Neurobehavioral functioning after cessation of manganese exposure: a follow-up after 14 years

Child and Adolescent Manganese Biomarkers and Adolescent Postural Balance in Marietta CARES Cohort Participants

BACKGROUND

Manganese (Mn) plays a significant role in both human health and global industries. Epidemiological studies of exposed populations demonstrate a dose-dependent association between Mn and neuromotor effects ranging from subclinical effects to a clinically defined syndrome. However, little is known about the relationship between early life Mn biomarkers and adolescent postural balance.

OBJECTIVES

This study investigated the associations between childhood and adolescent Mn biomarkers and adolescent postural balance in participants from the longitudinal Marietta Communities Actively Researching Exposures Study (CARES) cohort.

METHODS

Participants were recruited into CARES when they were 7-9 y old, and reenrolled at 13-18 years of age. At both time points, participants provided samples of blood, hair, and toenails that were analyzed for blood Mn and lead (Pb), serum cotinine, hair Mn, and toenail Mn. In adolescence, participants completed a postural balance assessment. Greater sway indicates postural instability (harmful effect), whereas lesser sway indicates postural stability (beneficial effect). Multivariable linear regression models were conducted to investigate the associations between childhood and adolescent Mn biomarkers and adolescent postural balance adjusted for age, sex, height-weight ratio, parent/caregiver intelligence quotient, socioeconomic status, blood Pb, and serum cotinine.

RESULTS

CARES participants who completed the adolescent postural balance assessment (n = 123 ) were 98% White and 54% female and had a mean age of 16 y (range: 13-18 y). In both childhood and adolescence, higher Mn biomarker concentrations were significantly associated with greater adolescent sway measures. Supplemental analyses revealed sex-specific associations; higher childhood Mn biomarker concentrations were significantly associated with greater sway in females compared with males.

DISCUSSION

This study found childhood and adolescent Mn biomarkers were associated with subclinical neuromotor effects in adolescence. This study demonstrates postural balance as a sensitive measure to assess the association between Mn biomarkers and neuromotor function. https://doi.org/10.1289/EHP13381.

An Overview of the Relationship Between Occupational Manganese Exposure and Parkinsonism

Manganese (Mn) is an essential element used in many industries, such as welding, foundries, the production of metal alloys, especially stainless steel, and the production of dry batteries, pesticides, paints, and explosives. Individuals are exposed to Mn through inhalation of fumes, dermal absorption, and ingestion. This metal is an essential trace element required for normal growth, development, and cellular homeostasis. It has also toxic effects on the central nervous system and can cause Parkinsonism symptoms in exposed patients. Studies on human and animal models reveal that neurons of the globus pallidus, the cerebellum, pons, red nucleus, the thalamus, cortex, and the anterior horn of the spinal cord could be affected by Mn toxicity. Although the diagnosis of manganese-induced Parkinsonism is primarily clinical, there are some supporting features on brain MRI images that may be helpful to objectively distinguish it. This study was designed to review the ways of exposure to Mn, clinical symptoms in case of exposure, and discover the relationship between exposure to Mn and Parkinsonism in the working population.

Neurocognitive function in adult residents of a mining district in Mexico after reducing manganese exposure: Follow-up after 11 years

BACKGROUND

Little is known about the neurotoxic effects of chronic exposure to airborne Mn once exposure has been reduced. The environmentally exposed and the reference adult populations evaluated in 2002 were followed, after an environmental management program (EMP) was implemented to reduce the exposure in a mining district in Mexico.

OBJECTIVE

The aim of this study was to compare the association between exposure to Mn and neurocognitive performance in environmentally exposed and reference groups of adults before and after EMP implementation.

METHODS

In 2013, the same battery of neurocognitive tests used in the initial study (2002) was applied to 58 adults exposed to airborne Mn and 30 adults from the reference community. A cumulative exposure index (CEI) was estimated for the study population before and after the EMP. Categorical outcomes were analyzed using logistic regression, and the resulting ORs were compared between studies. Continuous outcomes were analyzed using linear regression. All models were adjusted for age, years of education, socioeconomic status and blood lead levels.

RESULTS

Exposed adults from the post-EMP study showed an improvement in fine motor and verbal regulation of motor skills (OR < 1) compared to the exposed adults from the pre-EMP study (OR > 1). In both pre- and post-studies, the exposed adults showed a deterioration in their dynamic organization of motor activity compared to the reference group (p < 0.05); however, they showed no significant change in attention and working-memory performance.

DISCUSSION

After four years of a significant reduction in airborne Mn levels resulting from EMP implementation, chronically exposed adults showed an improvement in fine motor and verbal regulation of motor skills; however, the remaining areas of their motor and cognitive functions remained impaired.

Prevalence of Parkinsonism Among Foundry Workers in an Automobile Manufacturing Factory in Tehran

Background

Manganese, as an essential element, has neurotoxic effects on basal ganglia and causes parkinsonism, dystonia, and cognitive symptoms in exposed individuals. Transcranial sonography (TCS) is a noninvasive and easily accessible imaging modality for detecting the accumulation of trace elements in the basal ganglia.

Methodology

In a cross-sectional study of foundry workers of one of the automobile manufacturing companies in 2019, the prevalence of parkinsonism was assessed through neurological examination and brain parenchymal sonography or TCS. The prevalence of parkinsonism according to age, smoking, work experience, marital status, and exposure to manganese was determined.

Results

Among 83 male workers, the prevalence of parkinsonism according to neurological examination, substantia nigra hyperechogenicity on TCS, lentiform nucleus hyperechogenicity, and totally was 33.7%, 9.6%, 10.8%, and 42.2%, respectively. The association between the prevalence of parkinsonism and age, smoking, work experience, marital status, and manganese exposure was evaluated. Parkinsonism according to lentiform nucleus hyperechogenicity was associated with smoking (odds ratio [OR] (95% confidence interval [CI]) = 26.63 (2.38-178.71)) and work experience (OR (95% CI) = 7.18 (0.84-61.32)).

Conclusions

According to this study, the prevalence of parkinsonism based on neurological examination or brain sonography findings was 42.2%. The implementation of this combined screening method might facilitate earlier detection of affected individuals among manganese-exposed workers.

Biomarkers for occupational manganese exposure

Long-term inhalation exposure to manganese (Mn) metal or its inorganic compounds can result in manganism or subclinical neurofunctional deficits. Studies have described affected workers in Mn dioxide mining, Mn-containing ore crushing and milling facilities, manufacturing of dry-cell batteries, Mn steel and alloy production plants, and in welders. The objective of this study was to critically review existing evidence on the reliability of potential biomarkers of Mn exposure, specifically the relationship between inhalation exposure to Mn particulates in different occupational settings and Mn concentrations in blood and other biological fluids and tissues, with a particular focus on whole blood as a potentially useful medium for measuring internal tissue dose. We also examined available evidence on the relationship between Mn levels in blood and adverse clinical and subclinical neurotoxic outcomes. Three bibliographic databases were searched for relevant studies and identified references were screened by two independent reviewers. Of the 6338 unique references identified, 76 articles were retained for data abstraction. Findings indicate that the relationships between Mn in blood and both external Mn exposure indices and neurofunctional impairments are limited and inconsistent. Different sources of exposure to Mn compounds, heterogeneity in the methodological approaches, and inadequate reporting of essential information limited direct comparison of the reported findings. Among the Mn-exposure biomarkers considered in this review - including biomarkers in blood, plasma, serum, erythrocytes, urine, bone, toenails, fingernails, hair, saliva - biomarkers in whole blood may provide to be most useful in Mn biomonitoring and risk assessment.

Biomonitoring and bioaccessibility of environmental airborne manganese in relation to motor function in a healthy adult population

BACKGROUND/AIM

Santander, the capital of Cantabria, Spain (172,000 inhabitants) is 7 km from an industrial emission source (IES) of Mn located in a 10,000 inhabitants town (Maliaño) (annual air Mn arithmetic mean = 231.8 ng/m³; reference WHO guideline = 150 ng/m³). Our objective was to compare the motor function of adult healthy volunteers living in both places.

METHODS

Cross-sectional study analyzing 130 consecutive participants. Exposure to Mn was assessed in terms of source distance from the IES, by Personal Environmental Monitors (PEMs) carried for 24 h by participants consisting of a portable impactor connected to a personal pump, and by biomarkers (blood, hair and fingernails). The impactor allowed the separation of fine (PM2.5) and coarse (PM10-2.5) particles and for each particle size in-vitro bioaccessibility tests with biologically active fluids were performed to separate the soluble (bioaccessible) from the insoluble (non-bioaccessible) fraction. Mean Differences (MDs) adjusted for age, sex, and study level, were obtained for motor function tests results.

