Remodeling of the involucrin gene during primate evolution

Selective sweep for an enhancer involucrin allele identifies skin barrier adaptation out of Africa

The genetic modules that contribute to human evolution are poorly understood. Here we investigate positive selection in the Epidermal Differentiation Complex locus for skin barrier adaptation in diverse HapMap human populations (CEU, JPT/CHB, and YRI). Using Composite of Multiple Signals and iSAFE, we identify selective sweeps for LCE1A-SMCP and involucrin (IVL) haplotypes associated with human migration out-of-Africa, reaching near fixation in European populations. CEU-IVL is associated with increased IVL expression and a known epidermis-specific enhancer. CRISPR/Cas9 deletion of the orthologous mouse enhancer in vivo reveals a functional requirement for the enhancer to regulate Ivl expression in cis. Reporter assays confirm increased regulatory and additive enhancer effects of CEU-specific polymorphisms identified at predicted IRF1 and NFIC binding sites in the IVL enhancer (rs4845327) and its promoter (rs1854779). Together, our results identify a selective sweep for a cis regulatory module for CEU-IVL, highlighting human skin barrier evolution for increased IVL expression out-of-Africa.

Recent evolution of the human skin barrier

The skin is the first line of defense against the environment, with the epidermis as the outermost tissue providing much of the barrier function. Given its direct exposure to and encounters with the environment, the epidermis must evolve to provide an optimal barrier for the survival of an organism. Recent advances in genomics have identified a number of genes for the human skin barrier that have undergone evolutionary changes since humans diverged from chimpanzees. Here, we highlight a selection of key and innovative genetic findings for skin barrier evolution in our divergence from our primate ancestors and among modern human populations.

Basonuclin 2 has a function in the multiplication of embryonic craniofacial mesenchymal cells and is orthologous to disco proteins

Basonuclin 2 is a recently discovered zinc finger protein of unknown function. Its paralog, basonuclin 1, is associated with the ability of keratinocytes to multiply. The basonuclin zinc fingers are closely related to those of the Drosophila proteins disco and discorelated, but the relation between disco proteins and basonuclins has remained elusive because the function of the disco proteins in larval head development seems to have no relation to that of basonuclin 1 and because the amino acid sequence of disco, apart from the zinc fingers, also has no similarity to that of the basonuclins. We have generated mice lacking basonuclin 2. These mice die within 24 h of birth with a cleft palate and abnormalities of craniofacial bones and tongue. In the embryonic head, expression of the basonuclin 2 gene is restricted to mesenchymal cells in the palate, at the periphery of the tongue, and in the mesenchymal sheaths that surround the brain and the osteocartilagineous structures. In late embryos, the rate of multiplication of these mesenchymal cells is greatly diminished. Therefore, basonuclin 2 is essential for the multiplication of craniofacial mesenchymal cells during embryogenesis. Non-Drosophila insect databases available since 2008 reveal that the basonuclins and the disco proteins share much more extensive sequence and gene structure similarity than noted when only Drosophila sequences were examined. We conclude that basonuclin 2 is both structurally and functionally the vertebrate ortholog of the disco proteins. We also note the possibility that some human craniofacial abnormalities are due to a lack of basonuclin 2.

Ancient origin of the gene encoding involucrin, a precursor of the cross-linked envelope of epidermis and related epithelia

The cross-linked (cornified) envelope is a characteristic product of terminal differentiation in the keratinocyte of the epidermis and related epithelia. This envelope contains many proteins of which involucrin was the first to be discovered and shown to become cross-linked by a cellular transglutaminase. Involucrin has evolved greatly in placental mammals, but retains the glutamine repeats that make it a good substrate for the transglutaminase. Until recently, it has been impossible to detect involucrin outside the placental mammals, but analysis of the GenBank and Ensembl databases that have become available since 2006 reveals the existence of involucrin in marsupials and birds. We describe here the properties of these involucrins and the ancient history of their evolution.

Mouse ribosomal RNA genes contain multiple differentially regulated variants

Previous cytogenetic studies suggest that various rDNA chromosomal loci are not equally active in different cell types. Consistent with this variability, rDNA polymorphism is well documented in human and mouse. However, attempts to identify molecularly rDNA variant types, which are regulated individually (i.e., independent of other rDNA variants) and tissue-specifically, have not been successful. We report here the molecular cloning and characterization of seven mouse rDNA variants (v-rDNA). The identification of these v-rDNAs was based on restriction fragment length polymorphisms (RFLPs), which are conserved among individuals and mouse strains. The total copy number of the identified variants is less than 100 and the copy number of each individual variant ranges from 4 to 15. Sequence analysis of the cloned v-rDNA identified variant-specific single nucleotide polymorphisms (SNPs) in the transcribed region. These SNPs were used to develop a set of variant-specific PCR assays, which permitted analysis of the v-rDNAs' expression profiles in various tissues. These profiles show that three v-rDNAs are expressed in all tissues (constitutively active), two are expressed in some tissues (selectively active), and two are not expressed (silent). These expression profiles were observed in six individuals from three mouse strains, suggesting the pattern is not randomly determined. Thus, the mouse rDNA array likely consists of genetically distinct variants, and some are regulated tissue-specifically. Our results provide the first molecular evidence for cell-type-specific regulation of a subset of rDNA.

