Differential expression and cell envelope incorporation of small proline-rich protein 1 in different cornified epithelia

Strengthening the Skin Barrier by Using a Late Cornified Envelope 6A-Derived Biomimetic Peptide

Changes in the expression of cornified envelope (CE) components are a hallmark of numerous pathological skin conditions and aging, underlying the importance of this stratum corneum structure in the homeostasis of the epidermal barrier. We performed a detailed characterisation of LCE6A, a member of the Late Cornified Envelope protein family. Immunohistochemical and immunoblot experiments confirmed that LCE6A is expressed late during epidermal differentiation. Crosslinking assays of recombinant LCE6A performed either in situ on human skin sections or in vitro demonstrated that LCE6A is indeed a substrate of transglutaminases and crosslinked to CEs. LCE6A-derived peptides containing a glutamine-lysine sequence retained these properties of the full-length protein and reinforced the mechanical resistance of CE submitted to sonication. We designed P26, a LCE6A-derived biomimetic peptide that similarly reinforced CE in vitro, and evaluated its protective properties ex vivo, on human skin explants, and in two double blind and vehicle-controlled clinical trials. P26 was able to protect the skin from barrier disruption, to limit the damage resulting from a defective barrier, and could improve the signs of aging such as loss of skin firmness and increased skin roughness. Hence, our detailed characterisation of LCE6A as a component of the CE enabled us to develop a LCE6A-derived peptide, biologically active with a new and original mode of action that could be of great interest as a cosmetic ingredient and a pharmacologic agent.

Characterization of TG2 and TG1-TG2 double knock-out mouse epidermis

Transglutaminases (TGs) are a family of enzymes that catalyse the formation of isopeptide bonds between the γ-carboxamide groups of glutamine residues and the ε-amino groups of lysine residues leading to cross-linking reactions among proteins. Four members, TG1, TG2, TG3, and TG5, of the nine mammalian enzymes are expressed in the skin. TG1, TG3 and TG5 crosslinking properties are fundamental for cornified envelope assembly. In contrast, the role of TG2 in keratinization has never been studied at biochemical level in vivo. In this study, taking advantage of the TG2 knock-out (KO) and TG1 heterozygous mice, we generated and characterized the epidermis of TG1-TG2 double knock-out (DKO) mice. We performed morphological analysis of the epidermis and evaluation of the expression of differentiation markers. In addition, we performed analysis of the amino acid composition from isolated corneocytes. We found a significant change in amino acid composition in TG1KO cornified cell envelopes (CEs) while TG2KO amino acid composition was similar to wild-type CEs. Our results confirm a key role of TG1 in skin differentiation and CE assembly and demonstrate that TG2 is not essential for CE assembly and skin formation.

A keratin scaffold regulates epidermal barrier formation, mitochondrial lipid composition, and activity

Epidermal keratin filaments are important components and organizers of the cornified envelope and regulate mitochondrial metabolism by modulating their membrane composition.

Keratin intermediate filaments (KIFs) protect the epidermis against mechanical force, support strong adhesion, help barrier formation, and regulate growth. The mechanisms by which type I and II keratins contribute to these functions remain incompletely understood. Here, we report that mice lacking all type I or type II keratins display severe barrier defects and fragile skin, leading to perinatal mortality with full penetrance. Comparative proteomics of cornified envelopes (CEs) from prenatal KtyI^−/− and KtyII^−/−_K8 mice demonstrates that absence of KIF causes dysregulation of many CE constituents, including downregulation of desmoglein 1. Despite persistence of loricrin expression and upregulation of many Nrf2 targets, including CE components Sprr2d and Sprr2h, extensive barrier defects persist, identifying keratins as essential CE scaffolds. Furthermore, we show that KIFs control mitochondrial lipid composition and activity in a cell-intrinsic manner. Therefore, our study explains the complexity of keratinopathies accompanied by barrier disorders by linking keratin scaffolds to mitochondria, adhesion, and CE formation.

Caspase-14-deficient mice are more prone to the development of parakeratosis

Caspase-14 is an important protease in the proper formation of a fully functional skin barrier. Newborn mice that are deficient in caspase-14 exhibit increased transepidermal water loss and are highly sensitive to UVB-induced photodamage. Decreased caspase-14 expression and incomplete caspase-14 processing in lesional psoriatic parakeratotic stratum corneum has been reported previously. In this study, we show that caspase-14-deficient skin frequently displays incompletely cornified cells in the transitional zone between the granular and the cornified layers, pointing to a delay in cornification. We also demonstrate that after challenge of epidermal permeability barrier function by repetitive acetone treatment, a higher incidence of large parakeratotic plaques was observed in caspase-14-deficient skin. Furthermore, caspase-14-deficient mice are more prone than control mice to the development of parakeratosis upon induction of psoriasis-like dermatitis by imiquimod treatment. These results show that lack of caspase-14 expression predisposes to the development of parakeratosis and that caspase-14 has an important role in keratinocyte terminal differentiation and the maintenance of normal stratum corneum, especially in conditions causing epidermal hyperproliferation.

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.

