AhR Ligands, Malassezin, and Indolo[3,2-b]Carbazole are Selectively Produced by Malassezia furfur Strains Isolated from Seborrheic Dermatitis

Characteristics of Malassezia furfur at various pH and effects of Malassezia lipids on skin cells

Malassezia species are commensal and opportunistic fungi found in human skin. All Malassezia species lack fatty acid synthesis genes and survive by utilizing several lipases to degrade and absorb fatty acids from external lipid sources, but little research has been done on their optimal active pH and temperature. Our skin protects itself from external stimuli and maintains homeostasis, involving bacteria and fungi such as Malassezia species that inhabit our skin. Hence, dysbiosis in the skin microbiome can lead to various skin diseases. The skin's pH is slightly acidic compared to neutral, and changes in pH can affect the metabolism of Malassezia species. We used keratinocyte cell lines to determine the effect of lipids bio-converted by Malassezia furfur, Malassezia japonica, and Malassezia yamatoensis under pH conditions similar to those of healthy skin. Lipids bio-converted from Malassezia species were associated with the regulation of transcripts related to inflammation, moisturizing, and promoting elasticity. Therefore, to determine the effect of pH on lipid metabolism in M. furfur, which is associated with seborrheic dermatitis, changes in biomass, lipid content, and fatty acid composition were determined. The results showed that pH 7 resulted in low growth and reduced lipid content, which had a negative impact on skin health. Given that bio-converted Malassezia-derived lipids show positive effects at the slightly acidic pH typical of healthy skin, it is important to study their effects on skin cells under various pH conditions. KEY POINTS: • pH 6, Malassezia spp. bio-converted lipid have a positive effect on skin cells • Malassezia spp. have different lipid, fatty acid, and growth depending on pH • Malassezia spp. can play a beneficial role by secreting lipids to the outside.

Multi-omics approach to understand the impact of sun exposure on an in vitro skin ecosystem and evaluate a new broad-spectrum sunscreen

A reconstructed human epidermal model (RHE) colonized with human microbiota and sebum was developed to reproduce the complexity of the skin ecosystem in vitro. The RHE model was exposed to simulated solar radiation (SSR) with or without SPF50+ sunscreen (with UVB, UVA, long-UVA, and visible light protection). Structural identification of discriminant metabolites was acquired by nuclear magnetic resonance and metabolomic fingerprints were identified using reverse phase-ultra high-performance liquid chromatography-high resolution mass spectrometry, followed by pathway enrichment analysis. Over 50 metabolites were significantly altered by SSR (p < 0.05, log2 values), showing high skin oxidative stress (glutathione and purine pathways, urea cycle) and altered skin microbiota (branched-chain amino acid cycle and tryptophan pathway). 16S and internal transcribed spacer rRNA sequencing showed the relative abundance of various bacteria and fungi altered by SSR. This study identified highly accurate metabolomic fingerprints and metagenomic modifications of sun-exposed skin to help elucidate the interactions between the skin and its microbiota. Application of SPF50+ sunscreen protected the skin ecosystem model from the deleterious effects of SSR and preserved the physiological interactions within the skin ecosystem. These innovative technologies could thus be used to evaluate the effectiveness of sunscreen.

Microbiome: Role in Inflammatory Skin Diseases

As the body's largest organ, the skin harbors a highly diverse microbiota, playing a crucial role in resisting foreign pathogens, nurturing the immune system, and metabolizing natural products. The dysregulation of human skin microbiota is implicated in immune dysregulation and inflammatory responses. This review delineates the microbial alterations and immune dysregulation features in common Inflammatory Skin Diseases (ISDs) such as psoriasis, rosacea, atopic dermatitis(AD), seborrheic dermatitis(SD), diaper dermatitis(DD), and Malassezia folliculitis(MF).The skin microbiota, a complex and evolving community, undergoes changes in composition and function that can compromise the skin microbial barrier. These alterations induce water loss and abnormal lipid metabolism, contributing to the onset of ISDs. Additionally, microorganisms release toxins, like Staphylococcus aureus secreted α toxins and proteases, which may dissolve the stratum corneum, impairing skin barrier function and allowing entry into the bloodstream. Microbes entering the bloodstream activate molecular signals, leading to immune disorders and subsequent skin inflammatory responses. For instance, Malassezia stimulates dendritic cells(DCs) to release IL-12 and IL-23, differentiating into a Th17 cell population and producing proinflammatory mediators such as IL-17, IL-22, TNF-α, and IFN-α.This review offers new insights into the role of the human skin microbiota in ISDs, paving the way for future skin microbiome-specific targeted therapies.

γδ T cells respond directly and selectively to the skin commensal yeast Malassezia for IL-17-dependent fungal control

Stable microbial colonization of the skin depends on tight control by the host immune system. The lipid-dependent yeast Malassezia typically colonizes skin as a harmless commensal and is subject to host type 17 immunosurveillance, but this fungus has also been associated with diverse skin pathologies in both humans and animals. Using a murine model of Malassezia exposure, we show that Vγ4+ dermal γδ T cells expand rapidly and are the major source of IL-17A mediating fungal control in colonized skin. A pool of memory-like Malassezia-responsive Vγ4+ T cells persisted in the skin, were enriched in draining lymph nodes even after fungal clearance, and were protective upon fungal re-exposure up to several weeks later. Induction of γδT17 immunity depended on IL-23 and IL-1 family cytokine signalling, whereas Toll-like and C-type lectin receptors were dispensable. Furthermore, Vγ4+ T cells from Malassezia-exposed hosts were able to respond directly and selectively to Malassezia-derived ligands, independently of antigen-presenting host cells. The fungal moieties detected were shared across diverse species of the Malassezia genus, but not conserved in other Basidiomycota or Ascomycota. These data provide novel mechanistic insight into the induction and maintenance of type 17 immunosurveillance of skin commensal colonization that has significant implications for cutaneous health.

Anti-Malassezia Drug Candidates Based on Virulence Factors of Malassezia-Associated Diseases

Malassezia is a lipophilic unicellular fungus that is able, under specific conditions, to cause severe cutaneous and systemic diseases in predisposed subjects. This review is divided into two complementary parts. The first one discusses how virulence factors contribute to Malassezia pathogenesis that triggers skin diseases. These virulence factors include Malassezia cell wall resistance, lipases, phospholipases, acid sphingomyelinases, melanin, reactive oxygen species (ROS), indoles, hyphae formation, hydrophobicity, and biofilm formation. The second section describes active compounds directed specifically against identified virulence factors. Among the strategies for controlling Malassezia spread, this review discusses the development of aryl hydrocarbon receptor (AhR) antagonists, inhibition of secreted lipase, and fighting biofilms. Overall, this review offers an updated compilation of Malassezia species, including their virulence factors, potential therapeutic targets, and strategies for controlling their spread. It also provides an update on the most active compounds used to control Malassezia species.