A Synthesis Approach to Understanding Repeated Peptides Conserved in Mineralization Proteins

We created artificial proteins that contained repeats of a short peptide motif, Asn-Gly-Asx. In nature this motif is repeated within shell proteins as an idiosyncratic domain, while in vitro it has been shown to suppress calcification. The motif was embedded within peptide sequences that did or did not have the ability to form secondary structures, which provided the motif with a variety of physicochemical properties. Although a short synthetic peptide containing the motif did not inhibit calcification in vitro, some of the artificial proteins carrying repeats of the motif did show robust suppression of calcification. Artificial proteins lacking the motif did not exhibit suppressive activity. Likewise, one construct containing multiple repeats of the motifs also did not exert an inhibitory effect on calcification. Apparently, carrying the Asn-Gly-Asx motif is not, by itself, sufficient for expression of its cryptic activity; instead, certain physicochemical properties of the polypeptides mediate its manifestation. We anticipate that syntheses using "motif programming", such as the one described here, will shed light on the origin of repetitive sequences as well as on the evolution of biomineralization proteins.

Systematic repeat addition at a precise location in the coding region of the involucrin gene of wild mice reveals their phylogeny

The involucrin gene encodes a protein of terminally differentiated keratinocytes. Its segment of repeats, which represents up to 80% of the coding region, is highly polymorphic in mouse strains derived from wild progenitors. Polymorphism includes nucleotide substitutions, but is most strikingly due to the recent addition of a variable number of repeats at a precise location within the segment of repeats. Each mouse taxon examined showed consistent and distinctive patterns of evolution of its variable region: very rapid changes in most M. m. domesticus alleles, slow changes in M. m. musculus, and complete arrest in M. spretus. We conclude that changes in the variable region are controlled by the genetic background. One of the M. m. domesticus alleles (DIK-L), which is of M. m. musculus origin, has undergone a recent repeat duplication typical of M. m. domesticus. This suggests that the genetic background controls repeat duplications through trans-acting factors. Because the repeat pattern differs in closely related murine taxa, involucrin reveals with greater sensitivity than random nucleotide substitutions the evolutionary relations of the mouse and probably of all murids.

On the role of periodism in the origin of proteins

Two different views have been proposed for origins of genes (or proteins). One is that primordial genes evolved from random sequences. This view underlies the concept of modern in vitro evolution experiments that functional molecules (even proteins) evolved from random sequence-libraries. On the contrary, the second view reminds that "random sequences" would be an unusual state in which to find RNA or DNA, because it is their inherent nature to yield periodic structures during the course of semi-conservative replication. In this second view, the periodicity of DNA (or RNA) is responsible for emergence of primordial genes. Although recent reports on the variety of periodicities present in proteins, genes and genomes are consistent with the second view, it has yet to be experimentally tested. We assessed the significance of periodicities of DNA in the origin of genes by constructing such periodic DNAs. The results showed that periodic DNA produced ordered proteins at very high rates, which is in contrast to the fact that proteins with random sequences lack secondary structures. We concluded that periodicity played a pivotal role in the origin of many genes. The observation should pave the way for new experimental evolution systems for proteins.

Function of basonuclin in increasing transcription of the ribosomal RNA genes during mouse oogenesis

Active protein synthesis during early oogenesis requires accelerated transcription of ribosomal RNA genes (rDNAs). In response to this demand, rDNAs are amplified more than 1000-fold early in Xenopus oogenesis. Here, we report evidence that rDNA is not amplified in mouse oocytes, but these cells may instead employ the zinc-finger protein basonuclin, a putative rDNA transcription factor, to enhance rRNA synthesis. This conclusion is based on observations that basonuclin is localized in the nucleolus in the mouse oocyte early in its growth phase, when rRNA transcription is highly active; and that the binding sites of basonuclin zinc fingers on the human and mouse rDNA promoters are homologous. In a co-transfection assay, basonuclin can elevate transcription from an rDNA promoter, and its zinc-finger domain can inhibit RNA polymerase I transcription, as detected by a run-on assay, in growing mouse oocytes.

Targeted ablation of the murine involucrin gene

Involucrin is synthesized in abundance during terminal differentiation of keratinocytes. Involucrin is a substrate for transglutaminase and one of the precursors of the cross-linked envelopes present in the corneocytes of the epidermis and other stratified squamous epithelia. These envelopes make an important contribution to the physical resistance of the epidermis. We have generated mice lacking involucrin from embryonic stem cells whose involucrin gene had been ablated by homologous recombination. These mice developed normally, possessed apparently normal epidermis and hair follicles, and made cornified envelopes that could not be distinguished from those of wild-type mice. No compensatory increase of mRNA for other envelope precursors was observed.