RESULTS

Regarding Grooved Pegboard, overall mean time to complete the test was 59.31 and 65.27 seconds (Standard Deviation = 10.11 and 11.69) for dominant and nondominant hands respectively. Statistically significant higher times (indicating worse function) were observed when living near the IES in both hands but MDs of only 1.22 and 2.05 seconds were obtained after adjusting for the predefined confounders (p = 0.373 and 0.221 respectively). Regarding Mn levels in their PEMs (in both bioaccessible and non-bioaccessible coarse&fine fractions) higher times were computed in participants with higher levels for the bioaccessible-fine fraction, with a MD that diminished but still yielded statistical significance after controlling for confounding: adjusted MD = 3.01 more seconds; 95%CI (0.44-5.38), p = 0.022. Poorer results were also observed for fingernails levels. Regarding Finger Tapping Test, no statistically significant differences were found with the exception of Mn fingernails levels.

CONCLUSIONS

Our results suggest poorer motor function as assessed by Grooved Pegboard test in relation to "proximity to IES", "bioaccessible-fine fraction as determined by PEMs and "Mn fingernails levels". However, our findings were affected by confounding, and only the adjusted MD for the Mn bioaccessible-fine fraction remained of sufficient magnitude to maintain statistical significance.

Protective effect of Monoisoamyl-2, 3-Dimercaptosuccinic Acid against Manganese-induced Neurotoxicity in rats

BACKGROUND

Apart from being an essential heavy metal, manganese (Mn) serves as an important component of the antioxidant enzyme system in humans. Overexposure to manganese leads to the development of manganism, which is characterized by motor dysfunction along with neurodegeneration. The management of manganism often utilizes chelation therapy. In this regard, Monoisoamyl-2, 3-Dimercaptosuccinic Acid (MiADMSA) has been reported as a novel arsenic chelator, due to the presence of vicinal sulfhydril group. MiADMSA has been reported to reduce the level in divalent ions (like copper) therefore, it may be hypothesized that MiADMSA would be helpful in Mn-induced neurotoxicity.

OBJECTIVE

This study is envisaged to explore the protective effect of MiADMSA on Mn-induced neurotoxicity.

METHOD

Mn exposure was carried out by intraperitoneal administration of Mn (as manganese chloride, 10 mg/kg; i.p.). The animals were treated with MiADMSA (50 mg/kg; p.o.) either alone or in combination with Mn. The effect of different treatments on neurobehavioral functions was observed by assessing spontaneous locomotor activity, motor rotarod test, and depression-like behavior in the forced swim test. After behavioral evaluations, all the animals were sacrificed and the brain and liver were isolated for metal estimations.

RESULTS

Mn exposure leads to loss of motor coordination as observed in spontaneous locomotor activity and rotarod test. However, treatment with MiADMSA significantly improved motor impairments as compared to Mn exposed animals. Accumulation of Mn in the liver and brain has been recorded with Mn exposure; however, MiADMSA treatment significantly reduced the Mn content from the liver and brain.

CONCLUSION

The outcome of the study suggests that treatment with MiADMSA reversed Mn-induced neurotoxicity by reducing Mn load. Therefore, the use of MiADMSA may be suggested in manganese toxicity, after careful investigation.

Depression and anxiety in a manganese-exposed community

OBJECTIVE

To characterize the association between residential environmental manganese (Mn) exposure and depression and anxiety, given prior associations among occupationally-exposed workers.

METHODS

We administered the Beck Depression Inventory (BDI) and the State-Trait Anxiety Inventory (STAI) to 697 study participants in their preferred languages. These participants represented a population-based sample of residents aged ≥40 from two predominantly Black African communities in Gauteng province, South Africa: 605 in Meyerton, adjacent to a large Mn smelter, and 92 in Ethembalethu, a comparable non-exposed community. We investigated the associations between community (Meyerton vs. Ethembalethu) and severity of depression and anxiety, using linear regression, adjusting for age and sex. To document community-level differences in Mn exposure, we measured airborne PM_2.5-Mn.

RESULTS

Meyerton residents had BDI scores 5.63 points (95 % CI 3.07, 8.20) higher than Ethembalethu residents, with all questions contributing to this significant difference. STAI-state scores were marginally higher in Meyerton than Ethembalethu residents [2.12 (95 % CI -0.17, 4.41)], whereas STAI-trait scores were more similar between the communities [1.26 (95 % CI -0.82, 3.35)]. Mean PM_2.5-Mn concentration was 203 ng/m³ at a long-term fixed site in Meyerton and 10 ng/m³ in Ethembalethu.

CONCLUSION

Residence near Mn emission sources may be associated with greater depression symptomatology, and possibly current, but not lifetime, anxiety.

Severity of parkinsonism associated with environmental manganese exposure

BACKGROUND

Exposure to occupational manganese (Mn) is associated with neurotoxic brain injury, manifesting primarily as parkinsonism. The association between environmental Mn exposure and parkinsonism is unclear. To characterize the association between environmental Mn exposure and parkinsonism, we performed population-based sampling of residents older than 40 in Meyerton, South Africa (N = 621) in residential settlements adjacent to a large Mn smelter and in a comparable non-exposed settlement in Ethembalethu, South Africa (N = 95) in 2016-2020.

METHODS

A movement disorders specialist examined all participants using the Unified Parkinson Disease Rating Scale motor subsection part 3 (UPDRS3). Participants also completed an accelerometry-based kinematic test and a grooved pegboard test. We compared performance on the UPDRS3, grooved pegboard, and the accelerometry-based kinematic test between the settlements using linear regression, adjusting for covariates. We also measured airborne PM_2.5-Mn in the study settlements.

RESULTS

Mean PM_2.5-Mn concentration at a long-term fixed site in Meyerton was 203 ng/m³ in 2016-2017 - approximately double that measured at two other neighborhoods in Meyerton. The mean Mn concentration in Ethembalethu was ~ 20 times lower than that of the long-term Meyerton site. UPDRS3 scores were 6.6 (CI 5.2, 7.9) points higher in Meyerton than Ethembalethu residents. Mean angular velocity for finger-tapping on the accelerometry-based kinematic test was slower in Meyerton than Ethembalethu residents [dominant hand 74.9 (CI 48.7, 101.2) and non-dominant hand 82.6 (CI 55.2, 110.1) degrees/second slower]. Similarly, Meyerton residents took longer to complete the grooved pegboard, especially for the non-dominant hand (6.9, CI -2.6, 16.3 s longer).

CONCLUSIONS

Environmental airborne Mn exposures at levels substantially lower than current occupational exposure thresholds in the United States may be associated with clinical parkinsonism.

Association of exposure to manganese and fine motor skills in welders - Results from the WELDOX II study

The aim of this study was to evaluate the effect of exposure to manganese (Mn) on fine motor functions. A total of 48 welders and 30 unexposed workers as controls completed questionnaires, underwent blood examinations, and a motor test battery. The shift exposure of welders to respirable Mn was measured with personal samplers. For all subjects accumulations of Mn in the brain were assessed with T1-weighted magnetic resonance imaging. Welders showed normal motor functions on the Movement Disorder Society-Sponsored Revision of the Unified Parkinson Disease Rating Scale part III. Furthermore welders performed excellent on a steadiness test, showing better results than controls. However, welders were slightly slower than controls in motor tests. There was no association between fine motor test results and the relaxation rates R1 in globus pallidus and substantia nigra as MRI-based biomarkers to quantify Mn deposition in the brain.

Manganese Exposure and Neurologic Outcomes in Adult Populations

A review of published articles examining the effects of manganese exposure to workers and community residents shows adverse neurologic outcomes. Innovative biomarkers, including those from neuroimaging, were incorporated into many of these studies to assess both manganese exposure and neurologic outcomes. A variety of health effects were evaluated, including cognitive and motor impairments. Studies of community participants residing near manganese point sources show variability in outcomes, reflecting the complexities of exposure measurement, individual absorption, and assessment of neurologic effects. The aging population provides insight into the impacts of chronic exposure in younger populations.

Cognitive profile of patients with manganese-methcathinone encephalopathy

Manganese-methcathinone encephalopathy (MME) is a rare parkinsonian syndrome described in drug addicts who have self-injected a home-made mixture containing methcathinone and manganese. We assessed 14 patients with MME and compared their results with 14 matched control subjects. The patients had a parkinsonian syndrome with symmetrical bradykinesia, dystonias, and postural, gait and speech impairment, with moderate restrictions in activities of daily living. Their cognitive status was assessed with the Russian version of the Wechsler Adult Intelligence Scale (WAIS) and with tests of attention (Trail Making Test, Bourdon-Wiersma Dot Cancellation Test), memory (Auditory Verbal Learning Test, Rey-Osterrieth Complex Figure), motor skills (Grooved Pegboard), visuospatial skills (Money Road Map Test, Benton Judgment of Line Orientation), and executive abilities (Verbal Fluency, 5-Point Test, Wisconsin Card Sorting Test). Only a few significant differences emerged. After controlling for multiple comparisons, the results in the WAIS Object Assembly subtest, the Grooved Pegboard test (dominant and nondominant hand) and the Verbal Fluency test remained significant.

