Diagnosis of manganism and manganese neurotoxicity: A workshop report

With declining exposures to manganese (Mn) in occupational settings, there is a need for more sensitive exposure assessments and clinical diagnostic criteria for manganism and Mn neurotoxicity. To address this issue, a workshop was held on November 12-13, 2020, with international experts on Mn toxicity. The workshop discussions focused on the history of the diagnostic criteria for manganism, including those developed by the Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail (IRSST) in Quebec in 2005 and criteria developed by the Chinese government in 2002 and updated in 2006; the utility of biomarkers of exposure; recent developments in magnetic resonance imaging (MRI) for assessing Mn accumulation in the brain and diagnosing manganism; and potential future applications of metabolomics. The suggestions of the participants for updating manganism diagnostic criteria included the consideration of: i) A history of previous occupational and environmental exposure to Mn; ii) relevant clinical symptoms such as dystonia; iii) MRI imaging to document Mn accumulation in the neural tissues, including the basal ganglia; and iv) criteria for the differential diagnosis of manganism and other neurological conditions. Important research gaps include the characterization of Mn exposure and other co-exposures, exploration of the roles of different brain regions with MRI, understanding the complexity of metal ion transporters involved in Mn homeostasis, and a need for information on other neurotransmitter systems and brain regions underlying the pathophysiology of manganism.

Consequences of Disturbing Manganese Homeostasis

Manganese (Mn) is an essential trace element with unique functions in the body; it acts as a cofactor for many enzymes involved in energy metabolism, the endogenous antioxidant enzyme systems, neurotransmitter production, and the regulation of reproductive hormones. However, overexposure to Mn is toxic, particularly to the central nervous system (CNS) due to it causing the progressive destruction of nerve cells. Exposure to manganese is widespread and occurs by inhalation, ingestion, or dermal contact. Associations have been observed between Mn accumulation and neurodegenerative diseases such as manganism, Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. People with genetic diseases associated with a mutation in the gene associated with impaired Mn excretion, kidney disease, iron deficiency, or a vegetarian diet are at particular risk of excessive exposure to Mn. This review has collected data on the current knowledge of the source of Mn exposure, the experimental data supporting the dispersive accumulation of Mn in the brain, the controversies surrounding the reference values of biomarkers related to Mn status in different matrices, and the competitiveness of Mn with other metals, such as iron (Fe), magnesium (Mg), zinc (Zn), copper (Cu), lead (Pb), calcium (Ca). The disturbed homeostasis of Mn in the body has been connected with susceptibility to neurodegenerative diseases, fertility, and infectious diseases. The current evidence on the involvement of Mn in metabolic diseases, such as type 2 diabetes mellitus/insulin resistance, osteoporosis, obesity, atherosclerosis, and non-alcoholic fatty liver disease, was collected and discussed.

Manganese Neurotoxicity as a Stroke Mimic: A Case Report

Manganese (Mn)-induced cerebral toxicity is a rare neurological condition that can present as a stroke mimic in high-risk populations. We present a case of a 40-year-old male with no known comorbidities who was brought to the emergency department with complaints of nonprogressive slurred speech and left facial weakness for eight days. Further history revealed that he had been working as a welder in a steel factory for the past seven years without using proper personal protective equipment (PPE). On physical examination, an upper motor neuron (UMN) type weakness on the left side of his face and spastic dysarthria could be appreciated. Following a brain computed tomography (CT) scan that showed ill-defined hypodensities in the basal ganglia without any signs of a hemorrhage, he was admitted to the stroke unit for conservative management and further investigations. A magnetic resonance imaging (MRI) scan of the brain done later showed features of manganese deposition and absorption in the globus pallidus and corticospinal tracts, indicating a diagnosis of manganese-induced cerebral toxicity. His serum manganese levels obtained during admission were normal. He was managed conservatively with intravenous rehydration and was discharged after symptomatic improvement. He was counseled and educated regarding the importance of wearing protective equipment while at work to reduce further exposure to the metal. During his follow-up visit, his symptoms had considerably improved with proper adherence to workplace safety measures.

