An Endogenous Ligand of Aryl Hydrocarbon Receptor 6-Formylindolo[3,2-b]Carbazole (FICZ) Is a Signaling Molecule in Neurogenesis of Adult Hippocampal Neurons

Adverse neurodevelopment in children associated with prenatal exposure to fine particulate matter (PM2.5) - Possible roles of polycyclic aromatic hydrocarbons (PAHs) and mechanisms involved

Prenatal exposure to ambient fine particles (PM2.5) and polycyclic aromatic hydrocarbons (PAHs) has been associated with adverse birth outcomes including neurodevelopmental effects with cognitive and/or behavioral implications in early childhood. As a background we first briefly summarize human studies on PM2.5 and PAHs associated with adverse birth outcomes and modified neurodevelopment. Next, we add more specific information from animal studies and in vitro studies and elucidate possible biological mechanisms. More specifically we focus on the potential role of PAHs attached to PM2.5 and explore whether effects of these compounds may arise from disturbance of placental function or more directly by interfering with neurodevelopmental processes in the fetal brain. Possible molecular initiating events (MIEs) include interactions with cellular receptors such as the aryl hydrocarbon receptor (AhR), beta-adrenergic receptors (βAR) and transient receptor potential (TRP)-channels resulting in altered gene expression. MIE linked to the binding of PAHs to cytochrome P450 (CYP) enzymes and formation of reactive electrophilic metabolites are likely less important. The experimental animal and in vitro studies support the epidemiological findings and suggest steps involved in mechanistic pathways explaining the associations. An overall evaluation of the doses/concentrations used in experimental studies combined with the mechanistic understanding further supports the hypothesis that prenatal PAHs exposure may cause adverse outcomes (AOs) linked to human neurodevelopment. Several MIEs will likely occur simultaneously in various cells/tissues involving several key events (KEs) which relative importance will depend on dose, time, tissue, genetics, other environmental factors, and neurodevelopmental endpoint in study.

Involvement of Nrf2 Signaling in Lead-induced Toxicity

Nuclear factor erythroid 2-related factor 2 (Nrf2) is used as one of the main protective factors against various pathological processes, as it regulates cells resistant to oxidation. Several studies have extensively explored the relationship between environmental exposure to heavy metals, particularly lead (Pb), and the development of various human diseases. These metals have been reported to be able to, directly and indirectly, induce the production of reactive oxygen species (ROS) and cause oxidative stress in various organs. Since Nrf2 signaling is important in maintaining redox status, it has a dual role depending on the specific biological context. On the one hand, Nrf2 provides a protective mechanism against metal-induced toxicity; on the other hand, it can induce metalinduced carcinogenesis upon prolonged exposure and activation. Therefore, the aim of this review was to summarize the latest knowledge on the functional interrelation between toxic metals, such as Pb and Nrf2 signaling.

Deletion of AhR attenuates fear memory leaving other types of memory intact

The aryl hydrocarbon receptor (AhR), a classic "environmental sensor", has been found to play an important role in cognitive and emotional function. Recent studies showed AhR deletion led to an attenuated fear memory, providing a potential target against fear memory, whether it is the consequence of attenuated sense of fear or memory ability deficit or both is unclear. Here this study aims to work this out. The freezing time in contextual fear conditioning (CFC) reduced significantly in AhR knockout mice, indicating an attenuated fear memory. Hot plate test and acoustic startle reflex showed that AhR knockout did not change the pain threshold and hearing, excluded the possibility of sensory impairments. Results from NORT, MWM and SBT showed that deletion of AhR had little effects on other types of memory. But the anxiety-like behaviors reduced both in naïve or suffered (tested after CFC) AhR knockout mice, showing that AhR-deficient mice have a reduced basal and stressful emotional response. The basal low-frequency to high-frequency (LF/HF) ratio of the AhR knockout mice was significantly lower than that of the control group, indicating lower sympathetic excitability in the basal state, suggesting a low level of basal stress in the knockout mice. Before and after CFC, the LF/HF ratio of AhR-KO mice tended to be significantly lower than that of WT mice, and their heart rate was significantly lower; and the AhR-KO mice also has a decreased serum corticosterone level after CFC, suggesting a reduced stress response in AhR knockout mice. Altogether, the basal stress level and stress response were significant reduced in AhR knockout mice, which might contribute to the attenuated fear memory with little impairment on other types of memory, suggesting AhR as a "psychologic sensor" additional to "environmental sensor".