Verbal Memory and Learning in Schoolchildren Exposed to Manganese in Mexico

Manganese (Mn) is an essential nutrient for cellular function, but in high concentrations, it is neurotoxic. Environmental exposure to Mn has been associated with cognitive effects in children. This study aimed to assess the effect of environmental exposure to Mn on verbal memory and learning in schoolchildren residents from two municipalities in the state of Hidalgo, Mexico. Cross-sectional studies were conducted in 2006 and 2013 with a total of 265 schoolchildren of 7 to 11 years old. Children's Auditory Verbal Learning Test-2 (CAVLT-2) was used to assess verbal memory and learning. Mn exposure tertiles were defined according to hair manganese (MnH) levels determined by atomic absorption spectrophotometry. Linear regression models were used to estimate the association between MnH levels and CAVLT-2 scores. The models were adjusted by potential confounders. The lowest and highest exposure tertiles were defined below and above MnH levels of ≤ 0.72 and ≥ 3.96 μg/g, respectively. Mn exposure was significantly associated with an average of 5- to 9-point decrease in learning curves and summary CAVLT-2 scores in the highest tertile. This study adds to the evidence of decreased verbal memory and learning in schoolchildren environmentally exposed to manganese.

Metabolomic Responses to Manganese Dose in SH-SY5Y Human Neuroblastoma Cells

Manganese (Mn)-associated neurotoxicity has been well recognized. However, Mn is also an essential nutrient to maintain physiological function. Our previous study of human neuroblastoma SH-SY5Y cells showed that Mn treatment comparable to physiological and toxicological concentrations in human brain resulted in different mitochondrial responses, yet cellular metabolic responses associated with such different outcomes remain uncharacterized. Herein, SH-SY5Y cells were examined for metabolic responses discriminated by physiological and toxicological levels of Mn using high-resolution metabolomics (HRM). Before performing HRM, we examined Mn dose (from 0 to100 μM) and time effects on cell death. Although we did not observe any immediate cell death after 5 h exposure to any of the Mn concentrations assessed (0-100 μM), cell loss was present after a 24-h recovery period in cultures treated with Mn ≥ 50 μM. Exposure to Mn for 5 h resulted in a wide range of changes in cellular metabolism including amino acids (AA), neurotransmitters, energy, and fatty acids metabolism. Adaptive responses at 10 μM showed increases in neuroprotective AA metabolites (creatine, phosphocreatine, phosphoserine). A 5-h exposure to 100 µM Mn, a time before any cell death occurred, resulted in decreases in energy and fatty acid metabolites (hexose-1,6 bisphosphate, acyl carnitines). The results show that adjustments in AA metabolism occur in response to Mn that does not cause cell death while disruption in energy and fatty acid metabolism occur in response to Mn that results in subsequent cell death. The present study establishes utility for metabolomics analyses to discriminate adaptive and toxic molecular responses in a human in vitro cellular model that could be exploited in evaluation of Mn toxicity.

Biomonitoring of metals and trace elements in urine of central Ethiopian populations

Biomonitoring of chemical concentrations in humans is important for detecting, monitoring, and addressing a wide range of health threats. However, it is virtually absent across many African nations, including Ethiopia. This study aims to determine urinary concentrations for metals and trace elements in populations living in the central Ethiopian Rift Valley. The region is unindustralized, rural, and characterized by unique geologic rifting and volcanic activities that have produced vast pyroclastic materials, forming its aquifer and fertile agricultural soils. Millions of people in the region rely on wells for drinking water and are engaged in cereal-based subsistence agriculture. We enrolled a total of 386 residents aged 10-50 years old (201 females and 185 males). The levels of 23 elements except F^─ were quantified in water and urine samples by ICP-MS. Mean concentrations of B, F^─, Ca, and Mg were measured in mg/L levels, while concentrations of Mo, Zn, Sr, Rb, and Li ranged between 100 and 700 μg/L. Mean concentrations between 5 and 15 μg/L were found for Ni, Cu, and Mn, while Ag, Be, Cd, Co, Pb, Sb, Th, TI, and U were all below 5 μg/L. Arsenic and Al had mean concentrations between 30 and 50 μg/L. Mean urinary concentrations of Ca, Cu, Mg, Pb, Sr, and Zn were significantly higher in males than females, whereas Co and Mn were higher in females. Finally, younger individuals (10-30 years) had significantly higher mean concentrations of B, Cd, Co, Mg, Mo, and Pb than those between 31 and 50 years, whereas only Ca was higher in the older age group. The concentration ranges of B, Mo, Mn, TI, Li, Zn, and in particular F^─ (0.44-44.6 mg/L) and As (2.2-164 μg/L) in urine were higher than the reference ranges reported in healthy unexposed North American and European populations, while those for the remaining 16 elements were comparable to published reference ranges from such settings. The established concentration ranges are important to monitor future changes in exposure, and risk factors for disease, that might stem from the economic growth and industrialization that is currently underway in the region.

Airborne manganese exposure and neurobehavior in school-aged children living near a ferro-manganese alloy plant

Excessive exposure to Mn can lead to its accumulation in the brain with neurotoxic consequences. In children, elevated Mn has been associated with deficits in certain neuropsychological domains such as cognition, motor function, memory and attention, and in some instances, hyperactivity and behavioral problems. The aim of this study was to evaluate behavioral effects in school-aged children living near a ferro-manganese alloy plant and examine their association with Mn exposure. Occipital hair, toenails and blood samples were collected from 225 children (7-12 years old) enrolled in four elementary schools with different levels of exposure to Mn, based on dust Mn deposition rates. Full data set collection was completed and run from 165 children. Mn in hair (MnH), toenails (MnTn), blood (MnB) and blood lead levels (PbB) were determined by graphite furnace atomic absorption spectrometry. Children's behavior was assessed with the Child Behavior Check List (CBCL) reported by parents. Median levels and range of MnH, MnT and MnB were, respectively, 0.73 µg/g (0.16-8.79), 0.84 µg/g (0.15-9.29) and 8.98 μg/L (1.51-40.43). Median and range of PbB were 1.2 µg/dL (0.2-15.6). MnH and MnB were not associated with any scale of the CBCL behavior scores. We found a positive association between logMnTn and raw total CBCL score (β = 10.17, p = 0.034), adjusting for sex, age, maternal IQ and logPbB. Analyses using Generalized Additive Model showed non-linear associations between MnTn and externalizing behavior (p = 0.035), as well as with the related subscales: aggressive behavior (p = 0.045) and rule-breaking behavior (p = 0.024). Further positive associations were observed between MnTn and thought problems (p = 0.031) and social problems (p = 0.027). These findings corroborate previous studies showing an association between Mn exposures and externalizing behavior. Our results suggest that toenail Mn, as a biomarker of environmental exposure, is associated with disruptive behavior in children living near a ferro-manganese alloy plant.

Occupational Exposure to Manganese and Fine Motor Skills in Elderly Men: Results from the Heinz Nixdorf Recall Study

Objectives

Exposure to manganese (Mn) may cause movement disorders, but less is known whether the effects persist after the termination of exposure. This study investigated the association between former exposure to Mn and fine motor deficits in elderly men from an industrial area with steel production.

Methods

Data on the occupational history and fine motor tests were obtained from the second follow-up of the prospective Heinz Nixdorf Recall Study (2011-2014). The study population included 1232 men (median age 68 years). Mn in blood (MnB) was determined in archived samples (2000-2003). The association between Mn exposure (working as welder or in other at-risk occupations, cumulative exposure to inhalable Mn, MnB) with various motor functions (errors in line tracing, steadiness, or aiming and tapping hits) was investigated with Poisson and logistic regression, adjusted for iron status and other covariates. Odds ratios (ORs) with 95% confidence intervals (CIs) were estimated for substantially impaired dexterity (errors >90th percentile, tapping hits <10th percentile).

Results

The median of cumulative exposure to inhalable Mn was 58 µg m-3 years in 322 men who ever worked in at-risk occupations. Although we observed a partly better motor performance of exposed workers at group level, we found fewer tapping hits in men with cumulative Mn exposure >184.8 µg m-3 years (OR 2.15, 95% CI 1.17-3.94). MnB ≥ 15 µg l-1, serum ferritin ≥ 400 µg l-1, and gamma-glutamyl transferase ≥74 U l-1 were associated with a greater number of errors in line tracing.

