., ; Alibardi, ).…”

“…D–1P). The thickness and composition of the corneous layers vary among lineages generating a range of mechanical properties, from an almost complete inflexibility (turtle and crocodilian scutes, beta layer of lepidosaurians, claws) to a stiff but elastic material in feathers, hairs, and adhesive setae of specialized lizard scales, to a softer corneous material of the mammalian and avian epidermis or the alpha layer of lepidosaurian and turtle scales (Baden and Maderson, ; Parakkal and Alexander, ; Wu et al., ; Alibardi, , ; Figs. D–1P).…”

Identification and comparative analysis of the epidermal differentiation complex in snakes

“…The histological and ultrastructural features of squamate skin have been reported78920. However, only few aspects of the molecular architecture of alpha and beta-layers of squamate epidermis have been determined so far21222324.…”

“…Corneous beta-proteins, traditionally called beta-keratins, have been identified as components of the epidermis in snakes, like in other reptiles22252627. Keratin intermediate filament proteins, previously referred to as alpha-keratins, are the main cytoskeletal proteins in the epidermis and skin appendages of vertebrates.…”