Moderate UV Exposure Enhances Learning and Memory by Promoting a Novel Glutamate Biosynthetic Pathway in the Brain.

Sunlight exposure is known to affect mood, learning, and cognition. However, the molecular and cellular mechanisms remain elusive. Here, we show that moderate UV exposure elevated blood urocanic acid (UCA), which then crossed the blood-brain barrier. Single-cell mass spectrometry and isotopic labeling revealed a novel intra-neuronal metabolic pathway converting UCA to glutamate (GLU) after UV exposure. This UV-triggered GLU synthesis promoted its packaging into synaptic vesicles and its release at glutamatergic terminals in the motor cortex and hippocampus. Related behaviors, like rotarod learning and object recognition memory, were enhanced after UV exposure. All UV-induced metabolic, electrophysiological, and behavioral effects could be reproduced by the intravenous injection of UCA and diminished by the application of inhibitor or short hairpin RNA (shRNA) against urocanase, an enzyme critical for the conversion of UCA to GLU. These findings reveal a new GLU biosynthetic pathway, which could contribute to some of the sunlight-induced neurobehavioral changes.

Rapid reaction studies on the chemistry of flavin oxidation in urocanate reductase.

Urocanate reductase (UrdA) is a bacterial flavin-dependent enzyme that reduces urocanate to imidazole propionate, enabling bacteria to use urocanate as an alternative respiratory electron acceptor. Elevated serum levels of imidazole propionate are associated with the development of type 2 diabetes, and, since UrdA is only present in humans in gut bacteria, this enzyme has emerged as a significant factor linking the health of the gut microbiome and insulin resistance. Here, we investigated the chemistry of flavin oxidation by urocanate in the isolated FAD domain of UrdA (UrdA') using anaerobic stopped-flow experiments. This analysis unveiled the presence of a charge-transfer complex between reduced FAD and urocanate that forms within the dead time of the stopped-flow instrument (∼1 ms), with flavin oxidation subsequently occurring with a rate constant of ∼60 s-1. The pH dependence of the reaction and analysis of an Arg411Ala mutant of UrdA' are consistent with Arg411 playing a crucial role in catalysis by serving as the active site acid that protonates urocanate during hydride transfer from reduced FAD. Mutational analysis of urocanate-binding residues suggests that the twisted conformation of urocanate imposed by the active site of UrdA' facilitates urocanate reduction. Overall, this study provides valuable insight into the mechanism of urocanate reduction by UrdA.

A modified protocol for studying filaggrin degradation using a reconstructed human epidermis model under low and high humidity.

BACKGROUND: Filaggrin (FLG) is an essential protein that plays a vital role in maintaining skin barrier function and moisture levels, allowing the skin to adapt to dry environments. However, the precise temporal dynamics of FLG metabolism in the human epidermis remain poorly understood, and suitable tools to study these time-dependent effects are currently lacking. OBJECTIVE: To investigate the molecular mechanisms and time course of FLG metabolism and skin barrier function under high- and low-humidity conditions, utilizing a reconstructed epidermis model. METHODS: EpiSkin specimens cultured under humid or dry conditions for varying durations (2-48 h) were compared by assessing FLG degradation and skin barrier formation using immunofluorescence staining and western blotting. RESULTS: Under conditions of low humidity, the proteolysis of FLG in EpiSkin increased between 4 and 12 h and was accompanied by elevated levels of cysteine-aspartic protease (caspase)-14. The expression of peptidyl arginine deiminase 1 and calpain 1 also increased at 4 h. However, after 24 h, the expression of these three FLG-degrading proteins significantly decreased. Conversely, the levels of pyrrolidone-5-carboxylic acid and urocanic acid initially decreased at 2 h and then increased between 12 and 24 h. Additionally, the expression of skin barrier proteins, such as FLG, transglutaminase 5, loricrin and zonula occludens-1, decreased starting from 12 h. Notably, epidermal cell viability and activity were also inhibited. CONCLUSION: We propose a reliable and ethical model to study the temporal dynamics of FLG metabolism and its role in skin barrier function. Using a commercially reconstructed epidermis to mimic dry skin formation obviates the need for animal and human testing.

