Peptidoglycan recognition proteins Pglyrp3 and Pglyrp4 are encoded from the epidermal differentiation complex and are candidate genes for the Psors4 locus on chromosome 1q21

Identification of epidermal differentiation genes of the tuatara provides insights into the early evolution of lepidosaurian skin

The tuatara (Sphenodon punctatus) is the phylogenetically closest relative of squamates (including lizards and snakes) from which it diverged around 250 million years ago. Together, they constitute the clade Lepidosauria. Fully terrestrial vertebrates (amniotes) form their skin barrier to the environment under the control of a gene cluster, termed the epidermal differentiation complex (EDC). Here we identified EDC genes in the genome of the tuatara and compared them to those of other amniotes. The organization of the EDC and proteins encoded by EDC genes are most similar in the tuatara and squamates. A subcluster of lepidosaurian EDC genes encodes corneous beta-proteins (CBPs) of which three different types are conserved in the tuatara. Small proline-rich proteins have undergone independent expansions in the tuatara and some, but not all subgroups of squamates. Two genes encoding S100 filaggrin-type proteins (SFTPs) are expressed during embryonic skin development of the tuatara whereas SFTP numbers vary between 1 and 3 in squamates. Our comparative analysis of the EDC in the tuatara genome suggests that many molecular features of the skin that were previously identified in squamates have evolved prior to their divergence from the lineage leading to the tuatara.

Pathogenesis of psoriasis in the "omic" era. Part II. Genetic, genomic and epigenetic changes in psoriasis

Psoriasis is a multifactorial disease in which genetic, environmental and epigenetic factors regulating gene expression play a key role. In the “genomic era”, genome-wide association studies together with target genotyping platforms performed in different ethnic populations have found more than 50 genetic susceptible markers associated with the risk of psoriasis which have been identified so far. Up till now, the strongest association with the risk of the disease has been proved for HLA-C*06 gene. The majority of other psoriasis risk SNPs are situated near the genes encoding molecules involved in adaptive and innate immunity, and skin barrier function. Many contemporary studies indicate that the epigenetic changes: histone modification, promoter methylations, long non-coding and micro-RNA hyperexpression are considered as factors contributing to psoriasis pathogenesis as they regulate abnormal keratinocyte differentiation and proliferation, aberrant keratinocytes – inflammatory cells communication, neoangiogenesis and chronic inflammation. The circulating miRNAs detected in the blood may become specific markers in the diagnosis, prognosis and response to the treatment of the disease. The inhibition of expression in selected miRNAs may be a new promising therapy option for patients with psoriasis.

The gut and the inflammatory bowel diseases inside-out: extra-intestinal manifestations

An increasing deal of attention is being conveyed on the extra-intestinal manifestations (EIM) of inflammatory bowel diseases (IBD). We compiled the present review in an attempt to upgrade the accuracy of the classification of such polymorphic entities. We focused on three patterns. First, the conventional EIM localized to bone and joints, to the eye, to the biliary tree and to the skin. Second, the so-called IBD-like syndromes accompanied by bone marrow-derived anomalies of innate or acquired immunity. Third, specific disorders of the skin and of the lungs. EIM are thought to derive from an altered gut permeability, the release of cross-reacting antigens, and subsequent peripheral inflammation; T helper 17 cells boosted by a polymorphic interleukin 23 circuitry would be the main effectors of this chain. Inflammatory bowel disease-like pictures would derive from inborn errors of the immune response causing undue inflammation home to the gut. Monogenic IBD belong to this subset, and are of specific pediatric interest. Psoriasis, chronic obstructive pulmonary disease, and IBD are all inflammatory disorders of the barrier organs: skin, lungs, and gut. The demonstration that specific antigen hyper- or hyporesponsiveness raised at any of the three districts can modulate the response of the other two sites, has led to the innovative concept of a system-wide mucosal immunological organ. An improved knowledge of these entities has not only a speculative importance, but can also bear a clinical impact, insofar as EIM prove often more disabling than the underlying IBD itself.

