Does age matter? Comparison of neurobehavioral effects of paraquat exposure on postnatal and adult C57BL/6 mice

Role of OCT3 and DRP1 in the Transport of Paraquat in Astrocytes: A Mouse Study

BACKGROUND

Paraquat (PQ) is a pesticide, exposure to which has been associated with an increased risk of Parkinson's disease; however, PQ transport mechanisms in the brain are still unclear. Our previous studies indicated that the organic cation transporter 3 (OCT3) expressed on astrocytes could uptake PQ and protect the dopaminergic (DA) neurons from a higher level of extracellular PQ. At present, it is unknown how OCT3 levels are altered during chronic PQ exposure or aging, nor is it clear how the compensatory mechanisms are triggered by OCT3 deficiency. Dynamic related protein 1 (DRP1) was previously reported to ameliorate the loss of neurons during Parkinson's disease. Nowadays, mounting studies have revealed the functions of astrocyte DRP1, prompting us to hypothesize that DRP1 could regulate the PQ transport capacity of astrocytes.

OBJECTIVES

The present study aimed to further explore PQ transport mechanisms in the nigrostriatal system and identify pathways involved in extracellular PQ clearance.

METHODS

Models of PQ-induced neurodegeneration were established by intraperitoneal (i.p.) injection of PQ in wild-type (WT) and organic cation transporter-3-deficient (Oct3-/-) mice. DRP1 knockdown was achieved by viral tools in vivo and small interfering RNA (siRNA) in vitro. Extracellular PQ was detected by in vivo microdialysis. In vitro transport assays were used to directly observe the functions of different transporters. PQ-induced neurotoxicity was evaluated by tyrosine hydroxylase immunohistochemistry, in vivo microdialysis for striatal DA and behavior tests. Western blotting analysis or immunofluorescence was used to evaluate the expression levels and locations of proteins in vitro or in vivo.

RESULTS

Older mice and those chronically exposed to PQ had a lower expression of brain OCT3 and, following exposure to a 10-mg/kg i.p. PQ2+ loading dose, a higher concentration of extracellular PQ. DRP1 levels were higher in astrocytes and neurons of WT and Oct3-/- mice after chronic exposure to PQ; this was supported by finding higher levels of DRP1 after PQ treatment of dopamine transporter-expressing neurons with and without OCT3 inhibition and in primary astrocytes of WT and Oct3-/- mice. Selective astrocyte DRP1 knockdown ameliorated the PQ2+-induced neurotoxicity in Oct3-/- mice but not in WT mice. GL261 astrocytes with siRNA-mediated DRP1 knockdown had a higher expression of alanine-serine-cysteine transporter 2 (ASCT2), and transport studies suggest that extracellular PQ was transported into astrocytes by ASCT2 when OCT3 was absent.

DISCUSSION

The present study mainly focused on the transport mechanisms of PQ between the dopaminergic neurons and astrocytes. Lower OCT3 levels were found in the older or chronically PQ-treated mice. Astrocytes with DRP1 inhibition (by viral tools or mitochondrial division inhibitor-1) had higher levels of ASCT2, which we hypothesize served as an alternative transporter to remove extracellular PQ when OCT3 was deficient. In summary, our data suggest that OCT3, ASCT2 located on astrocytes and the dopamine transporter located on DA terminals may function in a concerted manner to mediate striatal DA terminal damage in PQ-induced neurotoxicity. https://doi.org/10.1289/EHP9505.

JAK2/STAT3 pathway regulates microglia polarization involved in hippocampal inflammatory damage due to acute paraquat exposure

OBJECTIVE

To explore the effects of acute paraquat (PQ) exposure on the phenotypic polarization of hippocampal microglia and its mechanism.

METHODS

An acute PQ exposure rat model was established. Male SD rats were exposed to 0, 5, 25, and 50 mg/kg PQ, and brain hippocampal tissue was collected after 1, 3, and 7 days of exposure, respectively. Hippocampal pathological changes were examined by H&E staining, and immunohistochemistry (IHC) was used to detect changes in the number of Iba-1-positive cells, the average number of endpoints, and the average process length. The protein expression of Iba-1 was detected by western blotting. BV-2 microglia were treated with 0, 0.01, 0.025, 0.05, or 0.1 μmol/L PQ for 24 h. ELISA and western blotting assays were performed to detect the expression of TNF-α and IL-1β in vivo and in vitro. The M1 microglia marker iNOS, the M2 microglia marker Arg-1, and the p-JAK2 and p-STAT3 protein were detected by western blotting. JAK2/STAT3 pathway activation role in regulating microglia phenotypic polarization was further validated in vivo and in vitro by JAK2-specific inhibitor AG490 administration.