Identification of regulatory elements by gene family footprinting and in vivo analysis

Gene families of recently duplicated but subsequently diverged genes provide an unique opportunity for comparative analysis of regulatory elements. We have studied the human SPRR gene family of small proline rich proteins involved in barrier function of stratified squamous epithelia. These genes are all expressed in normal human keratinocytes, but respond differently to environmental insults. Comparisons of the functional promoter regions allows the rapid identification of both conserved and of novel regulatory elements that appeared after gene duplication. Competitive electrophoretic mobility shift assays can be used to confirm their presence. Here we show the power of gene family footprinting by the identification of two novel elements in the SPRR3 promoter, not present in SPRR1A and SPRR2A. One of these elements binds a protein similar to GAAP-1, a pro-apoptotic activator of IRF-1 and p53. In vivo analysis shows that this element functions as an inhibitor of SPRR3 transcription. The second novel element functions as an activator of promoter activity and is characterized by its A/T rich sequence. The latter interacting protein indeed binds through contacts in the minor groove, and strikingly, depends on the presence of calcium for DNA interaction.

Suprabasal Dsg2 expression in transgenic mouse skin confers a hyperproliferative and apoptosis-resistant phenotype to keratinocytes

Desmoglein 2 (Dsg2), a component of the desmosomal cell-cell adhesion structure, has been linked to invasion and metastasis in squamous cell carcinomas. However, it is unknown whether – and if so how – Dsg2 contributes to the malignant phenotype of keratinocytes. In this study, we addressed the consequences of Dsg2 overexpression under control of the involucrin promoter (Inv-Dsg2) in the epidermis of transgenic mice. These mice exhibited epidermal hyperkeratosis with slightly disrupted early and late differentiation markers, but intact epidermal barrier function. However, Inv-Dsg2 transgene expression was associated with extensive epidermal hyperplasia and increased keratinocyte proliferation in basal and suprabasal epidermal strata. Cultured Inv-Dsg2 keratinocytes showed enhanced cell survival in the anchorage-independent state that was critically dependent on EGF receptor activation and NF-κB activity. Consistent with the hyperproliferative and apoptosis-resistant phenotype of Inv-Dsg2 transgenic keratinocytes, we observed enhanced activation of multiple growth and survival pathways, including PI 3-kinase/AKT, MEK-MAPK, STAT3 and NF-κB, in the transgenic skin in situ. Finally, Inv-Dsg2 transgenic mice developed intraepidermal skin lesions resembling precancerous papillomas and were more susceptible to chemically induced carcinogenesis. In summary, overexpression of Dsg2 in epidermal keratinocytes deregulates multiple signaling pathways associated with increased growth rate, anchorage-independent cell survival, and the development of skin tumors in vivo.

The cornified envelope: a model of cell death in the skin

The epidermis functions as a barrier against the environment by means of several layers of terminally differentiated, dead keratinocytes - the cornified layer, which forms the endpoint of epidermal differentiation and death. The cornified envelope replaces the plasma membrane of differentiating keratinocytes and consists of keratins that are enclosed within an insoluble amalgam of proteins, which are crosslinked by transglutaminases and surrounded by a lipid envelope. New insights into the molecular mechanisms and the physiological endpoints of cornification are increasing our understanding of the pathological defects of this unique form of programmed cell death, which is associated with barrier malfunctions and ichthyosis.

Enrichment of genes in the aortic intima that are associated with stratified epithelium: implications of underlying biomechanical and barrier properties of the arterial intima

BACKGROUND

Arteries and veins are exposed to different pressures and are easily distinguished by morphology. Although several recent studies have focused on differential gene expression between the arterial and venous endothelium, the molecular distinctions that give rise to the dramatic structural distinctions between arteries and veins, such as in the organization of the intima, are not known.

METHODS AND RESULTS

We used high-density oligonucleotide arrays to analyze the transcriptional profile of the mouse aorta and inferior vena cava (IVC), not restricting our analysis to the endothelium, to identify genes whose expression was enriched in aorta over other tissues and the IVC. By quantitative reverse transcription-polymerase chain reaction analysis, these genes have been shown to be highly expressed in the mouse aorta and were either expressed at low levels or were undetectable in the murine IVC. By immunofluorescence analysis of human tissue, we determined that a subset of these aorta-enriched proteins exhibited a primarily intima-restricted expression. Intimal expression of at least a subset of these genes, plakoglobin, galectin 7, sciellin, and SPRR3, was also detected in other types of arteries but not in veins. Furthermore, SPRR3 expression in the intima was primarily associated with atheromas. The proteins identified are functionally related in that they are known to also be enriched in stratified epithelia, where they play an important role in stress-bearing and barrier properties.

CONCLUSIONS

Vascular expression of these genes has not been reported previously. Our observations suggest that they may play a significant role in the mechanisms by which large arteries may adapt to biomechanical stress.

Assessment of splice variant-specific functions of desmocollin 1 in the skin

Desmocollin 1 (Dsc1) is part of a desmosomal cell adhesion receptor formed in terminally differentiating keratinocytes of stratified epithelia. The dsc1 gene encodes two proteins (Dsc1a and Dsc1b) that differ only with respect to their COOH-terminal cytoplasmic amino acid sequences. On the basis of in vitro experiments, it is thought that the Dsc1a variant is essential for assembly of the desmosomal plaque, a structure that connects desmosomes to the intermediate filament cytoskeleton of epithelial cells. We have generated mice that synthesize a truncated Dsc1 receptor that lacks both the Dsc1a- and Dsc1b-specific COOH-terminal domains. This mutant transmembrane receptor, which does not bind the common desmosomal plaque proteins plakoglobin and plakophilin 1, is integrated into functional desmosomes. Interestingly, our mutant mice did not show the epidermal fragility previously observed in dsc1-null mice. This suggests that neither the Dsc1a- nor the Dsc1b-specific COOH-terminal cytoplasmic domain is required for establishing and maintaining desmosomal adhesion. However, a comparison of our mutants with dsc1-null mice suggests that the Dsc1 extracellular domain is necessary to maintain structural integrity of the skin.