CONTEXTE: la filaggrine (FLG) est une protéine essentielle qui joue un rôle vital dans le maintien de la fonction de barrière cutanée et des taux d'humidité, permettant à la peau de s'adapter aux environnements secs. Cependant, la dynamique temporelle précise du métabolisme de la FLG dans l'épiderme humain reste mal comprise, et des outils appropriés pour étudier ces effets dépendant du temps font actuellement défaut. OBJECTIF: étudier les mécanismes moléculaires et l'évolution dans le temps du métabolisme de la FLG et de la fonction de barrière cutanée en milieux à humidité élevée et faible, en utilisant un modèle d'épiderme reconstruit. MÉTHODES: les échantillons EpiSkin cultivés en milieux humides ou secs pendant des durées variables (2 à 48 h) ont été comparés en évaluant la dégradation de la FLG et la formation d'une barrière cutanée à l'aide d'une coloration par immunofluorescence et d'un Western blot. RÉSULTATS: en milieux à faible humidité, la protéolyse de la FLG dans EpiSkin a augmenté entre 4 et 12 h et s'est accompagnée de taux élevés de cystéine­protéase aspartique (caspase)­14. L'expression du peptidyl arginine déiminase 1 et de la calpaïne 1 a également augmenté à 4 h. Cependant, après 24 h, l'expression de ces trois protéines de dégradation de la FLG a significativement diminué. Inversément, les taux d'acide pyrrolidone­5­carboxylique et d'acide urocanique ont initialement diminué au bout de 2 h, puis ont augmenté entre 12 et 24 h. En outre, l'expression des protéines de la barrière cutanée, telles que la FLG, la transglutaminase 5, la loricrine et le zonula occludens­1, a diminué à partir de 12 h. Notamment, la viabilité et l'activité des cellules épidermiques ont également été inhibées. CONCLUSION: nous proposons un modèle fiable et éthique pour étudier la dynamique temporelle du métabolisme de la FLG et son rôle dans la fonction de barrière cutanée. L'utilisation d'un épiderme reconstitué commercialement pour imiter la formation d'une peau sèche élimine la nécessité de réaliser des examens sur des animaux et des humains.

Actinic keratosis and surrounding skin exhibit changes in corneocyte surface topography and decreased levels of filaggrin degradation products.

Actinic keratosis (AK) is a frequent premalignant skin lesion mainly caused by chronic sun exposure. AK lesions are often surrounded by invisible, subclinical alterations, called field of cancerization (FoC). Definition of FoC is of importance for therapy management; however, the criteria and non-invasive tools to characterize FoC are lacking. Atomic force microscopy (AFM) proved to be a suitable tool for detection of changes in the corneocyte surface topography in inflammatory skin diseases, which share similar clinical features with AK such as hyper- and parakeratosis. Therefore, in this study we applied AFM to investigate AK and surrounding skin obtained by non-invasive collection of the stratum corneum (SC) with adhesive tapes. Furthermore, we determined degradation products of structural protein filaggrin (natural moisturizing factor, NMF), which previously showed association with the changes in corneocyte surface topography. Ten patients with multiple AK on the face were recruited from the outpatient clinic. SC samples were collected from the AK lesion, skin sites adjacent to the AK, 5 cm from the AK and retroauricular area. Corneocyte surface topography was determined by AFM, and NMF by liquid chromatography. The AK lesion showed alterations of the corneocyte surface topography characterized by an increased number of nanosize protrusions, which gradually decreased with the distance from the lesion. NMF levels show an inverse pattern. Atomic force microscopy showed to be a suitable tool to detect changes in the corneocyte surface topography on the AK lesion and surrounding skin in a non-invasive manner.

Loss-of-function mutations in filaggrin gene and malignant melanoma: a case-control study.