Detection of Shared Balancing Selection in the Absence of Trans-Species Polymorphism

Trans-species polymorphism has been widely used as a key sign of long-term balancing selection across multiple species. However, such sites are often rare in the genome and could result from mutational processes or technical artifacts. Few methods are yet available to specifically detect footprints of trans-species balancing selection without using trans-species polymorphic sites. In this study, we develop summary- and model-based approaches that are each specifically tailored to uncover regions of long-term balancing selection shared by a set of species by using genomic patterns of intraspecific polymorphism and interspecific fixed differences. We demonstrate that our trans-species statistics have substantially higher power than single-species approaches to detect footprints of trans-species balancing selection, and are robust to those that do not affect all tested species. We further apply our model-based methods to human and chimpanzee whole-genome sequencing data. In addition to the previously established major histocompatibility complex and malaria resistance-associated FREM3/GYPE regions, we also find outstanding genomic regions involved in barrier integrity and innate immunity, such as the GRIK1/CLDN17 intergenic region, and the SLC35F1 and ABCA13 genes. Our findings not only echo the significance of pathogen defense but also reveal novel candidates in maintaining balanced polymorphisms across human and chimpanzee lineages. Finally, we show that these trans-species statistics can be applied to and work well for an arbitrary number of species, and integrate them into open-source software packages for ease of use by the scientific community.

Peptidoglycan recognition proteins in insect immunity

Insects lack an acquired immune system and rely solely on the innate immune system to combat microbial infection. The innate immunity of insects mainly depends on the interaction between the host's pattern recognition receptor (PRR) and pathogen-associated molecular pattern (PAMP). The peptidoglycan recognition proteins (PGRPs) family is the most important pattern recognition receptor (PRR) for insects. It can recognize the main component of the cell wall of the pathogenic microorganism, peptidoglycan (PGN), and plays an important role in the innate immunity of insects. In this paper, the structure, classification, and function of PGRPs is summarized, and the role of PGRPs in the innate immunity of insects is also discussed.

The Genetic Basis of Psoriasis

Psoriasis is widely regarded as a multifactorial condition which is caused by the interaction between inherited susceptibility alleles and environmental triggers. In the last decade, technological advances have enabled substantial progress in the understanding of disease genetics. Genome-wide association studies have identified more than 60 disease susceptibility regions, highlighting the pathogenic involvement of genes related to Th17 cell activation. This pathway has now been targeted by a new generation of biologics that have shown great efficacy in clinical trials. At the same time, the study of rare variants of psoriasis has identified interleukin (IL)-36 cytokines as important amplifiers of Th17 signaling and promising targets for therapeutic intervention. Here, we review these exciting discoveries, which highlight the translational potential of genetic studies.

Current knowledge on psoriasis and autoimmune diseases

Psoriasis is a prevalent, chronic inflammatory disease of the skin, mediated by crosstalk between epidermal keratinocytes, dermal vascular cells, and immunocytes such as antigen presenting cells (APCs) and T cells. Exclusive cellular “responsibility” for the induction and maintenance of psoriatic plaques has not been clearly defined. Increased proliferation of keratinocytes and endothelial cells in conjunction with APC/T cell/monocyte/macrophage inflammation leads to the distinct epidermal and vascular hyperplasia that is characteristic of lesional psoriatic skin. Despite the identification of numerous susceptibility loci, no single genetic determinant has been identified as responsible for the induction of psoriasis. Thus, numerous other triggers of disease, such as environmental, microbial and complex cellular interactions must also be considered as participants in the development of this multifactorial disease. Recent advances in therapeutics, especially systemic so-called “biologics” have provided new hope for identifying the critical cellular targets that drive psoriasis pathogenesis. Recent recognition of the numerous co-morbidities and other autoimmune disorders associated with psoriasis, including inflammatory bowel disease, multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematosus suggest common signaling elements and cellular mediators may direct disease pathogenesis. In this review, we discuss common cellular pathways and participants that mediate psoriasis and other autoimmune disorders that share these cellular signaling pathways.