RESULTS

After acute PQ exposure, hippocampal neurons showed pathological changes such as loose arrangement and nuclear pyknosis, the number of Iba-1 positive cells and the expression of Iba-1 protein increased, and the average number of endpoints and average process length of microglia decreased. Histological examination revealed that compared with the control group, in the 50 mg/kg PQ group on the 3rd and 7th day, the expression of TNF-α, IL-1β, and iNOS significantly increased, while that of Arg-1 significantly decreased. p-JAK2 and p-STAT3 expression significantly increased in the 50 mg/kg PQ group on the 1st, 3rd, and 7th day. In vitro, compared with the control group, the expression of TNF-α, IL-1β, iNOS, p-JAK2, and p-STAT3 significantly increased, while Arg-1 expression was significantly reduced in the 0.025, 0.05, and 0.1 μmol/L PQ groups. After AG490 administration, the expression levels of p-JAK2, p-STAT3, iNOS, TNF-α, and IL-1β in the AG490 +PQ group were significantly inhibited in vivo and in vitro compared with the PQ-only group. On the contrary, Arg-1 expression was significantly increased.

CONCLUSION

Our results suggest that acute PQ exposure may induce M1-type polarization of hippocampal microglia by activating the JAK2/STAT3 pathway, which in turn releases pro-inflammatory factors such as TNF-α and IL-1β, leading to hippocampal inflammatory damage.

Paraquat Inhibits Autophagy Via Intensifying the Interaction Between HMGB1 and α-Synuclein

Paraquat, a widely used herbicide, is associated with an increased risk of Parkinson's disease (PD). PQ induces upregulation and accumulation of α-synuclein in neurons, which is one of the major pathological hallmarks of PD. Autophagy, as the major mechanism for the clearance of α-synuclein, is disrupted upon pesticide exposure as well as in PD patients. Meanwhile, HMGB1 is involved in autophagy dysfunction and particularly relevant to PD. However, whether PQ exposure affects HMGB1, α-synuclein, and autophagy function have rarely been reported. In this study, we found that PQ exposure impaired autophagy function via disturbing the complex formation of HMGB1 and Beclin1. Moreover, the expression of α-synuclein is modulated by HMGB1 and the interaction between HMGB1 and α-synuclein was intensified by PQ exposure. Taken together, our results revealed that HMGB1-mediated α-synuclein accumulation could competitively perturb the complex formation of HMGB1 and Beclin1, thereby inhibiting the autophagy function in SH-SY5Y cells.

Paraquat-activated BV-2 microglia induces neuroinflammatory responses in the neuron model through NF-κB signaling pathway

Paraquat (PQ), a non-selective contact herbicide, has been generally accepted as one of the environmental neurotoxicants. Despite the direct evidence that PQ could induce inflammation responses in microglia, little is known about the effects of the inflammatory microglia on neurons. Thus in the present study, mouse primary cortical neurons and PC12 cells, widely-used in vitro neuron models for neurotoxicity research were applied to investigate the neuroinflammatory effects of PQ-activated microglia on neurons. We observed that the secretion levels of TNF-α and IL-6 in PC12 cells were markedly increased upon treatment with the supernatants of inflammatory BV2 microglia, and NF-κB p65 protein expression was also elevated. Specific inhibition of NF-κB by PDTC dramatically attenuated the increase of TNF-α and IL-6 release. These results suggested that PQ-induced inflammatory microglia exerts secondary inflammatory effects on neurons through activation of NF-κB pathway.

Dysregulation of prostaglandine E2 and BDNF signaling mediated by estrogenic dysfunction induces primary hippocampal neuronal cell death after single and repeated paraquat treatment

Paraquat (PQ) produces hippocampal neuronal cell death and cognitive dysfunctions after unique and continued exposure, but the mechanisms are not understood. Primary hippocampal wildtype or βAPP-Tau silenced cells were co-treated with PQ with or without E2, N-acetylcysteine (NAC), NS-398 (cyclooxygenase-2 inhibitor), MF63 (PGES-1 inhibitor) and/or recombinant brain-derived neurotrophic factor (BDNF) during one- and fourteen-days to studied PQ effect on prostaglandin E2 (PGE2) and BDNF signaling and their involvement in hyperphosphorylated Tau (pTau) and amyloid-beta (Aβ) protein formation, and oxidative stress generation, that lead to neuronal cell loss through estrogenic disruption, as a possible mechanism of cognitive dysfunctions produced by PQ. Our results indicate that PQ overexpressed cyclooxygenase-2 that leads to an increase of PGE2 and alters the expression of EP1-3 receptor subtypes. PQ induced also a decrease of proBDNF and mature BDNF levels and altered P75^NTR and tropomyosin receptor kinase B (TrkB) expression. PQ induced PGE2 and BDNF signaling dysfunction, mediated through estrogenic disruption, leading to Aβ and pTau proteins synthesis, oxidative stress generation and finally to cell death. Our research provides relevant information to explain PQ hippocampal neurotoxic effects, indicating a probable explanation of the cognitive dysfunction observed and suggests new therapeutic strategies to protect against PQ toxic effects.

