Sodium p-Aminosalicylic Acid Reverses Sub-Chronic Manganese-Induced Impairments of Spatial Learning and Memory Abilities in Rats, but Fails to Restore γ-Aminobutyric Acid Levels

Treatment of manganese and lead poisoning with sodium para-aminosalicylic acid: A contemporary update

Sodium para-aminosalicylic acid (PAS-Na) treatment for manganese (Mn) intoxication has shown efficacy in experimental and clinical studies, giving rise to additional studies on its efficacy for lead (Pb) neurotoxicity and its associated mechanisms of neuroprotection. The difference between PAS-Na and other metal complexing agents, such as edetate calcium sodium (CaNa2-EDTA), is firstly that PAS-Na can readily pass through the blood-brain barrier (BBB), and complex and facilitate the excretion of manganese and lead. Secondly, PAS-Na has anti-inflammatory effects. Recent studies have broadened the understanding on the mechanisms associated with efficacy of PAS-Na. The latter has been shown to modulate multifarious manganese- and lead- induced neurotoxicity, via its anti-apoptotic and anti-inflammatory effects, as well as its ability to inhibit pyroptosis, and regulate abnormal autophagic processes. These observations provide novel scientific bases and new concepts for the treatment of lead, mercury, copper, thallium, as well as other toxic encephalopathies, and implicate PAS-Na as a compound with greater prospects for clinical medical application.

Mechanisms of manganese-induced neurotoxicity and the pursuit of neurotherapeutic strategies

Chronic exposure to elevated levels of manganese via occupational or environmental settings causes a neurological disorder known as manganism, resembling the symptoms of Parkinson's disease, such as motor deficits and cognitive impairment. Numerous studies have been conducted to characterize manganese's neurotoxicity mechanisms in search of effective therapeutics, including natural and synthetic compounds to treat manganese toxicity. Several potential molecular targets of manganese toxicity at the epigenetic and transcriptional levels have been identified recently, which may contribute to develop more precise and effective gene therapies. This review updates findings on manganese-induced neurotoxicity mechanisms on intracellular insults such as oxidative stress, inflammation, excitotoxicity, and mitophagy, as well as transcriptional dysregulations involving Yin Yang 1, RE1-silencing transcription factor, transcription factor EB, and nuclear factor erythroid 2-related factor 2 that could be targets of manganese neurotoxicity therapies. This review also features intracellular proteins such as PTEN-inducible kinase 1, parkin, sirtuins, leucine-rich repeat kinase 2, and α-synuclein, which are associated with manganese-induced dysregulation of autophagy/mitophagy. In addition, newer therapeutic approaches to treat manganese's neurotoxicity including natural and synthetic compounds modulating excitotoxicity, autophagy, and mitophagy, were reviewed. Taken together, in-depth mechanistic knowledge accompanied by advances in gene and drug delivery strategies will make significant progress in the development of reliable therapeutic interventions against manganese-induced neurotoxicity.

Therapeutic Effects of Sodium Para-Aminosalicylic Acid on Cognitive Deficits and Activated ERK1/2-p90RSK/NF-κB Inflammatory Pathway in Pb-Exposed Rats

Lead (Pb) is a toxic heavy metal and environmental pollutant that adversely affects the nervous system. However, effective therapeutic drugs for Pb-induced neurotoxicity have yet to be developed. In the present study, we investigated the ameliorative effect of sodium para-aminosalicylic acid (PAS-Na) on Pb-induced neurotoxicity. Male Sprague-Dawley rats were treated with (CH₃COO)₂ Pb•4H₂O (6 mg/kg) for 4 weeks, followed by 3 weeks of PAS-Na (100, 200, and 300 mg/kg). The results showed that subacute Pb exposure significantly decreased rats body-weight gains and increased liver coefficient, and impaired spatial learning and memory. HE staining showed that Pb damaged the structure of the hippocampus. Moreover, Pb activated the ERK1/2-p90^RSK/ NF-κB pathway concomitant with increased inflammatory cytokine IL-1β levels in rat hippocampus. PAS-Na reversed the Pb-induced increase in the liver coefficient as well as the learning and memory deficits. In addition, PAS-Na reduced the phosphorylation of ERK1/2, p90^RSK and NF-κB p65, decreasing IL-1β levels in hippocampus. Our findings indicated that PAS-Na showed efficacy in reversing Pb-induced rats cognitive deficits and triggered an anti-inflammatory response. Thus, PAS-Na may be a promising therapy for treating Pb-induced neurotoxicity.

