Role of an interdependent Wnt, GSK3-β/β-catenin and HB-EGF/EGFR mechanism in arsenic-induced hippocampal neurotoxicity in adult mice

We previously reported the neurotoxic effects of arsenic in the hippocampus. Here, we explored the involvement of Wnt pathway, which contributes to neuronal functions. Administering environmentally relevant arsenic concentrations to postnatal day-60 (PND60) mice demonstrated a dose-dependent increase in hippocampal Wnt3a and its components, Frizzled, phospho-LRP6, Dishevelled and Axin1 at PND90 and PND120. However, p-GSK3-β(Ser9) and β-catenin levels although elevated at PND90, decreased at PND120. Additionally, treatment with Wnt-inhibitor, rDkk1, reduced p-GSK3-β(Ser9) and β-catenin at PND90, but failed to affect their levels at PND120, indicating a time-dependent link with Wnt. To explore other underlying factors, we assessed epidermal growth factor receptor (EGFR) pathway, which interacts with GSK3-β and appears relevant to neuronal functions. We primarily found that arsenic reduced hippocampal phosphorylated-EGFR and its ligand, Heparin-binding EGF-like growth factor (HB-EGF), at both PND90 and PND120. Moreover, treatment with HB-EGF rescued p-GSK3-β(Ser9) and β-catenin levels at PND120, suggesting their HB-EGF/EGFR-dependent regulation at this time point. Additionally, rDkk1, LiCl (GSK3-β-activity inhibitor), or β-catenin protein treatments induced a time-dependent recovery in HB-EGF, indicating potential inter-dependent mechanism between hippocampal Wnt/β-catenin and HB-EGF/EGFR following arsenic exposure. Fluorescence immunolabeling then validated these findings in hippocampal neurons. Further exploration of hippocampal neuronal survival and apoptosis demonstrated that treatment with rDkk1, LiCl, β-catenin and HB-EGF improved Nissl staining and NeuN levels, and reduced cleaved-caspase-3 levels in arsenic-treated mice. Supportively, we detected improved Y-Maze and Passive Avoidance performances for learning-memory functions in these mice. Overall, our study provides novel insights into Wnt/β-catenin and HB-EGF/EGFR pathway interaction in arsenic-induced hippocampal neurotoxicity.

Arsenic Exposure Induces Neuro-immune Toxicity in the Cerebral Cortex and the Hippocampus via Neuroglia and NLRP3 Inflammasome Activation in C57BL/6 Mice

This study aimed to examine the immuntoxic effects of arsenic in the nervous system. Our results showed that arsenic increased corticocerebral and hippocampal weights (p < 0.05). Morris water maze tests revealed that arsenic significantly increased the time spent in latency to platform on the fourth day in 50 mg/L arsenic exposure and the fifth day in 25 and 50 mg/L arsenic exposure, as well as reduced the path length in target quadrant, time spent in target quadrant, and crossing times of the platform (p < 0.05). Hematoxylin-eosin staining showed that the vacuolated degeneration and pyknosis was found in the cerebral cortex and hippocampus of arsenic-treated mice. The mRNA levels of corticocerebral and hippocampal brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) were decreased in the 50 mg/L arsenic-treated group (p < 0.05). In addition, immunofluorescence staining showed that 25 and 50 mg/L arsenic all increased the expression of CD11b and glial fibrillary acidic protein (GFAP) in the cerebral cortex and hippocampus (p < 0.05). Arsenic markedly raised antigen-presenting molecule MHCII and CD40 mRNA levels in the cerebral cortex and hippocampus and upregulated the cell chemokine receptor 5 (CCR5) and CCR7 mRNA levels in the cerebral cortex at the 50 mg/L arsenic group, and increased the CCR7 mRNA levels in the hippocampus at the 25 and 50 mg/L arsenic groups (p < 0.05). Arsenic activated the nucleotide-binding domain-like receptor protein-3 (NLRP3) inflammasome, and enhanced its upstream promoter NF-κB protein level and downstream regulators IL-18 mRNA levels. Collectively, these results provide new evidences for the neuro-immune toxicity of arsenic.