Conclusions

We found evidence that exposure to inhalable Mn may carry a risk for dexterity deficits. Whether these deficits can be exclusively attributed to Mn remains to be elucidated, as airborne Mn is strongly correlated with iron in metal fumes, and high ferritin was also associated with errors in line tracing. Furthermore, hand training effects must be taken into account when testing for fine motor skills.

Exposure of Drosophila melanogaster to Mancozeb Induces Oxidative Damage and Modulates Nrf2 and HSP70/83

Mancozeb (MZ), a manganese- and zinc-containing ethylene-bis-dithiocarbamate, is a broad-spectrum fungicide. Harmful effects of this fungicide have been reported in nontarget organisms via a not fully understood mechanism. Drosophila melanogaster has provided remarkable contributions for toxicological studies. This work was aimed at evaluating the biochemical targets and implication of oxidative stress in MZ-mediated toxicity in drosophilas. Exposure of flies for fifteen days to MZ at 5 and 10 mg/mL through the diet impaired locomotor performance and induced fly mortality. In parallel, it caused lipid peroxidation and reactive oxygen species (ROS) formation and Mn overload. MZ inhibited superoxide dismutase and inducted catalase and glutathione S-transferase activities. Nitric oxide and reduced glutathione levels were significantly decreased by MZ. Heat shock proteins (HSP70 and HSP83) and Nrf2 mRNA levels were significantly augmented in MZ-exposed flies. Our study reinforced the use of Drosophila melanogaster as a reliable model for the study of biochemical targets of pesticides, and based on our data, MZ induced oxidative damage and Mn accumulation in a concentration-dependent manner. An adaptative cellular state was inducted by the lower concentration of pesticide, possibly contributing to the slighter damage observed.

Manganese and lead levels in settled dust in elementary schools are correlated with biomarkers of exposure in school-aged children

Previously, we showed that manganese (Mn) levels in settled dust in elementary schools increased at a rate of 34.1% per km closer to a ferro-manganese alloy plant in the rainy season. In this study, we investigated how this environmental pollution indicator varied in the dry season and if there was an association with Mn biomarker levels in school-aged children. Dust samples were collected with passive samplers (disposable Petri dishes) placed in interior and exterior environments of 14 elementary schools. Occipital hair, toenails and blood samples were collected from 173 students aged 7-12 years from three of these schools, with varying distance from the industrial plant. Mn and lead (Pb) levels were measured by graphite furnace atomic absorption spectrometry. Mn concentration geometric means (GM) in dust fall accumulation in interior environments of schools located at 2, 4, 6 and > 6 km-radii from the plant were 2212, 584, 625 and 224 μg Mn/m²/30 days, respectively. The modelled rate of change of dust Mn levels decreases by 59.8% for each km further from the plant. Pb levels in settled dust varied between 18 and 81 μg/m²/30 days with no association with distance from the plant. Blood lead levels median (range) were 1.2 μg/dL (0.2-15.6), of which 97.8% were <5 μg/dL. Mn in hair and toenails were 0.66 μg/g (0.16-8.79) and 0.86 μg/g (0.15-13.30), respectively. Mn loading rates were positively associated with log MnH (β = 1.42 × 10^-5, p < 0.001) after adjusting for children's age; and also with log MnTn (β = 2.31 × 10^-5, p < 0.001) independent of age. Mn loading rates explained 18.5% and 28.5% of the variance in MnH and MnTn levels, respectively. School-aged children exposure to Mn, independently of age, increases significantly with school proximity to the ferro-manganese alloy plant.

Determinants of Hair Manganese, Lead, Cadmium and Arsenic Levels in Environmentally Exposed Children

Biomarkers of environmental metal exposure in children are important for elucidating exposure and health risk. While exposure biomarkers for As, Cd, and Pb are relatively well defined, there are not yet well-validated biomarkers of Mn exposure. Here, we measured hair Mn, Pb, Cd, and As levels in children from the Mid-Ohio Valley to determine within and between-subject predictors of hair metal levels. Occipital scalp hair was collected in 2009-2010 from 222 children aged 6-12 years (169 female, 53 male) participating in a study of chemical exposure and neurodevelopment in an industrial region of the Mid-Ohio Valley. Hair samples from females were divided into three two centimeter segments, while males provided a single segment. Hair was cleaned and processed in a trace metal clean laboratory, and analyzed for As, Cd, Mn, and Pb by magnetic sector inductively coupled plasma mass spectrometry. Hair Mn and Pb levels were comparable (median 0.11 and 0.15 µg/g, respectively) and were ~10-fold higher than hair Cd and As levels (0.007 and 0.018 µg/g, respectively). Hair metal levels were higher in males compared to females, and varied by ~100-1000-fold between all subjects, and substantially less (<40-70%) between segments within female subjects. Hair Mn, Pb, and Cd, but not As levels systematically increased by ~40-70% from the proximal to distal hair segments of females. There was a significant effect of season of hair sample collection on hair Mn, Pb, and Cd, but not As levels. Finally, hair metal levels reported here are ~2 to >10-fold lower than levels reported in other studies in children, most likely because of more rigorous hair cleaning methodology used in the present study, leading to lower levels of unresolved exogenous metal contamination of hair.

Manganese Exposure and Cognition Across the Lifespan: Contemporary Review and Argument for Biphasic Dose-Response Health Effects

Manganese (Mn) is both an essential micronutrient and potential neurotoxicant. This dual role underlies a growing body of literature demonstrating that Mn exhibits a biphasic dose-response relationship with neurocognitive outcomes. We reviewed recent epidemiologic studies from 2007 to 2016 that investigated the relationship between Mn exposure and cognitive outcomes across the lifespan: early life, school-aged children, and adulthood. In total, 27 research articles were included in this review: 12 pediatric and 15 adult studies (10 occupational and five environmental exposures). The majority of these studies provided evidence of the negative effects of Mn exposure on cognition. The pediatric literature provides evidence that both high and low levels of Mn are negatively associated with intellectual development. Future Mn research should include examination of non-linear relationships and multiple neurotoxicants across the lifespan and particularly during critical developmental windows.

Manganese promotes intracellular accumulation of AQP2 via modulating F-actin polymerization and reduces urinary concentration in mice

Aquaporin-2 (AQP2) is a water channel protein expressed in principal cells (PCs) of the kidney collecting ducts (CDs) and plays a critical role in mediating water reabsorption and urine concentration. AQP2 undergoes both regulated trafficking mediated by vasopressin (VP) and constitutive recycling, which is independent of VP. For both pathways, actin cytoskeletal dynamics is a key determinant of AQP2 trafficking. We report here that manganese chloride (MnCl₂) is a novel and potent regulator of AQP2 trafficking in cultured cells and in the kidney. MnCl₂ treatment promoted internalization and intracellular accumulation of AQP2. The effect of MnCl₂ on the intracellular accumulation of AQP2 was associated with activation of RhoA and actin polymerization without modification of AQP2 phosphorylation. Although the level of total and phosphorylated AQP2 did not change, MnCl₂ treatment impeded VP-induced phosphorylation of AQP2 at its serine-256, -264, and -269 residues and dephosphorylation at serine 261. In addition, MnCl₂ significantly promoted F-actin polymerization along with downregulation of RhoA activity and prevented VP-induced membrane accumulation of AQP2. Finally, MnCl₂ treatment in mice resulted in significant polyuria and reduced urinary concentration, likely due to intracellular relocation of AQP2 in the PCs of kidney CDs. More importantly, the reduced urinary concentration caused by MnCl₂ treatment in animals was not corrected by VP. In summary, our study identified a novel effect of MnCl₂ on AQP2 trafficking through modifying RhoA activity and actin polymerization and uncovered its potent impact on water diuresis in vivo.

Manganese and neurobehavioral impairment. A preliminary risk assessment

Similar patterns of cognitive and motor deficits have been widely reported from manganese exposures in welding, metallurgical and chemical industry workers. A risk assessment was performed based on studies reported in the literature, extending some earlier work, and deriving new estimates of exposure response and excess risk. Many investigations of manganese neurological effects in humans have insufficient information to derive an exposure response; however, findings from a chemical manufacturer, two smelter and two welder populations permitted application of the benchmark dose procedure for continuous end-points. Small particles and aggregates of condensation fume (condensing vaporized metal, <0.1μm in diameter) appear to have a higher potency per unit mass than larger particles from dusts (>1.0μm). Consideration was given to long-term effects of continuous low exposures that instead of producing increasing toxicity attain a steady-state condition. Impairment was defined as excursions beyond the 5th percentile in a normal population and the concentrations of manganese predicted to result in 1% excess prevalence of impairment over different time periods were calculated. Over five years, exposures resulting in 1% excess prevalence of impairment (for purposes of discussion) were in the vicinity of 10μg/m³ for manganese fume and 25μg/m³ for larger particle dusts. These levels are below current recommendations for occupational limits on manganese exposure in the United States.

