Distribution of caspase-14 in epidermis and hair follicles is evolutionarily conserved among mammals

Comparative immunohistochemical analysis suggests a conserved role of EPS8L1 in epidermal and hair follicle barriers of mammals

The mammalian skin and its appendages depend on tightly coordinated differentiation of epithelial cells. Epidermal growth factor receptor (EGFR) pathway substrate 8 (EPS8) like 1 (EPS8L1) is enriched in the epidermis among human tissues and has also been detected in the epidermis of lizards. Here, we show by the analysis of single-cell RNA-sequencing data that EPS8L1 mRNA is co-expressed with filaggrin and loricrin in terminally differentiated human epidermal keratinocytes. Comparative genomics indicated that EPS8L1 is conserved in all main clades of mammals, whereas the orthologous gene has been lost in birds. Using a polyclonal antibody against EPS8L1, we performed an immunohistochemical screening of skin from diverse mammalian species and immuno-electron microscopy of human skin. EPS8L1 was detected predominantly in the granular layer of the epidermis in monotremes, marsupial, and placental mammals. The labeling was partly associated with cell membranes, and it was evident along the perimeter of keratinocytes at the transition with the cornified layer of the epidermis, similar to involucrin distribution. Basal, spinous, and the fully mature cornified layers lacked immunolabeling of EPS8L1. In addition to the epidermis, the hair follicle inner root sheath (IRS) was immunolabeled. Both epidermal granular layer and IRS contribute to the barrier function of the skin, suggesting that EPS8L1 is involved in the regulation of these barriers.

Comparative genomics of monotremes provides insights into the early evolution of mammalian epidermal differentiation genes

The function of the skin as a barrier against the environment depends on the differentiation of epidermal keratinocytes into highly resilient corneocytes that form the outermost skin layer. Many genes encoding structural components of corneocytes are clustered in the epidermal differentiation complex (EDC), which has been described in placental and marsupial mammals as well as non-mammalian tetrapods. Here, we analyzed the genomes of the platypus (Ornithorhynchus anatinus) and the echidna (Tachyglossus aculeatus) to determine the gene composition of the EDC in the basal clade of mammals, the monotremes. We report that mammal-specific subfamilies of EDC genes encoding small proline-rich proteins (SPRRs) and late cornified envelope proteins as well as single-copy EDC genes such as involucrin are conserved in monotremes, suggesting that they have originated in stem mammals. Monotremes have at least one gene homologous to the group of filaggrin (FLG), FLG2 and hornerin (HRNR) in placental mammals, but no clear one-to-one pairwise ortholog of either FLG, FLG2 or HRNR. Caspase-14, a keratinocyte differentiation-associated protease implicated in the processing of filaggrin, is encoded by at least 3 gene copies in the echidna. Our results reveal evolutionarily conserved and clade-specific features of the genetic regulation of epidermal differentiation in monotremes.

A Review on Caspases: Key Regulators of Biological Activities and Apoptosis

Caspases are proteolytic enzymes that belong to the cysteine protease family and play a crucial role in homeostasis and programmed cell death. Caspases have been broadly classified by their known roles in apoptosis (caspase-3, caspase-6, caspase-7, caspase-8, and caspase-9 in mammals) and in inflammation (caspase-1, caspase-4, caspase-5, and caspase-12 in humans, and caspase-1, caspase-11, and caspase-12 in mice). Caspases involved in apoptosis have been subclassified by their mechanism of action as either initiator caspases (caspase-8 and caspase-9) or executioner caspases (caspase-3, caspase-6, and caspase-7). Caspases that participate in apoptosis are inhibited by proteins known as inhibitors of apoptosis (IAPs). In addition to apoptosis, caspases play a role in necroptosis, pyroptosis, and autophagy, which are non-apoptotic cell death processes. Dysregulation of caspases features prominently in many human diseases, including cancer, autoimmunity, and neurodegenerative disorders, and increasing evidence shows that altering caspase activity can confer therapeutic benefits. This review covers the different types of caspases, their functions, and their physiological and biological activities and roles in different organisms.