Endothelial Notch Signaling Regulates the Function of the Retinal Pigment Epithelial Barrier via EC Angiocrine Signaling

The outer blood-retina barrier (oBRB), comprises tightly connected retinal pigment epithelium (RPE) cells, Bruch's membrane, and choroid blood vessels, and is essential for retinal health and normal visual function. Disruption of the RPE barrier and its dysfunction can lead to retinal disorders such as age-related macular degeneration (AMD). In the present study, we investigated the essential role of choroid endothelial cells (ECs) in the RPE barrier formation process and its dysfunction. We discovered that ECs promoted RPE barrier formation through angiocrine signaling. Through blocking or activating endothelial Notch signaling and conducting experiments in vitro and in vivo, we confirmed that endothelial Notch signaling regulated the expression of heparin-binding epidermal growth factor (HBEGF) and consequently impacted the expression and activity of matrix metalloproteinases (MMP)-9 in RPE cells. This modulation influenced the RPE extracellular matrix deposition, tight junctions and RPE barrier function. In in vivo experiments, the intravitreal administration of recombinant HBEGF (r-HBEGF) alleviated the RPE barrier disruption induced by subretinal injection (SI) or laser treatment and also rescued RPE barrier disruption in endothelial Notch-deficient mice. Our results showed that the endothelial Notch signaling drove HBEGF expression through angiocrine signaling and effectively improved RPE barrier function by regulating the MMP-9 expression in RPE cells. It suggests that the modulation of Notch signaling in the choroidal endothelium may offer a novel therapeutic strategy for retinal degenerative diseases.

The disruption of blood-brain barrier induced by long-term arsenic exposure is associated with the increase of MMP-9 and MMP-2: The characteristics are similar to those caused by senescence

Accumulating evidence suggests that Matrix metalloproteinase-9 (MMP-9) and -2 (MMP-2) are involved in the neuropathological processes by contributing to breaking the extracellular matrix and the tight junctions that constitute the blood-brain barrier (BBB). However, the influences of arsenic (As) on these two MMPs were inconsistent. In the cross-sectional study of 500 adults, serum MMP-2 and MMP-9 positively correlated with urine arsenic. And the positive correlation between urine tAs and serum MMP-9/2 was found in people older than 59 years. In vivo studies, we found that arsenic exposure or senescence might decrease number of neurons and neuritic density and increase serum and cortical MMP-9/2 levels. Furthermore, arsenic exposure or senescence could disrupt the tight junction of BBB and elevate MMP-9 and MMP-2 expression in the cerebral microvascular endothelium. The MMP-9 and MMP-2 are of particular interest when researching the link between arsenic exposure and nerve damage.

Mechanisms Associated with Cognitive and Behavioral Impairment Induced by Arsenic Exposure

Arsenic (As) is a metalloid naturally present in the environment, in food, water, soil, and air; however, its chronic exposure, even with low doses, represents a public health concern. For a long time, As was used as a pigment, pesticide, wood preservative, and for medical applications; its industrial use has recently decreased or has been discontinued due to its toxicity. Due to its versatile applications and distribution, there is a wide spectrum of human As exposure sources, mainly contaminated drinking water. The fact that As is present in drinking water implies chronic human exposure to this metalloid; it has become a worldwide health problem, since over 200 million people live where As levels exceed safe ranges. Many health problems have been associated with As chronic exposure including cancer, cardiovascular diseases, gastrointestinal disturbances, and brain dysfunctions. Because As can cross the blood-brain barrier (BBB), the brain represents a target organ where this metalloid can exert its long-term toxic effects. Many mechanisms of As neurotoxicity have been described: oxidative stress, inflammation, DNA damage, and mitochondrial dysfunction; all of them can converge, thus leading to impaired cellular functions, cell death, and in consequence, long-term detrimental effects. Here, we provide a current overview of As toxicity and integrated the global mechanisms involved in cognitive and behavioral impairment induced by As exposure show experimental strategies against its neurotoxicity.