Molecular cloning and expression of keratinocyte proline-rich protein, a novel squamous epithelial marker isolated during skin development

We describe a novel rat cDNA named keratinocyte proline-rich protein (KPRP) isolated by RNA differential display during skin development. We determine that KPRP is expressed in stratified squamous epithelium, and its approximately 2.8-kb cDNA encodes a 699-amino acid protein with high proline content (19%). KPRP is an insoluble protein, similar to most epidermal terminal differentiation-associated proteins. Immunoblot of the protein lysate from keratinocytes, using strong reducing conditions, demonstrates two KPRP bands of approximately 76 and 55 kDa size. KPRP is expressed in stratified squamous epithelia of skin, tongue, and esophagus. The initiation of KPRP expression in fetal rat skin at E17, E18, E19, E20, and E21 was analyzed by reverse transcription-PCR. Fetal skin at E19 and later expresses KPRP. In situ hybridization of skin from E18, E19, and 4-day-old neonatal rats demonstrates that interfollicular and follicular keratinocytes express KPRP. Anti-KPRP antibody demonstrates KPRP protein localizes to all layers of stratified epithelia in skin, tongue, and esophagus. In cultured dermal keratinocytes, KPRP is diffusely distributed throughout the cytoplasm with denser staining adjacent to the nuclear and plasma membranes. Additionally, immunoreactive intracellular granules are observed during keratinocyte detachment from their plastic substrate. Rat KPRP has 89% homology to a mouse genomic DNA sequence and 56% homology to a human hypothetical protein. We conclude that KPRP may be a new epidermal terminal differentiation-related protein expressed in stratified squamous epithelia. KPRP is expressed by fetal dermal keratinocytes during late gestation and is a new marker of maturing epidermis during fetal skin development.

Human papillomavirus type 11 alters the transcription and expression of loricrin, the major cell envelope protein

Human papillomavirus (HPV) does not induce lysis of infected keratinocytes, and the exact mechanisms of viral escape are not known. As keratinocytes differentiate, the cornified cell envelope (CCE) develops, providing a protective barrier to the host. We previously showed that the normally durable CCE in HPV 11-infected epithelium is fragile compared to CCEs in healthy epithelium. In this study, we examined uninfected and HPV 11-infected human genital epithelium for expression of loricrin, the major CCE protein in healthy epidermis. In HPV 11-infected human genital epithelium, detection of loricrin was reduced in immunoelectron microscopic and immunoblot assays, suggesting that loricrin incorporation into the CCE was reduced or that loricrin synthesis was reduced. Loricrin detection was reduced in immunohistochemical assays in areas of high viral replication. Mathematical modeling by least squares was performed using the amino acid composition of highly purified CCE fragments, confirming that loricrin was markedly reduced and that the small proline-rich proteins and cytokeratins were increased in those derived from HPV 11-infected epithelium compared to healthy genital epithelium. In RNase protection and RT-PCR assays, loricrin transcripts were markedly reduced in HPV 11-infected epithelium compared to uninfected epithelium. Loricrin transcripts were detectable by RNA in situ analysis in isolated cells of HPV 11-infected epithelium, but were absent in large regions of epithelium. We conclude that HPV 11 infection reduces transcription of the loricrin gene and that this leads to a reduction in loricrin incorporation into the CCE. Further studies will examine the effects of specific HPV gene products on transcription of loricrin and other CCE components, as it is likely that viral egress from the infected keratinocyte depends on these effects.

Quasi-normal cornified cell envelopes in loricrin knockout mice imply the existence of a loricrin backup system

The cornified cell envelope, a lipoprotein layer that assembles at the surface of terminally differentiated keratinocytes, is a resilient structure on account of covalent crosslinking of its constituent proteins, principally loricrin, which accounts for up to 60%-80% of total protein. Despite the importance of the cell envelope as a protective barrier, knocking out the loricrin gene in mice results in only mild syndromes. We have investigated the epidermis and forestomach epithelium of these mice by electron microscopy. In both tissues, corneocytes have normal-looking cell envelopes, despite the absence of loricrin, which was confirmed by immunolabeling, and the absence of the distinctive loricrin-containing keratohyalin granules (L-granules). Isolated cell envelopes were normal in thickness (approximately 15 nm) and mass per unit area (approximately 7.3 kDa per nm2); however, metal shadowing revealed an altered substructure on their cytoplasmic surface. Their amino acid compositions indicate altered protein compositions. Analysis of these data implies that the epidermal cell envelopes have elevated levels of the small proline-rich proteins, and cell envelopes of both kinds contain other protein(s) that, like loricrin, are rich in glycine and serine. These observations imply that, in the absence of loricrin, the mechanisms that govern cell envelope assembly function normally but employ different building-blocks.

Differentially expressed late constituents of the epidermal cornified envelope

Barrier activity of skin and internal barrier-forming epithelial linings are conferred by a lipid-corneocyte structure (stratum corneum in skin). The integrity of the corneocytes depends on the outer cornified envelope and is essential for maintenance of barrier function. During epidermal development and differentiation, proteins are sequentially incorporated into the envelope via action of epidermal transglutaminases in a well documented process. However, recent knockouts of major cornified envelope constituents have failed to disrupt barrier function significantly, suggesting that additional unidentified components are involved. We report a new gene cluster in the epidermal differentiation complex at human 1q21 encoding a family of 18 proteins that are substrates for epidermal transglutaminases. These proteins incorporate into the cornified envelope late in development and late in the process of envelope maturation during epidermal differentiation. The genes cluster within the epidermal differentiation complex according to expression pattern, i.e., epidermally expressed proteins cluster together while proteins from internal barrier-forming epithelia also cluster. We propose that these proteins modulate barrier activity over the surface of the animal, in a manner analogous to that proposed for the well characterized cornified envelope precursors, the small proline-rich proteins. To emphasize the incorporation of these proteins late in envelope assembly, we call the human proteins late envelope proteins.

