The Structural Basis for the Heterogeneity of Chick Down Feather Keratin. The Partial Amino Acid Sequence of Down Feather Kreatin

Dynamic evolution of the alpha (α) and beta (β) keratins has accompanied integument diversification and the adaptation of birds into novel lifestyles

BACKGROUND

Vertebrate skin appendages are constructed of keratins produced by multigene families. Alpha (α) keratins are found in all vertebrates, while beta (β) keratins are found exclusively in reptiles and birds. We have studied the molecular evolution of these gene families in the genomes of 48 phylogenetically diverse birds and their expression in the scales and feathers of the chicken.

RESULTS

We found that the total number of α-keratins is lower in birds than mammals and non-avian reptiles, yet two α-keratin genes (KRT42 and KRT75) have expanded in birds. The β-keratins, however, demonstrate a dynamic evolution associated with avian lifestyle. The avian specific feather β-keratins comprise a large majority of the total number of β-keratins, but independently derived lineages of aquatic and predatory birds have smaller proportions of feather β-keratin genes and larger proportions of keratinocyte β-keratin genes. Additionally, birds of prey have a larger proportion of claw β-keratins. Analysis of α- and β-keratin expression during development of chicken scales and feathers demonstrates that while α-keratins are expressed in these tissues, the number and magnitude of expressed β-keratin genes far exceeds that of α-keratins.

CONCLUSIONS

These results support the view that the number of α- and β-keratin genes expressed, the proportion of the β-keratin subfamily genes expressed and the diversification of the β-keratin genes have been important for the evolution of the feather and the adaptation of birds into multiple ecological niches.

Histidine-rich protein B of embryonic feathers is present in the transient embryonic layers of scutate scales

Based on its amino acid composition and N-terminal sequence, a polypeptide (HRP-B) has been identified as a member of the avian histidine-rich protein (HRP) family. An antiserum against HRP-B has been used to localize this polypeptide in developing feathers and scales of chick embryos. HRP-B was first detectable in the barb ridge cells of feathers at 13 days of incubation and progressively appeared in the distal/proximal and peripheral/central gradients observed previously for the feather-type beta keratins in developing feathers. The HRP-B polypeptide was detected only in the embryonic layers of scutate scales. It first appeared at 16 days of incubation and was not found in the differentiated beta strata of these scales. At no time during the development of reticulate scales or apteric skin regions did the epidermal cells or cells of the embryonic layers express HRP-B. The transient expression of HRP-B by the embryonic layers of the scutate scale epidermis is discussed in light of the feather-forming potential of the presumptive epidermis of the scutate scale-forming region.

Avian scale development

Germinative cells of the scutate scale epidermis from 15-day embryos are committed to appendage-specific, beta stratum formation in association with a foreign dermis. Commitment precedes the time (17 days of development) at which beta strata are actually present in their site-specific locations along the outer surface of each scutate scale. This observation suggested the possibility that commitment to beta stratum formation might be occurring as the outer epidermal surface of each scutate scale first becomes established between 12 and 13 days of development. It is at this time that the scale epidermis loses its ability to participate in feather morphogenesis and cell proliferation becomes restricted to a true stratum basale. To examined the ability of the presumptive scutate scale epidermis to generate beta strata in the absence of the inductive scutate scale dermis, scutate scale epidermis from 11-, 12-, and 13-day embryos was recombined with 15-day reticulate scale dermis and grown for 7 or 9 days. The dermis of reticulate scales does not induce beta stratum formation, but it does support differentiation of a beta stratum by the determined 15-day scutate scale epidermis. Using immunohistological and biochemical analyses of beta-keratins, we find that each of these presumptive scutate scale epidermises is competent to generate appendage-specific beta strata in the absence of the scutate scale dermis. This determination is occurring prior to scale ridge morphogenesis and differentiation of the epidermis into the distinct outer and inner epidermal surfaces of the scale ridge. The restricted distribution of beta strata to the apical domes of individual reticulate-like scales demonstrates that the germinative cells of the committed epidermises are responding to patterned cues. This study also suggests that all basal cells of the presumptive scutate scale epidermis are initially endowed with the ability to generate cells that form a beta stratum.

Avian keratin genes. II. Chromosomal arrangement and close linkage of three gene families

We describe the isolation and characterization of a set of overlapping cosmid clones that contain chicken keratin genes. The 100 kb (1 kb = 10(3) base-pairs) of DNA represented in these clones contains a cluster of 18 feather keratin genes spanning 53 kb of DNA. The feather keratin genes are spaced about 3 kb apart and at least 11 of them have the same transcriptional orientation. Southern analysis using oligonucleotide probes made from highly conserved portions of the 5' non-coding, intron and 3' non-coding regions, respectively, indicate that these sequences have been highly conserved among the gene family as a whole, with only one or two exceptions in each case. The presence of some regularly repeated restriction enzyme sites are indicative of tandem duplication events in the recent history of the feather keratin gene family. The feather keratin gene locus is flanked on both sides by related types of keratin genes. On the 5' side of the feather gene cluster are three keratin genes (designated feather-like) that are located 5 kb from the last feather keratin gene and are spaced about 4 kb apart. On the 3' side of the feather gene cluster, 21 kb from the last feather keratin gene, lies a cluster of four genes that encode claw keratins. These genes are spaced about 1 kb apart and appear to be divergently transcribed. Partial DNA sequence analysis of the feather-like gene lying proximal to the feather keratin gene cluster demonstrated that it encodes a protein of 115 amino acid residues that is 80% homologous to the feather keratins at both the DNA and amino acid sequence levels. The feather-like gene(s) are expressed in both embryonic and adult (post-hatch) chick feathers and at a very low level in embryonic scale tissue. These genes therefore form a new family of feather proteins that is distinct from the previously characterized feather keratins.