Pglyrp-Regulated Gut Microflora Prevotella falsenii, Parabacteroides distasonis and Bacteroides eggerthii Enhance and Alistipes finegoldii Attenuates Colitis in Mice

Dysbiosis is a hallmark of inflammatory bowel disease (IBD), but it is unclear which specific intestinal bacteria predispose to and which protect from IBD and how they are regulated. Peptidoglycan recognition proteins (Pglyrps) are antibacterial, participate in maintaining intestinal microflora, and modulate inflammatory responses. Mice deficient in any one of the four Pglyrp genes are more sensitive to dextran sulfate sodium (DSS)-induced colitis, and stools from Pglyrp-deficient mice transferred to wild type (WT) germ-free mice predispose them to much more severe colitis than stools from WT mice. However, the identities of these Pglyrp-regulated bacteria that predispose Pglyrp-deficient mice to colitis or protect WT mice from colitis are not known. Here we identified significant changes in β-diversity of stool bacteria in Pglyrp-deficient mice compared with WT mice. The most consistent changes in microbiome in all Pglyrp-deficient mice were in Bacteroidales, from which we selected four species, two with increased abundance (Prevotella falsenii and Parabacteroides distasonis) and two with decreased abundance (Bacteroides eggerthii and Alistipes finegoldii). We then gavaged WT mice with stock type strains of these species to test the hypothesis that they predispose to or protect from DSS-induced colitis. P. falsenii, P. distasonis, and B. eggerthii all enhanced DSS-induced colitis in both WT mice with otherwise undisturbed intestinal microflora and in WT mice with antibiotic-depleted intestinal microflora. By contrast, A. finegoldii (which is the most abundant species in WT mice) attenuated DSS-induced colitis both in WT mice with otherwise undisturbed intestinal microflora and in WT mice with antibiotic-depleted intestinal microflora, similar to the colitis protective effect of the entire normal microflora. These results identify P. falsenii, P. distasonis, and B. eggerthii as colitis-promoting species and A. finegoldii as colitis-protective species.

Genetic and epigenetic basis of psoriasis pathogenesis

Psoriasis is a chronic inflammatory skin disease whose prevalence varies among different populations worldwide. It is a complex multi-factorial disease and the exact etiology is largely unknown. Family based studies have indicated a genetic predisposition; however they cannot fully explain the disease pathogenesis. In addition to genetic susceptibility, environmental as well as gender and age related factors were also been found to be associated. Recently, imbalances in epigenetic networks are indicated to be causative elements in psoriasis. The present knowledge of epigenetic involvement, mainly the DNA methylation, chromatin modifications and miRNA deregulation is surveyed here. An integrated approach considering genetic and epigenetic anomalies in the light of immunological network may explore the pathogenesis of psoriasis.

The cutaneous innate immune response in patients with atopic dermatitis

Orchestrating when and how the cutaneous innate immune system should respond to commensal or pathogenic microbes is a critical function of the epithelium. The cutaneous innate immune system is a key determinant of the physical, chemical, microbial, and immunologic barrier functions of the epidermis. A malfunction in this system can lead to an inadequate host response to a pathogen or a persistent inflammatory state. Atopic dermatitis is the most common inflammatory skin disorder and characterized by abnormalities in both skin barrier structures (stratum corneum and tight junctions), a robust T(H)2 response to environmental antigens, defects in innate immunity, and an altered microbiome. Many of these abnormalities may occur as the consequence of epidermal dysfunction. The epidermis directly interfaces with the environment and, not surprisingly, expresses many pattern recognition receptors that make it a key player in cutaneous innate immune responses to skin infections and injury. This review will discuss the role epidermal innate receptors play in regulation of skin barriers and, where possible, discuss the relevance of these findings for patients with atopic dermatitis.

Genetic Association of Peptidoglycan Recognition Protein Variants with Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a common disease, includes Crohn's disease (CD) and ulcerative colitis (UC), and is determined by altered gut bacterial populations and aberrant host immune response. Peptidoglycan recognition proteins (PGLYRP) are innate immunity bactericidal proteins expressed in the intestine. In mice, PGLYRPs modulate bacterial populations in the gut and sensitivity to experimentally induced UC. The role of PGLYRPs in humans with CD and/or UC has not been previously investigated. Here we tested the hypothesis that genetic variants in PGLYRP1, PGLYRP2, PGLYRP3 and PGLYRP4 genes associate with CD and/or UC and with gender and/or age of onset of disease in the patient population. We sequenced all PGLYRP exons in 372 CD patients, 77 UC patients, 265 population controls, 210 familial CD controls, and 24 familial UC controls, identified all polymorphisms in these populations, and analyzed the variants for significant association with CD and UC. We identified 16 polymorphisms in the four PGLYRP genes that significantly associated with CD, UC, and/or subgroups of patient populations. Of the 16, 5 significantly associated with both CD and UC, 6 with CD, and 5 with UC. 12 significant variants result in amino acid substitutions and based on structural modeling several of these missense variants may have structural and/or functional consequences for PGLYRP proteins. Our data demonstrate that genetic variants in PGLYRP genes associate with CD and UC and may provide a novel insight into the mechanism of pathogenesis of IBD.