Bioprospecting the Skin Microbiome: Advances in Therapeutics and Personal Care Products

Bioprospecting is the discovery and exploration of biological diversity found within organisms, genetic elements or produced compounds with prospective commercial or therapeutic applications. The human skin is an ecological niche which harbours a rich and compositional diversity microbiome stemming from the multifactorial interactions between the host and microbiota facilitated by exploitable effector compounds. Advances in the understanding of microbial colonisation mechanisms alongside species and strain interactions have revealed a novel chemical and biological understanding which displays applicative potential. Studies elucidating the organismal interfaces and concomitant understanding of the central processes of skin biology have begun to unravel a potential wealth of molecules which can exploited for their proposed functions. A variety of skin-microbiome-derived compounds display prospective therapeutic applications, ranging from antioncogenic agents relevant in skin cancer therapy to treatment strategies for antimicrobial-resistant bacterial and fungal infections. Considerable opportunities have emerged for the translation to personal care products, such as topical agents to mitigate various skin conditions such as acne and eczema. Adjacent compound developments have focused on cosmetic applications such as reducing skin ageing and its associated changes to skin properties and the microbiome. The skin microbiome contains a wealth of prospective compounds with therapeutic and commercial applications; however, considerable work is required for the translation of in vitro findings to relevant in vivo models to ensure translatability.

Part 2: Understanding the role of Malassezia spp. in skin disorders: pathogenesis of Malassezia associated skin infections

INTRODUCTION

Malassezia is a major component of the skin microbiome, a lipophilic symbiotic organism of the mammalian skin, which can switch to opportunistic pathogens triggering multiple dermatological disorders in humans and animals. This phenomenon is favored by endogenous and exogenous host predisposing factors, which may switch Malassezia from a commensal to a pathogenic phenotype.

AREA COVERED

This review summarizes and discusses the most recent literature on the pathogenesis of Malassezia yeasts, which ultimately results in skin disorders with different clinical presentation. A literature search of Malassezia pathogenesis was performed via PubMed and Google scholar (up to May 2023), using the following keywords: Pathogenesis and Malassezia;host risk factors and Malassezia, Malassezia and skin disorders; Malassezia and virulence factors: Malassezia and metabolite production; Immunology and Malassezia.

EXPERT OPINION

Malassezia yeasts can maintain skin homeostasis being part of the cutaneous mycobiota; however, when the environmental or host conditions change, these yeasts are endowed with a remarkable plasticity and adaptation by modifying their metabolism and thus contributing to the appearance or aggravation of human and animal skin disorders.

Staphylococcus epidermidis isolates from atopic or healthy skin have opposite effect on skin cells: potential implication of the AHR pathway modulation

Introduction

Staphylococcus epidermidis is a commensal bacterium ubiquitously present on human skin. This species is considered as a key member of the healthy skin microbiota, involved in the defense against pathogens, modulating the immune system, and involved in wound repair. Simultaneously, S. epidermidis is the second cause of nosocomial infections and an overgrowth of S. epidermidis has been described in skin disorders such as atopic dermatitis. Diverse isolates of S. epidermidis co-exist on the skin. Elucidating the genetic and phenotypic specificities of these species in skin health and disease is key to better understand their role in various skin conditions. Additionally, the exact mechanisms by which commensals interact with host cells is partially understood. We hypothesized that S. epidermidis isolates identified from different skin origins could play distinct roles on skin differentiation and that these effects could be mediated by the aryl hydrocarbon receptor (AhR) pathway.

Methods

For this purpose, a library of 12 strains originated from healthy skin (non-hyperseborrheic (NH) and hyperseborrheic (H) skin types) and disease skin (atopic (AD) skin type) was characterized at the genomic and phenotypic levels.

Results and discussion

Here we showed that strains from atopic lesional skin alter the epidermis structure of a 3D reconstructed skin model whereas strains from NH healthy skin do not. All strains from NH healthy skin induced AhR/OVOL1 path and produced high quantities of indole metabolites in co-culture with NHEK; especially indole-3-aldehyde (IAld) and indole-3-lactic acid (ILA); while AD strains did not induce AhR/OVOL1 path but its inhibitor STAT6 and produced the lowest levels of indoles as compared to the other strains. As a consequence, strains from AD skin altered the differentiation markers FLG and DSG1. The results presented here, on a library of 12 strains, showed that S. epidermidis originated from NH healthy skin and atopic skin have opposite effects on the epidermal cohesion and structure and that these differences could be linked to their capacity to produce metabolites, which in turn could activate AHR pathway. Our results on a specific library of strains provide new insights into how S. epidermidis may interact with the skin to promote health or disease.

The aryl hydrocarbon receptor in immune regulation and autoimmune pathogenesis

As a ligand-activated transcription factor, the aryl hydrocarbon receptor (AhR) is activated by structurally diverse ligands derived from the environment, diet, microorganisms, and metabolic activity. Recent studies have demonstrated that AhR plays a key role in modulating both innate and adaptive immune responses. Moreover, AhR regulates innate immune and lymphoid cell differentiation and function, which is involved in autoimmune pathogenesis. In this review, we discuss recent advances in understanding the mechanism of activation of AhR and its mediated functional regulation in various innate immune and lymphoid cell populations, as well as the immune-regulatory effect of AhR in the development of autoimmune diseases. In addition, we highlight the identification of AhR agonists and antagonists that may serve as potential therapeutic targets for the treatment of autoimmune disorders.

Targeting the aryl hydrocarbon receptor by gut phenolic metabolites: A strategy towards gut inflammation

The Aryl Hydrocarbon Receptor (AHR) is a ligand-dependent transcription factor able to control complex transcriptional processes in several cell types, which has been correlated with various diseases, including inflammatory bowel diseases (IBD). Numerous studies have described different compounds as ligands of this receptor, like xenobiotics, natural compounds, and several host-derived metabolites. Dietary (poly)phenols have been studied regarding their pleiotropic activities (e.g., neuroprotective and anti-inflammatory), but their AHR modulatory capabilities have also been considered. However, dietary (poly)phenols are submitted to extensive metabolism in the gut (e.g., gut microbiota). Thus, the resulting gut phenolic metabolites could be key players modulating AHR since they are the ones that reach the cells and may exert effects on the AHR throughout the gut and other organs. This review aims at a comprehensive search for the most abundant gut phenolic metabolites detected and quantified in humans to understand how many have been described as AHR modulators and what could be their impact on inflammatory gut processes. Even though several phenolic compounds have been studied regarding their anti-inflammatory capacities, only 1 gut phenolic metabolite, described as AHR modulator, has been evaluated on intestinal inflammatory models. Searching for AHR ligands could be a novel strategy against IBD.