HMGB1 Mediates Paraquat-Induced Neuroinflammatory Responses via Activating RAGE Signaling Pathway

Paraquat (PQ), a widely characterized neurotoxicant, has been generally accepted as one of the environmental factors in the etiology of Parkinson's disease (PD). Despite the direct evidence that PQ could induce inflammatory responses in central nervous system, the putative adverse effects of PQ on the neuroimmune interactions have rarely been investigated. High-mobility group box 1 (HMGB1) has been proven to be relevant to the neuroinflammation involved in PD; however, whether and how HMGB1 exerts modulatory effects in nervous system upon PQ exposure remain elusive. Therefore, the present study investigated the underlying association between HMGB1 and PQ exposure in SH-SY5Y cells, which is a well-established in vitro model for PD research. We observed that HMGB1 was markedly increased in a concentration and time-dependent manner upon PQ exposure, and the elevated HMGB1 could be translocated into cytosol and then released to the extracellular milieu of SH-SY5Y cells. Knockdown of HMGB1 inhibited the activation of RAGE-P38-NF-κB signaling pathway and the expression of inflammation cytokines such as TNF-α and IL-6. These results suggested that HMGB1 is involved in the PQ-induced neuron death via activating RAGE signaling pathways and promoting neuroinflammatory responses.

Primary hippocampal estrogenic dysfunction induces synaptic proteins alteration and neuronal cell death after single and repeated paraquat exposure

The extensively utilized herbicide Paraquat (PQ) was reported to generate cognitive disorders and hippocampal neuronal cell death after unique and extended exposure. Although, most of the mechanisms that mediate these actions remain unknown. We researched whether PQ induces synaptic protein disruption, Tau and amyloid beta protein formation, oxidative stress generation, and hippocampal neuronal cell loss through anti-estrogen action in primary hippocampal neurons, after day and two weeks PQ treatment, as a probable mechanism of such learning and memory impairment. Our results reveal that PQ did not alter estrogen receptors (ERα and ERβ) gene expression, yet it decreased ER activation, which led to synaptic proteins disruption and amyloid beta proteins generation and Tau proteins hyperphosphorylation. Estrogenic signaling disruption induced by PQ also downregulated the NRF2 pathway leading to oxidative stress generation. Finally, PQ exposure induced cell death mediated by ER dysfunction partially through oxidative stress and amyloid beta proteins generation and Tau proteins hyperphosphorylation. The results presented provide a therapeutic strategy to protect against PQ toxic effects, possibly giving an explanation for the learning and memory impairment generated following PQ exposure.

Paraquat modulates microglia M1/M2 polarization via activation of TLR4-mediated NF-κB signaling pathway

Paraquat (PQ) is a widely characterized neurotoxicant able to induce a series of nervous system disorders, including neurobehavioral defects and neurodegenerative diseases. Despite the direct evidence that PQ could induce inflammatory responses in central nervous system and largely contribute to neurotoxicity, the putative adverse effects of PQ on the neuroimmune interactions have rarely been investigated. Therefore, the present study investigated underlying mechanisms of PQ-induced inflammatory response in BV-2 microglia cells. Proliferation, migration and phagocytosis of BV-2 cells upon PQ exposure were first investigated to demonstrate that PQ did stimulate BV-2 microglia into an active phenotype. Increased microglia M1 markers expression and decreased microglia M2 markers expression confirmed that PQ induces BV-2 cells towards M1 activation. The levels of pro-inflammatory cytokines were determined using ELISA and western blotting assays, showing that paraquat significantly promote the secretion of pro-inflammatory mediators such as tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β) and interleukin 6 (IL-6). The up-regulation of TLR4/MyD88 protein expressions and enhanced translocation of NF-κB p65 protein upon PQ exposure were further demonstrated. Taken together, our results suggested that PQ induces M1 microglia polarization by increased production of pro-inflammatory molecules, which could be explained by the activation of the TLR4-mediated NF-κB signaling pathway.