Neurotoxicity and the Global Worst Pollutants: Astroglial Involvement in Arsenic, Lead, and Mercury Intoxication

Environmental pollution is a global threat and represents a strong risk factor for human health. It is estimated that pollution causes about 9 million premature deaths every year. Pollutants that can cross the blood-brain barrier and reach the central nervous system are of special concern, because of their potential to cause neurological and development disorders. Arsenic, lead and mercury are usually ranked as the top three in priority lists of regulatory agencies. Against xenobiotics, astrocytes are recognised as the first line of defence in the CNS, being involved in virtually all brain functions, contributing to homeostasis maintenance. Here, we discuss the current knowledge on the astroglial involvement in the neurotoxicity induced by these pollutants. Beginning by the main toxicokinetic characteristics, this review also highlights the several astrocytic mechanisms affected by these pollutants, involving redox system, neurotransmitter and glucose metabolism, and cytokine production/release, among others. Understanding how these alterations lead to neurological disturbances (including impaired memory, deficits in executive functions, and motor and visual disfunctions), by revisiting the current knowledge is essential for future research and development of therapies and prevention strategies.

Arsenic Induces Blood‒Brain Barrier Disruption and Regulates T Lymphocyte Subpopulation Differentiation in the Cerebral Cortex and Hippocampus Associated with the Nrf2 Pathway In Vivo

Increasing evidence has confirmed that the nervous system shows innate and adaptive immunity, which also participates in nerve damage. This study aimed to explore the neuroimmune imbalance induced by arsenic and its possible mechanism. Mice were exposed to NaAsO₂ (0, 5, 10, 25, and 50 mg/L) for 1 month by drinking water. Y-maze and Morris water maze tests revealed that arsenic impaired learning and memory. The optical density of Evans blue showed a marked dose-dependent increase in the brain, and the mRNA and protein levels of the BBB tight junctions (TJs), occludin at 25 and 50 mg/L arsenic, and claudin-5 at 50 mg/L arsenic, were markedly decreased in the cerebral cortex. Arsenic downregulated occludin and claudin-5 mRNA expression at 50 mg/L and protein expression at 25 and 50 mg/L in the hippocampus. Immunohistochemical staining showed that 50 mg/L arsenic increased corticocerebral and hippocampal CD3⁺ T, CD4⁺ T, and CD8⁺ T cells; CD4 and CD8 proteins were increased with 25 and 50 mg/L arsenic. Arsenic decreased the corticocerebral and hippocampal Th1, Th17, and regulatory Treg transcription factors T-bet, Rorγt, and Foxp3 and the cytokine IFN-γ, IL-17, and TGF-β mRNA levels and increased the Th2 transcription factor GATA3 and cytokine IL-4 mRNA levels. Moreover, arsenic enhanced the expression of nuclear factor E2-related factor (Nrf2) and its downstream enzymes heme oxygenase-1 (HO-1) and glutathione-S-transferase (GST). In conclusion, these results demonstrate that arsenic exposure induces BBB dysfunction and T lymphocyte infiltration and affects CD4⁺ T lymphocyte differentiation, which may be associated with Nrf2 activation.

SOX2 modulated astrocytic process plasticity is involved in arsenic-induced metabolic disorders

Metabolic disorders induced by arsenic exposure have attracted great public concern. However, it remains unclear whether hypothalamus-based central regulation mechanisms are involved in this process. Here, we exposed mice to 100 μg/L arsenic in drinking water and established a chronic arsenic exposure model. Our study revealed that chronic arsenic exposure caused metabolic disorders in mice including impaired glucose metabolism and decreased energy expenditure. Arsenic exposure also impaired glucose sensing and the activation of proopiomelanocortin (POMC) neurons in the hypothalamus. In particular, arsenic exposure damaged the plasticity of hypothalamic astrocytic process. Further research revealed that arsenic exposure inhibited the expression of sex-determining region Y-Box 2 (SOX2), which decreased the expression level of insulin receptors (INSRs) and the phosphorylation of AKT. The conditional deletion of astrocytic SOX2 exacerbated arsenic-induced effects on metabolic disorders, the impairment of hypothalamic astrocytic processes, and the inhibition of INSR/AKT signaling. Furthermore, the arsenic-induced impairment of astrocytic processes and inhibitory effects on INSR/AKT signaling were reversed by SOX2 overexpression in primary hypothalamic astrocytes. Together, we demonstrated here that chronic arsenic exposure caused metabolic disorders by impairing SOX2-modulated hypothalamic astrocytic process plasticity in mice. Our study provides evidence of novel central regulatory mechanisms underlying arsenic-induced metabolic disorders and emphasizes the crucial role of SOX2 in regulating the process plasticity of adult astrocytes.