Seborrheic dermatitis: topical therapeutics and formulation design

Seborrheic dermatitis (SD) is a common dermatological disorder with symptoms that include skin flaking, erythema and pruritus. This review discusses the topical products available for treating SD, which target several aspects of disease pathobiology, including cutaneous microbial dysbiosis (driven by Malassezia yeast), inflammation, sebum production and skin barrier disruption. Among the various treatments available, zinc pyrithione (ZnPT) based products that exhibit anti-fungal action are the market leaders. A skin compartment approach is presented here for combining ZnPT exposure information with threshold levels for anti-fungal efficacy and toxicity, overall providing a comprehensive picture of ZnPT therapeutics and safety. While Malassezia yeast on the surface are effectively targeted, yeast residing beyond the superficial follicle may not receive adequate ZnPT for anti-fungal effect forming the basis for skin re-colonisation. Levels entering systemic circulation from topical delivery are well below toxic thresholds, however the elevated zinc levels within the viable epidermis warrants further investigation. Strategies to improve formulation design can be broadly classified as influencing 1) topical delivery, 2) therapeutic bioactivity, 3) skin mildness, and 4) sensory attributes. Successful SD treatment ultimately requires formulations that can balance efficacy, safety, and consumer appeal.

The Ah Receptor from Toxicity to Therapeutics: Report from the 5th AHR Meeting at Penn State University, USA, June 2022

The aryl hydrocarbon receptor (AHR) is a sensor of low-molecular-weight molecule signals that originate from environmental exposures, the microbiome, and host metabolism. Building upon initial studies examining anthropogenic chemical exposures, the list of AHR ligands of microbial, diet, and host metabolism origin continues to grow and has provided important clues as to the function of this enigmatic receptor. The AHR has now been shown to be directly involved in numerous biochemical pathways that influence host homeostasis, chronic disease development, and responses to toxic insults. As this field of study has continued to grow, it has become apparent that the AHR is an important novel target for cancer, metabolic diseases, skin conditions, and autoimmune disease. This meeting attempted to cover the scope of basic and applied research being performed to address possible applications of our basic knowledge of this receptor on therapeutic outcomes.

New insights into immunity to skin fungi shape our understanding of health and disease

Fungi represent an integral part of the skin microbiota. Their complex interaction network with the host shapes protective immunity during homeostasis. If host defences are breached, skin-resident fungi including Malassezia and Candida, and environmental fungi such as dermatophytes can cause cutaneous infections. In addition, fungi are associated with diverse non-infectious skin disorders. Despite their multiple roles in health and disease, fungi remain elusive and understudied, and the mechanisms underlying the emergence of pathological conditions linked to fungi are largely unclear. The identification of IL-17 as an important antifungal effector mechanism represents a milestone for understanding homeostatic antifungal immunity. At the same time, host-adverse, disease-promoting roles of IL-17 have been delineated, as in psoriasis. Fungal dysbiosis represents another feature of many pathological skin conditions with an unknown causal link of intra- and interkingdom interactions to disease pathogenesis. The emergence of new fungal pathogens such as Candida auris highlights the need for more research into fungal immunology to understand how antifungal responses shape health and diseases. Recent technological advances for genetically manipulating fungi to target immunomodulatory fungal determinants, multi-omics approaches for studying immune cells in the human skin, and novel experimental models open up a promising future for skin fungal immunity.

Dysbiosis: The first hit for digestive system cancer

Gastrointestinal cancer may be associated with dysbiosis, which is characterized by an alteration of the gut microbiota. Understanding the role of gut microbiota in the development of gastrointestinal cancer is useful for cancer prevention and gut microbiota-based therapy. However, the potential role of dysbiosis in the onset of tumorigenesis is not fully understood. While accumulating evidence has demonstrated the presence of dysbiosis in the intestinal microbiota of both healthy individuals and patients with various digestive system diseases, severe dysbiosis is often present in patients with digestive system cancer. Importantly, specific bacteria have been isolated from the fecal samples of these patients. Thus, the association between dysbiosis and the development of digestive system cancer cannot be ignored. A new model describing this relationship must be established. In this review, we postulate that dysbiosis serves as the first hit for the development of digestive system cancer. Dysbiosis-induced alterations, including inflammation, aberrant immune response, bacteria-produced genotoxins, and cellular stress response associated with genetic, epigenetic, and/or neoplastic changes, are second hits that speed carcinogenesis. This review explains the mechanisms for these four pathways and discusses gut microbiota-based therapies. The content included in this review will shed light on gut microbiota-based strategies for cancer prevention and therapy.

Malassezia: A Commensal, Pathogen, and Mutualist of Human and Animal Skin

Identified in the late nineteenth century as a single species residing on human skin, Malassezia is now recognized as a diverse genus comprising 18 species inhabiting not only skin but human gut, hospital environments, and even deep-sea sponges. All cultivated Malassezia species are lipid dependent, having lost genes for lipid synthesis and carbohydrate metabolism. The surging interest in Malassezia results from development of tools to improve sampling, culture, identification, and genetic engineering, which has led to findings implicating it in numerous skin diseases, Crohn disease, and pancreatic cancer. However, it has become clear that Malassezia plays a multifaceted role in human health, with mutualistic activity in atopic dermatitis and a preventive effect against other skin infections due to its potential to compete with skin pathogens such as Candida auris. Improved understanding of complex microbe-microbe and host-microbe interactions will be required to define Malassezia's role in human and animal health and disease so as to design targeted interventions.

