Development of a Cumulative Exposure Index (CEI) for Manganese and Comparison with Bone Manganese and Other Biomarkers of Manganese Exposure

The Manganese-Bone Connection: Investigating the Role of Manganese in Bone Health

The complex relationship between trace elements and skeletal health has received increasing attention in the scientific community. Among these minerals, manganese (Mn) has emerged as a key element affecting bone metabolism and integrity. This review examines the multifaceted role of Mn in bone health, including its effects on bone regeneration, mineralization, and overall skeletal strength. This review article is based on a synthesis of experimental models, epidemiologic studies, and clinical trials of the mechanisms of the effect of Mn on bone metabolism. Current research data show that Mn is actively involved in the processes of bone remodeling by modulating the activity of osteoblasts and osteoclasts, as well as the main cells that regulate bone formation and resorption. Mn ions have a profound effect on bone mineralization and density by intricately regulating signaling pathways and enzymatic reactions in these cells. Additionally, Mn superoxide dismutase (MnSOD), located in bone mitochondria, plays a crucial role in osteoclast differentiation and function, protecting osteoclasts from oxidative damage. Understanding the nuances of Mn's interaction with bone is essential for optimizing bone strategies, potentially preventing and managing skeletal diseases. Key findings include the stimulation of osteoblast proliferation and differentiation, the inhibition of osteoclastogenesis, and the preservation of bone mass through the RANK/RANKL/OPG pathway. These results underscore the importance of Mn in maintaining bone health and highlight the need for further research into its therapeutic potential.

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.

Bone manganese is a sensitive biomarker of ongoing elevated manganese exposure, but does not accumulate across the lifespan

Studies have established associations between environmental and occupational manganese (Mn) exposure and executive and motor function deficits in children, adolescents, and adults. These health risks from elevated Mn exposure underscore the need for effective exposure biomarkers to improve exposure classification and help detect/diagnose Mn-related impairments. Here, neonate rats were orally exposed to 0, 25, or 50 mg Mn/kg/day during early life (PND 1-21) or lifelong through ∼ PND 500 to determine the relationship between oral Mn exposure and blood, brain, and bone Mn levels over the lifespan, whether Mn accumulates in bone, and whether elevated bone Mn altered the local atomic and mineral structure of bone, or its biomechanical properties. Additionally, we assessed levels of bone Mn compared to bone lead (Pb) in aged humans (age 41-91) living in regions impacted by historic industrial ferromanganese activity. The animal studies show that blood, brain, and bone Mn levels naturally decrease across the lifespan without elevated Mn exposure. With elevated exposure, bone Mn levels were strongly associated with blood Mn levels, bone Mn was more sensitive to elevated exposures than blood or brain Mn, and Mn did not accumulate with lifelong elevated exposure. Elevated early life Mn exposure caused some changes in bone mineral properties, including altered local atomic structure of hydroxyapatite, along with some biomechanical changes in bone stiffness in weanlings or young adult animals. In aged humans, blood Mn ranged from 5.4 to 23.5 ng/mL; bone Mn was universally low, and decreased with age, but did not vary based on sex or female parity history. Unlike Pb, bone Mn showed no evidence of accumulation over the lifespan, and may not be a biomarker of cumulative long-term exposure. Thus, bone may be a useful biomarker of recent ongoing Mn exposure in humans, and may be a relatively minor target of elevated exposure.

The association of bone and blood manganese with motor function in Chinese workers