BACKGROUND: Loss-of-function mutations in filaggrin gene (FLG) have been suggested to increase the susceptibility of skin malignancies due to reduced levels of epidermal filaggrin and its degradation products, urocanic acid, which may be protective against ultraviolet irradiation. OBJECTIVE: We aimed to investigate the association between FLG mutation status and the occurrence of malignant melanoma (MM) in Danish adults. METHODS: The prevalence of FLG mutations in a sample of MM biopsies was compared with a FLG-genotyped cohort from two general population studies. Pearson's chi-squared and Fisher's exact tests were used to compare the two groups. RESULTS: A total of 867 MM biopsies and 9965 general population controls were genotyped, respectively. In the MM sample, two (0.23%) individuals were homozygous and 80 (9.4%) were heterozygous mutation carriers. In the general population controls, the prevalence of FLG mutations was 18 (0.18%) and 835 (8.4%) for homozygous and heterozygous mutations, respectively. Fisher's exact test and Pearson's chi-squared test yielded non-significant P-values when the groups were compared. CONCLUSION: FLG mutation was not associated with MM in the studied populations. This finding indicates that epidermal deficiency of filaggrin and its degradation products does not influence the risk of MM significantly.

[The epidermal barrier]. / Barrière épidermique.

The skin acts as an interface between the body and its surrounding environment. The epidermis, the surface layer of the skin, is chiefly responsible for this interactive protective function. The epidermal barrier may be subdivided into three defensive systems: the photoprotective barrier, the immune barrier, and the physical and chemical barrier of the stratum corneum or horny layer. To protect against harmful ultraviolet radiation, the epidermis has absorption factors such as melanin, produced by melanocytes, and urocanic acid, which is a degradation product of filaggrin. The epidermal immune defence system comprises an innate component, which is rapid but non-specific, together with adaptive response, which is systemic and antigen-specific, initiated by Langerhans cells. The stratum corneum, derived from terminal differentiation of epidermal keratinocytes, plays a key role as a physical and chemical permeability barrier. This horny layer is made up of corneocytes, covered with horny envelopes and linked to one another by corneodesmosomes and by extracellular matrix sheets. The epidermal barrier, which is constantly being renewed, is characterised by its extremely great capacity of adaptation to changing conditions in the environment.

Expression of Filaggrin and its Degradation Products in Human Skin Following Erythemal Doses of Ultraviolet B Irradiation.

Epidermal filaggrin level is affected by ultraviolet B irradiation in animal and experimental models. This study examined the effect of ultraviolet B irradiation on epidermal filaggrin and natural moisturizing factors in vivo in healthy adults (n = 22). Participants were irradiated with 2 minimal erythema doses of ultraviolet B on the skin. Biopsies and tape strips were collected from skin irradiated 24 and 72 h earlier and from non-irradiated skin (control). Real-time quantitative PCR on skin biopsies showed significantly reduced profilaggrin mRNA expression 24 h after irradiation (mean relative mRNA expression ± standard deviation: control, 3.86 ± 2.06 vs. 24 h, 1.52 ± 0.640; p = 0.02; n = 8). Immunohistochemistry showed aberrant spatial distribution of filaggrin protein 72 h after irradiation (n = 3). High-pressure liquid chromatography of tape extracts showed no change in mean total natural moisturizing factor levels after irradiation, but mean trans-urocanic acid was significantly reduced, as expected (n = 8). In conclusion, erythemal doses of ultraviolet B exert acute effects on profilaggrin mRNA and filaggrin protein in human skin in vivo.

The effect of epidermal levels of urocanic acid on 25-hydroxyvitamin D synthesis and inflammatory mediators upon narrowband UVB irradiation.

BACKGROUND/PURPOSE: Urocanic acid (UCA) absorbs ultraviolet (UV)B radiation in the epidermis which may interfere with phototherapy. Therefore, the influence of individual levels of UCA on immune reactivity and vitamin D synthesis induced by narrowband UVB radiation was assessed. METHODS: Twenty-eight subjects with irritant contact dermatitis of the hands were irradiated with suberythemal doses of narrowband UVB radiation on their unaffected lower forearms on three consecutive days. Stratum corneum tape strips and epidermal interstitial fluid (ISF) as well as blood samples were analyzed. RESULTS: Narrowband UVB irradiation led to the conversion of trans-UCA into its cis-isomer in the epidermis. The observed increase in 25-hydroxyvitamin D serum concentrations was inversely correlated with the baseline levels of trans-UCA. Furthermore, UVB irradiation induced significant changes in the levels of CXCL10/IP-10, CCL2/MCP-1, CCL4/MIP-1ß, and the IL-1RA/IL-1α ratio. The levels of IL-1α and CXCL9/MIG showed a trend toward increase. The changes in the levels of inflammatory and immunomodulatory mediators did not depend on baseline levels of trans-UCA. CONCLUSION: The results suggest that epidermal levels of trans-UCA affect vitamin D synthesis, but not cutaneous immune reactivity upon repeated exposure to suberythemal doses of narrowband UVB radiation. However, this requires further exploration.