Blockage of KHSRP-NLRP3 by MCC950 Can Reverse the Effect of Manganese-Induced Neuroinflammation in N2a Cells and Rat Brain

Manganese neurotoxicity has been reported to cause a neurodegenerative disease known as parkinsonism. Previous reports have shown that the expression of the KH-type splicing regulatory protein (KHSRP), a nucleic acid-binding protein, and NLRP3 is increased upon Mn exposure. However, the relation between these two during Mn toxicity has not been fully deduced. The mouse neuroblastoma (N2a) and SD rats are treated with LPS and MnCl2 to evaluate the expression of KHSRP and NLRP3. Further, the effect of the NLRP3 inhibitor MCC950 is checked on the expression of NLRP3, KHSRP and pro-inflammatory markers (TNFα, IL-18 and IL-1β) as well as the caspase-1 enzyme. Our results demonstrated an increment in NLRP3 and KHSRP expression post-MnCl2 exposure in N2a cells and rat brain, while on the other hand with LPS exposure only NLRP3 expression levels were elevated and KHSRP was found to be unaffected. An increased expression of KHSRP, NLRP3, pro-inflammatory markers and the caspase-1 enzyme was observed to be inhibited with MCC950 treatment in MnCl2-exposed cells and rats. Manganese exposure induces NLRP3 and KHSRP expression to induce neuroinflammation, suggesting a correlation between both which functions in toxicity-related pathways. Furthermore, MCC950 treatment reversed the role of KHSRP from anti-inflammatory to pro-inflammatory.

Intracerebral Transplantation of Neural Stem Cells Restores Manganese-Induced Cognitive Deficits in Mice

Manganese (Mn) is a potent neurotoxin known to cause long-lasting structural damage and progressive cognitive deficits in the brain. However, new therapeutic approaches are urgently needed since current treatments only target symptoms of Mn exposure. Recent studies have suggested a potential role for multipotent neural stem cells (NSCs) in the etiology of Mn-induced cognitive deficits. In this study, we evaluated the effect of direct intracerebral transplantation of NSCs on cognitive function of mice chronically exposed to MnCl₂, and further explored the distribution of transplanted NSCs in brain tissues. NSCs were isolated and bilaterally injected into the hippocampal regions or lateral ventricles of Mn-exposed mice. The results showed that many transplanted cells migrated far away from the injection sites and survived in vivo in the Mn-exposed mouse brain, implying enhanced neurogenesis in the host brain. We found that NSCs transplanted into either the hippocampal regions or the lateral ventricles significantly improved spatial learning and memory function of the Mn-exposed mice in the Morris water maze. Immunofluorescence analyses indicated that some surviving NSCs differentiated into neurons or glial cells, which may have become functionally integrated into the impaired local circuits, providing a possible cellular basis for the improvement of cognitive function in NSC-transplanted mice. Taken together, our findings confirm the Mn-induced impairment of neurogenesis in the brain and underscore the potential of treating Mn exposure by NSC transplantation, providing a practical therapeutic strategy against this type of neurotoxicity.

Manganese-Induced Neurotoxicity: New Insights Into the Triad of Protein Misfolding, Mitochondrial Impairment, and Neuroinflammation

Occupational or environmental exposure to manganese (Mn) can lead to the development of "Manganism," a neurological condition showing certain motor symptoms similar to Parkinson's disease (PD). Like PD, Mn toxicity is seen in the central nervous system mainly affecting nigrostriatal neuronal circuitry and subsequent behavioral and motor impairments. Since the first report of Mn-induced toxicity in 1837, various experimental and epidemiological studies have been conducted to understand this disorder. While early investigations focused on the impact of high concentrations of Mn on the mitochondria and subsequent oxidative stress, current studies have attempted to elucidate the cellular and molecular pathways involved in Mn toxicity. In fact, recent reports suggest the involvement of Mn in the misfolding of proteins such as α-synuclein and amyloid, thus providing credence to the theory that environmental exposure to toxicants can either initiate or propagate neurodegenerative processes by interfering with disease-specific proteins. Besides manganism and PD, Mn has also been implicated in other neurological diseases such as Huntington's and prion diseases. While many reviews have focused on Mn homeostasis, the aim of this review is to concisely synthesize what we know about its effect primarily on the nervous system with respect to its role in protein misfolding, mitochondrial dysfunction, and consequently, neuroinflammation and neurodegeneration. Based on the current evidence, we propose a 'Mn Mechanistic Neurotoxic Triad' comprising (1) mitochondrial dysfunction and oxidative stress, (2) protein trafficking and misfolding, and (3) neuroinflammation.