Modulating AHR function offers exciting therapeutic potential in gut immunity and inflammation

Aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a classical exogenous synthetic ligand of AHR that has significant immunotoxic effects. Activation of AHR has beneficial effects on intestinal immune responses, but inactivation or overactivation of AHR can lead to intestinal immune dysregulation and even intestinal diseases. Sustained potent activation of AHR by TCDD results in impairment of the intestinal epithelial barrier. However, currently, AHR research has been more focused on elucidating physiologic AHR function than on dioxin toxicity. The appropriate level of AHR activation plays a role in maintaining gut health and protecting against intestinal inflammation. Therefore, AHR offers a crucial target to modulate intestinal immunity and inflammation. Herein, we summarize our current understanding of the relationship between AHR and intestinal immunity, the ways in which AHR affects intestinal immunity and inflammation, the effects of AHR activity on intestinal immunity and inflammation, and the effect of dietary habits on intestinal health through AHR. Finally, we discuss the therapeutic role of AHR in maintaining gut homeostasis and relieving inflammation.

AhR and Wnt/β-Catenin Signaling Pathways and Their Interplay

As evolutionarily conserved signaling cascades, AhR and Wnt signaling pathways play a critical role in the control over numerous vital embryonic and somatic processes. AhR performs many endogenous functions by integrating its signaling pathway into organ homeostasis and into the maintenance of crucial cellular functions and biological processes. The Wnt signaling pathway regulates cell proliferation, differentiation, and many other phenomena, and this regulation is important for embryonic development and the dynamic balance of adult tissues. AhR and Wnt are the main signaling pathways participating in the control of cell fate and function. They occupy a central position in a variety of processes linked with development and various pathological conditions. Given the importance of these two signaling cascades, it would be interesting to elucidate the biological implications of their interaction. Functional connections between AhR and Wnt signals take place in cases of crosstalk or interplay, about which quite a lot of information has been accumulated in recent years. This review is focused on recent studies about the mutual interactions of key mediators of AhR and Wnt/β-catenin signaling pathways and on the assessment of the complexity of the crosstalk between the AhR signaling cascade and the canonical Wnt pathway.

Pteris laeta Wall. and Its New Phytochemical, Pterosinsade A, Promote Hippocampal Neurogenesis via Activating the Wnt Signaling Pathway

Pteris laeta Wall., as a traditional tea, is popular in Southwest China, but its role in preventing cognitive impairment is unclear. In this study, Pteris laeta Wall. extracts (PW) and its active compounds were evaluated for preventive effects on Alzheimer's disease (AD) in vivo and in vitro. The results showed that PW diminished oxidative stress damage and apoptosis of Aβ-induced HT22 cells and also rescued cognitive deficits, and ameliorated pathological injury and inflammatory response in APP/PS1 mice. Besides, a new pterosin sesquiterpene, named pterosinsade A (PA), and nine known compounds were discovered from the EtOAc extract that possessed the best neuroprotective activity. PA reduced apoptosis of APP-overexpressing neural stem cells and promoted their proliferation and neuronal differentiation. Meanwhile, PW and PA promoted hippocampal neurogenesis, which proved to be associated with activating the Wnt signaling pathway. These findings suggest that PW and PA are candidates for AD prevention.