Urocanate as a potential signaling molecule for bacterial recognition of eukaryotic hosts.

Host recognition is the crucial first step in infectious disease pathogenesis. Recognition allows pathogenic bacteria to identify suitable niches and deploy appropriate phenotypes for successful colonization and immune evasion. However, the mechanisms underlying host recognition remain largely unknown. Mounting evidence suggests that urocanate-an intermediate of the histidine degradation pathway-accumulates in tissues, such as skin, and acts as a molecule that promotes bacterial infection via molecular interaction with the bacterial regulatory protein HutC. In Gram-negative bacteria, HutC has long been known as a transcriptional repressor of hut genes for the utilization of histidine (and urocanate) as sources of carbon and nitrogen. Recent work on the opportunistic human pathogen Pseudomonas aeruginosa and zoonotic pathogen Brucella abortus shows that urocanate, in conjunction with HutC, plays a significant role in the global control of cellular metabolism, cell motility, and expression of virulence factors. We suggest that in addition to being a valuable source of carbon and nitrogen, urocanate may be central to the elicitation of bacterial pathogenesis.

The skin microbiome of caspase-14-deficient mice shows mild dysbiosis.

Caspase-14, an important proteinase involved in filaggrin catabolism, is mainly active in terminally differentiating keratinocytes, where it is required for the generation of skin natural moisturizing factors (NMFs). Consequently, caspase-14 deficient epidermis is characterized by reduced levels of NMFs such as urocanic acid and 2-pyrrolidone-5-carboxylic acid. Patients suffering from filaggrin deficiency are prone to develop atopic dermatitis, which is accompanied with increased microbial burden. Among several reasons, this effect could be due to a decrease in filaggrin breakdown products. In this study, we found that caspase-14(-/-) mice show enhanced antibacterial response compared to wild-type mice when challenged with bacteria. Therefore, we compared the microbial communities between wild-type and caspase-14(-/-) mice by sequencing of bacterial 16S ribosomal RNA genes. We observed that caspase-14 ablation leads to an increase in bacterial richness and diversity during steady-state conditions. Although both wild-type and caspase-14(-/-) skin were dominated by the Firmicutes phylum, the Staphylococcaceae family was reduced in caspase-14(-/-) mice. Altogether, our data demonstrated that caspase-14 deficiency causes the imbalance of the skin-resident bacterial communities.

Urocanate reductase: identification of a novel anaerobic respiratory pathway in Shewanella oneidensis MR-1.

Interpretation of the constantly expanding body of genomic information requires that the function of each gene be established. Here we report the genomic analysis and structural modelling of a previously uncharacterized redox-metabolism protein UrdA (SO_4620) of Shewanella oneidensis MR-1, which led to a discovery of the novel enzymatic activity, urocanate reductase. Further cloning and expression of urdA, as well as purification and biochemical study of the gene's product UrdA and redox titration of its prosthetic groups confirmed that the latter is indeed a flavin-containing enzyme catalysing the unidirectional reaction of two-electron reduction of urocanic acid to deamino-histidine, an activity not reported earlier. UrdA exhibits both high substrate affinity and high turnover rate (K(m) << 10 µM, k(cat) = 360 s(-1) ) and strong specificity in favour of urocanic acid. UrdA homologues are present in various bacterial genera, such as Shewanella, Fusobacterium and Clostridium, the latter including the human pathogen Clostridium tetani. The UrdA activity in S. oneidensis is induced by its substrate under anaerobic conditions and it enables anaerobic growth with urocanic acid as a sole terminal electron acceptor. The latter capability can provide the cells of UrdA-containing bacteria with a niche where no other bacteria can compete and survive.