Expansion of mouse involucrin by intra-allelic repeat addition

Involucrin, loricrin and the small proline-rich proteins (SPRRs) are precursors of the cornified envelope of terminally differentiated keratinocytes. The genes for these proteins are closely linked on mouse chromosome 3. Each of the proteins is encoded by a single exon and is largely composed of a segment of short tandem repeats. No size polymorphism of either loricrin or the SPRRs was observed. In contrast, involucrin was found in at least eight polymorphic forms of different size with molecular weights ranging from 51 to 82kDa. Two classes of involucrin alleles were identified. Size polymorphism of involucrin has resulted from the recent expansion of the segment of repeats in one class of alleles, but not in the other. In expanding alleles, repeats were added at a precise location within the segment of repeats, in a 5'-to-3' direction. A study of a large number of allele-specific markers, located on both sides of the site of repeat addition, revealed no evidence for recombination between any of the alleles examined. Expansion of the segment of repeats of the gene for mouse involucrin must result from an intra-allelic process controlled by a cis-acting element, active in one class of alleles, and inactive in the other.

alpha-Helical solenoid model for the human involucrin

Involucrin is a key component of the cross-linked envelope of terminally differentiated keratinocytes. The human molecule largely consists of 10 residue repeats and forms a thin 460 A long rod. Summarized experimental data and a detailed stereochemical analysis made with computer modeling resulted in a structural model for the involucrin molecule. The suggested structure is a left-handed alpha-helical solenoid built of a tandem array of helix-turn-helix folds. The structure enables us to explain the whole set of experimental data and residue conservations within the repeats. It is ideally suited to serve as a scaffold for cell envelope assembly and proposes a possible mode of the intermolecular interactions of involucrin during cell cornification.

Involucrin cross-linking by transglutaminase 1. Binding to membranes directs residue specificity

The transglutaminase 1 (TGase 1) enzyme is essential for the assembly of the cell envelope barrier in stratified squamous epithelia. It is usually bound to membranes, but to date most studies with it have involved solution assays. Here we describe an in vitro model system for characterizing the function of TGase 1 on the surface of synthetic lipid vesicles (SLV) of composition similar to eukaryote plasma membranes. Recombinant baculovirus-expressed human TGase 1 readily binds to SLV and becomes active in cross-linking above 10 microM Ca2+, in comparison to above 100 microM in solution assays, suggesting that the membrane surface is important for enzyme function. Involucrin also binds to SLV containing 12-18% phosphatidylserine and at Ca2+ concentrations above 1 microM. In reactions of involucrin with TGase 1 enzyme in solution, 80 of its 150 glutamines serve as donor residues. However, on SLV carrying both involucrin and TGase 1, only five glutamines serve as donors, of which glutamine 496 was the most favored. As controls, there was no change in specificity toward the glutamines of other substrates used by free or SLV-bound TGase 1 enzyme. We propose a model in which involucrin and TGase 1 bind to membranes shortly after expression in differentiating keratinocytes, but cross-linking begins only later as intracellular Ca2+ levels increase. Furthermore, the data suggest that the membrane surface regulates the steric interaction of TGase 1 with substrates such as involucrin to permit specific cross-linking for initiation of cell envelope barrier formation.

Complementary oligonucleotides and the origin of the mammalian involucrin gene

A large portion of the consensus sequence of four mammalian involucrin genes comprises four pairs of complementary oligonucleotides. One of the oligonucleotides in each pair is rich in purines and the other in pyrimidines. The perfect complementarity of a number of pairs apparently resulted from a concerted single nucleotide substitution in each pairing oligonucleotide from their respective consensus. This suggests that one member of the pair was derived from the other member of the same pair by a template-dependent replication mechanism. Such a replication mechanism differs from the one that creates tandem repeats by producing short complementary sequence on the same strand of DNA.

The involucrin gene of the tree shrew: recent repeat additions and the relocation of cysteine codons

The coding region of the involucrin gene of Tupaia glis has been cloned and sequenced. It resembles the involucrin coding region of other non-anthropoid mammals in possessing a segment of related, short tandem repeats at a defined location, but in Tupaia, there has been recent serial duplication of a repeat into which a cysteine codon had earlier been introduced. As a result of the duplication, there is a total of as many as six cysteine codons in the segment of repeats, a number larger than for any other species yet examined. In Ratttus there has been a comparable but independent addition of cysteine codons, and both Tupaia and Rattus have eliminated an otherwise conserved cysteine codon 75 located close to but outside the segment of repeats. In Tupaia, this elimination probably occurred by gene conversion. Also independently, the gene of Canis has added cysteine codons to the segment of repeats but has not yet lost cysteine 75. It is proposed that the gain and the loss of cysteine codons are parts of a multi-stage program of cysteine relocation.

