Inhibition of Aryl Hydrocarbon Receptor Attenuates Hyperglycemia-Induced Hematoma Expansion in an Intracerebral Hemorrhage Mouse Model

Aryl hydrocarbon receptor impairs circadian regulation in Alzheimer's disease: Potential impact on glymphatic system dysfunction

Circadian clocks maintain diurnal rhythms of sleep-wake cycle of 24 h that regulate not only the metabolism of an organism but also many other periodical processes. There is substantial evidence that circadian regulation is impaired in Alzheimer's disease. Circadian clocks regulate many properties known to be disturbed in Alzheimer's patients, such as the integrity of the blood-brain barrier (BBB) as well as the diurnal glymphatic flow that controls waste clearance from the brain. Interestingly, an evolutionarily conserved transcription factor, that is, aryl hydrocarbon receptor (AhR), impairs the function of the core clock proteins and thus could disturb diurnal rhythmicity in the BBB. There is abundant evidence that the activation of AhR signalling inhibits the expression of the major core clock proteins, such as the brain and muscle arnt-like 1 (BMAL1), clock circadian regulator (CLOCK) and period circadian regulator 1 (PER1) in different experimental models. The expression of AhR is robustly increased in the brains of Alzheimer's patients, and protein level is enriched in astrocytes of the BBB. It seems that AhR signalling inhibits glymphatic flow since it is known that (i) activation of AhR impairs the function of the BBB, which is cooperatively interconnected with the glymphatic system in the brain, and (ii) neuroinflammation and dysbiosis of gut microbiota generate potent activators of AhR, which are able to impair glymphatic flow. I will examine current evidence indicating that activation of AhR signalling could disturb circadian functions of the BBB and impair glymphatic flow and thus be involved in the development of Alzheimer's pathology.

Empirical analysis of lead neurotoxicity mode of action and its application in health risk assessment

The mode of action (MOA) framework is proposed to inform a biological link between chemical exposures and adverse health effects. Despite a significant increase in knowledge and awareness, the application of MOA in human health risk assessment (RA) remains limited. This study aims to discuss the adoption of MOA for health RA within a regulatory context, taking our previously proposed but not yet validated MOA for lead neurotoxicity as an example. We first conducted a quantitative weight of evidence (qWOE) assessment, which revealed that the MOA has a moderate confidence. Then, targeted bioassays were performed within an in vitro blood-brain barrier (BBB) model to quantitatively validate the scientific validity of key events (KEs) in terms of essentiality and concordance of empirical support (dose/temporal concordance), which increases confidence in utilizing the MOA for RA. Building upon the quantitative validation data, we further conducted benchmark dose (BMD) analysis to map dose-response relationships for the critical toxicity pathways, and the lower limit of BMD at a 5% response (BMDL5) was identified as the point of departure (POD) value for adverse health effects. Notably, perturbation of the Aryl Hydrocarbon Receptor (AHR) signaling pathway exhibited the lowest POD value, measured at 0.0062 μM. Considering bioavailability, we further calculated a provisional health-based guidance value (HBGV) for children's lead intake, determining it to be 2.56 μg/day. Finally, the health risk associated with the HBGV was assessed using the hazard quotient (HQ) approach, which indicated that the HBGV established in this study is a relative safe reference value for lead intake. In summary, our study described the procedure for utilizing MOA in health RA and set an example for MOA-based human health risk regulation.

Autonomic dysfunction and treatment strategies in intracerebral hemorrhage

AIMS

Autonomic dysfunction with central autonomic network (CAN) damage occurs frequently after intracerebral hemorrhage (ICH) and contributes to a series of adverse outcomes. This review aims to provide insight and convenience for future clinical practice and research on autonomic dysfunction in ICH patients.

DISCUSSION

We summarize the autonomic dysfunction in ICH from the aspects of potential mechanisms, clinical significance, assessment, and treatment strategies. The CAN structures mainly include insular cortex, anterior cingulate cortex, amygdala, hypothalamus, nucleus of the solitary tract, ventrolateral medulla, dorsal motor nucleus of the vagus, nucleus ambiguus, parabrachial nucleus, and periaqueductal gray. Autonomic dysfunction after ICH is closely associated with neurological functional outcomes, cardiac complications, blood pressure fluctuation, immunosuppression and infection, thermoregulatory dysfunction, hyperglycemia, digestive dysfunction, and urogenital disturbances. Heart rate variability, baroreflex sensitivity, skin sympathetic nerve activity, sympathetic skin response, and plasma catecholamine concentration can be used to assess the autonomic functional activities after ICH. Risk stratification of patients according to autonomic functional activities, and development of intervention approaches based on the restoration of sympathetic-parasympathetic balance, would potentially improve clinical outcomes in ICH patients.

CONCLUSION

The review systematically summarizes the evidence of autonomic dysfunction and its association with clinical outcomes in ICH patients, proposing that targeting autonomic dysfunction could be potentially investigated to improve the clinical outcomes.