Topical antihistamines display potent anti-inflammatory activity linked in part to enhanced permeability barrier function

Systemic antagonists of the histamine type 1 and 2 receptors (H1/2r) are widely used as anti-pruritics and central sedatives, but demonstrate only modest anti-inflammatory activity. Because many inflammatory dermatoses result from defects in cutaneous barrier function, and because keratinocytes express both Hr1 and Hr2, we hypothesized that H1/2r antagonists might be more effective, if they were used topically to treat inflammatory dermatoses. Topical H1/2r antagonists additively enhanced permeability barrier homeostasis in normal mouse skin by: i) stimulation of epidermal differentiation, leading to thickened cornified envelopes; and ii) enhanced epidermal lipid synthesis and secretion. Since barrier homeostasis was enhanced to a comparable extent in mast cell-deficient mice, with no further improvement following application of topical H1/2r antagonists, H1/2r antagonists likely oppose mast cell-derived histamine. In four immunologically-diverse, murine disease models, characterized by either inflammation alone (acute irritant contact dermatitis, acute allergic contact dermatitis), or by prominent barrier abnormalities (subacute allergic contact dermatitis, atopic dermatitis), topical H1/2r agonists aggravated, while H1/2r antagonists improved inflammation and/or barrier function. The apparent ability of topical H1r/2r antagonists to target epidermal H1/2r could translate into increased efficacy in the treatment of inflammatory dermatoses, likely due to decreased inflammation and enhanced barrier function. These results could shift current paradigms of antihistamine utilization from a predominantly-systemic to a topical approach.

Staphylococcus prevails in the skin microbiota of long-term immunodeficient mice

Host-commensal relationships in the skin are a complex system governed by variables related to the host, the bacteria and the environment. A disruption of this system may lead to new steady states, which, in turn, may lead to disease. We have studied one such disruption by characterizing the skin microbiota in healthy and immunodepressed (ID) mice. A detailed anatomopathological study failed to reveal any difference between the skin of healthy and ID mice. We sequenced the 16S rDNA V1-V2 gene region to saturation in 10 healthy and 10 ID 8 week-old mice, and found than all of the healthy and two of the ID mice had bacterial communities that were similar in composition to that of human skin, although, presumably because of the uniform raising conditions, less interindividual variation was found in mice. However, eight ID mice showed microbiota dominated by Staphylococcus epidermidis. Quantitative PCR amplification of 16S rDNA gene and of the Staphylococcus-specific TstaG region confirmed the previous results and indicated that the quantitative levels of Staphylococcus were similar in both groups while the total number of 16S copies was greater in the healthy mice. Thus, it is possible that, under long-term immunodeficiency, which removes the acquired but not the native immune system, S.epidermidis may inhibit the growth of other bacteria but does not cause a pathogenic state.

Peptidoglycan recognition protein Pglyrp2 protects mice from psoriasis-like skin inflammation by promoting regulatory T cells and limiting Th17 responses

Skin protects the body from the environment and is an important component of the innate and adaptive immune systems. Psoriasis is a frequent inflammatory skin disease of unknown cause determined by multigenic predisposition, environmental factors, and aberrant immune response. Peptidoglycan recognition proteins (Pglyrps) are expressed in the skin, and we report in this article that they modulate sensitivity in an experimentally induced mouse model of psoriasis. We demonstrate that Pglyrp2(-/-) mice (but not Pglyrp3(-/-) and Pglyrp4(-/-) mice) are more sensitive to the development of 12-O-tetradecanoylphorbol 13-acetate-induced psoriasis-like inflammation, whereas Pglyrp1(-/-) mice are less sensitive. The mechanism underlying this increased sensitivity of Pglyrp2(-/-) mice to 12-O-tetradecanoylphorbol 13-acetate-induced psoriasis-like inflammation is reduced recruitment of regulatory T cells to the skin and enhanced production and activation of Th17 cells in the skin in Pglyrp2(-/-) mice, which results in more severe inflammation and keratinocyte proliferation. Thus, in wild type mice, Pglyrp2 limits overactivation of Th17 cells by promoting accumulation of regulatory T cells at the site of inflammation, which protects the skin from the exaggerated inflammatory response.