Sodium P-aminosalicylic Acid Attenuates Manganese-Induced Neuroinflammation in BV2 Microglia by Modulating NF-κB Pathway

Exposure to high levels of manganese (Mn) leads to brain Mn accumulation, and a disease referred to as manganism. Activation of microglia plays an important role in Mn-induced neuroinflammation. Sodium p-aminosalicylic acid (PAS-Na) is a non-steroidal anti-inflammatory drug that inhibits Mn-induced neuroinflammation. The aim of the current study was to explore the role of NF-κB in the protective mechanism of PAS-Na on Mn-induced neuroinflammation in BV2 microglial experimental model. We treated BV2 microglia with 200 μM Mn for 24 h followed by 48 h treatment with graded concentrations of PAS-Na, using an NF-kB inhibitor, JSH-23, as a positive control. MTT results established that 200 and 400 μM PAS-Na treatment increased the Mn-induced cell viability reduction. NF-κB (P65) mRNA expression and the phosphorylation of p65 were increased in Mn-treated BV2 cell, and suppressed by PAS-Na, analogous to the effect of JSH-23 pretreatment. Furthermore, PAS-Na significantly reduced the contents of the inflammatory cytokine TNF-α and IL-1β, both of which were increased by Mn treatment. The current results show that PAS-Na attenuated Mn-induced inflammation by abrogating the activation of the NF-κB signaling pathways and reduced the release of pro-inflammatory cytokines.

Molecular Targets of Manganese-Induced Neurotoxicity: A Five-Year Update

Understanding of the immediate mechanisms of Mn-induced neurotoxicity is rapidly evolving. We seek to provide a summary of recent findings in the field, with an emphasis to clarify existing gaps and future research directions. We provide, here, a brief review of pertinent discoveries related to Mn-induced neurotoxicity research from the last five years. Significant progress was achieved in understanding the role of Mn transporters, such as SLC39A14, SLC39A8, and SLC30A10, in the regulation of systemic and brain manganese handling. Genetic analysis identified multiple metabolic pathways that could be considered as Mn neurotoxicity targets, including oxidative stress, endoplasmic reticulum stress, apoptosis, neuroinflammation, cell signaling pathways, and interference with neurotransmitter metabolism, to name a few. Recent findings have also demonstrated the impact of Mn exposure on transcriptional regulation of these pathways. There is a significant role of autophagy as a protective mechanism against cytotoxic Mn neurotoxicity, yet also a role for Mn to induce autophagic flux itself and autophagic dysfunction under conditions of decreased Mn bioavailability. This ambivalent role may be at the crossroad of mitochondrial dysfunction, endoplasmic reticulum stress, and apoptosis. Yet very recent evidence suggests Mn can have toxic impacts below the no observed adverse effect of Mn-induced mitochondrial dysfunction. The impact of Mn exposure on supramolecular complexes SNARE and NLRP3 inflammasome greatly contributes to Mn-induced synaptic dysfunction and neuroinflammation, respectively. The aforementioned effects might be at least partially mediated by the impact of Mn on α-synuclein accumulation. In addition to Mn-induced synaptic dysfunction, impaired neurotransmission is shown to be mediated by the effects of Mn on neurotransmitter systems and their complex interplay. Although multiple novel mechanisms have been highlighted, additional studies are required to identify the critical targets of Mn-induced neurotoxicity.