Exons lost and found. Unusual evolution of a seminal vesicle transglutaminase substrate

The GP1G gene codes for three of the four abundant androgen-regulated secretory proteins produced by the guinea pig seminal vesicle. Sequencing of the entire 6.3-kilobase gene and comparison with other mammalian seminal vesicle secretory protein genes reveals a common three-exon, two-intron organization. However, significant sequence similarity between this group of genes is largely limited to their 5'-flanking regions and first exons, which code almost exclusively for signal peptides in each case. The first intron of GP1G does contain a region with high similarity to the coding exon of a human seminal vesicle secretory protein gene, semenogelin II. The 3' half of the GP1G gene appears to share a common ancestry with the human SKALP/elafin gene. Sequences related to the elafin promoter, coding, untranslated regions, and introns are clearly identifiable within the GP1G sequence. The elafin gene codes for a serine protease inhibitor and is expressed in a variety of different human tissues. To determine if the GP1G gene was also active outside of the seminal vesicle, RNA from a variety of guinea pig tissues was hybridized to a GP1G cDNA probe. At least three novel RNA bands hybridizing to the GP1G probe were detected in testis RNA samples, and GP1G-related mRNAs were also found in other tissues. These data suggest that these seminal vesicle secretory proteins may have functional roles outside the reproductive system.

The recurring evolution of lipoprotein(a). Insights from cloning of hedgehog apolipoprotein(a)

The lipoprotein Lp(a), a major inherited risk factor for atherosclerosis, consists of a low density lipoprotein-like particle containing apolipoprotein B-100 plus the distinguishing component apolipoprotein(a) (apo(a)). Human apo(a) contains highly repeated domains related to plasminogen kringle four plus single kringle five and protease-like domains. Apo(a) is virtually confined to primates, and the gene may have arisen during primate evolution. One exception is the occurrence of an Lp(a)-like particle in the hedgehog. Cloning of the hedgehog apo(a)-like gene shows that it is distinctive in form and evolutionary history from human apo(a), but that it has acquired several common features. It appears that the primate and hedgehog apo(a) genes evolved independently by duplication and modification of different domains of the plasminogen gene, providing a novel type of "convergent" molecular evolution.

Effect of 1,25 dihydroxyvitamin D3 and calcium on growth and differentiation and on c-fos and p53 gene expression in normal human keratinocytes

Calcium enhances keratinocyte differentiation, and 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) is both antiproliferative and prodifferentiative in many cell types, including normal human keratinocytes. In the present study, we examined the combined effects of calcium and 1,25(OH)2D3 on parameters of growth and differentiation and on c-fos and p53 gene expression in normal human keratinocytes. Exposure of normal human keratinocytes to 1,25(OH)2D3 markedly reduced [3H] thymidine incorporation and cell number at low and high medium Ca++ concentrations. Simultaneously, cells in the G0/G1 phase of the cell cycle increased significantly and those in the S phase fell precipitously. 1,25(OH)2D3 and calcium also induced keratinocyte differentiation independently, as assessed by immunocytochemistry and by induction of involucrin mRNA. Both Ca++ and 1,25(OH)2D3 were shown, by nuclear run-on assays, to increase involucrin gene transcription. A rapid, transient elevation in c-fos protooncogene expression preceded these effects when epidermal growth factor was present alone. When 1,25(OH)2D3 was added to quiescent keratinocytes, there was a marked augmentation of c-fos mRNA accumulation at low and high medium Ca++ concentrations. Varying medium Ca++ concentrations had no effect on c-fos mRNA levels. Increasing medium Ca++ concentrations from 0.15 to 2.0 mM produced marked elevations of p53 mRNA accumulation and of the rate of p53 gene transcription, whereas 1,25(OH)2D3 had no effect. These results, therefore, suggest that 1,25(OH)2D3 and calcium act in concert to modulate the expression of two important cell-cycle-associated genes, which may be important components in the initial programming of growth and differentiation of normal human keratinocytes.

Homopolymer length variation in the Drosophila gene mastermind

Runs of identical amino acids encoded by triplet repeats (homopolymers) are components of numerous proteins, yet their role is poorly understood. Large numbers of homopolymers are present in the Drosophila melanogaster mastermind (mam) protein surrounding several unique charged amino acid clusters. Comparison of mam sequences from D. virilis and D. melanogaster reveals a high level of amino acid conservation in the charged clusters. In contrast, significant divergence is found in repetitive regions resulting from numerous amino acid replacements and large insertions and deletions. It appears that repetitive regions are under less selective pressure than unique regions, consistent with the idea that homopolymers act as flexible spacers separating functional domains in proteins. Notwithstanding extensive length variation in intervening homopolymers, there is extreme conservation of the amino acid spacing of specific charge clusters. The results support a model where homopolymer length variability is constrained by natural selection.

Expression patterns of loricrin in various species and tissues

In this study we analyzed the expression patterns of loricrin in various species and tissues using immunohistochemistry, immunoblotting and Northern blots. Loricrin is a glycine-, serine- and cysteine-rich protein expressed very late in epidermal differentiation in the granular layers of normal mouse and human epidermis. Later on in differentiation, loricrin becomes crosslinked as a major component into the cornified cell envelope by the formation of N epsilon-(gamma-glutamyl)lysine isopeptide bonds. This process either occurs directly or by the intermediate accumulation in L-keratohyaline granules of mouse epidermis and human acrosyringia. Loricrin was identified in all mammalian species analyzed by virtue of its highly conserved carboxy-terminal sequences revealing an electric mobility of approximately 60 kDa in rodents, rabbit and cow and of approximately 35 kDa in lamb and human on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Loricrin is expressed in the granular layer of all mammalian orthokeratinizing epithelia tested including oral, esophageal and fore-stomach mucosa of rodents, tracheal squamous metaplasia of vitamin A deficient hamster and estrogen induced squamous vaginal epithelium of ovary ectomized rats. Loricrin is also expressed in a few parakeratinizing epithelia such as BBN [N-butyl-N-(4-hydroxybutyl)nitrosamine]-induced murine bladder carcinoma and a restricted subset of oral and single vaginal epithelial cells in higher mammals. Our results provide further evidence that the program of squamous differentiation in internal epithelia of the upper alimentary tract in rodents and higher mammals differ remarkably. In addition, we also have noted the distinct distribution patterns of human loricrin and involucrin, another major precursor protein of the cornified cell envelope.