Differential effects of peptidoglycan recognition proteins on experimental atopic and contact dermatitis mediated by Treg and Th17 cells

Skin protects the body from the environment and is an important component of the innate and adaptive immune systems. Atopic dermatitis and contact dermatitis are among the most frequent inflammatory skin diseases and are both determined by multigenic predisposition, environmental factors, and aberrant immune response. Peptidoglycan Recognition Proteins (Pglyrps) are expressed in the skin and we report here that they modulate sensitivity to experimentally-induced atopic dermatitis and contact dermatitis. Pglyrp3(-/-) and Pglyrp4(-/-) mice (but not Pglyrp2(-/-) mice) develop more severe oxazolone-induced atopic dermatitis than wild type (WT) mice. The common mechanism underlying this increased sensitivity of Pglyrp3(-/-) and Pglyrp4(-/-) mice to atopic dermatitis is reduced recruitment of Treg cells to the skin and enhanced production and activation Th17 cells in Pglyrp3(-/-) and Pglyrp4(-/-) mice, which results in more severe inflammation and keratinocyte proliferation. This mechanism is supported by decreased inflammation in Pglyrp3(-/-) mice following in vivo induction of Treg cells by vitamin D or after neutralization of IL-17. By contrast, Pglyrp1(-/-) mice develop less severe oxazolone-induced atopic dermatitis and also oxazolone-induced contact dermatitis than WT mice. Thus, Pglyrp3 and Pglyrp4 limit over-activation of Th17 cells by promoting accumulation of Treg cells at the site of chronic inflammation, which protects the skin from exaggerated inflammatory response to cell activators and allergens, whereas Pglyrp1 has an opposite pro-inflammatory effect in the skin.

Review: Mammalian peptidoglycan recognition proteins (PGRPs) in innate immunity

Peptidoglycan recognition proteins (PGRPs or PGLYRPs) are innate immunity proteins that are conserved from insects to mammals, recognize bacterial peptidoglycan, and function in antibacterial immunity and inflammation. Mammals have four PGRPs - PGLYRP1, PGLYRP2, PGLYRP3, and PGLYRP4. They are secreted proteins expressed in polymorphonuclear leukocytes (PGLYRP1), liver (PGLYRP2), or on body surfaces, mucous membranes, and in secretions (saliva, sweat) (PGLYRP3 and PGLYRP4). All PGRPs recognize bacterial peptidoglycan. Three PGRPs, PGLYRP1, PGLYRP3, and PGLYRP4 are directly bactericidal for both Gram-positive and Gram-negative bacteria and have no enzymatic activity, whereas PGLYRP2 is an N-acetylmuramoyl-L-alanine amidase that hydrolyzes bacterial cell wall peptidoglycan. Peptidoglycan recognition proteins influence host- pathogen interactions not only through their antibacterial or peptidoglycan-hydrolytic properties, but also through their pro-inflammatory and anti-inflammatory properties that are independent of their hydrolytic and antibacterial activities. The PGRPs likely play a role both in antibacterial defenses and several inflammatory diseases. They modulate local inflammatory responses in tissues (such as arthritic joints) and there is evidence for association of PGRPs with inflammatory diseases, such as psoriasis.

Broad defects in epidermal cornification in atopic dermatitis identified through genomic analysis

BACKGROUND

Psoriasis and atopic dermatitis (AD) are common, complex inflammatory skin diseases. Both diseases display immune infiltrates in lesions and epidermal growth/differentiation alterations associated with a defective skin barrier. An incomplete understanding of differences between these diseases makes it difficult to compare human disease pathology to animal disease models.

OBJECTIVE

To characterize differences between these diseases in expression of genes related to epidermal growth/differentiation and inflammatory circuits.

METHODS

We performed genomic profiling of mRNA in chronic psoriasis (n = 15) and AD (n = 18) skin lesions compared with normal human skin (n = 15).