Paraquat Preferentially Induces Apoptosis of Late Stage Effector Lymphocyte and Impairs Memory Immune Response in Mice

Paraquat (PQ) is a toxic non-selective herbicide. To date, the effect of PQ on memory immune response is still unknown. We investigated the impact of PQ on memory immune response. Adult C57BL/6 mice were subcutaneously injected with 2 mg/kg PQ, 20 mg/kg PQ or vehicle control every three days for two weeks. A single injection of keyhole limpet hemocyanin (KLH) at day four after the initial PQ treatment was used to induce a primary immune response; a second KLH challenge was performed at three months post the first KLH immunization to induce a secondary immune response. In steady state, treatment with 20 mg/kg PQ reduced the level of serum total IgG, but not that of IgM; treatment with 20 mg/kg PQ decreased the number of effector and memory lymphocytes, but not naïve or inactivated lymphocytes. During the primary immune response to KLH, treatment with 20 mg/kg PQ did not influence the proliferation of lymphocytes or expression of co-stimulatory molecules. Instead, treatment with 20 mg/kg PQ increased the apoptosis of lymphocytes at late stage, but not early stage of the primary immune response. During the secondary immune response to KLH, treatment with 20 mg/kg PQ reduced the serum anti-KLH IgG and KLH-responsive CD4 T cells and B cells. Moreover, effector or activated lymphocytes were more sensitive to PQ-induced apoptosis in vitro. Treatment with 2 mg/kg PQ did not impact memory immune response to KLH. Thus, treatment with 20 mg/kg PQ increased apoptosis of late stage effector cells to yield less memory cells and thereafter impair memory immune response, providing a novel understanding of the immunotoxicity of PQ.

Lipopolysaccharide-Induced Striatal Nitrosative Stress and Impaired Social Recognition Memory Are Not Magnified by Paraquat Coexposure

Systemic inflammation triggered by lipopolysaccharide (LPS) administration disrupts blood-brain barrier (BBB) homeostasis in animal models. This event leads to increased susceptibility of several encephalic structures to potential neurotoxicants present in the bloodstream. In this study, we investigated the effects of alternate intraperitoneal injections of LPS on BBB permeability, social recognition memory and biochemical parameters in the striatum 24 h and 60 days after treatments. In addition, we investigated whether the exposure to a moderate neurotoxic dose of the herbicide paraquat could potentiate LPS-induced neurotoxicity. LPS administration caused a transient disruption of BBB integrity, evidenced by increased levels of exogenously administered sodium fluorescein in the striatum. Also, LPS exposure caused delayed impairment in social recognition memory (evaluated at day 38 after treatments) and increase in the striatal levels of 3-nitrotyrosine. These events were observed in the absence of significant changes in motor coordination and in the levels of tyrosine hydroxylase (TH) in the striatum and substantia nigra. PQ exposure, which caused a long-lasting decrease of striatal mitochondrial complex I activity, did not modify LPS-induced behavioral and striatal biochemical changes. The results indicate that systemic administration of LPS causes delayed social recognition memory deficit and striatal nitrosative stress in adult mice and that the coexposure to a moderately toxic dose of PQ did not magnify these events. In addition, PQ-induced inhibition of striatal mitochondrial complex I was also not magnified by LPS exposure, indicating the absence of synergic neurotoxic effects of LPS and PQ in this experimental model.

Primary hippocampal neuronal cell death induction after acute and repeated paraquat exposures mediated by AChE variants alteration and cholinergic and glutamatergic transmission disruption

Paraquat (PQ) is a widely used non-selective contact herbicide shown to produce memory and learning deficits after acute and repeated exposure similar to those induced in Alzheimer's disease (AD). However, the complete mechanisms through which it induces these effects are unknown. On the other hand, cholinergic and glutamatergic systems, mainly in the hippocampus, are involved on learning, memory and cell viability regulation. An alteration of hippocampal cholinergic or glutamatergic transmissions or neuronal cell loss may induce these effects. In this regard, it has been suggested that PQ may induce cell death and affect cholinergic and glutamatergic transmission, which alteration could produce neuronal loss. According to these data, we hypothesized that PQ could induce hippocampal neuronal loss through cholinergic and glutamatergic transmissions alteration. To prove this hypothesis, we evaluated in hippocampal primary cell culture, the PQ toxic effects after 24h and 14 consecutive days exposure on neuronal viability and the cholinergic and glutamatergic mechanisms related to it. This study shows that PQ impaired acetylcholine levels and induced AChE inhibition and increased CHT expression only after 14days exposure, which suggests that acetylcholine levels alteration could be mediated by these actions. PQ also disrupted glutamate levels through induction of glutaminase activity. In addition, PQ induced, after 24h and 14days exposure, cell death on hippocampal neurons that was partially mediated by AChE variants alteration and cholinergic and gultamatergic transmissions disruption. Our present results provide new view of the mechanisms contributing to PQ neurotoxicity and may explain cognitive dysfunctions observed after PQ exposure.