Pathobiology of actinic keratosis: ultraviolet-dependent keratinocyte proliferation.

Actinic keratoses are proliferations of transformed neoplastic keratinocytes in the epidermis that are the result of cumulative ultraviolet (UV) radiation from sun exposure. They are commonly found on sites of sun-exposed skin such as the face, balding scalp, and back of the hand. Although UV exposure does exert certain beneficial effects on the skin, excessive exposure to UV radiation induces multiple cascades of molecular signaling events at the cellular level that produce inflammation, immunosuppression, failure of apoptosis, and aberrant differentiation. Cumulatively, these actions result in mutagenesis and, ultimately, carcinogenesis. This article provides a brief overview of the key mediators that are implicated in the pathobiology of actinic keratosis. Three evolutionary possibilities exist for these keratoses in the absence of treatment: (1) spontaneous remission, which can be common; (2) remaining stable, without further progression; or (3) transformation to invasive squamous cell carcinoma, which may metastasize. Because the effects of UV radiation on the skin are complex, it is not yet fully clear how all of the mediators of actinic keratosis progression are interrelated. Nonetheless, some represent potential therapeutic targets, because it is clear that directing therapy to the effects of UV radiation at a number of different levels could interrupt and possibly reverse the mechanisms leading to malignant transformation.

Facile and efficient syntheses of a series of N-benzyl and N-biphenylmethyl substituted imidazole derivatives based on (E)-urocanic acid, as angiotensin II AT1 receptor blockers.

In the present work, a facile and efficient route for the synthesis of a series of N-substituted imidazole derivatives is described. Docking studies have revealed that N-substituted imidazole derivatives based on (E)-urocanic acid may be potential antihypertensive leads. Therefore, new AT1 receptor blockers bearing either the benzyl or the biphenylmethyl moiety at the N-1 or N-3 position, either the (E)-acrylate or the propanoate fragment and their related acids at the C-4 position as well as a halogen atom at the C-5 position of the imidazole ring, were synthesized. The newly synthesized analogues were evaluated for binding to human AT1 receptor. The biological results showed that this class of molecules possesses moderate or no activity, thus not always confirming high docking scores. Nonetheless, important conclusions can be derived for their molecular basis of their mode of action and help medicinal chemists to design and synthesize more potent ones. An aliphatic group as in losartan seems to be important for enhancing binding affinity and activity.

Variation in transport explains polymorphism of histidine and urocanate utilization in a natural Pseudomonas population.

Phenotypic variation is a fundamental requirement for evolution by natural selection. While evidence of phenotypic variation in natural populations abounds, its genetic basis is rarely understood. Here we report variation in the ability of plant-colonizing Pseudomonas to utilize histidine, and its derivative, urocanate, as sole sources of carbon and nitrogen. From a population of 164 phyllosphere-colonizing Pseudomonas strains, 77% were able to utilize both histidine and urocanate (His(+) , Uro(+) ) as growth substrates, whereas the remainder could utilize histidine, but not urocanate (His(+) , Uro(-) ), or vice versa (His(-) , Uro(+) ). An in silico analysis of the hut locus, which determines capacity to utilize both histidine and urocanate, from genome-sequenced Pseudomonas strains, showed significant variation in the number of putative transporters. To identify transporter genes specific for histidine and urocanate, we focused on a single genotype of Pseudomonas fluorescens, strain SBW25, which is capable of utilizing both substrates. Site-directed mutagenesis, combined with [(3) H]histidine transport assays, shows that hutT(u) encodes a urocanate-specific transporter; hutT(h) encodes the major high-affinity histidine transporter; and hutXWV encodes an ABC-type transporter that plays a minor role in histidine uptake. Introduction of cloned copies of hutT(h) and hutT(u) from SBW25 into strains incapable of utilizing either histidine, or urocanate, complemented the defect, demonstrating a lack of functional transporters in these strains. Taken together our data show that variation in transport systems, and not in metabolic genes, explains a naturally occurring phenotypic polymorphism.

Trans-urocanic acid facilitates spatial memory, implications for Alzheimer's disease.