Welding-related brain and functional changes in welders with chronic and low-level exposure

Although an essential nutrient, manganese (Mn) can be toxic at high doses. There is, however, uncertainty regarding the effects of chronic low-level Mn-exposure. This review provides an overview of Mn-related brain and functional changes based on studies of a cohort of asymptomatic welders who had lower Mn-exposure than in most previous work. In welders with low-level Mn-exposure, we found: 1) Mn may accumulate in the brain in a non-linear fashion: MRI R1 (1/T1) signals significantly increased only after a critical level of exposure was reached (e.g., ≥300 welding hours in the past 90days prior to MRI). Moreover, R1 may be a more sensitive marker to capture short-term dynamic changes in Mn accumulation than the pallidal index [T1-weighted intensity ratio of the globus pallidus vs. frontal white matter], a traditional marker for Mn accumulation; 2) Chronic Mn-exposure may lead to microstructural changes as indicated by lower diffusion tensor fractional anisotropy values in the basal ganglia (BG), especially when welding years exceeded more than 30 years; 3) Mn-related subtle motor dysfunctions can be captured sensitively by synergy metrics (indices for movement stability), whereas traditional fine motor tasks failed to detect any significant differences; and 4) Iron (Fe) also may play a role in welding-related neurotoxicity, especially at low-level Mn-exposure, evidenced by higher R2* values (an estimate for brain Fe accumulation) in the BG. Moreover, higher R2* values were associated with lower phonemic fluency performance. These findings may guide future studies and the development of occupation- and public health-related polices involving Mn-exposure.

Methylphenidate alleviates manganese-induced impulsivity but not distractibility

Recent studies from our lab have demonstrated that postnatal manganese (Mn) exposure in a rodent model can cause lasting impairments in fine motor control and attention, and that oral methylphenidate (MPH) treatment can effectively treat the dysfunction in fine motor control. However, it is unknown whether MPH treatment can alleviate the impairments in attention produced by Mn exposure. Here we used a rodent model of postnatal Mn exposure to determine whether (1) oral MPH alleviates attention and impulse control deficits caused by postnatal Mn exposure, using attention tasks that are variants of the 5-choice serial reaction time task, and (2) whether these treatments affected neuronal dendritic spine density in the medial prefrontal cortex (mPFC) and dorsal striatum. Male Long-Evans rats were exposed orally to 0 or 50Mn/kg/d throughout life starting on PND 1, and tested as young adults (PND 107-115) on an attention task that specifically tapped selective attention and impulse control. Animals were treated with oral MPH (2.5mg/kg/d) throughout testing on the attention task. Our findings show that lifelong postnatal Mn exposure impaired impulse control and selective attention in young adulthood, and that a therapeutically relevant oral MPH regimen alleviated the Mn-induced dysfunction in impulse control, but not selective attention, and actually impaired focused attention in the Mn group. In addition, the effect of MPH was qualitatively different for the Mn-exposed versus control animals across a range of behavioral measures of inhibitory control and attention, as well as dendritic spine density in the mPFC, suggesting that postnatal Mn exposure alters catecholaminergic systems modulating these behaviors. Collectively these findings suggest that MPH may hold promise for treating the behavioral dysfunction caused by developmental Mn exposure, although further research is needed with multiple MPH doses to determine whether a dose can be identified that ameliorates the dysfunction in both impulse control and selective attention, without impairing focused attention.

Manganese exposure facilitates microglial JAK2-STAT3 signaling and consequent secretion of TNF-a and IL-1β to promote neuronal death

Chronic manganese (Mn) exposure can lead to neuroinflammation and neurological deficit, which resemble idiopathic Parkinson's disease (IPD). However, the precise mechanisms underlying Mn exposure-induced neurotoxicity remain incompletely understood. Microglia can become hyperactivated and plays a vital role in neuroinflammation and consequent neurodegeneration in response to pro-inflammatory stimuli. In the present study, we found that HAPI microglial cells exhibited increased secretion of pro-inflammatory TNF-α and IL-1β following Mn exposure in dose- and time-dependent manners. In addition, we showed that Mn exposure could trigger the activation of JAK2/STAT3 signaling pathway in microglia. Notably, Mn-induced secretion of TNF-α and IL-1β was significantly attenuated by the treatment of JAK2 inhibitor. Finally, through incubating PC12 neuronal cells with Mn-treated microglial conditioned medium, we demonstrated that Mn-induced secretion of microglial TNF-α and IL-1β facilitated neuronal apoptosis. Thus, we speculate that Mn exposure might trigger JAK2-STAT3 signal pathway in microglia, leading to resultant neuroinflammation and neuronal loss.

A tissue dose-based comparative exposure assessment of manganese using physiologically based pharmacokinetic modeling-The importance of homeostatic control for an essential metal

A physiologically-based pharmacokinetic (PBPK) model (Schroeter et al., 2011) was applied to simulate target tissue manganese (Mn) concentrations following occupational and environmental exposures. These estimates of target tissue Mn concentrations were compared to determine margins of safety (MOS) and to evaluate the biological relevance of applying safety factors to derive acceptable Mn air concentrations. Mn blood concentrations measured in occupational studies permitted verification of the human PBPK models, increasing confidence in the resulting estimates. Mn exposure was determined based on measured ambient air Mn concentrations and dietary data in Canada and the United States (US). Incorporating dietary and inhalation exposures into the models indicated that increases in target tissue concentrations above endogenous levels only begin to occur when humans are exposed to levels of Mn in ambient air (i.e. >10μg/m³) that are far higher than those currently measured in Canada or the US. A MOS greater than three orders of magnitude was observed, indicating that current Mn air concentrations are far below concentrations that would be required to produce the target tissue Mn concentrations associated with subclinical neurological effects. This application of PBPK modeling for an essential element clearly demonstrates that the conventional application of default factors to "convert" an occupational exposure to an equivalent continuous environmental exposure, followed by the application of safety factors, is not appropriate in the case of Mn. PBPK modeling demonstrates that the relationship between ambient Mn exposures and dose-to-target tissue is not linear due to normal tissue background levels and homeostatic controls.

"Manganese-induced neurotoxicity: a review of its behavioral consequences and neuroprotective strategies"

Manganese (Mn) is an essential heavy metal. However, Mn’s nutritional aspects are paralleled by its role as a neurotoxicant upon excessive exposure. In this review, we covered recent advances in identifying mechanisms of Mn uptake and its molecular actions in the brain as well as promising neuroprotective strategies. The authors focused on reporting findings regarding Mn transport mechanisms, Mn effects on cholinergic system, behavioral alterations induced by Mn exposure and studies of neuroprotective strategies against Mn intoxication. We report that exposure to Mn may arise from environmental sources, occupational settings, food, total parenteral nutrition (TPN), methcathinone drug abuse or even genetic factors, such as mutation in the transporter SLC30A10. Accumulation of Mn occurs mainly in the basal ganglia and leads to a syndrome called manganism, whose symptoms of cognitive dysfunction and motor impairment resemble Parkinson’s disease (PD). Various neurotransmitter systems may be impaired due to Mn, especially dopaminergic, but also cholinergic and GABAergic. Several proteins have been identified to transport Mn, including divalent metal tranporter-1 (DMT-1), SLC30A10, transferrin and ferroportin and allow its accumulation in the central nervous system. Parallel to identification of Mn neurotoxic properties, neuroprotective strategies have been reported, and these include endogenous antioxidants (for instance, vitamin E), plant extracts (complex mixtures containing polyphenols and non-characterized components), iron chelating agents, precursors of glutathione (GSH), and synthetic compounds that can experimentally afford protection against Mn-induced neurotoxicity.

Synergy as a new and sensitive marker of basal ganglia dysfunction: A study of asymptomatic welders

BACKGROUND

Multi-digit synergies, a recently developed, theory-based method to quantify stability of motor action, are shown to reflect basal ganglia dysfunction associated with parkinsonian syndromes. In this study, we tested the hypothesis that multi-digit synergies may capture early and subclinical basal ganglia dysfunction. We chose asymptomatic welders to test the hypothesis because the basal ganglia are known to be most susceptible to neurotoxicity caused by welding-related metal accumulation (such as manganese and iron).