RESULTS

As expected, clear disease classifications could be constructed on the basis of expected immune polarity (T(H)1, T(H)2, T(H)17) differences. However, even more striking differences were identified in epidermal differentiation programs that could be used for precise disease classifications. Although both psoriasis and AD skin lesions displayed regenerative epidermal hyperplasia, which is a general alteration in epidermal growth, keratinocyte terminal differentiation was differentially polarized. In AD, we found selective defects in expression of multiple genes encoding the cornified envelope, with the largest alteration in loricrin (expressed at 2% of the level of normal skin). At the ultrastructural level, the cornified envelope in AD was broadly defective with highly decreased compaction of corneocytes and reduced intercellular lipids. Hence, the entire keratinocyte terminal differentiation program (cytoplasmic compaction, cornification, and lipid release) is defective in AD, potentially underlying the immune differences.

CONCLUSION

Our study shows that although alterations in barrier responses exist in both diseases, epidermal differentiation is differentially polarized, with major implications for primary disease pathogenesis.

Variation in genes of the epidermal differentiation complex in German atopic dermatitis patients

The filaggrin (FLG) gene is one of the most widely replicated susceptibility genes for atopic dermatitis (AD) so far. Yet, FLG mutations cannot fully account for the original linkage peak on chromosome 1q21, a region comprising the so-called epidermal differentiation complex (EDC). Since the EDC contains numerous genes relevant for epidermal differentiation, we sought to evaluate variation in other genes located in this region in a German AD case-control cohort. Thirty-two single nucleotide polymorphisms (SNPs) in 21 genes across the EDC were genotyped in 402 unrelated AD patients and 325 non-atopic controls by means of restriction enzyme digestion or TaqMan assays. Allele and genotype frequencies were tested for differences between patients and controls by logistic regression. Haplotype frequencies were evaluated using the famhap software. Except for the already known association with FLG, we did not identify any additional significant associations of EDC genes with AD. Thus, in this German cohort, there is no evidence that additional genes in the EDC region apart from FLG contribute substantially to AD pathogenesis.

Novel peptide therapeutics for treatment of infections

As antibiotic resistance increases worldwide, there is an increasing pressure to develop novel classes of antimicrobial compounds to fight infectious disease. Peptide therapeutics represent a novel class of therapeutic agents. Some, such as cationic antimicrobial peptides and peptidoglycan recognition proteins, have been identified from studies of innate immune effector mechanisms, while others are completely novel compounds generated in biological systems. Currently, only selected cationic antimicrobial peptides have been licensed, and only for topical applications. However, research using new approaches to identify novel antimicrobial peptide therapeutics, and new approaches to delivery and improving stability, will result in an increased range of peptide therapeutics available in the clinic for broader applications.

Association of skin barrier genes within the PSORS4 locus is enriched in Singaporean Chinese with early-onset psoriasis

Psoriasis (OMIM#177900) is a common polygenic skin disorder affecting approximately 2% of the northern European population and 0.1% of the Han Chinese. Psoriasis patients suffer from chronic skin inflammation, manifested by erythematous scaly lesions. PSORS1-PSORS9 have been confirmed as psoriasis susceptibility loci in independent genetic studies on predominantly Caucasian populations, with psoriasis susceptibility loci (PSORS1, PSORS9) and additional loci at 9q33-34 and 2p22.3-11.2 reported in Han Chinese patients. In this study, we show the association of PSORS4 with psoriasis in Singaporean Chinese. Dense genotyping of single-nucleotide polymorphism-tagging candidate genes within the epidermal differentiation complex revealed significant association in the proximity of the involucrin gene (IVL); the strongest association was seen in early-onset psoriasis patients (P=0.0014). A follow-up genome-wide association screen localized the psoriasis susceptibility region to approximately 360 kb along chromosome 1 in the vicinity of IVL, small proline-rich region (SPRR) and proline-rich region 9 (PRR9) genes. The study of interactions between the causative variant(s) in this locus will provide insights into a possible role for epidermal barrier formation in the pathogenesis of psoriasis.