Trans-urocanic acid (trans-UCA) is an isomer of cis-UCA and is widely distributed in the brain, predominantly in the hippocampus and prefrontal cortex. Previous studies have investigated the role of trans-UCA in non-spatial memory; however, its influence on spatial memory remains unclear. In the present study, network pharmacology strategy and behavioral testing were used to evaluate the role of trans-UCA in spatial memory and predict its possible mechanism. The results showed that there are 40 intersecting targets between trans-UCA and spatial memory identified by several databases and Venn diagram, indicating that trans-UCA may be involved in spatial memory. Behavioral results show that trans-UCA facilitates spatial working memory in the Y-maze test as well as spatial recognition memory acquisition, consolidation and retrieval in an object location recognition (OLR) task. Furthermore, PPI (protein-protein interaction) network analysis, GO (gene ontology) and KEGG (Kyoto encyclopedia of genes and genomes) pathway enrichment analyses show that the molecular mechanisms underlying the enhancing effect of trans-UCA on spatial memory are mainly associated with the regulation of insulin, mitogen-activated protein kinase (MAPK) and nuclear factor Kappa B (NF-κB) signaling pathways, serotonergic synapse and arginine and proline metabolism. The results of this study suggest that trans-UCA facilitates spatial memory in the Y-maze test and OLR task and may offer therapeutic potential for Alzheimer's disease (AD). The underlying mechanisms predicted by network pharmacology should be further verified.

Cis-urocanic acid inhibits SAPK/JNK signaling pathway in UV-B exposed human corneal epithelial cells in vitro.

PURPOSE: The cornea is sensitive to ultraviolet B (UV-B) radiation-induced oxidative stress and inflammation. Its clinical manifestations are photokeratitis and climatic droplet keratopathy. Urocanic acid (UCA) is a major endogenous UV-absorbing chromophore in the epidermis and it is also an efficacious immunosuppressant. We have previously shown that cis-UCA can suppress UV-B-induced interleukin-6 and -8 secretion and cytotoxicity in human corneal epithelium (HCE) cells. In the current study, we further wanted to investigate the effects of cis-UCA on UV-B-induced inflammatory and apoptotic responses in HCE-2 cells, focusing on the nuclear factor kappa B (NF-κB) and AP-1 (subunits c-Fos and c-Jun) signaling pathways. METHODS: After exposing HCE-2 cells to UV-B and cis-UCA, DNA binding of c-Fos, c-Jun and NF-κB was measured with ELISA. In addition, the endogenous levels of phosphorylated stress-activated protein kinase/c-Jun N-terminal kinase (phospho-SAPK/JNK) and phospho-c-Jun were determined. The proliferative capacity of HCE-2 cells was also quantified, and the cytotoxicity of the cis-UCA and UV-B treatments was monitored by measuring the release of lactate dehydrogenase enzyme in the culture medium. RESULTS: UV-B irradiation induced the binding of transcription factors c-Jun, c-Fos, and NF-κB to DNA. Cis-UCA inhibited the binding of c-Jun and c-Fos but not that of NF-κB. Moreover, UV-B increased the levels of phospho-c-Jun and phospho-JNK, and the expression of both was attenuated by cis-UCA. Cis-UCA also alleviated the UV-B-induced apoptosis and proliferative decline in human corneal cells. CONCLUSIONS: The results from this study suggest that cis-UCA suppresses JNK signaling pathway, which provides potential for treating UV-B-induced inflammatory defects in human corneal cells.

Levels of filaggrin degradation products are influenced by both filaggrin genotype and atopic dermatitis severity.