Serum Epidermal Growth Factor is Low in Schizophrenia and Not Affected by Antipsychotics Alone or Combined With Electroconvulsive Therapy

BACKGROUND

Epidermal growth factor (EGF) is implicated in the pathogenesis of schizophrenia, suggesting possible value as a biomarker for disease severity or treatment response. However, basal EGF levels and changes during treatment are inconsistent across studies. The goal of this study is to compare serum EGF in schizophrenia patients before and after treatment with antipsychotics alone or combined with electroconvulsive therapy (ECT).

METHOD

Patients meeting DSM-IV diagnostic criteria for schizophrenia were recruited from June 2013 to December 2015 (n = 186) and followed up after 8 weeks of treatment with antipsychotics alone (n = 119, drug group) or combined with ECT (n = 67, ECT group). Serum EGF levels were measured by ELISA and compared among patients and 74 healthy control subjects. Psychopathology and clinical effects were assessed using the Positive and Negative Syndrome Scale (PANSS).

RESULTS

Basal serum EGF was significantly lower in the entire patient cohort compared to healthy controls (P < 0.05). Repeated-measures ANOVA showed no main effect of time (F = 1.273; P = 0.261), time × group interaction (F = 1.228; P = 0.270), main effect of clinical response (F = 0.191; P = 0.663), or group × clinical interaction (F = 1.765; P = 0.186) on serum EGF. Serum EGF levels did not change significantly following antipsychotic drug or combined therapy (P > 0.05). Additionally, neither basal EGF nor EGF change was associated with the clinical response to drug or combined treatment (P > 0.05). However, baseline serum EGF was weakly associated with PANSS positive score (pretreatment: r = 0.206, posttreatment: r = 0.201) and general symptom score (pretreatment: r = -0.244). Serum EGF was also associated with duration of illness (pretreatment: r = 0.285, posttreatment: r = -0.231).

CONCLUSIONS

Serum EGF concentration is low in schizophrenia but is unchanged following treatment with antipsychotics alone or combined with ECT, regardless of clinical response. Thus, serum EGF is not a surrogate biomarker for treatment response and is unlikely to be involved in the therapeutic mechanisms of antipsychotics or ECT.

H2S restores the cardioprotective effects of ischemic post-conditioning by upregulating HB-EGF/EGFR signaling

Hydrogen sulfide (H₂S) reduces ischemia/reperfusion (I/R) injury and apoptosis and restores the cardioprotective effects of ischemic post-conditioning (PC) in aged cardiomyocytes by inhibiting oxidative stress and endoplasmic reticulum stress and increasing autophagy. However, the mechanism is unclear. In the present study, we observed a loss of PC-mediated cardioprotection of aged cardiomyocytes. NaHS (a H₂S donor) exerted significant protective effects against H/R-induced cell damage, apoptosis, production of cleaved caspase-3 and caspase-9, and release of cytochrome c. NaHS also reversed the H/R-induced reduction in cell viability and increased HB-EGF expression, cellular HB-EGF content, and EGFR phosphorylation. Additionally, NaHS increased expression of Bcl-2, c-myc, c-fos and c-jun, and the phosphorylation of ERK1/2, PI3K, Akt and GSK-3β. PC alone did not provide protection to H/R-treated aged cardiomyocytes, but it was significantly restored by supplementation of NaHS. The beneficial effects of NaHS during PC were inhibited by EGFR knockdown, AG1478 (EGFR inhibitor), PD98059 (ERK1/2 inhibitor) or LY294002 (PI3K inhibitor). These results suggest that exogenous H₂S restores PC-mediated cardioprotection by up-regulating HB-EGF/EGFR signaling, which activates the ERK1/2-c-myc (and fos and c-jun) and PI3K-Akt- GSK-3β pathways in the aged cardiomyocytes.