Making the head: Caspases in life and death

The term apoptosis, as a way of programmed cell death, was coined a half century ago and since its discovery the process has been extensively investigated. The anatomy and physiology of the head are complex and thus apoptosis has mostly been followed in separate structures, tissues or cell types. This review aims to provide a comprehensive overview of recent knowledge concerning apoptosis-related molecules involved in the development of structures of head with a particular focus on caspases, cysteine proteases having a key position in apoptotic pathways. Since many classical apoptosis-related molecules, including caspases, are emerging in several non-apoptotic processes, these were also considered. The largest organ of the head region is the brain and its development has been extensively investigated, including the roles of apoptosis and related molecules. Neurogenesis research also includes sensory organs such as the eye and ear, efferent nervous system and associated muscles and glands. Caspases have been also associated with normal function of the skin and hair follicles. Regarding mineralised tissues within craniofacial morphogenesis, apoptosis in bones has been of interest along with palate fusion and tooth development. Finally, the role of apoptosis and caspases in angiogenesis, necessary for any tissue/organ development and maintenance/homeostasis, are discussed. Additionally, this review points to abnormalities of development resulting from improper expression/activation of apoptosis-related molecules.

Study of the protective effects of cosmetic ingredients on the skin barrier, based on the expression of barrier-related genes and cytokines

Background

Sensitive skin is the result of a complex process that is closely linked to the damage of the skin barrier. There are no recognized methods for evaluating the efficacy of anti-allergy products.

Methods

In this study, a model of skin barrier damage was created by treating HaCaT cells with 60 μg/ml of sodium dodecyl sulfate for 48 h. The protective effects of nine cosmetic ingredients, including oat extract (S1), on the skin barrier were investigated based on the gene expression levels of aquaporin3 (AQP3), filaggrin (FLG), caspase-14 (CASP14), and human tissue kallikrein7 (KLK7), as well as those of various interleukins (IL) and vascular endothelial growth factor (VEGF).

Results

Among the nine ingredients, S1 had a good protective effect on the function of the skin barrier. It promoted the expression of AQP3, FLG, and CASP14, while inhibiting the expression of KLK7 in HaCaT cells, at a concentration of 0.06%. It also maintained IL-6, IL-8, and VEGF at appropriate levels while promoting the proliferation and differentiation of HaCaT cells.

Conclusions

The above indicators allow for the preliminary establishment of a method to evaluate the efficacy of the barrier protection ability of sensitive skin.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11033-021-06918-5.

Caspase-14-From Biomolecular Basics to Clinical Approach. A Review of Available Data

Caspase-14 is a unique member of the caspase family-a family of molecules participating in apoptosis. However, it does not affect this process but regulates another form of programmed cell death-cornification, which is characteristic of the epidermis. Therefore, it plays a crucial role in the formation of the skin barrier. The cell death cycle has been a subject of interest for researchers for decades, so a lot of research has been done to expand the understanding of caspase-14, its role in cell homeostasis and processes affecting its expression and activation. Conversely, it is also an interesting target for clinical researchers searching for its role in the physiology of healthy individuals and its pathophysiology in particular diseases. A summary was done in 2008 by Denecker et al., concentrating mostly on the biotechnological aspects of the molecule and its physiological role. However, a lot of new data have been reported, and some more practical and clinical research has been conducted since then. The majority of studies tackled the issue of clinical data presenting the role of caspase in the etiopathology of many diseases such as retinal dysfunctions, multiple malignancies, and skin conditions. This review summarizes the available knowledge on the molecular and, more interestingly, the clinical aspects of caspase-14. It also presents how theoretical science may pave the way for medical research. Methods: The authors analyzed publications available on PubMed until 21 March 2021, using the search term "caspase 14".

KLK5 and KLK7 Ablation Fully Rescues Lethality of Netherton Syndrome-Like Phenotype

Netherton syndrome (NS) is a severe skin disease caused by the loss of protease inhibitor LEKTI, which leads to the dysregulation of epidermal proteases and severe skin-barrier defects. KLK5 was proposed as a major protease in NS pathology, however its inactivation is not sufficient to rescue the lethal phenotype of LEKTI-deficient mice. In this study, we further elucidated the in vivo roles of the epidermal proteases in NS using a set of mouse models individually or simultaneously deficient for KLK5 and KLK7 on the genetic background of a novel NS-mouse model. We show that although the ablation of KLK5 or KLK7 is not sufficient to rescue the lethal effect of LEKTI-deficiency simultaneous deficiency of both KLKs completely rescues the epidermal barrier and the postnatal lethality allowing mice to reach adulthood with fully functional skin and normal hair growth. We report that not only KLK5 but also KLK7 plays an important role in the inflammation and defective differentiation in NS and KLK7 activity is not solely dependent on activation by KLK5. Altogether, these findings show that unregulated activities of KLK5 and KLK7 are responsible for NS development and both proteases should become targets for NS therapy.