Rosmarinus officinalis L. Leaf Extracts and Their Metabolites Inhibit the Aryl Hydrocarbon Receptor (AhR) Activation In Vitro and in Human Keratinocytes: Potential Impact on Inflammatory Skin Diseases and Skin Cancer

Aryl hydrocarbon receptor (AhR) activation by environmental agents and microbial metabolites is potentially implicated in a series of skin diseases. Hence, it would be very important to identify natural compounds that could inhibit the AhR activation by ligands of microbial origin as 6-formylindolo[3,2-b]carbazole (FICZ), indirubin (IND) and pityriazepin (PZ) or the prototype ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Five different dry Rosmarinus officinalis L. extracts (ROEs) were assayed for their activities as antagonists of AhR ligand binding with guinea pig cytosol in the presence of [³H]TCDD. The methanolic ROE was further assayed towards CYP1A1 mRNA induction using RT-PCR in human keratinocytes against TCDD, FICZ, PZ, and IND. The isolated metabolites, carnosic acid, carnosol, 7-O-methyl-epi-rosmanol, 4′,7-O-dimethylapigenin, and betulinic acid, were assayed for their agonist and antagonist activity in the presence and absence of TCDD using the gel retardation assay (GRA). All assayed ROE extracts showed similar dose-dependent activities with almost complete inhibition of AhR activation by TCDD at 100 ppm. The methanol ROE at 10 ppm showed 99%, 50%, 90%, and 85% inhibition against TCDD, FICZ, IND, and PZ, respectively, in human keratinocytes. Most assayed metabolites exhibited dose-dependent antagonist activity. ROEs inhibit AhR activation by TCDD and by the Malassezia metabolites FICZ, PZ, and IND. Hence, ROE could be useful for the prevention or treatment of skin diseases mediated by activation of AhR.

Non-bacteria microbiome (virus, fungi, and archaea) in gastrointestinal cancer

The gastrointestinal tract houses millions of microbes collectively referred to as the gut microbiome. The gut microbes comprise of bacteria, viruses, fungi, archaea, and microscopic eukaryotes, which co-evolved or colonize the gut forming complex symbiotic and mutualistic relationships. A state of homeostasis is required between host and gut microbiome relationship to maintain several host beneficial processes. Alterations in the taxonomic and functional composition of the gut microbes are associated with several human diseases including gastrointestinal cancers. Owed to their overwhelming abundance and ease of characterization, several studies focus on the role of bacteria in gastrointestinal cancers. There is however growing evidence that non-bacteria gut microbes are associated with the pathogenesis of gastrointestinal cancers. This review details the association of non-bacteria gut microbes including fungi, viruses, and archaea and their potential manipulation in the prevention and treatment of human gastrointestinal cancers.

Role of AHR Ligands in Skin Homeostasis and Cutaneous Inflammation

Aryl hydrocarbon receptor (AHR) is an important regulator of skin barrier function. It also controls immune-mediated skin responses. The AHR modulates various physiological functions by acting as a sensor that mediates environment–cell interactions, particularly during immune and inflammatory responses. Diverse experimental systems have been used to assess the AHR’s role in skin inflammation, including in vitro assays of keratinocyte stimulation and murine models of psoriasis and atopic dermatitis. Similar approaches have addressed the role of AHR ligands, e.g., TCDD, FICZ, and microbiota-derived metabolites, in skin homeostasis and pathology. Tapinarof is a novel AHR-modulating agent that inhibits skin inflammation and enhances skin barrier function. The topical application of tapinarof is being evaluated in clinical trials to treat psoriasis and atopic dermatitis. In the present review, we summarize the effects of natural and synthetic AHR ligands in keratinocytes and inflammatory cells, and their relevance in normal skin homeostasis and cutaneous inflammatory diseases.

Targeted Delivery of Zinc Pyrithione to Skin Epithelia

Zinc pyrithione (ZnPT) is an anti-fungal drug delivered as a microparticle to skin epithelia. It is one of the most widely used ingredients worldwide in medicated shampoo for treating dandruff and seborrheic dermatitis (SD), a disorder with symptoms that include skin flaking, erythema and pruritus. SD is a multi-factorial disease driven by microbiol dysbiosis, primarily involving Malassezia yeast. Anti-fungal activity of ZnPT depends on the cutaneous availability of bioactive monomeric molecular species, occurring upon particle dissolution. The success of ZnPT as a topical therapeutic is underscored by the way it balances treatment efficacy with formulation safety. This review demonstrates how ZnPT achieves this balance, by integrating the current understanding of SD pathogenesis with an up-to-date analysis of ZnPT pharmacology, therapeutics and toxicology. ZnPT has anti-fungal activity with an average in vitro minimum inhibitory concentration of 10-15 ppm against the most abundant scalp skin Malassezia species (Malassezia globosa and Malassezia restrica). Efficacy is dependent on the targeted delivery of ZnPT to the skin sites where these yeasts reside, including the scalp surface and hair follicle infundibulum. Imaging and quantitative analysis tools have been fundamental for critically evaluating the therapeutic performance and safety of topical ZnPT formulations. Toxicologic investigations have focused on understanding the risk of local and systemic adverse effects following exposure from percutaneous penetration. Future research is expected to yield further advances in ZnPT formulations for SD and also include re-purposing towards a range of other dermatologic applications, which is likely to have significant clinical impact.

Marine Alkaloid Pityriacitrin and Its Analogues: Discovery, Structures, Synthetic Methods and Biological Properties

Pityriacitrin is a natural marine alkaloid with a typical β-carboline scaffold, and which has been demonstrated to exhibit diverse biological functions. The special structural features for pityriacitrin lead to the increasing research interest and the emergence of versatile derivatives, and many pityriacitrin analogues have been isolated or synthesized over the past decades. The structural diversity and evolved biological activity of these natural alkaloids can offer opportunities for the development of highly potential novel drugs with a new mechanism of action, and therefore, the aim of this brief review is to describe the discovery, synthesis, and biological properties of natural pityriacitrin and its derivatives, as well as the isolation source.

The aryl hydrocarbon receptor at the forefront of host-microbe interactions in the skin: A perspective on current knowledge gaps and directions for future research and therapeutic applications

The skin is home to a community of skin microbiota including bacteria, viruses and fungi, which are widely accepted to be of importance for skin homeostasis but also associated with skin diseases. Detailed knowledge on the skin microbiota composition and its changes in a number of skin diseases is available. Yet, specific interactions between microbes and the host skin cells or how they communicate with each other are less well understood. To identify, understand and eventually therapeutically exploit causal relationships of microbial dysbiosis with disease, studies are required that address the receptors and mediators involved in host‐microbe interactions. In this perspective article, we provide an outlook on one of such receptors, namely the aryl hydrocarbon receptor (AHR). The AHR is well known for being a ligand‐activated transcription factor regulating the proliferation, differentiation and function of many cell types present in the skin. Its targeting by anti‐inflammatory therapeutics such as coal tar and Tapinarof is effective in atopic dermatitis and psoriasis. AHR signalling is activated upon binding of wide variety of small chemicals or ligands, including microbiota‐derived metabolites. New evidence has emerged pointing towards a key role for epidermal AHR signalling through skin microbiota‐derived metabolites. In response, AHR‐driven expression of antimicrobial peptides and stratum corneum formation may alter the skin microbiota composition. This a self‐perpetuating feedback loop calls for novel therapeutic intervention strategies for which we herein discuss the requirements in future mechanistic studies.