Methamphetamine reduces expressions of tight junction proteins, rearranges F-actin cytoskeleton and increases the blood brain barrier permeability via the RhoA/ROCK-dependent pathway

Methamphetamine (METH) is a psychostimulant with severe neurotoxicity, which is related to an increase of blood-brain barrier (BBB) permeability. However, the exact mechanisms have not been fully illuminated. In the present study, male Sprague Dawley rats were treated with METH or saline with 8 injections (i.p.) at 12-h intervals and sacrificed 24 h after the last METH injection. To evaluate BBB permeability, 6 rats were administered with Evans blue (EB) by tail vein injection 1 h prior to sacrifice. EB levels significantly increased in both left and right frontal lobes in METH-treated rats, suggesting increase of BBB permeability, which was proved by the rearrangement of F-actin cytoskeleton and decreased expressions of tight junction (TJ) proteins in hippocampus. Over-expressions of RhoA, ROCK, myosin light chain (MLC), cofilin, phosphorylation (p)-MLC, p-cofilin and matrix metalloproteinase (MMP)-9 were observed, indicating activated RhoA/ROCK pathway. Rat brain microvascular endothelial cells (RBMECs) were isolated and treated with inhibitors of RhoA and ROCK followed by METH. Pretreatments of the inhibitors significantly decreased expressions of RhoA, ROCK, MLC, cofilin, p-MLC and p-cofilin, increased expressions of TJ proteins, suppressed F-actin cytoskeleton rearrangement and reduced the permeability of RBMECs. These results suggested that METH increased BBB permeability through activating the RhoA/ROCK pathway, which resulted in F-actin cytoskeleton rearrangement and down-regulation of TJ proteins.

Rosiglitazone up-regulates glial fibrillary acidic protein via HB-EGF secreted from astrocytes and neurons through PPARγ pathway and reduces apoptosis in high-fat diet-fed mice

The anti-diabetic drug and peroxisome proliferator-activated receptor-gamma (PPARγ) agonist, rosiglitazone, alters astrocyte activation; however, its mechanism remains less-known. We hypothesized participation of epidermal growth factor receptor (EGFR), known to control astrocyte reactivity. We first detected that rosiglitazone promoted glial fibrillary acidic protein (GFAP) expression in primary astrocytes as well as the mouse cerebral cortex, associated with increased EGFR activation. Screening for EGFR ligands revealed a rosiglitazone-mediated increase of heparin-binding epidermal growth factor (HB-EGF) in astrocytes, resulting in HB-EGF release into culture medium and mouse cerebrospinal fluid too. Treatment with HB-EGF-siRNA and EGFR inhibitors showed that the rosiglitazone-induced HB-EGF and p-EFGR were interdependent, which participated in GFAP increase. Interestingly, we observed that rosiglitazone could induce cellular and secreted-HB-EGF in neurons also, contributing toward the activated EGFR-induced GFAP in astrocytes. Probing whether these effects of rosiglitazone were PPARγ-linked, revealed potential PPARγ-responsive elements within HB-EGF gene. Moreover, gel-shift, site-directed mutagenesis, chromatin-immunoprecipitation and luciferase-reporter assays demonstrated a PPARγ-dependent HB-EGF transactivation. Subsequently, we examined effects of rosiglitazone in a high-fat diet-fed diabetes mouse model, and supporting observations in the normal cortical cells, identified a rosiglitazone-induced GFAP, astrocyte and neuronal HB-EGF and secreted-HB-EGF in the cerebral cortex of diabetic mice. Moreover, assessing relevance of increased HB-EGF and GFAP revealed an anti-apoptotic role of rosiglitazone in the cerebral cortex, supported by a GFAP-siRNA as well as HB-EGF-siRNA-mediated increase in cleaved-caspase 3 and 9 levels in the rosiglitazone-treated astrocyte-neuron coculture. Overall, our study indicates that rosiglitazone may protect the brain, via a PPARγ-dependent HB-EGF/EGFR signaling and increased GFAP.