Sodium para-aminosalicylic acid inhibits manganese-induced NLRP3 inflammasome-dependent pyroptosis by inhibiting NF-κB pathway activation and oxidative stress

Background

The activation of NOD-like receptor protein 3 (NLRP3) inflammasome-dependent pyroptosis has been shown to play a vital role in the pathology of manganese (Mn)-induced neurotoxicity. Sodium para-aminosalicylic acid (PAS-Na) has a positive effect on the treatment of manganism. However, the mechanism is still unclear. We hypothesized that PAS-Na might act through NLRP3.

Methods

The microglial cell line BV2 and male Sprague-Dawley rats were used to investigate the impacts of PAS-Na on Mn-induced NLRP3 inflammasome-dependent pyroptosis. The related protein of the NF-κB pathway and NLRP3-inflammasome-dependent pyroptosis was detected by western blot. The reactive oxygen species and mitochondrial membrane potential were detected by immunofluorescence staining and flow cytometry. The activation of microglia and the gasdermin D (GSDMD) were detected by immunofluorescence staining.

Results

Our results showed that Mn treatment induced oxidative stress and activated the NF-κB pathway by increasing the phosphorylation of p65 and IkB-α in BV2 cells and in the basal ganglia of rats. PAS-Na could alleviate Mn-induced oxidative stress damage by inhibiting ROS generation, increasing mitochondrial membrane potential and ATP levels, thereby reducing the phosphorylation of p65 and IkB-α. Besides, Mn treatment could activate the NLRP3 pathway and promote the secretion of IL-18 and IL-1β, mediating pyroptosis in BV2 cells and in the basal ganglia and hippocampus of rats. But an inhibitor of NF-κb (JSH-23) treatment could significantly reduce LDH release, the expression of NLRP3 and Cleaved CASP1 protein and IL-1β and IL-18 mRNA level in BV2 cells. Interestingly, the effect of PAS-Na treatment in Mn-treated BV2 cells is similar to those of JSH-23. Besides, immunofluorescence results showed that PAS-Na reduced the increase number of activated microglia, which stained positively for GSDMD.

Conclusion

PAS-Na antagonized Mn-induced NLRP3 inflammasome dependent pyroptosis through inhibiting NF-κB pathway activation and oxidative stress.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-020-02018-6.

Manganese-induced neurodegenerative diseases and possible therapeutic approaches

INTRODUCTION

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and prion disease represent important public health concerns. Exposure to high levels of  heavy metals such as manganese (Mn) may contribute to their development.

AREAS COVERED

In this critical review, we address the role of Mn in the etiology of neurodegenerative diseases and discuss emerging treatments of Mn overload, such as chelation therapy. In addition, we discuss natural and synthetic compounds under development as prospective therapeutics. Moreover, bioinformatic approaches to identify new potential targets and therapeutic substances to reverse the neurodegenerative diseases are discussed.

EXPERT OPINION

Here, the authors highlight the importance of better understanding the molecular mechanisms of toxicity associated with neurodegenerative diseases, and the role of Mn in these diseases. Additional emphasis should be directed to the discovery of new agents to treat Mn-induced diseases, since present day chelator therapies have limited bioavailability. Furthermore, the authors encourage the scientific community to develop research using libraries of compounds to screen those compounds that show efficacy in regulating brain Mn levels. In addition, bioinformatics may provide novel insight for pathways and clinical treatments associated with Mn-induced neurodegeneration, leading to a new direction in Mn toxicological research.

Effects of Sub-Acute Manganese Exposure on Thyroid Hormone and Glutamine (Gln)/Glutamate (Glu)-γ- Aminobutyric Acid (GABA) Cycle in Serum of Rats