The aryl hydrocarbon receptor as a mediator of host-microbiota interplay

Increasing evidence suggests a significant role for microbiota dependent metabolites and co-metabolites, acting as aryl hydrocarbon receptor (AHR) ligands, to facilitate bidirectional communication between the host and the microbiota and thus modulate physiology. Such communication is particularly evident within the gastrointestinal tract. Through binding to or activating the AHR, these metabolites play fundamental roles in various physiological processes and likely contribute to the maintenance of intestinal homeostasis. In recent years, tryptophan metabolites were screened to identify physiologically relevant AHR ligands or activators. The discovery of specific microbiota-derived indole-based metabolites as AHR ligands may provide insight concerning how these metabolites affect interactions between gut microbiota and host intestinal homeostasis and how this relates to chronic GI disease and overall health. A greater understanding of the mechanisms that modulate the production of such metabolites and associated AHR activity may be utilized to effectively treat inflammatory diseases and promote human health. Here, we review microbiota-derived AHR ligands generated from tryptophan that modulate host-gut microbiota interactions and discuss possible intervention strategies for potential therapies in the future.

Overview of the Potential Role of Malassezia in Gut Health and Disease

Malassezia is the most prevalent fungus identified in the human skin microbiota; originally described at the end of the nineteenth century, this genus is composed of at least 14 species. The role of Malassezia on the skin remains controversial because this genus has been associated with both healthy skin and pathologies (dermatitis, eczema, etc.). However, with the recent development of next-generation sequencing methods, allowing the description of the fungal diversity of various microbiota, Malassezia has also been identified as a resident fungus of diverse niches such as the gut or breast milk. A potential role for Malassezia in gut inflammation and cancer has also been suggested by recent studies. The aim of this review is to describe the findings on Malassezia in these unusual niches, to investigate what is known of the adaptation of Malassezia to the gut environment and to speculate on the role of this yeast in the host physiology specifically related to the gastrointestinal tract.

Host Immunity to Malassezia in Health and Disease

The microbiota plays an integral role in shaping physical and functional aspects of the skin. While a healthy microbiota contributes to the maintenance of immune homeostasis, dysbiosis can result in the development of diverse skin pathologies. This dichotomous feature of the skin microbiota holds true not only for bacteria, but also for fungi that colonize the skin. As such, the yeast Malassezia, which is by far the most abundant component of the skin mycobiota, is associated with a variety of skin disorders, of which some can be chronic and severe and have a significant impact on the quality of life of those affected. Understanding the causative relationship between Malassezia and the development of such skin disorders requires in-depth knowledge of the mechanism by which the immune system interacts with and responds to the fungus. In this review, we will discuss recent advances in our understanding of the immune response to Malassezia and how the implicated cells and cytokine pathways prevent uncontrolled fungal growth to maintain commensalism in the mammalian skin. We also review how the antifungal response is currently thought to affect the development and severity of inflammatory disorders of the skin and at distant sites.

Critical synthesis of available data in Malassezia folliculitis and a systematic review of treatments

Folliculitis is an inflammatory process involving the hair follicle, frequently attributed to infectious causes. Malassezia, an established symbiotic yeast that can evolve to a skin pathogen with opportunistic attributes, is a common source of folliculitis, especially when intrinsic (e.g. immunosuppression) or extrinsic (high ambient temperature and humidity, clothing) impact on the hair follicle and the overlying skin microenvironment. Our aim was to critically review the pathophysiology and clinical characteristics of Malassezia folliculitis, to describe laboratory methods that facilitate diagnosis and to systematically review treatment options. Malassezia folliculitis manifests as a pruritic, follicular papulopustular eruption distributed on the upper trunk. It commonly affects young to middle-aged adults and immunosuppressed individuals. Inclusion into the differential diagnosis of folliculitis is regularly oversighted, and the prerequisite-targeted diagnostic procedures are not always performed. Sampling by tape stripping or comedo extractor and microscopic examination of the sample usually identifies the monopolar budding yeast cells of Malassezia without the presence of hyphae. However, confirmation of the diagnosis with anatomical association with the hair follicle is performed by biopsy. For systematic review of therapies, PubMed was searched using the search string "(malassezia" [MeSH Terms] OR "malassezia" [All Fields] OR pityrosporum [All Fields]) AND "folliculitis" [MeSH Terms] and EMBASE was searched using the search string: 'malassezia folliculitis.mp OR pityrosporum folliculitis.mp'. In total, 28 full-length studies were assessed for eligibility and 21 were selected for inclusion in therapy evaluation. Conclusively Malassezia folliculitis should be considered in the assessment of truncal, follicular skin lesions. Patient's history, comorbidities and clinical presentation are usually indicative, but microscopically and histological examination is needed to confirm the diagnosis. Adequate samples obtained with comedo extractor and serial sections in the histological material are critical for proper diagnosis. Therapy should include systemic or topical measures for the control of the inflammation, as well as the prevention of recurrences.

Malassezia and Staphylococcus dominate scalp microbiome for seborrheic dermatitis

Seborrheic dermatitis (SD) is a common disease of the human scalp that causes physical damage and psychological problems for patients. Studies have indicated that dysbiosis of the scalp microbiome results in SD. However, the specific fungal and bacterial microbiome changes related to SD remain elusive. To further investigate the fungal and bacterial microbiome changes associated with SD, we recruited 57 SD patients and 53 healthy individuals and explored their scalp microbiomes using next generation sequencing and the QIIME and LEfSe bioinformatics tools. Skin pH, sebum secretion, hydration, and trans-epidermal water loss (TWEL) were also measured at the scalp. We found no statistically significant differences between the normal and lesion sites in SD patients with different subtypes of dandruff and erythema. However, the fungal and bacterial microbiome could differentiate SD patients from healthy controls. The presence of Malassezia and Aspergillus was both found to be potential fungal biomarkers for SD, while Staphylococcus and Pseudomonas were found to be potential bacterial biomarkers. The fungal and bacterial microbiome were divided into three clusters through co-abundance analysis and their correlations with host factors indicated the interactions and potential cooperation and resistance between microbe communities and host. Our research showed the skin microbe dysbiosis of SD and highlighted specific microorganisms that may serve as potential biomarkers of SD. The etiology of SD is multi-pathogenetic-dependent on the linkage of several microbes with host. Scalp microbiome homeostasis could be a promising new target in the risk assessment, prevention, and treatment of SD disease.