Activation of aryl hydrocarbon receptor (AhR) in Alzheimer's disease: role of tryptophan metabolites generated by gut host-microbiota

Gut microbiota in interaction with intestinal host tissues influences many brain functions and microbial dysbiosis has been linked with brain disorders, such as neuropsychiatric conditions and Alzheimer's disease (AD). L-tryptophan metabolites and short-chained fatty acids (SCFA) are major messengers in the microbiota-brain axis. Aryl hydrocarbon receptors (AhR) are main targets of tryptophan metabolites in brain microvessels which possess an enriched expression of AhR protein. The Ah receptor is an evolutionarily conserved, ligand-activated transcription factor which is not only a sensor of xenobiotic toxins but also a pleiotropic regulator of both developmental processes and age-related tissue degeneration. Major microbiota-produced tryptophan metabolites involve indole derivatives, e.g., indole 3-pyruvic acid, indole 3-acetaldehyde, and indoxyl sulfate, whereas indoleamine and tryptophan 2,3-dioxygenases (IDO/TDO) of intestine host cells activate the kynurenine (KYN) pathway generating KYN metabolites, many of which are activators of AhR signaling. Chronic kidney disease (CKD) increases the serum level of indoxyl sulfate which promotes AD pathogenesis, e.g., it disrupts integrity of blood-brain barrier (BBB) and impairs cognitive functions. Activation of AhR signaling disturbs vascular homeostasis in brain; (i) it controls blood flow via the renin-angiotensin system, (ii) it inactivates endothelial nitric oxide synthase (eNOS), thus impairing NO production and vasodilatation, and (iii) it induces oxidative stress, stimulates inflammation, promotes cellular senescence, and enhances calcification of vascular walls. All these alterations are evident in cerebral amyloid angiopathy (CAA) in AD pathology. Moreover, AhR signaling can disturb circadian regulation and probably affect glymphatic flow. It seems plausible that dysbiosis of gut microbiota impairs the integrity of BBB via the activation of AhR signaling and thus aggravates AD pathology. KEY MESSAGES: Dysbiosis of gut microbiota is associated with dementia and Alzheimer's disease. Tryptophan metabolites are major messengers from the gut host-microbiota to brain. Tryptophan metabolites activate aryl hydrocarbon receptor (AhR) signaling in brain. The expression of AhR protein is enriched in brain microvessels and blood-brain barrier. Tryptophan metabolites disturb brain vascular integrity via AhR signaling. Dysbiosis of gut microbiota promotes inflammation and AD pathology via AhR signaling.

Jujuboside A promotes proliferation and neuronal differentiation of APPswe-overexpressing neural stem cells by activating Wnt/β-catenin signaling pathway

Mobilization of hippocampal neurogenesis has been considered as a potential strategy for the treatment of neurodegenerative diseases, including Alzheimer's disease (AD). In present study, we evaluated both the neuroprotective effects and the effects on the proliferation and differentiation of APP-overexpressing neural stem cells (APP-NSCs) by Jujuboside A (JuA) in vitro. Our results demonstrated that JuA (50 μM) decreased apoptosis and suppressed oxidative stress damage of APP-NSCs. JuA (50 μM) upregulated the secretion of brain-derived neurotrophic factor and promoted the proliferation and neuronal differentiation of APP-NSCs. Moreover, JuA (50 μM) upregulated Wnt-3a and β-catenin protein expression, and enhanced the expression of downstream genes Ccnd1, Neurod1 and Prox1. However, XAV-939, an inhibitor of the Wnt/β-catenin signaling pathway, inhibited these positive effects of JuA. Taken together, these findings suggest that JuA promote proliferation and neuronal differentiation of APP-NSCs partly by activating the Wnt/β-catenin signaling pathway. We hope that this study will provide a viable strategy for the treatment of AD.