Avian keratin genes. I. A molecular analysis of the structure and expression of a group of feather keratin genes

The nucleotide sequence of the four complete chicken feather keratin genes A to D contained in the previously isolated recombinant lambda CFK1 has been determined. All four genes have a very similar structure; each gene encodes a polypeptide of 97 amino acid residues and contains an intron in the 5' non-coding region, 37 base-pairs from the cap site. Comparison of the previously determined feather keratin gene C sequence to genes A, B and D indicates that a high level of gene correction has occurred in the protein coding and 5' non-coding regions, which show more than 90% homology, whereas the intron and 3' non-coding regions are by contrast poorly conserved with one or two exceptions. The dramatic conservation of the 5' non-coding region between the feather keratin sequences and an unrelated but co-expressed gene encoding a histidine-rich protein suggests that this segment may play an important role in transcriptional regulation. In addition, both gene types contain an identically positioned intron in the 5' non-coding region. Northern blots performed using gene-specific probes show that the four characterized genes A to D plus gene E, which is partially contained in the recombinant lambda CFK1, are all expressed in feather tissue from 14-day old chick embryos. In addition, we report that a scale keratin gene (originally isolated from a scale complementary DNA library) is expressed at a low level in the embryonic feather.

Differentiation of embryonic chick feather-forming and scale-forming tissues in transfilter cultures

The dermal-epidermal tissue interaction in the chick embryo, leading to the formation of feathers and scales, provides a good experimental system to study the transfer between tissues of signals which specify cell type. At certain times in development, the dermis controls whether the epidermis forms feathers or scales, each of which are characterized by the synthesis of specific beta-keratins. In our culture system, a dermal effect on epidermal differentiation can still be observed, even when the tissues are separated by a Nuclepore filter, although development is abnormal. Epidermal morphological and histological differentiation in transfilter cultures are distinct and recognizable, more closely resembling feather or scale development, depending on the regional origin of the dermis. Differentiation is more advanced when epidermis is cultured transfilter from scale dermis than from feather dermis, as assessed by morphology and histology, as well as the expression of the tissue-specific gene products, the beta-keratins. Two-dimensional polyacrylamide gel analysis of the beta-keratins reveals that scale dermis cultured transfilter from either presumptive scale or feather epidermis induces the production of 7 of the 9 scale-specific beta-keratins that we have identified. Feather dermis, although less effective in activating the feather gene program when cultured transfilter from either presumptive feather or scale epidermis, is able to turn on the synthesis of 3 to 6 of the 18 feather-specific beta-keratins that we have identified. However, scale epidermis in transfilter recombinants with feather dermis also continues to synthesize many of the scale-specific beta-keratins. Using transmission and scanning electron microscopy, we detect no cell contact between tissues separated by a 0.2-micron pore diameter Nuclepore filter, while 0.4-micron filters readily permit cell processes to traverse the filter. We find that epidermal differentiation is the same with either pore size filter. Furthermore, we do not detect a basement membrane in transfilter cultures, implying that neither direct cell contact between dermis and epidermis, nor a basement membrane between the tissues is required for the extent of epidermal differentiation that we observe.

Isolation and characterisation of keratin mRNA from the scale epidermis of the embryonic chick

The keratin polypeptides of the epidermis from the leg scale region of 17-day-old embryonic chicks were extracted as S-carboxymethylated derivatives and characterised by electrophoresis on SDS and pH 9.5 urea gels including a combination of both in two dimensions. Proteins were isolated that gave X-ray diffraction patterns typical of alpha- and beta- (avian feather) keratins. An mRNA fraction was isolated from 17-day-old scale tissue by guanidinium chloride extraction and sucrose gradient fractionation. The mRNA was translated in the wheat germ system to give a major product indistinguishable from the molecular weight class (Mr 14 500) of scale beta-keratin polypeptides. A cDNA library was constructed in pBR322 from a 15 S mRNA subfraction and two recombinant clones were selected by their strong hybridisation to cDNA prepared from the 15 S mRNA. The sequencing of these has yielded details of the relatedness of two scale keratin genes including their 3' untranslated regions. Almost half of the protein sequences of the two homologous scale keratins has been deduced and a notable feature of the scale keratin structure appears to be the presence of at least two sequence domains consisting of 13 amino acid repeats.