METHODS

Twenty right-handed welders and 13 matched controls were invited to perform single- and multi-finger pressing tasks using the fingers of the right or left hand. Unified Parkinson's Disease Rating Scale and Grooved Pegboard scores were used to gauge gross and fine motor dysfunction, respectively. High-resolution (3T) T1-weighted, T2-weighted, T1 mapping, susceptibility, and diffusion tensor MRIs were obtained to reflect manganese, iron accumulation, and microstructural changes in basal ganglia. The synergy index stabilizing total force and anticipatory synergy adjustments were computed, compared between groups, and correlated with estimates of basal ganglia manganese [the pallidal index, R1 (1/T1)], iron [R2* (1/T2*)], and microstructural changes [fractional anisotropy and mean diffusivity].

RESULTS

There were no significant differences in Unified Parkinson's Disease Rating Scale (total or motor subscale) or Grooved Pegboard test scores between welders and controls. The synergy index during steady-state accurate force production was decreased significantly in the left hand of welders compared to controls (p=0.004) but did not reach statistical significance in the right hand (p=0.16). Anticipatory synergy adjustments, however, were not significantly different between groups. Among welders, higher synergy indices in the left hand were associated significantly with higher fractional anisotropy values in the left globus pallidus (R=0.731, p<0.001) but not with the pallidal index, R1, or R2* values in the basal ganglia.

CONCLUSIONS

These data suggest that multi-digit synergy metrics may serve as preclinical markers for basal ganglia dysfunction in welders and other populations at risk for neurodegenerative diseases involving parkinsonian symptoms. This finding may have important clinical, scientific, and public/occupational health implications.

Setting evidence-based occupational exposure limits for manganese

In 2004, a review by the Institute of Environment and Health (IEH) made recommendations on occupational exposure limits (OELs) for manganese and its inorganic compounds for inhalable and respirable fractions respectively. These OELs were based on a detailed comprehensive evaluation of all the scientific data available at that time. Since then, more published studies have become available and a number of occupational standard-setting committees (EU SCOEL, US ACGIH-TLV, and German MAK) have proposed OEL's for manganese and its inorganic compounds that are somewhat lower that those proposed in the 2004 review. Based on current understanding, the key toxicological and human health issues that are likely to influence a health-based recommendation relate to: neurotoxicology; reproductive and developmental toxicology; and mutagenicity/carcinogenicity. Of these, it is generally considered that neurotoxicity presents the most sensitive endpoint. As such, many of the studies that have been reported since the IEH review have sought to use those neurofunctional tests that appear to be particularly sensitive at identifying the subtle neurological changes thought to associate with manganese toxicity. These recent studies have, however, continued to be limited to a significant extent by reliance on cross-sectional designs and also by use of unreliable exposure estimation methods. Consequently the strength of the potential association between manganese exposure and these subtle subclinical cognitive or neuromotor changes is still poorly characterised and the relevance of these minor differences in terms of either their clinical or quality of life consequences remains unknown. Based upon the overall evidence, it is concluded that the 8-h time weighted averages (TWA) for respirable (0.05mg/m³ as Mn) and inhalable (0.2mg/m³ as Mn) fractions as recommended by the SCOEL in 2011 are the most methodologically-sound, as they are based on the best available studies, most suited to the development of health-based OELs for both respirable and inhalable fractions. The dose-response characterisation informed by the examined studies used can be considered to establish a true human NOAEL for all the neurofunctional endpoints examined within the selected studies.

Evaluating suspected work-related neurologic disorders (clinical diagnosis)

The clinical diagnosis of work-related neurologic disorders is essentially one of exclusion because symptoms and signs are often nonspecific. The clinical reasoning requires a three-step approach: (1) establish the characteristics of the presenting disease; (2) ascertain that observed clinical features are consistent with those caused by the suspected agent(s); and (3) assess occupational exposures. A detailed history is of paramount importance in evaluating patients with suspected work-related neurologic disorders as it is in other clinical contexts, especially because in some circumstances it may represent the only criterion to establish causality. Thus, besides characterization of neurologic symptoms, including their location, quality, timecourse, and possible other associated symptoms, the work environment of the patient should be understood in full detail. In this respect, when a neurotoxin is suspected, then the history collection can be guided by the knowledge of the likely syndromes it produces. Similarly, physical examination should be directed to the target of toxicity/entrapment based on information from the work history. Although specific sites and elements of the nervous system may be affected depending on the offending agent, most neurotoxic disorders are characterized by generalized rather than focal neurologic abnormalities. Laboratory toxicologic tests have limited application for the etiologic diagnosis of neurotoxic disorders, except in cases of acute poisoning and in patients exposed to neurotoxic chemicals with prolonged half-life. In most cases examination takes place after the end of exposure, when the offending chemical is no longer detectable in body fluids. Electrophysiologic studies, in particular evoked potentials, electromyography, and conduction velocities, are important to confirm the organic basis of symptoms, particularly to detect subclinical or early neurologic involvement and to reduce the number of disorders to be considered in the differential diagnoses. In general, imaging studies with computed tomography and magnetic resonance are of limited utility in the evaluation of suspected neurotoxic disorders, except for helping to exclude other causes of the patient's clinical state. Improved conditions and safer practices in the workplace have led to a gradual shift in application of neuropsychologic evaluation from the assessment of severe neurotoxic damage to the evaluation of mild subclinical disturbances, and these tests are nowadays extensively used in screening workers exposed to neurotoxicants. Tools used in the screening of large groups of workers exposed to neurotoxicants may differ from those used in the clinic. Whereas some are obviously impractical, such as physical examination, others, such as, for instance, toxicologic tests, are used for biologic monitoring of exposure to ascertain compliance with occupational exposure limits.

Airborne manganese as dust vs. fume determining blood levels in workers at a manganese alloy production plant

The appropriate exposure metrics for characterizing manganese (Mn) exposure associated with neurobehavioral effects have not been established. Blood levels of Mn (B-Mn) provide a potentially important intermediate marker of Mn airborne exposures. Using data from a study of a population of silicon- and ferro-manganese alloy production workers employed between 1973 and 1991, B-Mn levels were modeled in relation to prior Mn exposure using detailed work histories and estimated respirable Mn concentrations from air-sampling records. Despite wide variation in exposure levels estimated for individual jobs, duration of employment (exposure) was itself a strong predictor of B-Mn levels and strongest when an 80-day half-life was applied to contributions over time (t=6.95, 7.44, respectively; p<10(-5)). Partitioning exposure concentrations based on process origin into two categories: (1) "large" respirable particulate (Mn-LRP) derived mainly from mechanically generated dust, and (2) "small" respirable particulate (Mn-SRP) primarily electric furnace condensation fume, revealed that B-Mn levels largely track the small, fume exposures. With a half-life of 65 days applied in a model with cumulative exposure terms for both Mn-LRP (t=-0.16, p=0.87) and Mn-SRP (t=6.45, p<10(-5)), the contribution of the large-size fraction contribution was negligible. Constructing metrics based on the square root of SRP exposure concentrations produced a better model fit (t=7.87 vs. 7.44, R(2)=0.2333 vs. 0.2157). In a model containing both duration (t=0.79, p=0.43) and (square root) fume (t=2.47, p=0.01) metrics, the duration term was a weak contributor. Furnace-derived, small respirable Mn particulate appears to be the primary contributor to B-Mn levels, with a dose-rate dependence in a population chronically exposed to Mn, with air-concentrations declining in recent years. These observations may reflect the presence of homeostatic control of Mn levels in the blood and other body tissues and be useful in assessing Mn exposures for evaluating neurotoxic effects.

Respiratory manganese particle size, time-course and neurobehavioral outcomes in workers at a manganese alloy production plant

The progression of manganism with chronic exposure to airborne manganese (Mn) is not well understood. Here, we further investigate the findings on exposure and neurobehavioral outcomes of workers from a silico- and ferromanganese production plant and non-exposed workers from the same community in 1990 and 2004, using a variety of exposure metrics that distinguish particle size and origin within the range of respirable airborne exposures. Mn exposure matrices for large respirable particulate (Mn-LRP, dust) and small respirable particulate (Mn-SRP, fume), based on process origins, were used together with detailed work histories since 1973 (plant opening), to construct exposure metrics including burdens and cumulative burdens with various clearance half-lives. For three out of eight 1990 neurobehavioral tests analyzed with linear regression models, duration of Mn exposure was the best predictor: Luria-Nebraska Neuropsychological Battery - Motor Scale, Trail-Making B and Finger Tapping. The Luria-Nebraska Motor Scale had the strongest association (t ∼ 5.0, p < 10(-6)). For outcomes on three other tests, the duration and Mn-SRP metrics were comparable: Trail Making Test A, Cancellation H and Stroop Color-Word Test (color/word subtest). Delayed Word Recall was best predicted by Mn-SRP (based on square root or truncated air-concentrations). The Word score on the Stroop Color-Word Test was the only outcome for which Mn-LRP was the leading predictor (t = -2.92, p = 0.003), while performance on the WAIS-R Digit Span Test was not significantly predicted by any metric. For outcomes evaluated in both 1990 and 2004, a mixed-effect linear regression model was used to examine estimates of within-individual trends. Duration and Mn-SRP were associated with performance on the Luria-Nebraska Motor Scale, as well as with other outcomes that appeared to have both reversible and progressive features, including Trail Making A and B, Cancellation H and Delayed Word Recall. With the mixed-effect model, Digit Span exhibited a significant irreversible association with exposure duration (t = -2.34, p = 0.021) and Mn-SRP (square root; t = -2.38, p = 0.019) metrics. The strong prediction using duration of exposure is consistent with effective homeostatic regulation of tissue-level Mn in the observed exposure range of respirable Mn (< 0.2mg/m(3)).