Structural organization and regulation of the small proline-rich family of cornified envelope precursors suggest a role in adaptive barrier function

The protective barrier provided by stratified squamous epithelia relies on the cornified cell envelope (CE), a structure synthesized at late stages of keratinocyte differentiation. It is composed of structural proteins, including involucrin, loricrin, and the small proline-rich (SPRR) proteins, all encoded by genes localized at human chromosome 1q21. The genetic characterization of the SPRR locus reveals that the various members of this multigene family can be classified into two distinct groups with separate evolutionary histories. Whereas group 1 genes have diverged in protein structure and are composed of three different classes (SPRR1 (2x), SPRR3, and SPRR4), an active process of gene conversion has counteracted diversification of the protein sequences of group 2 genes (SPRR2 class, seven genes). Contrasting with this homogenization process, all individual members of the SPRR gene family show specific in vivo and in vitro expression patterns and react selectively to UV irradiation. Apparently, creation of regulatory rather than structural diversity has been the driving force behind the evolution of the SPRR gene family. Differential regulation of highly homologous genes underlines the importance of SPRR protein dosage in providing optimal barrier function to different epithelia, while allowing adaptation to diverse external insults.

Analysis of epidermal-type transglutaminase (TGase 3) expression in mouse tissues and cell lines

In the formation of the cornified cell envelope in the epidermis, epidermal-type transglutaminase (TGase 3) cross-links a variety of structural proteins. However, its expression in other tissue has not been investigated. Furthermore, no cell line expressing TGase 3 has been found. The tissue distribution of TGase 3 in mice was investigated using reverse-transcription polymerase chain reaction (RT-PCR) and Western blotting analyses. TGase 3 mRNA was expressed in the brain, stomach, spleen, small intestine, testis, skeletal muscle and skin. The stomach and testis expressed TGase 3 protein in size similar to that observed in the epidermis. Screening various cell lines, a gastric human cancer cell line, MKN-1 and mouse neuroblast cell line, neuro2a, were found to express TGase 3.

Transgenic mice expressing a mutant form of loricrin reveal the molecular basis of the skin diseases, Vohwinkel syndrome and progressive symmetric erythrokeratoderma

Mutations in the cornified cell envelope protein loricrin have been reported recently in some patients with Vohwinkel syndrome (VS) and progressive symmetric erythrokeratoderma (PSEK). To establish a causative relationship between loricrin mutations and these diseases, we have generated transgenic mice expressing a COOH-terminal truncated form of loricrin that is similar to the protein expressed in VS and PSEK patients. At birth, transgenic mice (ML.VS) exhibited erythrokeratoderma with an epidermal barrier dysfunction. 4 d after birth, high-expressing transgenic animals showed a generalized scaling of the skin, as well as a constricting band encircling the tail and, by day 7, a thickening of the footpads. Histologically, ML. VS transgenic mice also showed retention of nuclei in the stratum corneum, a characteristic feature of VS and PSEK. Immunofluorescence and immunoelectron microscopy showed the mutant loricrin protein in the nucleus and cytoplasm of epidermal keratinocytes, but did not detect the protein in the cornified cell envelope. Transfection experiments indicated that the COOH-terminal domain of the mutant loricrin contains a nuclear localization signal. To determine whether the ML.VS phenotype resulted from dominant-negative interference of the transgene with endogenous loricrin, we mated the ML.VS transgenics with loricrin knockout mice. A severe phenotype was observed in mice that lacked expression of wild-type loricrin. Since loricrin knockout mice are largely asymptomatic (Koch, P.K., P. A. de Viragh, E. Scharer, D. Bundman, M.A. Longley, J. Bickenbach, Y. Kawachi, Y. Suga, Z. Zhou, M. Huber, et al., J. Cell Biol. 151:389-400, this issue), this phenotype may be attributed to expression of the mutant form of loricrin. Thus, deposition of the mutant protein in the nucleus appears to interfere with late stages of epidermal differentiation, resulting in a VS-like phenotype.

Lessons from loricrin-deficient mice: compensatory mechanisms maintaining skin barrier function in the absence of a major cornified envelope protein

The epidermal cornified cell envelope (CE) is a complex protein-lipid composite that replaces the plasma membrane of terminally differentiated keratinocytes. This lamellar structure is essential for the barrier function of the skin and has the ability to prevent the loss of water and ions and to protect from environmental hazards. The major protein of the epidermal CE is loricrin, contributing approximately 70% by mass. We have generated mice that are deficient for this protein. These mice showed a delay in the formation of the skin barrier in embryonic development. At birth, homozygous mutant mice weighed less than control littermates and showed skin abnormalities, such as congenital erythroderma with a shiny, translucent skin. Tape stripping experiments suggested that the stratum corneum stability was reduced in newborn Lor(-/-) mice compared with wild-type controls. Isolated mutant CEs were more easily fragmented by sonication in vitro, indicating a greater susceptibility to mechanical stress. Nevertheless, we did not detect impaired epidermal barrier function in these mice. Surprisingly, the skin phenotype disappeared 4-5 d after birth. At least one of the compensatory mechanisms preventing a more severe skin phenotype in newborn Lor(-/-) mice is an increase in the expression of other CE components, such as SPRRP2D and SPRRP2H, members of the family of "small proline rich proteins", and repetin, a member of the "fused gene" subgroup of the S100 gene family.