Tissue-specific and differentiation-appropriate expression of the human involucrin gene in transgenic mice: an abnormal epidermal phenotype

Involucrin is a precursor of the keratinocyte cornified envelope that is specifically expressed in the suprabasal layers of the epidermis and other stratifying squamous epithelia. To study involucrin gene expression and the function of involucrin, we expressed a 6 kb DNA fragment of the human involucrin gene, containing approximately 2.5 kb of upstream sequence and 0.5 kb of downstream sequence, in transgenic mice. The transgene produces a 68 kDa protein that is detected by a human involucrin-specific antibody, and is expressed in a tissue-specific and differentiation-appropriate manner (i.e., expression is confined to the suprabasal layers of the epidermis, extocervix, trachea, esophagus and conjunctiva). Soluble involucrin levels are two to four times higher in transgenic epidermal keratinocytes compared to human foreskin keratinocytes. Newborn heterozygous animals have a normal birth weight and a normal appearing epidermis and hair growth begins at 4 to 5 days of age (i.e., the same time as hair growth in non-transgenic animals). In a subpopulation of the newborn homozygous animals birth weight is reduced, the epidermis is scaly and hair growth begins late, at around 9 to 10 days of age. In addition, the hair tends to stand erect on both heterozygous and homozygous adult animals giving the appearance of diffuse alopecia. Immunofluorescent and electron microscopy localize involucrin in the hair follicle and cornified envelope, respectively. These results suggest that overexpression of involucrin may cause abnormalities in hair follicle structure/function and cornified envelope structure. These animals provide a new model for the study of cornified envelope structure and function.

Involucrin--structure and role in envelope assembly

Recent findings have revealed much about the structure of involucrin. These findings make it possible to propose specific models regarding the role of involucrin and the mechanism of its crosslinking as an envelope precursor. These models provide clearly testable hypotheses that are expected to provide additional insights into the mechanism of cornified envelope assembly.

Mapping of the trichohyalin gene: co-localization with the profilaggrin, involucrin, and loricrin genes

The chromosomal location of the gene encoding the human hair follicle protein trichohyalin has been determined by in situ hybridization. The human gene has been localized to the region 1q21.1-1q23 (probably 1q21.3) using a sheep trichohyalin cDNA probe. The genes encoding three other epithelial proteins, namely, profilaggrin, involucrin, and loricrin, are also located in the same region of chromosome 1, which, together with their similar gene and protein structures, suggests that the four proteins form a novel superfamily of epithelial structural proteins.

Basonuclin: a keratinocyte protein with multiple paired zinc fingers

A cDNA clone has been prepared from mRNA of cultured human keratinocytes. The sequence of the cDNA reveals that in the C-terminal two-thirds of the corresponding protein (basonuclin), there are three separated pairs of adjacent zinc fingers. The amino acid sequence of each pair is homologous to that of the single pair of zinc fingers of the Drosophila transcription factor encoded by disco. Near the C-terminal end of basonuclin and on the surface of a putative alpha-helix, there is a stripe of serine residues similar to that of the transcription factor PRDII-BF1. Basonuclin possesses a sequence of six amino acids quite similar to one present in the myogenic family of proteins, including Myf5; this sequence is located in the omega loop of the myogenic proteins but within a zinc finger of the keratinocyte protein. As basonuclin is present mainly in the nuclei of the basal cell layer, its regulatory function is likely to be exerted prior to the process of terminal differentiation.

Characterisation of eight monoclonal antibodies to involucrin

Involucrin is a precursor of the insoluble protein envelope that is assembled in the outermost layers of the epidermis. The coding sequence of the protein contains a number of short tandem repeats that have been greatly altered during mammalian evolution. We have characterised eight mouse monoclonal antibodies raised against human involucrin, all of which bind to the protein in immunoprecipitation, immunoblot and immunohistochemical preparations. Each antibody was screened for cross-reactivity with gorilla, owl monkey, dog and pig involucrin and with a fragment of the human protein, expressed in lambda gt 11, that includes the entire early region of the modern segment of repeats. Three antibodies recognised involucrin in all of these assays. Four antibodies recognised primate involucrins and the lambda gt 11 fragment. One antibody, which showed cross-reactivity with lower molecular weight proteins, only recognised primate involucrins and therefore bound outside the early region of the modern segment. Since the antibodies can be used to detect involucrin both biochemically and histologically, in a range of species, they will have applications in further studies of the expression, function and evolution of the protein.