Histology and ultrastructure of transitional changes in skin morphology in the juvenile and adult four-striped mouse (Rhabdomys pumilio)

The four-striped mouse has a grey to brown coloured coat with four characteristic dark stripes interspersed with three lighter stripes running along its back. The histological differences in the skin of the juvenile and adult mouse were investigated by Haematoxylin and Eosin and Masson Trichrome staining, while melanocytes in the skin were studied through melanin-specific Ferro-ferricyanide staining. The ultrastructure of the juvenile skin, hair follicles, and melanocytes was also explored. In both the juvenile and adult four-striped mouse, pigment-containing cells were observed in the dermis and were homogeneously dispersed throughout this layer. Apart from these cells, the histology of the skin of the adult four-striped mouse was similar to normal mammalian skin. In the juvenile four-striped mouse, abundant hair follicles of varying sizes were observed in the dermis and hypodermis, while hair follicles of similar size were only present in the dermis of adult four-striped mouse. Ultrastructural analysis of juvenile hair follicles revealed that the arrangement and differentiation of cellular layers were typical of a mammal. This study therefore provides unique transition pattern in the four-striped mouse skin morphology different from the textbook description of the normal mammalian skin.

Expression of caspase 14 and filaggrin in oral squamous carcinoma

Caspase 14 is one of the latter discovered members of the caspase enzyme family and, although sharing sequence homologies with the other caspases, it is not involved in apoptosis. Together with its co-factor filaggrin, it plays an important role in skin barrier formation. It is already known that caspase 14 proteins are reduced during neoplastic dedifferentiation in cervical intraepithelial neoplasms and in invasive cervical carcinomas. Oral squamous carcinoma tissues have not been systematically evaluated for caspase 14 expression yet. Formalin-fixed and paraffin-embedded samples from oral squamous carcinomas (n = 36 tumours from 34 patients), metastases (n = 15) and controls (leukoplakia, n = 10) were analysed by immunohistochemistry. In carcinomas, human papilloma virus (HPV) infection was tested by PCR. Here we demonstrate that, in oral epithelia, caspase 14 is expressed mainly by cells of the intermediate and superficial cell layers while filaggrin is expressed only in keratinising foci in leukoplakia. Caspase 14 and filaggrin are co-localised. In invasive oral carcinomas, reduced expression of caspase 14 was detectable in 47 % of tumours but was not associated with keratinisation, tumour differentiation or HPV infection. Filaggrin was detectable in a subfraction of tumours (56 %) and was restricted to keratinising areas of the carcinomas. In summary, in contrast to cervical carcinomas, partial loss of caspase 14 is not associated with dedifferentiation in neoplastic lesions of the oral mucosa or HPV infection.

Expression of caspase-14 and keratin-19 in the human epidermis and appendages during fetal skin development

Caspase-14 is a seemingly non-apoptotic caspase involved in keratinocyte differentiation and cornification of the skin. Keratin-19 is an epithelial marker and a potential marker of epidermal stem cells that is expressed during human fetal skin development. We examined the immunohistochemical expression of caspase-14 in relation to CK-19 in the human fetal skin during development and perinatally, to assess their role in human skin maturation. Skin samples were received at autopsy. In the fetal epidermis, caspase-14 was predominantly expressed in the more differentiated layers, gradually disappearing from the basal layer toward term. By contrast, keratin-19 expression gradually decreased with epidermal maturation through gestation (rho = -0.949; p = 0.0001) and was a marker of the germinative layers. Keratin-19 was preserved in scarce basal cell nests at term and postnatally. Caspase-14 and keratin-19 were inversely expressed in the differentiating epidermal layers through gestation (p < 0.0001). Concerning the appendages, in hair follicles and sebaceous glands, caspase-14 located preferentially in the more differentiated layers of the inner root sheath, whereas keratin-19 was expressed in the outer sheath. Eccrine sweat glands showed a variable pattern of caspase-14 and keratin-19 expression. In conclusion, caspase-14 emerged as a marker of human skin differentiation during development, while keratin-19 marked the germinative epithelial layers in the fetal epidermis and appendages and possibly the nests of epidermal stem cells.