On the role of the epidermal differentiation complex in ichthyosis vulgaris, atopic dermatitis and psoriasis

Undisturbed epidermal differentiation is crucial for an intact skin barrier function. The epidermal differentiation complex (EDC) is a cluster of genes on chromosome 1q21 encoding proteins that fulfil important functions in terminal differentiation in the human epidermis, including filaggrin, loricrin, S100 proteins and others. Recently, evidence emerged that variation within EDC genes plays an important role in the pathogenesis of three common skin disorders, ichthyosis vulgaris, atopic dermatitis (AD) and psoriasis. Two loss-of-function mutations in the filaggrin (FLG) gene, R501X and 2282del4, were identified as causative for ichthyosis vulgaris in 15 affected European families, and the mode of inheritance was found to be semidominant. As ichthyosis vulgaris and AD often occur concomitantly in affected individuals, these two mutations were subsequently investigated in AD patients and found to be strongly associated with the disease. Following this first report, seven replication studies have been performed that all confirm an association of these two mutations with AD (or AD subtypes) in several European cohorts. Additionally, two unique loss-of-function mutations in the FLG gene were identified in Japanese ichthyosis vulgaris families and found to be associated with AD in a Japanese cohort. Thus, the FLG mutations are among the most consistently replicated associations for AD. Additionally, linkage analysis has suggested that variation within the EDC might also predispose for psoriasis but the exact susceptibility variation(s) have not yet been elucidated. Taken together, these findings convincingly demonstrate the important role of barrier dysfunction in various common skin disorders.

Peptidoglycan recognition proteins: pleiotropic sensors and effectors of antimicrobial defences

Peptidoglycan recognition proteins (PGRPs) are innate immunity molecules that are present in most invertebrate and vertebrate animals. All PGRPs function in antimicrobial defence and are homologous to the prokaryotic peptidoglycan-lytic type 2 amidases. However, only some PGRPs have the catalytic activity that protects the host from excessive inflammation, and most PGRPs have diversified to carry out other host-defence functions. Insect and mammalian PGRPs defend host cells against infection through very different mechanisms. Insect PGRPs activate signal transduction pathways in host cells or trigger proteolytic cascades in the haemolymph, both of which generate antimicrobial effectors. By contrast, mammalian PGRPs are directly bactericidal. Here, we review these contrasting modes of action.

Filaggrin mutations in children with severe atopic dermatitis

Atopic dermatitis (AD) results from strong genetic and environmental interactions. AD shows genetic linkage to Chromosome 1q21. This region contains the epidermal differentiation complex (EDC), which consists of genes that form essential components of epidermal surfaces. Filaggrin (FLG) is one of these. Mutations in FLG/(R501X and 2282del4) are reported to be strongly associated with AD and to influence asthma accompanying AD. We investigated these effects in families recruited through a child with severe AD. We genotyped two panels of families, totalling 426, containing 990 affected and unaffected children. We found significant associations with AD (P=0.0001), asthma (P=0.006), and atopy (P=0.002). The FLG mutations were present in 26.7% of patients with AD, but were also present in 14.4% of children without AD. They were weakly associated with disease severity. The variants were not independently associated with asthma. The overall LOD score for genetic linkage of markers to the region was 3.57. This fell to 2.03 after accounting for the FLG mutations, indicating the presence of other genetic variants influencing AD at this locus. Our results provide further confirmation of the importance of mutations in FLG and the skin barrier in AD pathogenesis. The results indicate that investigations of other genes within the EDC should be undertaken.

The peptidoglycan recognition proteins (PGRPs)

Peptidoglycan recognition proteins (PGRPs) are found in insects, mollusks, echinoderms, and vertebrates, and they protect animals against infections. The four mammalian family members are either bactericidal proteins or amidases that hydrolyze bacterial peptidoglycan.