The Role of Gut Microbiota in Blood-Brain Barrier Disruption after Stroke

Growing evidence has proved that alterations in the gut microbiota have been linked to neurological disorders including stroke. Structural and functional disruption of the blood-brain barrier (BBB) is observed after stroke. In this context, there is pioneering evidence supporting that gut microbiota may be involved in the pathogenesis of stroke by regulating the BBB function. However, only a few experimental studies have been performed on stroke models to observe the BBB by altering the structure of gut microbiota, which warrant further exploration. Therefore, in order to provide a novel mechanism for stroke and highlight new insights into BBB modification as a stroke intervention, this review summarizes existing evidence of the relationship between gut microbiota and BBB integrity and discusses the mechanisms of gut microbiota on BBB dysfunction and its role in stroke.

Host-Microbiome Interactions: Tryptophan Metabolism and Aromatic Hydrocarbon Receptors after Traumatic Brain Injury

Traumatic brain injury refers to the damage caused to intracranial tissues by an external force acting on the head, leading to both immediate and prolonged harmful effects. Neuroinflammatory responses play a critical role in exacerbating the primary injury during the acute and chronic phases of TBI. Research has demonstrated that numerous neuroinflammatory responses are mediated through the "microbiota-gut-brain axis," which signifies the functional connection between the gut microbiota and the brain. The aryl hydrocarbon receptor (AhR) plays a vital role in facilitating communication between the host and microbiota through recognizing specific ligands produced directly or indirectly by the microbiota. Tryptophan (trp), an indispensable amino acid in animals and humans, represents one of the key endogenous ligands for AhR. The metabolites of trp have significant effects on the functioning of the central nervous system (CNS) through activating AHR signalling, thereby establishing bidirectional communication between the gut microbiota and the brain. These interactions are mediated through immune, metabolic, and neural signalling mechanisms. In this review, we emphasize the co-metabolism of tryptophan in the gut microbiota and the signalling pathway mediated by AHR following TBI. Furthermore, we discuss the impact of these mechanisms on the underlying processes involved in traumatic brain injury, while also addressing potential future targets for intervention.

Apolipoprotein E mimetic peptide COG1410 alleviates blood‑brain barrier injury in a rat model of ischemic stroke

Blood‑brain barrier (BBB) damage is one of the main causes of poor outcomes and increased mortality rates following cerebral ischemia‑reperfusion injury. Apolipoprotein E (ApoE) and its mimetic peptide have been previously reported to exhibit potent neuroprotective properties in various central nervous system disease models. Therefore, the present study aimed to investigate the possible role of the ApoE mimetic peptide COG1410 in cerebral ischemia‑reperfusion injury and its potential underlying mechanism. Male SD rats were subjected to 2 h middle cerebral artery occlusion followed by 22 h reperfusion. Evans blue leakage and IgG extravasation assays results revealed that COG1410 treatment significantly reduced BBB permeability. In addition, in situ zymography and western blotting were used to prove that COG1410 was able to downregulate the activities of MMPs and upregulate the expression of occludin in the ischemic brain tissue samples. Subsequently, COG1410 was found to significantly reverse microglia activation while also suppressing inflammatory cytokine production, according to immunofluorescence signal of Iba‑1 and CD68 and protein expression of COX‑2. Consequently, this neuroprotective mechanism mediated by COG1410 was further tested using the BV2 cell line in vitro, which was exposed to oxygen glucose deprivation followed by reoxygenation. The mechanism of COG1410 was found to be mediated, as least partly, through the activation of triggering receptor expressed on myeloid cells 2. In conclusion, the data suggest that COG1410 can alleviate BBB injury and neuroinflammation following ischemic stroke.

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.

Kynurenine/Aryl Hydrocarbon Receptor Modulates Mitochondria-Mediated Oxidative Stress and Neuronal Apoptosis in Experimental Intracerebral Hemorrhage

Aims: Oxidative stress and neuronal apoptosis play crucial roles in the pathological processes of secondary injury after intracerebral hemorrhage (ICH). Aryl hydrocarbon receptor (AHR), together with its endogenous ligand kynurenine, is known to mediate free radical accumulation and neuronal excitotoxicity in central nervous systems. Herein, we investigate the pathological roles of kynurenine/AHR after ICH. Results: Endogenous AHR knockout alleviated reactive oxygen species accumulation and neuronal apoptosis in ipsilateral hemisphere at 48 h after ICH in mice. The ICH insult resulted in an increase of total and nucleus AHR protein levels and AHR transcriptional activity. Inhibition of AHR provided both short- and long- term neurological benefits by attenuating mitochondria-mediated oxidative stress and neuronal apoptosis after ICH in mice. RhoA-Bax signaling activated mitochondrial death pathway and participated in deleterious actions of AHR. Finally, we reported that exogenous kynurenine aggravated AHR activation and mediated the brain mentioned earlier. Male animals were used in the experiments. Innovation: We show for the first time that kynurenine/AHR mediates mitochondria death and free radical accumulation, at least partially via the RhoA/Bax signaling pathway. Pharmacological antagonists of AHR and kynurenine may ameliorate neurobehavioral function and improve the prognosis of patients with ICH. Conclusion: Kynurenine/AHR may serve as a potential therapeutic target to attenuate mitochondria-mediated oxidative stress and neuronal cells impairment in patients with ICH. Antioxid. Redox Signal. 37, 1111-1129.