Manganese (Mn) is an essential element. However, Mn overexposure is associated with motor dysfunction. This cross-sectional study assessed the association between bone Mn (BnMn) and whole blood Mn (BMn) with motor function in 59 Chinese workers. BnMn and BMn were measured using a transportable in vivo neutron activation analysis system and inductively coupled plasma mass spectrometry, respectively. Motor function (manual coordination, postural sway, postural hand tremor, and fine motor function) was assessed using the Coordination Ability Test System (CATSYS) and the Purdue Pegboard. Relationships between Mn biomarkers and motor test scores were analyzed with linear regression models adjusted for age, education, current employment, and current alcohol consumption. BMn was significantly inversely associated with hand tremor intensity (dominant hand (β=-0.04, 95 % confidence interval (CI):-0.07, -0.01; non-dominant hand β=-0.05, 95 % CI:-0.08, -0.01) hand tremor center frequency (non-dominant hand β=-1.61, 95 % CI:-3.03, -0.19) and positively associated with the Purdue Pegboard Assembly Score (β = 4.58, 95 % CI:1.08, 8.07). BnMn was significantly inversely associated with finger-tapping performance (non-dominant hand β=-0.02, 95 % CI:-0.04,-0.004), mean sway (eyes closed and foam β=-0.68, 95 % CI:-1.31,-0.04), and positively associated with hand tremor center frequency (dominant hand, β = 0.40, 95 % CI:0.002, 0.80). These results suggest BMn is related to better postural hand tremor and fine motor control and BnMn is related to worse motor coordination and postural hand tremor but better (i.e., less) postural sway. The unexpected positive results might be explained by choice of biomarker or confounding by work-related motor activities. Larger, longitudinal studies in this area are recommended.

Association Between Occupational Exposure to Formaldehyde and Cognitive Impairment

OBJECTIVE

To our knowledge, no study has investigated the effect of exposure to formaldehyde on cognition in the general population. Our objective was to examine the association between occupational exposure to formaldehyde and cognitive impairment in middle-aged and young- old adults (≥45 years).

METHODS

In the French CONSTANCES cohort, cognitive function was assessed with a standardized battery of seven cognitive tests to evaluate global cognitive function, episodic verbal memory, language abilities and executive functions (e.g., Digit Symbol Substitution Test, DSST). A global cognitive score was created using principal component analysis. Cognitive impairment was assessed in reference to norms of neuropsychological battery according to age, sex and education. Lifetime exposure to formaldehyde was assessed using a French job-exposure matrix created in the framework of the Matgéné project. After performing multiple imputation, separate modified Poisson regression models were used to evaluate the association between cognitive impairment (<25^th percentile) and formaldehyde exposure (exposed/never exposed), exposure duration, cumulative exposure index (CEI), and combination of CEI and time of last exposure.

RESULTS

Among 75 322 participants (median age: 57.5 years, women: 53%), 8% were exposed to formaldehyde during their professional life. These participants were at higher risk of global cognitive impairment (for global cognitive score: adjusted relative risk, aRR, 1.17, 95% confidence interval, CI: 1.11-1.23), after adjusting for confounders (age, sex, education, income, solvent exposure, Effort-Reward Imbalance, night-shift, repetitive, and noisy work). They were at higher risk of cognitive impairment for all cognitive domains explored. Longer exposure duration and high CEI were associated with cognitive impairment, with a dose-effect relationship for exposure duration. Recent exposure was associated with impairment in all cognitive domains. Time did not fully attenuate formaldehyde-associated cognitive deficits especially in highly exposed individuals (for DSST: high past exposure aRR 1.23, 95%CI: 1.11-1.36; high recent exposure: aRR 1.24, 95%CI: 1.13-1.35).

CONCLUSION

Our findings highlight the long-term detrimental effect of formaldehyde exposure on cognitive health in a relatively young population.

Manganese Intoxication Recovery and the Expression Changes of Park2/Parkin in Rats