Excessive manganese (Mn) exposure may adversely affect the central nervous system, and cause an extrapyramidal disorder known as manganism. The glutamine (Gln)/glutamate (Glu)-γ-aminobutyric acid (GABA) cycle and thyroid hormone system may be involved in Mn-induced neurotoxicity. However, the effect of Mn on the Gln/Glu-GABA cycle in the serum has not been reported. Herein, the present study aimed to investigate the effects of sub-acute Mn exposure on the Gln/Glu-GABA cycle and thyroid hormones levels in the serum of rats, as well as their relationship. The results showed that sub-acute Mn exposure increased serum Mn levels with a correlation coefficient of 0.733. Furthermore, interruption of the Glu/Gln-GABA cycle in serum was found in Mn-exposed rats, as well as thyroid hormone disorder in the serum via increasing serum Glu levels, and decreasing serum Gln, GABA, triiodothyronine (T3) and thyroxine (T4) levels. Additionally, results of partial correlation showed that there was a close relationship between serum Mn levels and the detected indicators accompanied with a positive association between GABA and T3 levels, as well as Gln and T4 levels in the serum of Mn-exposed rats. Unexpectedly, there was no significant correlation between serum Glu and the serum T3 and T4 levels. In conclusion, the results demonstrated that both the Glu/Gln-GABA cycle and thyroid hormone system in the serum may play a potential role in Mn-induced neurotoxicity in rats. Thyroid hormone levels, T3 and T4, have a closer relationship with GABA and Gln levels, respectively, in the serum of rats.

The Impact of Environmental Mn Exposure on Insect Biology

Manganese (Mn) is an essential trace element that acts as a metal co-factor in diverse biochemical and cellular functions. However, chronic environmental exposure to high levels of Mn is a well-established risk factor for the etiology of severe, atypical parkinsonian syndrome (manganism) via its accumulation in the basal ganglia, pallidum, and striatum brain regions, which is often associated with abnormal dopamine, GABA, and glutamate neural signaling. Recent studies have indicated that chronic Mn exposure at levels that are below the risk for manganism can still cause behavioral, cognitive, and motor dysfunctions via poorly understood mechanisms at the molecular and cellular levels. Furthermore, in spite of significant advances in understanding Mn-induced behavioral and neuronal pathologies, available data are primarily for human and rodents. In contrast, the possible impact of environmental Mn exposure on brain functions and behavior of other animal species, especially insects and other invertebrates, remains mostly unknown both in the laboratory and natural habitats. Yet, the effects of environmental exposure to metals such as Mn on insect development, physiology, and behavior could also have major indirect impacts on human health via the long-term disruptions of food webs, as well as direct impact on the economy because of the important role insects play in crop pollination. Indeed, laboratory and field studies indicate that chronic exposures to metals such as Mn, even at levels that are below what is currently considered toxic, affect the dopaminergic signaling pathway in the insect brain, and have a major impact on the behavior of insects, including foraging activity of important pollinators such as the honey bee. Together, these studies highlight the need for a better understanding of the neuronal, molecular, and genetic processes that underlie the toxicity of Mn and other metal pollutants in diverse animal species, including insects.

Neuroprotective and Therapeutic Strategies for Manganese-Induced Neurotoxicity

Manganese (Mn) is an essential element required for growth, development and general maintenance of health. However, chronic or high occupational and environmental exposure to excessive levels of Mn has long been known to lead to a progressive neurological disorder similar to Parkinsonism. Manganism patients display a variety of symptoms, including mental, cognitive and behavioural impediments, as well as motor dysfunctions that are associated with basal ganglia dysfunction. Taking into account the pharmacokinetics and Mn-related toxicity mechanisms, several neuroprotective compounds and therapeutic approaches have been investigated to assess their efficacy in mitigating its neurotoxicity. Here, we will briefly address some of the toxic mechanisms of Mn, followed by neuroprotective strategies and therapeutic approaches aiming to reduce or treat Mn induced neurotoxicity. Natural and synthetic antioxidants, anti-inflammatory compounds, ATP/ADP ratio protectors and glutamate protectors have been introduced in view of decreasing Mn-induced neurotoxicity. In addition, the efficacy and mechanisms of several therapeutic interventions such as levodopa, ethylene-diamine-tetraacetic acid (EDTA) and para-aminosalicylic acid (PAS), aimed at ameliorating Mn neurotoxic symptoms in humans, will be reviewed.