Nicotinamide adenine dinucleotide prevents neuroaxonal degeneration induced by manganese in cochlear organotypic cultures

Manganese (Mn) is an essential trace mineral for normal growth and development. Persistent exposures to high atmospheric levels of Mn have deleterious effects on CNS and peripheral nerves including those associated with the auditory system. Nicotinamide adenine dinucleotide (NAD) is a coenzyme which functions in the electron transfer system within the mitochondria. One of the most notable protective functions of NAD is to delay axonal degenerations caused by various neurodegenerative injuries. We hypothesized that NAD might also protect auditory nerve fibers (ANF) and SGN from Mn injury. To test this hypothesis, cochlear organotypic cultures were treated with different doses of Mn (0.5-3.0 mM) alone or combined with 20 mM NAD. Results demonstrate that the percentage of hair cells, ANF and SGN decreased with increasing Mn concentration. The addition of 20 mM NAD did not significantly reduce hair cells loss in the presence of Mn, whereas the density of ANF and SGN increased significantly in the presence of NAD. NAD suppressed Mn-induced TUNEL staining and caspase activation suggesting it prevents apoptotic cell death. These results suggest that excess Mn has ototoxic and neurotoxic effects on the auditory system and that NAD may prevent Mn-induced axonal degeneration and avoid or delay hearing loss caused by excess Mn exposure.

Screening for early detection of parkinsonism using a self-administered questionnaire: a cross-sectional epidemiologic study

BACKGROUND

Manganese (Mn) is a common component of welding fume. Exposure to Mn fume has been associated with parkinsonism. A simple and reliable screening tool to evaluate Mn exposed workers for neurotoxic injury would have broad occupational health application.

METHODS

This study investigated 490 occupational welders recruited from a trade union list. Subjects were examined by a movement disorders specialist using the Unified Parkinson Disease Rating Scale motor subsection 3 (UPDRS3). Parkinsonism, intermediate, and normal groups were defined as UPDRS3 score ≥ 15, 6-15, and <6, respectively. Workers completed a health status questionnaire (PDQ39) and a Parkinson disease (PD) Symptoms Questionnaire. Areas under receiver operator curve (AUC) were analyzed based on these scores, adjusted for age, smoking, race, gender, and neurologist, using normal as the reference.

RESULTS

The AUC was 0.79 (95% confidence interval [CI]=0.73-0.84) for PDQ39 and 0.78 (95% CI=0.72-0.85) for PD Symptoms Questionnaire score. At 70% sensitivity, the specificity for PDQ39 score and PD Symptoms Questionnaire score for the prediction of parkinsonism was 73.1% and 80.1%, respectively.

CONCLUSIONS

These results suggest the questionnaires have reasonably good sensitivity and specificity to predict parkinsonism in Mn exposed workers. These questionnaires could be a valuable first step in a tiered screening approach for Mn exposed workers.

Neurobehavioral deficits and parkinsonism in occupations with manganese exposure: a review of methodological issues in the epidemiological literature

Exposure to manganese (Mn) is associated with neurobehavioral effects. There is disagreement on whether commonly occurring exposures in welding, ferroalloy, and other industrial processes produce neurologically significant neurobehavioral changes representing parkinsonism. A review of methodological issues in the human epidemiological literature on Mn identified: (1) studies focused on idiopathic Parkinson disease without considering manganism, a parkinsonian syndrome; (2) studies with healthy worker effect bias; (3) studies with problematic statistical modeling; and (4) studies arising from case series derived from litigation. Investigations with adequate study design and exposure assessment revealed consistent neurobehavioral effects and attributable subclinical and clinical signs and symptoms of impairment. Twenty-eight studies show an exposure-response relationship between Mn and neurobehavioral effects, including 11 with continuous exposure metrics and six with three or four levels of contrasted exposure. The effects of sustained low-concentration exposures to Mn are consistent with the manifestations of early manganism, i.e., consistent with parkinsonism. This is compelling evidence that Mn is a neurotoxic chemical and there is good evidence that Mn exposures far below the current US standard of 5.0 mg/m³ are causing impairment.

Intranasal exposure to manganese disrupts neurotransmitter release from glutamatergic synapses in the central nervous system in vivo

Chronic exposure to aerosolized manganese induces a neurological disorder that includes extrapyramidal motor symptoms and cognitive impairment. Inhaled manganese can bypass the blood-brain barrier and reach the central nervous system by transport down the olfactory nerve to the brain's olfactory bulb. However, the mechanism by which Mn disrupts neural function remains unclear. Here we used optical imaging techniques to visualize exocytosis in olfactory nerve terminals in vivo in the mouse olfactory bulb. Acute Mn exposure via intranasal instillation of 2-200 μg MnCl(2) solution caused a dose-dependent reduction in odorant-evoked neurotransmitter release, with significant effects at as little as 2 μg MnCl(2) and a 90% reduction compared to vehicle controls with a 200 μg exposure. This reduction was also observed in response to direct electrical stimulation of the olfactory nerve layer in the olfactory bulb, demonstrating that Mn's action is occurring centrally, not peripherally. This is the first direct evidence that Mn intoxication can disrupt neurotransmitter release, and is consistent with previous work suggesting that chronic Mn exposure limits amphetamine-induced dopamine increases in the basal ganglia despite normal levels of dopamine synthesis (Guilarte et al., J Neurochem 2008). The commonality of Mn's action between glutamatergic neurons in the olfactory bulb and dopaminergic neurons in the basal ganglia suggests that a disruption of neurotransmitter release may be a general consequence wherever Mn accumulates in the brain and could underlie its pleiotropic effects.

Effect of intranasal manganese administration on neurotransmission and spatial learning in rats

The effect of intranasal manganese chloride (MnCl(2)·4H(2)O) exposure on spatial learning, memory and motor activity was estimated in Morris water maze task in adult rats. Three-month-old male Wistar rats received for 2weeks MnCl(2)·4H(2)O at two doses the following: 0.2mg/kg b.w. (Mn0.2) or 0.8mg/kg b.w. (Mn0.8) per day. Control (Con) and manganese-exposed groups were observed for behavioral performance and learning in water maze. ANOVA for repeated measurements did not show any significant differences in acquisition in the water maze between the groups. However, the results of the probe trial on day 5, exhibited spatial memory deficits following manganese treatment. After completion of the behavioral experiment, the regional brain concentrations of neurotransmitters and their metabolites were determined via HPLC in selected brain regions, i.e. prefrontal cortex, hippocampus and striatum. ANOVA demonstrated significant differences in the content of monoamines and metabolites between the treatment groups compared to the controls. Negative correlations between platform crossings on the previous platform position in Southeast (SE) quadrant during the probe trial and neurotransmitter turnover suggest that impairment of spatial memory and cognitive performance after manganese (Mn) treatment is associated with modulation of the serotonergic, noradrenergic and dopaminergic neurotransmission in the brain. These findings show that intranasally applied Mn can impair spatial memory with significant changes in the tissue level and metabolism of monoamines in several brain regions.

Manganese exposure and cognitive deficits: a growing concern for manganese neurotoxicity

This symposium comprised five oral presentations dealing with recent findings on Mn-related cognitive and motor changes from epidemiological studies across the life span. The first contribution highlighted the usefulness of functional neuroimaging of the central nervous system (CNS) to evaluate cognitive as well as motor deficits in Mn-exposed welders. The second dealt with results of two prospective studies in Mn-exposed workers or welders showing that after decrease of Mn exposure the outcome of reversibility in adverse CNS effects may differ for motor and cognitive function and, in addition the issue of plasma Mn as a reliable biomarker for Mn exposure in welders has been addressed. The third presentation showed a brief overview of the results of an ongoing study assessing the relationship between environmental airborne Mn exposure and neurological or neuropsychological effects in adult Ohio residents living near a Mn point source. The fourth paper focused on the association between blood Mn and neurodevelopment in early childhood which seems to be sensitive to both low and high Mn concentrations. The fifth contribution gave an overview of six studies indicating a negative impact of excess environmental Mn exposure from air and drinking water on children's cognitive performance, with special attention to hair Mn as a potential biomarker of exposure. These studies highlight a series of questions about Mn neurotoxicity with respect to cognitive processes, forms and routes of exposure, adequate biomarkers of exposure, gender differences, susceptibility and exposure limits with regard to age.