Ordered structure acquisition by the N- and C-terminal domains of the small proline-rich 3 protein

The cell envelope (CE) is a vital structure for barrier function in terminally differentiated dead stratified squamous epithelia. It is assembled by transglutaminase (TGase) cross-linking of several proteins, including hSPR3 in certain specialized epithelia normally subjected to mechanical trauma. Biochemical studies show that hSPR3 serves as a complete substrate for TGase1, TGase2, and TGase3. Multiple adjacent glutamines and lysines of only head-and-tail domain sequences are used by each enzyme for cross-linking. Structural data suggest that the hSPR3 central repeats, as well as hSPR1 and hSPR 2, are highly flexible and mobile; thus, the TGases might not be able to recognize the residues localized on the repeats as adequate substrate. To investigate this hypothesis further and to complete the structural investigation of hSPR3, we performed circular dichroism (CD) studies on peptides corresponding to the N- and C-terminal domain. CD spectra have also been carried out in the presence of different concentrations of the structure-promoting agent cosolvent trifluoroethanol (TFE), which mimics a partial hydrophobic environment found in vivo in or next to the membrane. In fact, this agent increases the dielectric constant of water proportionally, depending on its concentration, and confers structuring properties to the solution, to peptides and proteins that have a structuring propensity. The results indicate that in both the N-terminal and C-terminal, peptides acquire a more ordered structure as a function of the TFE concentration in water. This ability of both N- and C-terminal domain to acquire a more stable ordered conformation might be relevant for SPR3 to act as substrate of TGases. Indeed, only the N- and C-terminus is cross-linked by TGase1 and 3.

Distinct roles for amino- and carboxyl-terminal sequences of SPRR1 protein in the formation of cross-linked envelopes of conducting airway epithelial cells

The small proline-rich protein, SPRR1, is a marker gene whose expression in conducting airway epithelium is elevated under a variety of conditions that enhance squamous differentiation. The purpose of this study is to elucidate the nature of the SPRR1 sequence involved in cross-linked envelope formation in a tissue/cell type, such as conducting airway epithelium, that normally does not express squamous function except after injury or maintenance in culture. For this, a Flag-SPRR1 fusion protein expression system has been developed. Using the liposome-mediated gene transfer technique on passage 1 culture of human tracheobronchial epithelial (TBE) cells, the Flag-SPRR1 fusion protein can be expressed and detected immunologically by both anti-Flag and anti-SPRR1 antibodies. The incorporation of Flag-SPRR1 fusion protein into cross-linked envelopes can be demonstrated when transfected human passage 1 TBE cultures are treated with phorbol 12-myristate 13-acetate and high calcium (1.5 mM). By deletion and site-directed mutagenesis, two distinct roles of the amino- and carboxyl-terminal sequences of SPRR1 have been demonstrated. First, we demonstrated that the amino-terminal sequence of SPRR1 protein is required for the incorporation of the fusion protein into cross-linked envelopes, whereas a deletion on the carboxyl-terminal region or on the middle repetitive unit has no effect. Interestingly, insertion of a 24-amino acid peptide of monkey MUC2 repetitive sequence in the amino-terminus of SPRR1 protein had a stimulatory effect. Site-directed mutagenesis on the following amino acid residues, Lys(7), Gln(88), and Lys(89), which were found previously to participate in the cross-linked envelope formation of keratinocytes, had no detrimental effect on the incorporation. However, mutations on Gln clusters, such as Gln(4)-Gln(6) and Gln(22)-Gln(25), had detrimental effects on the incorporation. These results suggest an amino-terminal sequence-dependent and multiple cross-linked sites for the incorporation of Flag-SPRR1 fusion protein into cross-linked envelopes of cultured human TBE cells. Second, we demonstrated that the carboxyl terminus of SPRR1 protein is required for a high level of Flag-fusion protein expression. A deletion in the carboxyl region or a mutation on the last lysine residue of the carboxyl end had a detrimental effect on the level of Flag-SPRR1 fusion protein expressed in transfected cells. In contrast, there was only a slight decrease in the level of expression if the amino-terminus was deleted. Interestingly, the efficiency for fusion protein to incorporate into cross-linked envelopes was elevated by the mutation at the carboxyl end. These results suggest distinct roles, perhaps coordinately, for both amino- and carboxyl-terminal sequences in the regulation of the life cycle of SPRR1 protein in cultured TBE cells.

Initiation of assembly of the cell envelope barrier structure of stratified squamous epithelia

The cell envelope (CE) is a specialized structure that is important for barrier function in terminally differentiated stratified squamous epithelia. The CE is formed inside the plasma membrane and becomes insoluble as a result of cross-linking of constituent proteins by isopeptide bonds formed by transglutaminases. To investigate the earliest stages of assembly of the CE, we have studied human epidermal keratinocytes induced to terminally differentiate in submerged liquid culture as a model system for epithelia in general. CEs were harvested from 2-, 3-, 5-, or 7-d cultured cells and examined by 1) immunogold electron microscopy using antibodies to known CE or other junctional proteins and 2) amino acid sequencing of cross-linked peptides derived by proteolysis of CEs. Our data document that CE assembly is initiated along the plasma membrane between desmosomes by head-to-tail and head-to-head cross-linking of involucrin to itself and to envoplakin and perhaps periplakin. Essentially only one lysine and two glutamine residues of involucrin and two glutamines of envoplakin were used initially. In CEs of 3-d cultured cells, involucrin, envoplakin, and small proline-rich proteins were physically located at desmosomes and had become cross-linked to desmoplakin, and in 5-d CEs, these three proteins had formed a continuous layer extending uniformly along the cell periphery. By this time >15 residues of involucrin were used for cross-linking. The CEs of 7-d cells contain significant amounts of the protein loricrin, typically expressed at a later stage of CE assembly. Together, these data stress the importance of juxtaposition of membranes, transglutaminases, and involucrin and envoplakin in the initiation of CE assembly of stratified squamous epithelia.