Balbiani ring 1 gene in Chironomus tentans. Sequence organization and dynamics of a coding minisatellite

Balbiani ring (BR) genes in diptera encode large secretory proteins and are classical model systems for studies of gene expression. In Chironomus tentans, four closely related BR genes, BR 1, BR 2.1, BR 2.2 and BR 6 form a gene family. The BR genes have been partially characterized and are known to contain long arrays of tandemly arranged repeat units with an hierarchical repeat organization. Here, we report the sequence organization of the complete transcribed part of the BR 1 gene in C. tentans. The gene contains five exons and four introns. Three of the introns are located at the 5' end and the fourth at the 3' end of the gene. Exon 4 is approximately 35,000 bases long and is built completely from tandemly organized repeats. We show that this long repeat block contains two types of related repeat units, beta and gamma. Each type forms a large uninterrupted array, a 5' beta array and a 3' gamma array with a sharp border between them. In the hierarchical repeat structure in each repeat array, all repeats are virtually identical at one level of repetition, but shown differences at the next level. The whole repeat block in the BR 1 gene fluctuates in size between different alleles, but not by more than 10%. In contrast, within the block, the beta and gamma arrays vary in length between 8000 and 29,000 bases in an inverse fashion, together keeping the overall length requirement. We propose that the length of exon 4 is conserved by selection of cross-over products of a given length, and that the internal hierarchical sequence organization in the BR 1 gene is a consequence of the combined action of several different sequence turnover mechanisms, all dependent on the unequal pairing of homologous sequences at different, competing levels of repetition.

Characterization of sciellin, a precursor to the cornified envelope of human keratinocytes

The cornified envelope, located beneath the plasma membrane of terminally differentiated keratinocytes, is formed as protein precursors are cross-linked by a membrane associated transglutaminase. This report characterizes a new precursor to the cornified envelope. A monoclonal antibody derived from mice immunized with cornified envelopes of human cultured keratinocytes stained the periphery of more differentiated cells in epidermis and other stratified squamous epithelia including hair and nails. The epitope was widely conserved among mammals as determined by immunohistochemical and Western analysis. Immunoelectron microscopy localized the epitope to the cell periphery in the upper stratum spinosum and granulosum of epidermis. In the hair follicle, the epitope was present in the internal root sheath and in the infundibulum, the innermost aspect of the external root sheath. The antibody recognized a protein of relative mobility (M(r)) 82,000, pI 7.8. The protein was a transglutaminase substrate as shown by a dansylcadaverine incorporation assay. Purified cornified envelopes absorbed the reactivity of the antibody to the partially purified protein and cleavage of envelopes by cyanogen bromide resulted in release of immunoreactive fragments. The protein was soluble only in denaturing buffers such as 8 M urea or 2% sodium dodecyl-sulfate (SDS). Partial solubility could be achieved in 50 mM TRIS pH 8.3 plus 0.3 M NaCl (high salt buffer); the presence of a reducing agent did not affect solubility. Extraction of cultured keratinocytes in 8 M urea and subsequent dialysis against 50 mM TRIS pH 8.3 buffer resulted in precipitation of the protein with the keratin filaments. Dialysis against high salt buffer prevented precipitation of the protein. The unique solubility properties of this protein suggest that it aggregates with itself and/or with keratin filaments. The possible role of the protein in cornified envelope assembly is discussed. We have named this protein Sciellin (from the old english "sciell" for shell).

Involucrin gene of tarsioids and other primates: alternatives in evolution of the segment of repeats

The involucrin genes of the prosimian primates and of the anthropoid primates possess nonhomologous segments of repeats located at two different sites, P and M, within the coding region. The involucrin gene of the tarsioids alone contains repeats at both sites, for it derived repeats at site P from a common ancestor of tarsioids and prosimians and a repeat at site M from a later common ancestor of tarsioids and anthropoids. After their divergence from the tarsioids, the anthropoids added many more repeats to site M and excised the older segment of repeats from site P; in contrast, the tarsioids stopped adding repeats at site M, retained the earlier segment of repeats at site P, and enlarged it. In the revision of their involucrin genes, the two lineages followed alternative routes. The mechanisms by which the revisions took place have been subject to abrupt onset or termination.

Isolation of two genes encoding putative protein kinases regulated during Dictyostelium discoideum development

Two Dictyostelium discoideum protein kinase(PK)-encoding cDNAs (Dd PK1 and Dd PK2) have been isolated by hybridization with an oligodeoxyribonucleotide derived from a highly conserved region of eukaryotic PKs. The two nucleotide (nt) sequences encode new putative serine/threonine-specific PKs. Dd PK1 is a partial cDNA covering the entire catalytic domain. The derived amino acid (aa) sequence is about 30% identical to both cAMP-dependent protein kinase (cAPK) and protein kinase C. The Dd PK2 sequence was extended through the isolation of a genomic fragment encoding a complete putative protein. A single intron is present, as deduced from sequence comparison with the cDNA. The catalytic domain appears more closely related to the catalytic subunit of cAPK (54% sequence identity). However, our nt sequence potentially codes for a much larger protein (648 vs. about 350 aa for most cAPKs) with a N-terminal half containing long homopolymers of threonines, glutamines and asparagines. Similar repeats occur at the C terminus of Dd PK1, Dd PK1 is expressed in vegetatively growing cells and during development. Dd PK1 RNA decreases after 6 h of starvation to re-accumulate once the cells have aggregated. Dd PK2 transcripts, present at a low amount in growing cells, rise upon starvation. A switch to a shorter form of transcripts occurs between 3 and 6 h into development.