Manganese neurotoxicity: behavioral disorders associated with dysfunctions in the basal ganglia and neurochemical transmission

Manganese (Mn) is an essential element required for many physiological functions. While it is essential at physiological levels, excessive accumulation of Mn in the brain causes severe dysfunctions in the central nervous system known as manganism. Manganism is an extrapyramidal disorder characterized by motor disturbances associated with neuropsychiatric and cognitive disabilities similar to Parkinsonism. As the primary brain regions targeted by Mn are the basal ganglia, known to be involved in the pathophysiology of extrapyramidal disorders, this review will examine the impact of Mn exposure on the basal ganglia circuitry and neurotransmitters in relation to motor and non-motor disorders. The collected data from recent available studies in humans and experimental animal models provide new information about the mechanisms by which Mn affects behavior, neurotransmitters, and basal ganglia function observed in manganism. The effects of the alterations of metals on basal ganglia and neurochemical functioning are critical to develop effective modalities not only for the treatment of vulnerable populations (e.g., Mn-exposed workers) but also for understanding the etiology of neurodegenerative diseases where brain metal imbalances are involved, such as Parkinson's disease. We examine the impact of manganese (Mn) exposure on the basal ganglia circuitry and neurotransmitters in relation with motor and non-motor disorders. The collected data from available studies show that when accumulated in the globus pallidus, Mn influences the subthalamic (STN) and substantia nigra (SN) neurons, which are at the origin of changes in the thalamus and the cortex.

Common Polymorphisms in the Solute Carrier SLC30A10 are Associated With Blood Manganese and Neurological Function

Manganese (Mn) is an essential nutrient in humans, but excessive exposure to Mn may cause neurotoxicity. Despite homeostatic regulation, Mn concentrations in blood vary considerably among individuals. We evaluated if common single-nucleotide polymorphisms (SNPs) in SLC30A10, which likely encodes an Mn transporter, influence blood Mn concentrations and neurological function. We measured blood Mn concentrations by ICP-MS or atomic absorption spectroscopy and genotyped 2 SLC30A10 non-coding SNPs (rs2275707 and rs12064812) by TaqMan PCR in cohorts from Bangladesh (N = 406), the Argentinean Andes (N = 198), and Italy (N = 238). We also measured SLC30A10 expression in whole blood by TaqMan PCR in a sub-group (N = 101) from the Andean cohort, and neurological parameters (sway velocity and finger-tapping speed) in the Italian cohort. The rs2275707 variant allele was associated with increased Mn concentrations in the Andes (8%, P = .027) and Italy (10.6%, P = .012), but not as clear in Bangladesh (3.4%, P = .21; linear regression analysis adjusted for age, gender, and plasma ferritin). This allele was also associated with increased sway velocity (15%, P = .033; adjusted for age and sex) and reduced SLC30A10 expression (−24.6%, P = .029). In contrast, the rs12064812 variant homozygous genotype was associated with reduced Mn concentrations, particularly in the Italian cohort (−18.4%, P = .04), and increased finger-tapping speed (8.7%, P = .025). We show that common SNPs in SLC30A10 are associated with blood Mn concentrations in 3 unrelated cohorts and that their influence may be mediated by altered SLC30A10 expression. Moreover, the SNPs appeared to influence neurological functions independent of blood Mn concentrations, suggesting that SLC30A10 could regulate brain Mn levels.

Manganese-Induced Parkinsonism and Parkinson's Disease: Shared and Distinguishable Features

Manganese (Mn) is an essential trace element necessary for physiological processes that support development, growth and neuronal function. Secondary to elevated exposure or decreased excretion, Mn accumulates in the basal ganglia region of the brain and may cause a parkinsonian-like syndrome, referred to as manganism. The present review discusses the advances made in understanding the essentiality and neurotoxicity of Mn. We review occupational Mn-induced parkinsonism and the dynamic modes of Mn transport in biological systems, as well as the detection and pharmacokinetic modeling of Mn trafficking. In addition, we review some of the shared similarities, pathologic and clinical distinctions between Mn-induced parkinsonism and Parkinson’s disease. Where possible, we review the influence of Mn toxicity on dopamine, gamma aminobutyric acid (GABA), and glutamate neurotransmitter levels and function. We conclude with a survey of the preventive and treatment strategies for manganism and idiopathic Parkinson’s disease (PD).