Role of metallic pollutants in neurodegeneration: effects of aluminum, lead, mercury, and arsenic in mediating brain impairment events and autism spectrum disorder

Autism spectrum disorder (ASD) is a developmental disorder of the brain characterized by shortfall in the social portfolio of an individual and abbreviated interactive and communication aspects rendering stereotypical behavior and pitfalls in a child's memory, thinking, and learning capabilities. The incidence of ASD has accelerated since the past decade, portraying environment as one of the primary assets, comprising of metallic components aiming to curb the neurodevelopmental pathways in an individual. Many regulations like Clean Air Act and critical steps taken by countries all over the globe, like Sweden and the USA, have rendered the necessity to study the effects of environmental metallic components on ASD progression. The review focuses on the primary metallic components present in the environment (aluminum, lead, mercury, and arsenic), responsible for accelerating ASD symptoms by a set of general mechanisms like oxidative stress reduction, glycolysis suppression, microglial activation, and metalloprotein disruption, resulting in apoptotic signaling, neurotoxic effects, and neuroinflammatory responses. The effect of these metals can be retarded by certain protective strategies like chelation, dietary correction, certain agents (curcumin, mangiferin, selenium), and detoxification enhancement, which can necessarily halt the neurodegenerative 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.

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.

Knockdown of TLR4 Represses the Paraquat-Induced Neuroinflammation and Microglial M1 Polarization

Paraquat (PQ) is associated with multiple nervous system disorders including Parkinson's disease. Despite the evidence that PQ could induce inflammatory responses in the central nervous system and largely contribute to neurotoxicity, the mechanisms of PQ-induced neuroinflammation are not yet fully understood. Toll-like receptor 4 (TLR4) could recognize various pathogens and initiate inflammation processes. Therefore, we investigated the role of TLR4 in PQ-induced neuroinflammation by using murine microglial immortalized BV-2 cell line. Normal microglia and TLR4-knockdown microglia were treated with PQ to evaluate signal transduction molecular expression, inflammatory responses, and microglial functions. Compared with normal microglia, PQ-induced production of pro-inflammatory cytokines was significantly reduced in TLR4-knockdown microglia. Levels of M1 markers were decreased, while levels of M2 markers were increased upon PQ exposure, confirming that TLR4 depletion inhibited the microglial M1 polarization. Besides, the migration and phagocytosis capability reduced by PQ were to some extent recovered in TLR4-knockdown microglia. Taken together, our results suggested that TLR4 mediated the neuroinflammatory responses in microglia and the depletion of TLR4 protects against PQ neurotoxicity.