Expression, regulation, and function of the SPR family of proteins. A review

The small, proline-rich (SPR) genes consist of three subclasses closely linked on human chromosome 1, a region referred to as the epidermal differentiation complex. SPR genes consist of two exons, with the second exon containing the entire open reading frame. SPRs are expressed in all squamous tissues of the skin, scalp, footpad, vaginal epithelia, and most of the epithelial lining of the digestive tract, including the lip, tongue, esophagus, and forestomach. Although SPR1 is absent in normal mucociliary epithelium of the respiratory tract, epithelia that undergo squamous differentiation in response to vitamin-A deficiency or to injury owing to exposure to environmental toxicants express SPR1. High levels of SPR1 are detected in various diseases and cancers of the skin or respiratory epithelia and in nonkeratinizing papillary adenocarcinomas. SPR expression can be regulated by transcriptional factors, by posttranscriptional factors, or by factors that affect SPR1 mRNA translation or protein turnover. Furthermore, regulation can be affected by the state of cell proliferation. The presence of SPR1 in most of these epithelia, and the absence of SPR3 in normal skin, suggest that these subclasses have distinct functions. Various approaches to the study of the cross-linked envelope (CE) components in identifying SPR1 and SPR2 and in suggesting that SPRs are one of the precursor proteins of the CE. However, expression of SPR1 in nonsquamous tissues and cell lines indicates a function not associated with squamous differentiation. Several studies have demonstrated that SPR1 antibodies react with nuclear proteins and that SPR1 is expressed in cells before entering the G0 phase of the cell cycle. Future studies should clarify the role of SPRs by modifying their contents in CE, and should identify SPR-associated proteins to clarify the cell growth-related role of SPR1.

Transglutaminase cross-linking properties of the small proline-rich 1 family of cornified cell envelope proteins. Integration with loricrin

Small proline-rich 1 (SPR1) proteins are important for barrier function in stratified squamous epithelia. To explore their properties, we expressed in bacteria a recombinant human SPR1 protein and isolated native SPR1 proteins from cultured mouse keratinocytes. By circular dichroism, they possess no alpha or beta structure but have some organized structure associated with their central peptide repeat domain. The transglutaminase (TGase) 1 and 3 enzymes use the SPR1 proteins as complete substrates in vitro but in different ways: head domain A sequences at the amino terminus were used preferentially for cross-linking by TGase 3, whereas those in head domain B sequences were used for cross-linking by TGase 1. The TGase 2 enzyme cross-linked SPR1 proteins poorly. Together with our data base of 141 examples of in vivo cross-links between SPRs and loricrin, this means that both TGase 1 and 3 are required for cross-linking SPR1 proteins in epithelia in vivo. Double in vitro cross-linking experiments suggest that oligomerization of SPR1 into large polymers can occur only by further TGase 1 cross-linking of an initial TGase 3 reaction. Accordingly, we propose that TGase 3 first cross-links loricrin and SPRs together to form small interchain oligomers, which are then permanently affixed to the developing CE by further cross-linking by the TGase 1 enzyme. This is consistent with the known consequences of diminished barrier function in TGase 1 deficiency models.

Structure and evolution of the human SPRR3 gene: implications for function and regulation

SPRR3, a member of the SPRR family of cornified envelope precursor proteins, is expressed in oral and esophageal epithelia, where it is strictly linked to keratinocyte terminal differentiation. This gene is characterized by intragenic duplications that have created the characteristic proline-rich repeats in the coding sequence, an alternative noncoding exon, and a 200-bp polypyrimidine tract in the promoter region. Mutational analysis of the promoter region and transient transfection in normal human keratinocytes showed that in addition to the polypyrimidine tract, multiple regulatory elements are involved in differentiation-specific expression. These elements include a high-affinity Ets binding site bound by ESE-1, an AP-1 site (TRE) recognized by the Jun/Fos family of transcription factors, and an ATF/CRE bound by Jun/Fos and ATF factors. The repositioning of the SPRR3 Ets binding site during evolution has a major effect on the relative contribution of this site to promoter activity.

Mouse Sprr2 genes: a clustered family of genes showing differential expression in epithelial tissues

Small proline-rich (SPR) proteins are structural components of the cornified cell envelope of stratified squamous epithelia. They are subdivided into three families, i.e., SPR1, SPR2, and SPR3, of which the SPR2 family is the most complex. To understand the significance of this complexity, we have isolated 11 mouse Sprr2 genes, constructed a provisional physical map of the Sprr2 locus on mouse Chromosome 3, and examined the expression patterns of the Sprr2 genes in mouse epithelial tissues. The 11 Sprr2 sequences are highly conserved with a central domain containing a variable number of repeats. In situ hybridization showed the Sprr2 expression to be confined to epithelia. RT-PCR using primers specific for each of the 11 Sprr2 members demonstrated varying degrees of expression among the individual Sprr2 members in different tissues. The correlation between the physical location of the genes in the Sprr2 locus and their expression patterns suggests multiple levels of controlled expression.