Peptidoglycan recognition proteins (PGRPs) are innate immunity molecules present in insects, mollusks, echinoderms, and vertebrates, but not in nematodes or plants. PGRPs have at least one carboxy-terminal PGRP domain (approximately 165 amino acids long), which is homologous to bacteriophage and bacterial type 2 amidases. Insects have up to 19 PGRPs, classified into short (S) and long (L) forms. The short forms are present in the hemolymph, cuticle, and fat-body cells, and sometimes in epidermal cells in the gut and hemocytes, whereas the long forms are mainly expressed in hemocytes. The expression of insect PGRPs is often upregulated by exposure to bacteria. Insect PGRPs activate the Toll or immune deficiency (Imd) signal transduction pathways or induce proteolytic cascades that generate antimicrobial products, induce phagocytosis, hydrolyze peptidoglycan, and protect insects against infections. Mammals have four PGRPs, which are secreted; it is not clear whether any are directly orthologous to the insect PGRPs. One mammalian PGRP, PGLYRP-2, is an N-acetylmuramoyl-L-alanine amidase that hydrolyzes bacterial peptidoglycan and reduces its proinflammatory activity; PGLYRP-2 is secreted from the liver into the blood and is also induced by bacteria in epithelial cells. The three remaining mammalian PGRPs are bactericidal proteins that are secreted as disulfide-linked homo- and hetero-dimers. PGLYRP-1 is expressed primarily in polymorphonuclear leukocyte granules and PGLYRP-3 and PGLYRP-4 are expressed in the skin, eyes, salivary glands, throat, tongue, esophagus, stomach, and intestine. These three proteins kill bacteria by interacting with cell wall peptidoglycan, rather than permeabilizing bacterial membranes as other antibacterial peptides do. Direct bactericidal activity of these PGRPs either evolved in the vertebrate (or mammalian) lineage or is yet to be discovered in insects.

Peptidoglycan: a major aetiological factor for psoriasis?

Peptidoglycan (PG), a major cell-wall component of Gram-positive bacteria, has been detected within antigen-presenting cells in various inflammatory conditions, including psoriasis. The additional presence of T-helper 1 cells specific for streptococcal or staphylococcal PG in psoriasis skin lesions implicates PG as an important T-cell stimulator for the disease. PG is a major target for the innate immune system, and associations between genetic polymorphisms of recognition receptors for PG and various auto-inflammatory diseases have been identified. The location of these genes within four linkage sites for psoriasis raises the possibility that an altered innate recognition of PG might contribute to the enhanced T-cell response to the bacterial antigen. These observations suggest that PG is a major aetiological factor for psoriasis and emphasize the importance of PG in bacterial-infection-induced inflammatory disease.

Mammalian PGRPs: novel antibacterial proteins

Peptidoglycan recognition proteins (PGRPs) are innate immunity molecules conserved from insects to mammals. Insects have up to 19 PGRPs, which activate Toll or Imd signal transduction pathways or induce proteolytic cascades that generate antimicrobial products, induce phagocytosis, hydrolyse peptidoglycan, and protect insects against infections. Mammals have four PGRPs, which were hypothesized to function as signal-transducing pattern recognition receptors. However, all mammalian PGRPs are secreted, usually as disulphide-linked homo- and heterodimers. One mammalian PGRP, PGLYRP-2, is an N-acetylmuramoyl-L-alanine amidase that hydrolyses bacterial peptidoglycan and reduces its proinflammatory activity. PGLYRP-2 is secreted from liver into blood, and is also induced by bacteria in epithelial cells. The three remaining mammalian PGRPs are bactericidal or bacteriostatic proteins. PGLYRP-1 is expressed primarily in the granules of polymorphonuclear leucocytes (PMNs) , and PGLYRP-3 and PGLYRP-4 are expressed in the skin, eyes, salivary glands, throat, tongue, esophagus, stomach and intestine, and protect the host against infections. They kill bacteria by interacting with their cell wall peptidoglycan, rather than permeabilizing their membranes. These PGRPs therefore are a new class of bactericidal and bacteriostatic proteins that have different structure, mechanism of action, and expression pattern from currently known vertebrate antimicrobial peptides. Direct bactericidal activity of these PGRPs either evolved in vertebrates or mammals, or it is yet to be discovered in insects.

Epidermal barrier formation and recovery in skin disorders

Skin is at the interface between the complex physiology of the body and the external, often hostile, environment, and the semipermeable epidermal barrier prevents both the escape of moisture and the entry of infectious or toxic substances. Newborns with rare congenital barrier defects underscore the skin's essential role in a terrestrial environment and demonstrate the compensatory responses evoked ex utero to reestablish a barrier. Common inflammatory skin disorders such as atopic dermatitis and psoriasis exhibit decreased barrier function, and recent studies suggest that the complex response of epidermal cells to barrier disruption may aggravate, maintain, or even initiate such conditions. Either aiding barrier reestablishment or dampening the epidermal stress response may improve the treatment of these disorders. This Review discusses the molecular regulation of the epidermal barrier as well as causes and potential treatments for defects of barrier formation and proposes that medical management of barrier disruption may positively affect the course of common skin disorders.