Comparative immunohistochemical analyses on the modes of cell death/keratinization in epidermal cyst, trichilemmal cyst, and pilomatricoma

Keratinization is a kind of cell death called terminal differentiation and includes various patterns such as epidermal keratinization (EK), trichilemmal keratinization (TK), and shadow cell differentiation (SCD), whereas these have not been comparatively investigated from a standpoint of cell death. In the present study, surgically extirpated specimens of epidermal cyst, trichilemmal cyst, and pilomatricoma (10 cases in each) were subjected to immunohistochemistry for single-strand DNA (ssDNA), gamma-H2AX, cleaved caspase-3, cleaved lamin A, caspase-14, and CD138 to compare the modes of cell death and keratinization pattern. Transitional cells in pilomatricoma were immunoreactive, although not in whole part, for ssDNA and gamma-H2AX, and negative for cleaved caspase-3 and cleaved lamin A. Epidermal and trichilemmal cyst were negative for these 4 markers, except for ssDNA or cleaved lamin A in a small number of parakeratotic cells in a few cases. The keratinizing component showed caspase-14(+)/CD138(-) in epidermal cyst, caspase-14(-)/CD138(+) in trichilemmal cyst, and caspase-14(-)/CD138(-) in pilomatricoma. These results indicate that EK, TK, and SCD have a common property of apoptosis-like programmed cell death without caspase-3 activation or nuclear fragmentation. Meanwhile, they show different characteristics one another as follows: (A), DNA double-strand breaks occur in the transitional cells of SCD but not in EK/TK; and (B), EK, TK, and SCD can be distinguished by expression pattern of caspase-14 and CD138 in the keratinizing component.

Gene expression profile of the skin in the 'hairpoor' (HrHp) mice by microarray analysis

Background

The transcriptional cofactor, Hairless (HR), acts as one of the key regulators of hair follicle cycling; the loss of function mutations is the cause of the expression of the hairless phenotype in humans and mice. Recently, we reported a new Hr mutant mouse called 'Hairpoor' (Hr^Hp). These mutants harbor a gain of the function mutation, T403A, in the Hr gene. This confers the overexpression of HR and Hr^Hp is an animal model of Marie Unna hereditary hypotrichosis in humans. In the present study, the expression profile of Hr^Hp/Hr^Hp skin was investigated using microarray analysis to identify genes whose expression was affected by the overexpression of HR.

Results

From 45,282 mouse probes, differential expressions in 43 (>2-fold), 306 (>1.5-fold), and 1861 genes (>1.2-fold) in skin from Hr^Hp/Hr^Hp mice were discovered and compared with skin from wild-type mice. Among the 1861 genes with a > 1.2-fold increase in expression, further analysis showed that the expression of eight genes known to have a close relationship with hair follicle development, ascertained by conducting real-time PCR on skin RNA produced during hair follicle morphogenesis (P0-P14), indicated that four genes, Wif1, Casp14, Krt71, and Sfrp1, showed a consistent expression pattern with respect to HR overexpression in vivo.

Conclusion

Wif1 and Casp14 were found to be upregulated, whereas Krt71 and Sfrp1 were downregulated in cells overexpressing HR in transient transfection experiments on keratinocytes, suggesting that HR may transcriptionally regulate these genes. Further studies are required to understand the mechanism of this regulation by the HR cofactor.

Caspase-14 reveals its secrets

Caspase-14 is a unique member of the evolutionarily conserved family of cysteinyl aspartate-specific proteinases, which are mainly involved in inflammation and apoptosis. However, recent evidence also implicates these proteases in proliferation and differentiation. Although most caspases are ubiquitously expressed, caspase-14 expression is confined mainly to cornifying epithelia, such as the skin. Moreover, caspase-14 activation correlates with cornification, indicating that it plays a role in terminal keratinocyte differentiation. The determination of in vitro conditions for caspase-14 activity paved the way to identifying its substrates. The recent development of caspase-14-deficient mice underscored its importance in the correct degradation of (pro)filaggrin and in the formation of the epidermal barrier that protects against dehydration and UVB radiation. Here, we review the current knowledge on caspase-14 in skin homeostasis and disease.