Structural and transglutaminase substrate properties of the small proline-rich 2 family of cornified cell envelope proteins

The small proline-rich (SPR) proteins are components of the cornified cell envelope of stratified squamous epithelia and become cross-linked to other proteins by transglutaminases (TGases). The SPR2 family is the most complex, as it consists of several differentially expressed members of the same size. To explore their physical and cross-linking properties, we have expressed in bacteria a human SPR2 family member, and purified it to homogeneity. By circular dichroism, it possesses no alpha or beta structure but has some organized structure associated with the central peptide repeat domain. The TGase 1, 2, and 3 enzymes expressed in epithelia use the recombinant SPR2 protein as a complete substrate in vitro, but with widely differing kinetic efficiencies, and in different ways. With TGase 1, only one glutamine on the head domain and one lysine on the tail domain were used for limited interchain cross-linking. With TGase 3, multiple head and tail domain residues were used for extensive interchain cross-linking. The total usage of glutamine and lysine residues in vitro by TGase 3 was similar to that seen in earlier in vivo studies. We conclude that SPR2 proteins are cross-linked in epithelia primarily by the TGase 3 enzyme, a minor extent by TGase 1, and probably not by TGase 2.

Small proline-rich proteins are cross-bridging proteins in the cornified cell envelopes of stratified squamous epithelia

The cornified cell envelope (CE) is a specialized structure which contributes barrier function to stratified squamous epithelial cells. It is composed of an amalgam of several structural proteins that are rendered insoluble by isopeptide bond crosslinking by transglutaminases. One set of the structural proteins present in CEs of most such epithelia are the small proline rich (SPR) proteins, which are a family of about 12 related structural proteins. We have recovered a large number of peptides containing isopeptide crosslinks, including 236 involving SPR proteins, following proteolysis of CEs isolated from foreskin epidermal tissue and cultured epidermal keratinocytes. Analysis of this database has provided novel information on their function. First, we found that SPRs became crosslinked to many other structural proteins within the CE. Second, multiple glutamine and lysine residues located only on the amino- and carboxy-termini of the SPR proteins were involved in crosslinking, so that the two ends are functionally equivalent. Third, the SPRs functioned as cross-bridging proteins, by directly adjoining other CE structural proteins. In the specialized case of the epidermal CE, the SPRs cross-bridged between loricrin. In cultured keratinocytes which make little loricrin and serve as a model for internal stratified squamous epithelia, the SPRs formed extensive cross-bridges among themselves. Thus SPRs are ubiquitous cross-bridging proteins whose differential expression patterns apparently reflect specific barrier requirements of different epithelia.

Ceramides are bound to structural proteins of the human foreskin epidermal cornified cell envelope

An important component of barrier function in human epidermis is contributed by ceramides that are bound by ester linkages to undefined proteins of the cornified cell envelope (CE). In this paper, we have examined the protein targets for the ceramide attachment. By partial saponification of isolated foreskin epidermal CEs followed by limited proteolysis, we have recovered several lipopeptides. Biochemical and mass spectroscopic characterization revealed that all contained near stoichiometric amounts of ceramides of masses ranging from about 690 to 890 atomic mass units, of which six quantitatively major species were common. The array of ceramides was similar to that obtained from pig skin, the composition of which is known, thereby providing strong indirect data for their fatty acid and sphingosine compositions. The recovered peptides accounted for about 20% of the total foreskin CE ceramides. By amino acid sequencing, about 35% of the peptides were derived from ancestral glutamine-glutamate-rich regions of involucrin, an important CE structural protein. Another 18% derived from rod domain sequences of periplakin and envoplakin, which are also known or suspected CE proteins. Other peptides were too short for unequivocal identification. Together, these data indicate that involucrin, envoplakin, periplakin, and possibly other structural proteins serve as substrates for the attachment of ceramides by ester linkages to the CE for barrier function in human epidermis.

Biochemical evidence that small proline-rich proteins and trichohyalin function in epithelia by modulation of the biomechanical properties of their cornified cell envelopes

The cornified cell envelope (CE) is a specialized structure involved in barrier function in stratified squamous epithelia, and is assembled by transglutaminase cross-linking of several proteins. Murine forestomach epithelium undergoes particularly rigorous mechanical trauma, and these CEs contain the highest known content of small proline-rich proteins (SPRs). Sequencing analyses of these CEs revealed that SPRs function as cross-bridgers by joining other proteins by use of multiple adjacent glutamines and lysines on only the amino and carboxyl termini and in functionally non-polar ways. Forestomach CEs also use trichohyalin as a novel cross-bridging protein. We performed mathematical modeling of amino acid compositions of the CEs of mouse and human epidermis of different body sites. Although the sum of loricrin + SPRs was conserved, the amount of SPRs varied in relation to the presumed physical requirements of the tissues. Our data suggest that SPRs could serve as modifiers of a composite CE material composed of mostly loricrin; we propose that increasing amounts of cross-bridging SPRs modify the structure of the CE, just as cross-linking proteins strengthen other types of tissues. In this way, different epithelia may use varying amounts of the cross-bridging SPRs to alter the biomechanical properties of the tissue in accordance with specific physical requirements and functions.