The interplay of aryl hydrocarbon receptor/WNT/CTNNB1/Notch signaling pathways regulate amyloid beta precursor mRNA/protein expression and effected the learning and memory of mice

The amyloid beta precursor protein (APP) plays a pathophysiological role in the development of Alzheimer's disease as well as a physiological role in neuronal growth and synaptogenesis. The aryl hydrocarbon receptor (AhR)/WNT/Catenin Beta 1 (CTNNB1)/Notch signaling pathways stamp in many functions, including development and growth of neurons. However, the regulatory role of AhR-/WNT-/CTNNB1-/Notch-induced APP expression and its influence on hippocampal-dependent learning and memory deficits is not clear. Male BALB/C mice received 6-formylindolo[3,2-b]carbazole (an AhR agonist), CH223191(an AhR antagonist), DAPT (an inhibitor of Notch signaling), and XAV-939 (a WNT pathway inhibitor) at a single dose of 100 μg/kg, 1, 5 , and 5 mg/kg of body weight, respectively, via intraperitoneal injection alone or in combination. Gene expression analyses and protein assay were performed on the 7th and 29th days. To assess the hippocampal-dependent memory, all six mice also underwent contextual fear conditioning on the 28th day after treatments. Our results showed that endogenous ligand of AhR has a regulatory effect on APP gene. Also, the interaction of AhR/WNT/CTNNB1 has a positive regulatory effect, but Notch has a negative regulatory effect on the mRNA and protein expression of APP, which have a correlation with mice's learning skills and memory.

Influence of cellular redox environment on aryl hydrocarbon receptor ligands induced melanogenesis

Many environmental pollutants, natural compounds, as well as endogenous chemicals exert their biological/toxicological effects by reacting with the aryl hydrocarbon receptor (AhR). Previous evidence shed new light on the role of AhR in skin physiology by regulating melanin production. In this study, we investigated the effect of oxidative imbalance induced by AhR ligands on the melanogenesis process in B16 murine melanoma cells. Exposure to 6-formylindolo[3,2-b] carbazole (FICZ) or benzo-α-pyrene (BαP) led to enhanced expression of CTNNB1, MITF, and TYR genes following increased tyrosinase enzyme activity and melanin content in an AhR-dependent manner. Analysis of the presence of reactive oxygen species (ROS) as well as reduced glutathione (GSH) / oxidized glutathione (GSSG) ratio revealed that treatment with AhR ligands is associated with oxidative stress which can be ameliorated with NAC (N-acetyl cysteine) or diphenyleneiodonium chloride (DPI). On the other hand, NAC and DPI enhanced melanogenesis induced by AhR ligands by reducing the level of ROS. We have shown for the first time that a cellular redox status is a critical event during AhR ligand-induced melanogenesis.

The Role of AhR in the Hallmarks of Brain Aging: Friend and Foe

In recent years, aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, has been considered to be involved in aging phenotypes across several species. This receptor is a highly conserved biosensor that is activated by numerous exogenous and endogenous molecules, including microbiota metabolites, to mediate several physiological and toxicological functions. Brain aging hallmarks, which include glial cell activation and inflammation, increased oxidative stress, mitochondrial dysfunction, and cellular senescence, increase the vulnerability of humans to various neurodegenerative diseases. Interestingly, many studies have implicated AhR signaling pathways in the aging process and longevity across several species. This review provides an overview of the impact of AhR pathways on various aging hallmarks in the brain and the implications for AhR signaling as a mechanism in regulating aging-related diseases of the brain. We also explore how the nature of AhR ligands determines the outcomes of several signaling pathways in brain aging processes.

Aryl hydrocarbon receptor: Its roles in physiology

Aryl hydrocarbon receptor (AHR) was initially discovered as a cellular protein involved in mediating the detoxification of xenobiotic compounds. Extensive research in the past two decades has identified several families of physiological ligands and uncovered important functions of AHR in normal development and homeostasis. Deficiency in AHR expression disrupts major signaling systems and transcriptional programs, which appear to be responsible for the development of numerous developmental abnormalities including cardiac hypertrophy and epidermal hyperplasia. This mini review primarily summarizes recent advances in our understanding of AHR functions in normal physiology with an emphasis on the cardiovascular, gastrointestinal, integumentary, nervous, and immunomodulatory systems.