Malassezia-Associated Skin Diseases, the Use of Diagnostics and Treatment

Yeasts of the genus, Malassezia, formerly known as Pityrosporum, are lipophilic yeasts, which are a part of the normal skin flora (microbiome). Malassezia colonize the human skin after birth and must therefore, as commensals, be normally tolerated by the human immune system. The Malassezia yeasts also have a pathogenic potential where they can, under appropriate conditions, invade the stratum corneum and interact with the host immune system, both directly but also through chemical mediators. The species distribution on the skin and the pathogenetic potential of the yeast varies between different Malassezia related diseases such as head and neck dermatitis, seborrheic dermatitis, pityriasis versicolor, and Malassezia folliculitis. The diagnostic methods used to confirm the presence of Malassezia yeasts include direct microcopy, culture based methods (often a combination of morphological features of the isolate combined with biochemical test), molecular based methods such as Polymerase Chain Reaction techniques, and Matrix Assisted Laser Desorption/Ionization—Time Of Flight mass spectrometry and the chemical imprint method Raman spectroscopy. Skin diseases caused by Malassezia are usually treated with antifungal therapy and if there are associated inflammatory skin mechanisms this is often supplemented by anti-inflammatory therapy. The aim of this paper is to provide an overview of Malassezia related skin disease, diagnostic methods and treatment options.

Role of the Aryl Hydrocarbon Receptor in Environmentally Induced Skin Aging and Skin Carcinogenesis

The skin is constantly exposed to a variety of environmental threats, including solar electromagnetic radiation, microbes, airborne particulate matter, and chemicals. Acute exposure to these environmental factors results in the activation of different signaling pathways that orchestrate adaptive stress responses to maintain cell and tissue homeostasis. Chronic exposure of skin to these factors, however, may lead to the accumulation of damaged macromolecules and loss of cell and tissue integrity, which, over time, may facilitate aging processes and the development of aging-related malignancies. One transcription factor that is expressed in all cutaneous cells and activated by various environmental stressors, including dioxins, polycyclic aromatic hydrocarbons, and ultraviolet radiation, is the aryl hydrocarbon receptor (AHR). By regulating keratinocyte proliferation and differentiation, epidermal barrier function, melanogenesis, and immunity, a certain degree of AHR activity is critical to maintain skin integrity and to adapt to acute stress situations. In contrast, a chronic activation of cutaneous AHR signaling critically contributes to premature aging and the development of neoplasms by affecting metabolism, extracellular matrix remodeling, inflammation, pigmentation, DNA repair, and apoptosis. This article provides an overview of the detrimental effects associated with sustained AHR activity in chronically stressed skin and pinpoints AHR as a promising target for chemoprevention.

A Biomimetic, One-Step Transformation of Simple Indolic Compounds to Malassezia-Related Alkaloids with High AhR Potency and Efficacy

Malassezia furfur isolates from diseased skin preferentially biosynthesize compounds which are among the most active known aryl-hydrocarbon receptor (AhR) inducers, such as indirubin, tryptanthrin, indolo[3,2-b]carbazole, and 6-formylindolo[3,2-b]carbazole. In our effort to study their production from Malassezia spp., we investigated the role of indole-3-carbaldehyde (I3A), the most abundant metabolite of Malassezia when grown on tryptophan agar, as a possible starting material for the biosynthesis of the alkaloids. Treatment of I3A with H₂O₂ and use of catalysts like diphenyldiselenide resulted in the simultaneous one-step transformation of I3A to indirubin and tryptanthrin in good yields. The same reaction was first applied on simple indole and then on substituted indoles and indole-3-carbaldehydes, leading to a series of mono- and bisubstituted indirubins and tryptanthrins bearing halogens, alkyl, or carbomethoxy groups. Afterward, they were evaluated for their AhR agonist activity in recombinant human and mouse hepatoma cell lines containing a stably transfected AhR-response luciferase reporter gene. Among them, 3,9-dibromotryptanthrin was found to be equipotent to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as an AhR agonist, and 3-bromotryptanthrin was 10-times more potent than TCDD in the human HG2L7.5c1 cell line. In contrast, 3,9-dibromotryptanthrin and 3-bromotryptanthrin were ∼4000 and >10,000 times less potent than TCDD in the mouse H1L7.5c3 cell line, respectively, demonstrating that they are species-specific AhR agonists. Involvement of the AhR in the action of 3-bromotryptanthrin was confirmed by the ability of the AhR antagonists CH223191 and SR1 to inhibit 3-bromotryptanthrin-dependent reporter gene induction in human HG2L7.5c1 cells. In conclusion, I3A can be the starting material used by Malassezia for the production of both indirubin and tryptanthrin through an oxidation mechanism, and modification of these compounds can produce some highly potent, efficacious and species-selective AhR agonists.

Cross-regulatory Circuit Between AHR and Microbiota

BACKGROUND

The gut microbes have a close symbiotic relationship with their host. Interactions between host and the microbiota affect the nutritional, immunological, and physiological status of the host. The Aryl Hydrocarbon Receptor (AHR) is a ligand activated transcription factor that mediates the toxicity of xenobiotics. Recently, the relationship between the gut microbiota and AHR has attracted the attention of many researchers.

METHODS

We undertook a structured search of bibliographic databases for peer-reviewed research literature.

RESULTS

We found and reviewed 49 peer-reviewed papers dealing with the major aspects related to the crosstalk between AHR and microbiota. The AHR influences the intestinal microbiota population and mediates host-microbe homeostasis. Interestingly, the gut microbiota also produces ligands of AHR from bacterial metabolism and thereby activates the AHR signaling pathway. Concusion: This review presents current knowledge of the cross-regulatory circuit between the AHR and intestinal microbiota. The findings of this review confirm the importance of AHR-microbiota interactions in health and disease.