In vitro degradation of the inner root sheath in human hair follicles lacking sebaceous glands

BACKGROUND

Cultured hair follicles lacking sebaceous glands do not appear to degrade the inner root sheath (IRS), suggesting that the gland may be involved in this process.

OBJECTIVES

To examine this supposition in cultured hair follicles.

METHODS

Pilosebaceous units were isolated from hair follicles cultured in vitro, and IRS degradation was studied by histology.

RESULTS

When grown in culture, the fibres of follicles lacking sebaceous glands were encased in a layer of translucent tissue. During hair growth in vitro this tissue remained intact at the distal end of the follicle but disappeared further down towards the bulb and then reappeared towards the proximal end. Transection within the region lacking this tissue resulted in the release of a naked hair fibre and the production of hair with no attached tissue upon subsequent hair growth. The translucent tissue represented the IRS, thereby demonstrating that this tissue is indeed degraded in vitro. Histological comparison with freshly isolated pilosebaceous units indicated that IRS degradation in vitro strongly resembled the process that occurs in vivo.

CONCLUSIONS

These data suggest that the sebaceous gland does not itself participate in IRS degradation. Indeed, this phenomenon appears to be a function of the follicle itself and is probably intimately linked with the processes of cellular proliferation, differentiation and death that occur during hair biogenesis.

Nine procaspases are expressed in normal human epidermis, but only caspase-14 is fully processed

BACKGROUND

During normal stratification of the epidermis, keratinocytes undergo a complex programme of terminal differentiation. This programmed cell death results in corneocyte accumulation to form the stratum corneum (SC). Terminal differentiation and apoptosis share numerous features such as elimination of nuclei and organelles, changes in cell shape, and activation of transglutaminases and proteases. Caspases are cysteine proteases that play a central role in apoptosis. Therefore they may also be involved in the terminal differentiation of keratinocytes.

OBJECTIVES

To identify the caspases expressed in normal human epidermis and to define their pattern of expression and activation.

METHODS

We analysed mRNAs from human epidermis by reverse transcription-polymerase chain reaction (RT-PCR), skin cryosections by immunohistological methods, and epidermal protein extracts by Western blotting.

RESULTS

The mRNAs encoding caspase-1, -2, -3, -4, -6, -7, -8, -9, -10 and -14 were detected by RT-PCR. Accordingly, the immunohistological analyses showed clear expression in the epidermis of the corresponding proteins except caspase-2 and caspase-8, with only a weak expression of caspase-9. Moreover, procaspase-1, -2, -3, -4, -6, -7, -9, -10 and -14, and the fully processed caspase-14, were immunodetected in total epidermis extracts. However, only procaspase-1 and the processed caspase-14 were detected in extracts of superficial SC. In addition to these two proteins, procaspase-4 was detected in extracts of superficial SC obtained from lesional psoriatic skin.

CONCLUSIONS

This study, the first exhaustive description of caspase expression and processing in normal human epidermis, indicates that in vivo granular keratinocytes express nine procaspases, and in addition the activated form of caspase-14. This confirms that only caspase-14 is involved in keratinocyte differentiation, and suggests that keratinocytes are ready to induce apoptosis in response to cutaneous damage.

Activation of PKCδ and p38δ MAPK during okadaic acid dependent keratinocyte apoptosis

There is substantial interest in identifying agents that differentially activate keratinocyte differentiation versus apoptosis. Okadaic acid (OA) is a tumor promoter in mouse skin that also stimulates apoptosis of murine keratinocytes. OA also enhances human keratinocyte differentiation; however, the impact of OA treatment on apoptosis in these cells has not been examined. We show that OA promotes normal human keratinocyte apoptosis as evidenced by increased accumulation of cells having sub-G1/S DNA content, decreased mitochondrial integrity, increased annexin V binding, increased cytoplasmic cytochrome c level, and increased procaspase 3 and PARP cleavage. Cyclin A, cyclin D1, cdk2, cdk4, p53 and p21 levels are reduced. These changes are associated with release of the PKCdelta catalytic domain and increased phosphorylation of PKCdelta-T(505)-responses consistent with PKCdelta activation. In contrast, phosphorylation of PKCdelta-Y(311) is not increased. The apoptotic response is enhanced in OA treated cells in the presence of p38delta, a PKCdelta target. OA treatment selectively activated p38delta, and OA-dependent apoptosis is not inhibited by treatment with the p38alpha/beta inhibitor, SB203580. These findings are consistent with the idea that the response is mediated by p38delta. Our data indicate that OA is an agent that regulates both keratinocyte differentiation and apoptosis, and that this regulation is mediated via activation of a PKCdelta/p38delta signaling cascade.