Cornified cell envelope assembly: a model based on electron microscopic determinations of thickness and projected density

In stratifying squamous epithelia, the cornified cell envelope (CE), a peripheral layer of crosslinked protein, is assembled sequentially from precursor proteins initially dispersed in the cytoplasm. Its major component is loricrin (37 kDa in mouse), which contributes from approx. 60% to >80% of the protein mass in different tissues. Despite its importance to the mechanical resilience and impenetrability of these tissues, detailed information has not been obtained on CE structure, even on such basic properties as its thickness or uniformity across a given CE or from tissue to tissue. To address this issue, we have studied CEs isolated from three murine epithelia, namely epidermis, forestomach and footpad, by electron microscopy of metal-shadowed specimens and scanning transmission electron microscopy (STEM) of unstained specimens. The former data reveal that the cytoplasmic surface is smoothly textured whereas the extracellular surface is corrugated, and that the average thickness is 15.3+/−1.2 nm, and strikingly uniform. Measurements of mass-per-unit-area from the STEM images yielded values of approx. 7.0+/−0.8 kDa/nm2, which were remarkably consistent over all three tissues. These data imply that the mature CE has a uniquely defined thickness. To explain its uniformity, we postulate that loricrin forms a molecular monolayer, not a variable number of multiple layers. In this scenario, the packing density is one loricrin monomer per 7 nm2, and loricrin should have an elongated shape, 2.5-3.0 nm wide by approx. 11 nm long. Moreover, we anticipate that any inter-tissue variations in the mechanical properties of CEs should depend more on protein composition and cross-linking pattern than on the thickness of the protein layer deposited.

Epidermal differentiation and squamous metaplasia: from stem cell to cell death

Epidermal differentiation is a multi-step process defined by a cascade of interrelated changes in the expression of growth-regulatory and differentiation-specific genes (Fig. 1). Irreversible growth arrest is an early event in epidermal differentiation which occurs when cells transit from the basal to the innermost suprabasal layer of the skin and begin to express squamous-specific genes. In culture, interferon gamma, phorbol esters, confluence and growth in suspension are effective signals to induce irreversible growth arrest and differentiation. The induction of differentiation-specific genes occurs either concomitantly with or following growth arrest and is believed to be linked to the molecular events that control irreversible growth arrest. Such a link has been demonstrated in other cell systems undergoing terminal differentiation, such as myogenesis and adipogenesis. Genes encoding proteins involved in the formation of the cross-linked envelope are one set of squamous-specific genes which are induced in the suprabasal layers and include transglutaminase I and III, involucrin, loricrin and cornifins/small proline-rich proteins. Squamous-specific genes exhibit not only different patterns of tissue-specific expression but are also induced at different stages during differentiation, suggesting that transcription of individual genes is regulated by distinct mechanisms. The latter is supported by the identification of different sets of regulatory elements controlling the transcription of these genes. The importance of understanding both the mechanisms that regulate growth arrest and the differentiation program is emphasized by the association found between specific skin diseases and genetic alterations in growth-regulatory genes as well as differentiation markers. In addition, studies into those mechanisms will provide insight into the control of squamous metaplasia and the development of squamous cell carcinomas.

Spatial distributions of sulfur-rich proteins in cornifying epithelia

We have used energy-filtered electron microscopy and electron energy loss spectroscopy (EELS) to characterize the distributions of sulfur-rich proteins in granular layer cells of squamous stratifying epithelia and their redistribution in the cornified layer, with particular attention to assembly of the cornified cell envelope (CE). Our measurements provide quantitative information that complements highly specific but qualitative data from immunocytochemistry. Spatial distributions of sulfur, phosphorus, and nitrogen were mapped in unstained thin sections of mouse epidermis and forestomach, using a postcolumn energy filter. Nitrogen images were indicative of total protein while phosphorus images provided a control to validate the algorithms used to calculate the elemental maps. Sulfur was found at high levels in round L-granules in the granulocyte cytoplasm and in the cornified CE, correlating with the presence of the protein, loricrin ( approximately 7% Cys/Met residues). EELS confirmed these observations quantitatively: either L-granules consist exclusively of loricrin or any additional components must have an equally high net sulfur content. These data also confirm the large fraction (approximately 75%) of loricrin in the CE, as inferred from modeling of its amino acid composition. We also observed extracellular deposits between cornified squames in fetal mouse epidermis that we call peripheral granules. Their sulfur content is at least as high as that of L-granules but they do not label with anti-loricrin antibodies, suggesting the presence of another sulfur-rich protein.

Direct evidence that involucrin is a major early isopeptide cross-linked component of the keratinocyte cornified cell envelope

Involucrin was the first protein to be identified as a likely constituent of the insoluble cornified cell envelope (CE) of stratified squamous epithelia. However, to date, direct isolation from CEs of involucrin cross-linked by way of the transglutaminase-induced isopeptide bond has not been reported. We have treated human foreskin CEs with methanol/KOH (saponification) to hydrolyze off much of the lipids. By immunogold electron microscopy, this exposed large amounts of involucrin epitopes as well as of desmoplakin, a desmosomal structural protein. About 20% of the total CE protein could be solubilized by proteolytic digestion after saponification, of which involucrin was the most abundant. Subsequent amino acid sequencing revealed many peptides involving involucrin cross-linked either to itself or to a variety of other known CE protein components, including cystatin alpha, desmoplakin, elafin, keratins, members of the small proline-rich superfamily, loricrin, and unknown proteins related to the desmoplakin family. Specific glutamines or lysines of involucrin were used to cross-link the different proteins, such as glutamines 495 and 496 to desmoplakin, glutamine 288 to keratins, and lysines 468, 485, and 508 and glutamines 465 and 489 for interchain involucrin cross-links. Many identical peptides were obtained from immature CEs isolated from the inner living cell layers of foreskin epidermis. The multiple cross-linked partners of involucrin provide experimental confirmation that involucrin is an important early scaffold protein in the CE. Further, these data suggest that there is significant redundancy in the structural organization of the CE.