Dichotomous response of Malassezia-infected macrophages to Malassezia pachydermatis and Malassezia furfur

Malassezia pachydermatis and Malassezia furfur are lipophilic yeasts of the cutaneous microbiome, although these organisms are occasionally responsible for serious invasive infections in neonates. Since phagocytosis is an important mechanism mediating the adaptive immune response, here we evaluated the phagocytosis capacity and production of nitric oxide and cytokine by macrophages after challenged with M. furfur CBS-1878 and M. pachydermatis CBS-1696. The phagocytic indexes was determined using RAW 264.7 cultivated or not with M. furfur or M. pachydermatis in the concentrations of 5:1 or 2:1 (yeasts:macrophages ratio) for 6 h, 24 h, and 48 h following the challenges. Evaluation of nitric oxide and pro- and anti-inflammatory cytokines (interleukin [IL]-2, IL-4, IL-6, IL-10, IL-17A, interferon (IFN)-γ and tumor necrosis factor [TNF]-α) by Griess method and flow cytometry, respectively, were performed in the different intervals by collecting the cell culture supernatant. Results showed a higher phagocytic index in the 5:1 ratio in 24 h for both species. Malassezia pachydermatis-infected macrophages had superior phagocytic indexes than M. furfur-infected macrophages. Phagocytosis evaluation at 48 h showed significant microorganisms proliferation and macrophages death, particularly in macrophages infected with M. pachydermatis, suggesting yeast evasion mechanism. Significant variations in the nitric oxide production were observed in macrophages infected with both species. Levels of TNF-α and IL-4 cytokines have increased in M. furfur and M. pachydermatis macrophage-infected cultures, respectively. The low microbicidal activity and the presence of pro- and anti-inflammatory cytokines reinforce the dichotomous character of the relation of these yeasts with the host, acting as a commensal in the cutaneous microbiome or causing infection.

Human microbial ecology and the rising new medicine

The first life forms on earth were Prokaryotic, and the evolution of all Eukaryotic life occurred with the help of bacteria. Animal-associated microbiota also includes members of the archaea, fungi, protists, and viruses. The genomes of this host-associated microbial life are called the microbiome. Across the mammalian tree, microbiomes guarantee the development of immunity, physiology, and resistance to pathogens. In humans, all surfaces and cavities are colonized by a microbiome, maintained by a careful balance between the host response and its colonizers-thus humans are considered now supraorganisms. These microbiomes supply essential ecosystem services that benefit health through homeostasis, and the loss of the indigenous microbiota leads to dysbiosis, which can have significant consequences to disease. This educational review aims to describe the importance of human microbial ecology, explain the ecological terms applied to the study of the human microbiome, developments within the cutting-edge microbiome field, and implications to diagnostic and treatment.

Malassezia-derived aryl hydrocarbon receptor ligands enhance the CCL20/Th17/soluble CD163 pathogenic axis in extra-mammary Paget's disease

Malassezia yeast play a role in the pathogenesis of chronic dermatitis, especially in apocrine areas, by polarizing the local immunologic background to a Th2/Th17 state through aryl hydrocarbon receptor (AhR)-dependent pathways. Extra-mammary Paget's disease (EMPD) is an adenocarcinoma of apocrine origin, and except for cases associated with Malassezia yeast and their metabolites, the lesions typically develop in areas not exposed to environmental material. The purpose of this study was to investigate (a) the immunomodulatory effects of Malassezia metabolites on normal human keratinocytes (NHKCs), focusing on interleukin (IL)-17 and related cytokines/chemokines (IL-23, IL-36γ, CCL20), (b) the expression of these factors in lesion-affected skin in EMPD and (c) the activation of tumor-associated macrophages (TAMs) by these factors. Malassezia metabolites augmented the expression of cytochrome P450, family 1, subfamily A, polypeptide 1 (CYP1A1), CCL20 and IL-36γ mRNA in NHKCs in vitro. In lesion-affected skin of patients with EMPD, epidermal keratinocytes expressed CYP1A1 and CCL20. In addition, Paget cells expressed CCL20 and IL-23. IL-17-producing cells were distributed adjacent to Paget cells. Compared to healthy donors, patients with EMPD exhibited significantly increased serum levels of soluble (s)CD163, CXCL5, CXCL10 and CCL20. In addition, serum levels of sCD163 decreased significantly following tumor resection. Our study demonstrates a possible mechanism for the development of EMPD involving AhR-mediated signalling by epidermal keratinocytes and RANKL-induced recruitment of Th17 cells and TAMs.

Extracellular vesicles derived from Malassezia furfur stimulate IL-6 production in keratinocytes as demonstrated in in vitro and in vivo models

BACKGROUND

Malassezia is one of the commensal microorganisms colonized on human skin and has been shown to be related to several inflammatory cutaneous disorders. Previous studies indicated that Malassezia. sympodialis (M. sympodialis) can produce extracellular vesicles, however, the immunoregulatory function of Malassezia extracellular vesicles on keratinocytes has not been studied.

OBJECTIVE

To investigate the extracellular vesicular production capability of Malassezia. furfur (M. furfur) and examine their immunoregulatory effects both in vitro and in vivo.

METHODS

Extracellular vesicles derived from M. furfur were isolated by sequential ultracentrifugation procedure. Their structure and diameter were determined by negative stain TEM and NTA, respectively. Confocal microscopy was used to visualize the internalization of these nanoparticles into HaCaT cells and mice epidermal keratinocytes. The expressions of inflammatory cytokines were screened using PCR Array assay and validated in vitro by qPCR and ELISA assays. In vivo cytokine production was measured by the IHC method. The role of NF-κB in such process was evaluated in HaCaT cells by western blot assay.

RESULTS

Our results showed that M. furfur produced ovoid-shaped nanoparticles, which could be then internalized into HaCaT cells, as well as mice epidermal keratinocytes. IL-6 expression was significantly enhanced in response to extracellular vesicular stimulation both in vitro and in vivo, in which process the activation of NF-κB was involved.

CONCLUSION

M. furfur has the ability to release extracellular vesicles, which can be internalized into keratinocytes and promote the production of IL-6 with the involvement of NF-κB dependent pathway. Such findings reveal some important new insights into Malassezia pathogenesis and therapy.

Malassezia pachydermatis up-regulates AhR related CYP1A1 gene and epidermal barrier markers in human keratinocytes

Cytochrome P450 CYP1A1 and CYP1B1 enzymes are regulated by the aryl hydrocarbon receptor (AhR), a transcription factor activated by a variety of ligands among which Malassezia metabolites. In this study, we analyzed the modulation of CYP1A1, CYP1B1, and AhR in human keratinocytes infected with different strains of Malassezia pachydermatis, as well as the upregulation of some genes involved in the epidermal homeostasis. We demonstrated that all the strains induced AhR activation and its nuclear translocation in HaCaT cells infected for 24 h, compared to untreated cells. The expression of CYP1A1 and CYP1B1, prototypical markers of the AhR signaling pathway, were upregulated with the level of CYP1A1 mRNA approximately 100-fold greater than that for CYP1B1. Filaggrin, involucrin, and TGaseI, proteins involved in epidermal differentiation, were all modulated by Malassezia pachydermatis strains, with the strongest induction observed for filaggrin. By contrast, quinone oxidoreductase 1 (NQO1), which is part of the antioxidant defense system involved in detoxification, was not modulated in our experimental model. In conclusions, our findings suggest that Malassezia pachydermatis infection of human keratinocytes induces activation of the AhR, and increases the expression of its responsive genes and markers of epidermal differentiation, paving the way for occurrence/exacerbation of pathological skin conditions.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Cervicovaginal Fungi and Bacteria Associated With Cervical Intraepithelial Neoplasia and High-Risk Human Papillomavirus Infections in a Hispanic Population