Cytochemical, biochemical and molecular aspects of the process of keratinization in the epidermis of reptilian scales

The characteristics of scaled skin of reptiles is one of their main features that distinguish them from the other amniotes, birds and mammals. The different scale patterns observed in extant reptiles result from a long evolutive history that allowed each species to adapt to its specific environment. The present review deals with comparative aspects of epidermal keratinization in reptiles, chelonians (turtles and tortoises), lepidosaurian (lizards, snakes, sphenodontids), archosaurians (crocodilians). Initially the morphology and cytology of reptilian scales is outlined to show the diversity in the epidermis among different groups. The structural proteins (alpha-keratins and associated proteins), and enzymes utilized to form the corneous layer of the epidermis are presented. Aside cytokeratins (alpha-keratins), used for making the cytoskeleton, reptilian alpha-keratinocytes produce interkeratin (matrix) and corneous cell envelope proteins. Keratin bundles and degraded cell organelles constitute most of the corneous material of alpha-keratinocytes. Matrix, histidine-rich and sulfur-rich proteins are produced in the soft epidermis and accumulated in the cornified cell envelope. Main emphasis is given to the composition and to the evolution of the hard keratins (beta-keratins). Beta-keratins constitute the hard corneous material of scales. These small proteins are synthesized in beta-keratinocytes and are accumulated into small packets that rapidly merge into a compact corneous material and form densely cornified layers. Beta-keratins are smaller proteins (8-20 kDa) in comparison to alpha-keratins (40-70 kDa), and this size may determine their dense packing in corneocytes. Both glycine-sulfur-rich and glycine-proline-rich proteins have been so far sequenced in the corneous material of scales in few reptilian species. The latter keratins possess C- and N-amino terminal amino acid regions with sequence homology with those of mammalian hard keratins. Also, reptilian beta-keratins possess a central core with homology with avian scale/feather keratins. Multiple genes code for these proteins and their discovery and sequentiation is presently an active field of research. These initial findings however suggest that ancient reptiles already possessed some common genes that have later diversified to produce the specific keratin-associated proteins in their descendants: extant reptiles, birds and mammals. The evolution of these small proteins in lepidosaurians, chelonians and archosaurians represent the next step to understand the evolution of cornification in reptiles and derived amniotes (birds and mammals).

Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

This review presents comparative aspects of epidermal keratinization in vertebrates, with emphasis on the evolution of the stratum corneum in land vertebrates. The epidermis of fish does not contain proteins connected with interkeratin matrix and corneous cell envelope formation. Mucus-like material glues loose keratin filaments. In amphibians a cell corneous envelope forms but matrix proteins, aside from mucus/glycoproteins, are scarce or absent. In reptiles, birds, and mammals specific proteins associated with keratin become relevant for the production of a resistant corneous layer. In reptiles some matrix, histidine-rich and sulfur-rich corneous cell envelope proteins are produced in the soft epidermis. In avian soft epidermis low levels of matrix and cornified proteins are present while lipids become abundant. In mammalian keratinocytes, interkeratin proteins, cornified cell envelope proteins, and transglutaminase are present. Topographically localized areas of dermal-epidermal interactions in amniote skin determine the formation of skin derivatives such as scales, feathers, and hairs. New types of keratin and associated proteins are produced in these derivatives. In reptiles and birds beta-keratins form the hard corneous material of scales, claws, beaks, and feathers. In mammals, small sulfur-rich and glycine-tyrosine-rich proteins form the corneous material of hairs, horns, hooves, and claws. Molecular studies on reptilian beta-keratins show they are glycine-rich proteins. They have C- and N-terminal amino acid regions homologous to those of mammalian proteins and a central core with homology to avian scale/feather keratins. These findings suggest that ancient reptiles already possessed some common genes that later diversified to produce some keratin-associated protein in extant reptiles and birds, and others in mammals. The evolution of these small proteins represents the more recent variation of the process of cornification in vertebrates.