BACKGROUND: Filaggrin, coded by FLG, is the main source of several major components of natural moisturizing factor (NMF) in the stratum corneum (SC), including pyrrolidone carboxylic acid (PCA) and urocanic acid (UCA). Loss-offunction mutations in FLG lead to reduced levels of filaggrin degradation products in the SC. It has recently been suggested that expression of filaggrin may additionally be influenced by the atopic inflammatory response. In this study, we investigated the levels of several breakdown products of filaggrin in the SC in healthy controls (CTRL) and patients with atopic dermatitis (AD) in relation to FLG null allele status. We examined the relationship between NMF (defined here as the sum of PCA and UCA) and AD severity. METHODS: The SC levels of filaggrin degradation products including PCA, UCA, histidine (HIS) and tyrosine were determined in 24 CTRL and 96 patients with moderate-to-severe AD. All subjects were screened for 11 FLG mutations relevant for the study population. RESULTS: The levels of PCA, UCA and HIS correlated with FLG genotype. Furthermore, these levels were higher in the CTRL when compared to AD patients with no FLG mutations. Multiple regression analysis showed that NMF levels were independently associated with FLG genotype and severity of disease. CONCLUSION: Decreased NMF is a global feature of moderate-to-severe AD; within AD, FLG genotype is the major determinant of NMF, with disease severity as a secondary modifier. NMF components are reliably determined by a noninvasive and relatively inexpensive tape stripping technique.

Effect of filaggrin breakdown products on growth of and protein expression by Staphylococcus aureus.

BACKGROUND: Colonization of the skin by Staphylococcus aureus in individuals with atopic dermatitis exacerbates inflammation. Atopic dermatitis is associated with loss-of-function mutations in the filaggrin (FLG) gene, accompanied by reduced levels of filaggrin breakdown products on the skin. OBJECTIVE: To assess the affect of growth in the presence of the filaggrin breakdown products urocanic acid (UCA) and pyrrolidone carboxylic acid (PCA) on fitness of and protein expression by S aureus. METHODS: S aureus was grown for 24 hours in the presence of UCA and PCA, and the density of the cultures was monitored by recording OD(600) values. Cell wall extracts and secreted proteins of S aureus were isolated and analyzed by SDS-PAGE. Cell wall-associated proteins known to be involved in colonization and immune evasion including clumping factor B, fibronectin binding proteins, protein A, iron-regulated surface determinant A, and the serine-aspartate repeat proteins were examined by Western immunoblotting. RESULTS: Acidification of growth media caused by the presence of UCA and PCA resulted in reduced growth rates and reduced final cell density of S aureus. At the lower pH, reduced expression of secreted and cell wall-associated proteins, including proteins involved in colonization (clumping factor B, fibronectin binding protein A) and immune evasion (protein A), was observed. Decreased expression of iron-regulated surface determinant A due to growth with filaggrin breakdown products appeared to be independent of the decreased pH. CONCLUSION: S aureus grown under mildly acidic conditions such as those observed on healthy skin expresses reduced levels of proteins that are known to be involved in immune evasion.

Structural characterization of the microbial enzyme urocanate reductase mediating imidazole propionate production.

The human microbiome can produce metabolites that modulate insulin signaling. Type 2 diabetes patients have increased circulating concentrations of the microbially produced histidine metabolite, imidazole propionate (ImP) and administration of ImP in mice resulted in impaired glucose tolerance. Interestingly, the fecal microbiota of the patients had increased capacity to produce ImP, which is mediated by the bacterial enzyme urocanate reductase (UrdA). Here, we describe the X-ray structures of the ligand-binding domains of UrdA in four different states, representing the structural transitions along the catalytic reaction pathway of this unexplored enzyme linked to disease in humans. The structures in combination with functional data provide key insights into the mechanism of action of UrdA that open new possibilities for drug development strategies targeting type 2 diabetes.

The dark side of daylight: photoaging and the tumor microenvironment in melanoma progression.

Continued thinning of the atmospheric ozone, which protects the earth from damaging ultraviolet radiation (UVR), will result in elevated levels of UVR reaching the earth's surface, leading to a drastic increase in the incidence of skin cancer. In addition to promoting carcinogenesis in skin cells, UVR is a potent extrinsic driver of age-related changes in the skin known as "photoaging." We are in the preliminary stages of understanding of the role of intrinsic aging in melanoma, and the tumor-permissive effects of photoaging on the skin microenvironment remain largely unexplored. In this Review, we provide an overview of the impact of UVR on the skin microenvironment, addressing changes that converge or diverge with those observed in intrinsic aging. Intrinsic and extrinsic aging promote phenotypic changes to skin cell populations that alter fundamental processes such as melanogenesis, extracellular matrix deposition, inflammation, and immune response. Given the relevance of these processes in cancer, we discuss how photoaging might render the skin microenvironment permissive to melanoma progression.