Coexpression of cytokeratins, involucrin, and blood group antigens in oral squamous cell carcinomas

The well and poorly differentiated oral squamous carcinomas preferentially express proteins, blood group antigens, and contain associated dendritic Langerhans' cells. Keratin pearls in well-differentiated carcinomas simulate the differentiation pathway of the normal oral squamous epithelium, whereas poorly differentiated carcinomas do not and appear more heterogeneous. Terminally keratinized cells correlate with involucrin and expression of blood group antigens in keratin pearls, a feature that differs from the nonkeratinizing normal epithelium in which such carcinomas arise. Dendritic Langerhans' cells are reduced in number in squamous carcinomas.

A new member of a secretory protein gene family in the dipteran Chironomus tentans has a variant repeat structure

We describe the structure of a gene expressed in the salivary gland cells of the dipteran Chironomus tentans and show that it encodes 1 of the approximately 15 secretory proteins exported by the gland cells. This sp115,140 gene consists of approximately 65 copies of a 42-bp sequence in a central uninterrupted core block, surrounded by short nonrepetitive regions. The repeats within the gene are highly similar to each other, but divergent repeats are present in a pattern which suggests that the repeat structure has been remodeled during evolution. The 42-bp repeat in the gene is a simple variant of the more complex repeat unit present in the Balbiani ring genes, encoding four of the other secretory proteins. The structure of the sp115,140 gene suggests that related repeat structures have evolved from a common origin and resulted in the set of genes whose secretory proteins interact in the assembly of the secreted protein fibers.

Identification of a major keratinocyte cell envelope protein, loricrin

During epidermal cell cornification, the deposition of a layer of covalently cross-linked protein on the cytoplasmic face of the plasma membrane forms the cell envelope. We have isolated and characterized cDNA clones encoding a major differentiation product of mouse epidermal cells, which has an amino acid composition similar to that of purified cell envelopes. Transcripts of this gene are restricted to the granular layer and are as abundant as the differentiation-specific keratins, K1 and K10. An antiserum against a C-terminal peptide localizes this protein in discrete granules in the stratum granulosum and subsequently at the periphery of stratum corneum cells. Immunofluorescence and immunoelectron microscopy detect this epitope only on the inner surface of purified cell envelopes. Taken together, these results suggest that it is a major component of cell envelopes. On the basis of its presumed function, this protein is named loricrin.

Filaggrin, an intermediate filament-associated protein: structural and functional implications from the sequence of a cDNA from rat

Filaggrin is an intermediate filament-associated protein that is involved in aggregation of keratin filaments in fully cornified cells of the mammalian epidermis, and is an important marker for epidermal differentiation. In this report, the sequence of a rat cDNA clone coding for a portion of the polymeric precursor, profilaggrin, is presented. The cDNA is 2,314 bp long with 1,875 bp of coding region ending with an A-T-rich 3' noncoding region. Genomic analysis indicates that the profilaggrin gene consists of 20 +/- 2 repeats of 1,218 bp of sequence coding for 406 amino acids, making the mRNA at least 25-27 kb in length. Each repeat consists of a filaggrin domain and a linker sequence with an estimated size of 380 and 26 amino acids, respectively. High levels of profilaggrin mRNA are found only in keratinizing epithelia. Comparison of the rat filaggrin sequence with that of mouse and human filaggrin and with the sequence of phosphorylated peptides from mouse profilaggrin indicates that the proteins share extensive amino acid sequence similarities, especially in the two phosphorylated regions. Proteolytic processing sites are also quite similar in rat and mouse. The three species show blocks of sequence that are similar in length and composition which alternate with sequences that are variable in length. This analysis suggests that the evolution of the present-day filaggrins has been constrained by maintenance of phosphorylation sites and overall amino acid composition. The cDNAs for the profilaggrins are similar in structure, reflecting genes that have simple repeating structures and lack introns within their coding regions. Mouse and rat profilaggrin terminate with a nonpolar sequence atypical of the rest of the coding region, and have similar 3' noncoding regions. To explain these observations, a novel evolutionary model is proposed.

Involucrin-like proteins in non-primates

A monoclonal and two polyclonal antibodies to human involucrin were used to look for involucrin epitopes in other species. All antibodies react strongly with the same proteins of monkey, and both polyclonal antibodies react with specific proteins of cow and dog. One of the polyclonal antibodies also reacts with proteins of sheep, guinea pig, rat, and finback whale. The immunoreactive proteins from cow and dog could be purified using a procedure developed for human involucrin. The reaction with the purified dog protein could be blocked by purified human involucrin. The results suggest that involucrin-like proteins have a wider species distribution than originally appreciated.