Role of glial cells in manganese neurotoxicity

The objectives of this focused review are to (i) provide a systematic overview of recent advances pertaining to the role of glia, namely microglia and astrocytes, in the neuropathology associated with excessive exposure to manganese (Mn), (ii) highlight possible mechanisms and factors involved in Mn-modulated, glia-derived neuroinflammation, and (iii) discuss the implications of excessive neuroinflammation on neuronal injury within the context of Mn overexposure. As this is not meant to be a comprehensive review on the topic of Mn neurotoxicity, the reader may wish to refer to several broader and more comprehensive reviews. After a brief introduction to Mn neurotoxicity, we first discuss the role of glial cells in neurodegeneration. Next, we review existing in vitro and in vivo studies that implicate Mn as a modulator of glial activation and ensuing neuroinflammation. This is followed by an examination of recognized and potential mechanisms that are involved in the modulation of glial inflammatory output by Mn; here the common pathways activated by Mn in glial and neuronal cells, including outcomes of such activation, are also addressed. We finish with a discussion of the implications of Mn-modulated glial activation for neuronal survival and with a list of data gaps in the topic that need to be filled in the future.

Environmental exposure to manganese and motor function of children in Mexico

OBJECTIVES

Occupational manganese (Mn) exposure has been associated with motor deficits in adult workers, but data on the potential effects of environmental exposure to Mn on the developing motor function for a children population is scarce. The aim of this study was to evaluate the association between exposure to Mn and motor function of school aged children.

METHODS

We conducted a cross-sectional study selecting 195 children (100 exposed and 95 unexposed) between 7 and 11 years old. The following tests were used to evaluate the motor function: Grooved pegboard, finger tapping, and Santa Ana test. Mn exposure was assessed by blood (MnB) and hair concentrations (MnH). We constructed linear regression models to evaluate the association between exposure to Mn and the different test scores adjusting for age, sex, maternal education, hemoglobin and blood lead.

RESULTS

The median concentration of MnH and MnB was significantly higher in exposed (12.6 μg/g and 9.5 μg/L) compared to unexposed children (0.6 μg/g and 8.0 μg/L). The exposed children on average performed the grooved pegboard test faster, but made more errors, although these results did not reach statistical significance with neither one of the Mn exposure biomarkers. MnB showed an inverse association on the execution of the finger tapping test (average in 5 trials β -0.4, p=0.02), but no association was observed with MnH.

CONCLUSIONS

A subtle negative association of Mn exposure on motor speed and coordination was shown. In adults, the main effect of environmental Mn exposure has been associated with motor skills, but these results suggest that such alterations are not the main effect on children.

Preweaning Mn exposure leads to prolonged astrocyte activation and lasting effects on the dopaminergic system in adult male rats

Little is known about the effects of manganese (Mn) exposure over neurodevelopment and whether these early insults result in effects lasting into adulthood. To determine if early Mn exposure produces lasting neurobehavioral and neurochemical effects, we treated neonate rats with oral Mn (0, 25, or 50 mg Mn/kg/d over PND 1-21) and evaluated (1) behavioral performance in the open arena in the absence (PND 97) and presence (PND 98) of a d-amphetamine challenge, (2) brain dopamine D1 and D2-like receptors and dopamine transporter densities in the prefrontal cortex, striatum, and nucleus accumbens (PND 107), and (3) astrocyte marker glial fibrillary acidic protein (GFAP) levels in these same brain regions (PND 24 and 107). We found that preweaning Mn exposure did not alter locomotor activity or behavior disinhibition in adult rats, though Mn-exposed animals did exhibit an enhanced locomotor response to d-amphetamine challenge. Preweaning Mn exposure led to increased D1 and D2 receptor levels in the nucleus accumbens and prefrontal cortex, respectively, compared with controls. We also found increased GFAP expression in the prefrontal cortex in Mn-exposed PND 24 weanlings, and increased GFAP levels in prefrontal cortex, medial striatum and nucleus accumbens of adult (PND 107) rats exposed to preweaning Mn, indicating an effect of Mn exposure on astrogliosis that persisted and/or progressed to other brain regions in adult animals. These data show that preweaning Mn exposure leads to lasting molecular and functional impacts in multiple brain regions of adult animals, long after brain Mn levels returned to normal.

APLP1, Alzheimer's-like pathology and neurodegeneration in the frontal cortex of manganese-exposed non-human primates

Chronic manganese (Mn) exposure produces a neurological syndrome with psychiatric, cognitive and parkinsonian features. Gene expression studies in the frontal cortex of Cynomolgus macaques exposed to different doses of Mn showed gene expression changes associated with cell cycle regulation, DNA repair, apoptosis, ubiquitin-proteasome system, protein folding, cholesterol homeostasis, axonal/vesicular transport and inflammation. Amyloid-beta (A-beta) precursor-like protein 1 (APLP1), a member of the amyloid precursor family, was the most highly up-regulated gene. Immunohistochemistry confirmed increased APLP1 expression and revealed the presence of A-beta diffuse plaques. Cortical neurons and white matter fibers from Mn-exposed animals exhibited accumulation of silver grains indicative of on-going degeneration. Cortical neurons also expressed nuclear hypertrophy, intracytoplasmic vacuoles, and apoptotis stigmata. The levels of p53 were increased in neurons and glial cells in Mn-exposed tissue. Analysis of another amyloidogenic protein, alpha-synuclein, also exhibited aggregation in the gray and white matter from Mn-exposed animals. In summary, chronic Mn exposure in non-human primates produces a cellular stress response leading to neurodegenerative changes, diffuse A-beta plaques and alpha-synuclein aggregation in the frontal cortex. These changes may help explain the cognitive and working memory deficits expressed by these animals.

Proposal for a revised Reference Concentration (RfC) for manganese based on recent epidemiological studies

In 1993, based on observations of subclinical neurological effects in workers, the United States Environmental Protection Agency (US EPA) published a Reference Concentration (RfC) of 0.05 microg/m(3) for manganese (Mn). The geometric mean exposure concentration, 150 microg/m(3) respirable Mn, was considered the lowest observable adverse effect level (LOAEL), and uncertainty factors (UFs) were applied to account for sensitive populations, database limitations, a LOAEL, subchronic exposure, and potential differences in toxicity of different forms of Mn. Based on a review of more recent literature, we propose two alternate Mn RfCs. Of 12 more recent occupational studies of eight cohorts with chronic exposure durations, examining subclinical neurobehavioral effects, predominantly on the motor system, three were considered appropriate for development of an RfC. All three studies yielded no observable adverse effect levels (NOAELs) of approximately 60 microg/m(3) respirable Mn. Converting the occupational NOAEL to a human equivalent concentration (HEC) of 21microg/m(3) (for continuous exposure) and applying a UF of 10 to account for intraspecies variability yielded an RfC of 2microg/m(3). We also derived a similar RfC (7 microg/m(3)) using an Mn benchmark dose (BMD) as the point of departure. Overall confidence in both RfCs is medium.

Effects of chronic manganese exposure on working memory in non-human primates

Human exposure to manganese (Mn) has been associated with a variety of cognitive deficits including learning and memory deficits. However, results from epidemiological studies have been inconsistent in describing the nature of such cognitive deficits. The present study was conducted to evaluate the effects of chronic Mn exposure on memory functioning in non-human primates and to correlate behavioral outcome with brain Mn levels in an attempt to explain outcome variability seen in prior studies. Cynomolgus macaque monkeys were trained to perform memory-related tasks (spatial working memory, non-spatial working memory, reference memory) and exposed to manganese sulfate (15-20 mg/kg/week) over an exposure period lasting 227.5+/-17.3 days. Blood manganese levels were in the upper range of levels reported for human environmental, medical or occupational exposures. By the end of the manganese exposure period, animals developed mild deficits in spatial working memory, more significant deficits in non-spatial working memory and no deficits in reference memory. Linear regression analyses showed that for most brain regions sampled, there was a significant inverse relationship between working memory task performance and brain Mn concentration. These results suggest that chronic exposure to levels of manganese achieved in this study may have detrimental effects on working memory and that Mn levels achieved in several brain regions are inversely related to working memory performance.