The human cervicovaginal microbiota resides at an interface between the host and the environment and may affect susceptibility to disease. Puerto Rican women have high human papillomavirus (HPV) infection and cervical cancer rates. We hypothesized that the population structure of the cervicovaginal bacterial and fungal biota changed with cervical squamous intraepithelial lesions and HPV infections. DNA was extracted from cervix, introitus, and anal sites of 62 patients attending high-risk San Juan clinics. The 16S rRNA V4 region and ITS-2 fungal regions were amplified and sequenced using Illumina technology. HPV genotyping was determined by reverse hybridization with the HPV SPF10-LiPA25 kit. HPV prevalence was 84% of which ∼44% subjects were infected with high-risk HPV, ∼35% were co-infected with as many as 9 HPV types and ∼5% were infected with exclusively low-risk HPV types. HPV diversity did not change with cervical dysplasia. Cervical bacteria were more diverse in patients with CIN3 pre-cancerous lesions. We found enrichment of Atopobium vaginae and Gardnerella vaginalis in patients with CIN3 lesions. We found no significant bacterial biomarkers associated with HPV infections. Fungal diversity was significantly higher in cervical samples with high-risk HPV and introitus samples of patients with Atypical Squamous Cells of Undetermined Significance (ASCUS). Fungal biomarker signatures for vagina and cervix include Sporidiobolaceae and Sacharomyces for ASCUS, and Malassezia for high-risk HPV infections. Our combined data suggests that specific cervicovaginal bacterial and fungal populations are related to the host epithelial microenvironment, and could play roles in cervical dysplasia.

The mammalian mycobiome: A complex system in a dynamic relationship with the host

Mammalian barrier surfaces are densely populated by symbiont fungi in much the same way the former are colonized by symbiont bacteria. The fungal microbiota, otherwise known as the mycobiota, is increasingly recognized as a critical player in the maintenance of health and homeostasis of the host. Here we discuss the impact of the mycobiota on host physiology and disease, the factors influencing mycobiota composition, and the current technologies used for identifying symbiont fungal species. Understanding the tripartite interactions among the host, mycobiota, and other members of the microbiota, will help to guide the development of novel prevention and therapeutic strategies for a variety of human diseases.

This article is categorized under:

Physiology > Mammalian Physiology in Health and Disease

Laboratory Methods and Technologies > Genetic/Genomic Methods

Models of Systems Properties and Processes > Organismal Models

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

AHR signaling in the development and function of intestinal immune cells and beyond

The intestinal immune system is challenged daily with the task of recognizing and eliminating pathogens while simultaneously tolerating dietary and commensal antigens. All components must effectively coordinate to differentiate a continual barrage of environmental cues and mount appropriate responses dependent on the nature of the stimuli encountered. Playing a pivotal role, the aryl hydrocarbon receptor (AHR) is a chemical sensor that detects both dietary and microbial cues and is important for development, maintenance, and function of several types of intestinal immune cells, particularly innate lymphoid cells (ILCs) and T cells. In this review, we will highlight recent advances in our knowledge of the role of AHR signaling in ILCs, T cells, B cells, and dendritic cells.

Putting It All Together to Understand the Role of Malassezia spp. in Dandruff Etiology

Dandruff is a common scalp condition causing both a discomfort and an undesired social image. Various studies dating from early 1900s have investigated the condition, but understanding of underlying mechanisms and etiology of the condition is still in its infancy. Formation of dandruff is a common but complex event which has been associated with numerous causal factors. Physiological conditions such as pH, water content, or sebum secretion are some of the host-related factors. An imbalance between these factors can disturb the physiological equilibrium of the scalp that can lead to dandruff formation. However, severity of the condition is strongly related to the lipophilic yeast of the skin microbiota, Malassezia spp. On the other hand, there are recent publications highlighting the role of other scalp microbiota members on dandruff formation. This review investigates the processes leading to the formation of dandruff to provide an etiological description of the condition, with a focus on Malassezia spp.

Upregulation of FLG, LOR, and IVL Expression by Rhodiola crenulata Root Extract via Aryl Hydrocarbon Receptor: Differential Involvement of OVOL1

Rhodiola species are antioxidative, salubrious plants that are known to inhibit oxidative stress induced by ultraviolet and γ-radiation in epidermal keratinocytes. As certain phytochemicals activate aryl hydrocarbon receptors (AHR) or OVO-like 1 (OVOL1) to upregulate the expression of epidermal barrier proteins such as filaggrin (FLG), loricrin (LOR), and involucrin (IVL), we investigated such regulation by Rhodiola crenulata root extract (RCE). We demonstrated that RCE induced FLG and LOR upregulation in an AHR-OVOL1-dependent fashion. However, RCE-mediated IVL upregulation was AHR-dependent but OVOL1-independent. Coordinated upregulation of skin barrier proteins by RCE via AHR may be beneficial in the management of barrier-disrupted inflammatory skin diseases such as atopic dermatitis.

Towards Targeting the Aryl Hydrocarbon Receptor in Cystic Fibrosis

Tryptophan (trp) metabolism is an important regulatory component of gut mucosal homeostasis and the microbiome. Metabolic pathways targeting the trp can lead to a myriad of metabolites, of both host and microbial origins, some of which act as endogenous low-affinity ligands for the aryl hydrocarbon receptor (AhR), a cytosolic, ligand-operated transcription factor that is involved in many biological processes, including development, cellular differentiation and proliferation, xenobiotic metabolism, and the immune response. Low-level activation of AhR by endogenous ligands is beneficial in the maintenance of immune health and intestinal homeostasis. We have defined a functional node whereby certain bacteria species contribute to host/microbial symbiosis and mucosal homeostasis. A microbial trp metabolic pathway leading to the production of indole-3-aldehyde (3-IAld) by lactobacilli provided epithelial protection while inducing antifungal resistance via the AhR/IL-22 axis. In this review, we highlight the role of AhR in inflammatory lung diseases and discuss the possible therapeutic use of AhR ligands in cystic fibrosis.