The Balbiani ring 3 gene in Chironomus tentans has a diverged repetitive structure split by many introns

A set of approximately 15 secretory proteins is synthesized by the salivary gland cells in the midge Chironomus tentans. These proteins are secreted but do not form insoluble fibers until they are transported out of the gland lumen. A Balbiani ring (BR) gene family consisting of four genes (BR1, BR2.1, BR2.2 and BR6) have previously been shown to encode four of these proteins, sp-I a to d, with relative molecular weights of 1 x 10(6). Each BR gene contains an uninterrupted block in which about 100 repeats are tandemly arranged. The repeats are virtually identical and efficient homogenization mechanisms must operate within each block. Here we describe a new BR gene, the BR3 gene, which according to structural similarities may belong to the BR gene family, but at the same time exhibits a strikingly different structure. The gene encodes a 10.9 kb transcript that contains 38 introns and is spliced into a 5.5 kb mRNA. The mRNA is translated into a cysteine-rich 185 kDa major component of the gland secretion. The coding sequence in the gene is built from diverged repeats in which mainly the cysteine codons are preserved and the sequence is split by the introns into 17 to 678-bp long exons. The introns are located at defined positions in relation to the repeat structure. In sharp contrast to the uninterrupted array of identical repeats in the BR1-BR6 genes, the repeats in the BR3 gene are not efficiently homogenized and have diverged extensively from each other. We propose that the splitting of the repeat structure into variable sized exons prevents homogenizations dependent on unequal aligning of homologous sequences.

The pancornulins: a group of basic low molecular weight proteins in mammalian epidermis and epithelium that may function as cornified envelope precursors

1. A monoclonal antibody (HCE-2) to human epidermal and epithelial cornified envelopes identified a group of soluble basic protein precursors. 2. Using HCE-2, envelope-like staining was observed in the epidermis and stratified squamous epithelium of a number of mammalian species. 3. Basic polypeptides reactive to HCE-2 varied in size and number among the different animals. 4. In those species studied, HCE-2-reactive peptides were substrates for transglutaminase and protease treatment of cornified envelopes released HCE-2-reactive degradation products. 5. These results suggest a new family of proteins in mammalian epidermis that may function as cornified envelope precursors.

Expression of murine epidermal differentiation markers is tightly regulated by restricted extracellular calcium concentrations in vitro

Epidermal differentiation is characterized by a series of coordinated morphological and biochemical changes which result in a highly specialized, highly organized, stratified squamous epithelium. Among the specific markers expressed in differentiating epidermis are (a) two early spinous cell proteins, keratins 1 and 10 (K1 and K10); and (b) two later granular cell proteins, filaggrin and a cornified envelope precursor (CE). In vitro, epidermal basal cells are selectively cultured in 0.05 mM Ca2+ medium, and terminal differentiation is induced when the Ca2+ concentration is increased to 1 mM. However, only a small fraction of the cells express the markers K1, K10, CE, or filaggrin in the higher Ca2+ medium. To explore the factors required for marker expression, cultured epidermal cells were exposed to intermediate Ca2+ concentrations and extracts were analyzed using specific antibody and nucleic acid probes for the four markers of interest. These studies revealed that marker expression was enhanced at a restricted concentration of Ca2+ in the medium of 0.10-0.16 mM. At this Ca2+ concentration, both protein and mRNA levels for each marker were substantially increased, whereas at higher or lower Ca2+ concentrations they were diminished or undetected. The percentage of cells expressing each marker was increased two- to threefold in the permissive Ca2+ medium as determined by immunofluorescence analysis. This optimal level of Ca2+ was required both to initiate and sustain marker expression. At the permissive Ca2+ concentration, expression of the markers was sequential and similar to the order of appearance in vivo. K1 was expressed within 8-12 h and K10 was expressed in the ensuing 12-24-h period. CE and filaggrin were expressed in the subsequent 24 h. Inhibition of K1 expression by cycloheximide suggested that an inducible protein was involved. Other investigators have determined that a shallow Ca2+ gradient exists in epidermis, where the basal cells and spinous cells are in a Ca2+ environment substantially below serum Ca2+ levels. These in vitro results suggest that the Ca2+ environment is a fundamental regulator of expression of epidermal differentiation markers and provide an explanation for the existence of the Ca2+ gradient in vivo.

Involucrin in the epidermal cells of subprimates

The protein involucrin is a precursor of the cross-linked envelope that forms during terminal differentiation of the keratinocyte. Most of the human involucrin molecule consists of a segment of homologous repeats of a sequence of 10 amino acids. A similar segment is present in the involucrin of other higher primates, but not in lower animals. We show here that the older part of the involucrin molecule (the ancestral segment) is present in the epidermal cells of subprimates. This has been demonstrated with antisera prepared against different peptides of the ancestral segment of the human protein. No single antiserum detects involucrin of all subprimate species, but probably all involucrins can be detected using antiserum against some sequence in the ancestral segment. Although the involucrin gene has been extensively remodeled in higher primates, its origins extend lower in the animal kingdom.