Occupational overexposure to manganese (Mn) produces Parkinson's disease-like manganism. Acute Mn intoxication in rats causes dopaminergic neuron loss, impairment of motor activity and reduction of the expression of Park2/Parkin. The expression of Park2/Parkin is also reduced. Whether these changes are reversible after cessation of Mn exposure is unknown, and is the goal of this investigation. Adult male rats were injected with Mn²⁺ at doses 1 mg/kg and 5 mg/kg in the form of MnCl₂·4H₂O, every other day for one-month to produce acute Mn neurotoxicity. For a half of rats Mn exposure was suspended for recovery for up to 5 months. Mn neurotoxicity was evaluated by the accumulation of Mn in blood and brain, behavioral activities, dopaminergic neuron loss, and the expression of Park2/Parkin in the blood cells and brain. Dose-dependent Mn neurotoxicity in rats was evidenced by Mn accumulation, rotarod impairments, reduction of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra, decreased level of Park2 mRNA in the blood and brain, and decreased Parkin protein in the brain. After cessation of Mn exposure, the amount of Park2 mRNA in the blood started to increase one month after the recovery. After 5-month of recovery, blood and brain Mn returned to normal, rotarod activity recovered, the reduction of TH-positive dopaminergic neurons ameliorated, and the level of Park2 mRNA in the blood and Park2/Parkin in the midbrain and striatum were returned to the normal. Mn neurotoxicity in rats is reversible after cessation of Mn exposure. The level of Park2 mRNA in the blood could be used as a novel biomarker for Mn exposure and recovery.

Title Changes in the Mineral Composition of Rat Femoral Bones Induced by Implantation of LNCaP Prostate Cancer Cells and Dietary Supplementation

Prostate cancer (PCa) is the second most frequent cancer in men and the fifth most common cause of death worldwide, with an estimated 378,553 deaths in 2020. Prostate cancer shows a strong tendency to form metastatic foci in the bones. A number of interactions between cancer cells attacking bones and cells of the bone matrix lead to destruction of the bone and growth of the tumour. The last few decades have seen increased interest in the precise role of minerals in human health and disease. Tumour cells accumulate various minerals that promote their intensive growth. Bone, as a storehouse of elements, can be a valuable source of them for the growing tumour. There are also reports suggesting that the presence of some tumours, e.g., of the breast, can adversely affect bone structure even in the absence of metastasis to this organ. This paper presents the effect of chronic dietary intake of calcium, iron and zinc, administered in doses corresponding maximally to twice their level in a standard diet, on homeostasis of selected elements (Ca, K, Zn, Fe, Cu, Sr, Ni, Co, Mn and Mo) in the femoral bones of healthy rats and rats with implanted cancer cells of the LNCaP line. The experiment was conducted over 90 days. After the adaptation period, the animals were randomly divided into four dietary groups: standard diet and supplementation with Zn, Fe and Ca. Every dietary group was divided into experimental group (with implanted cancer cells) and control group (without implanted cancer cells). The cancer cells (LnCaP) were implanted intraperitoneally in the amount 1 × 106 to the rats at day 90 of their lifetime. Bone tissue was dried and treated with microwave-assisted mineral digestation. Total elemental content was quantified by ICP-MS. Student's t-test and Anova or Kruskal-Wallis tests were applied in order to compare treatment and dietary groups. In the case of most of the diets, especially the standard diet, the femoral bones of rats with implanted LNCaP cells showed a clear downward trend in the content of the elements tested, which may be indicative of slow osteolysis taking place in the bone tissue. In the group of rats receiving the standard diet, there were significant reductions in the content of Mo (by 83%), Ca (25%), Co (22%), Mn (13%), K (13%) and Sr (9%) in the bone tissue of rats with implanted LNCaP cells in comparison with the control group receiving the same diet but without LNCaP implantation. Supplementation of the rat diet with calcium, zinc and iron decreased the frequency of these changes relative to the standard diet, which may indicate that the diet had an inhibitory effect on bone resorption in conditions of LNCaP implantation. The principal component analysis (PCA) score plot confirms the pronounced effect of implanted LNCaP cells and the standard diet on bone composition. At the same time, supplementation with calcium, zinc and iron seems to improve bone composition. The microelements that most often underwent quantitative changes in the experimental conditions were cobalt, manganese and molybdenum.

Manganese increases Aβ and Tau protein levels through proteasome 20S and heat shock proteins 90 and 70 alteration, leading to SN56 cholinergic cell death following single and repeated treatment

Manganese (Mn) produces cholinergic neuronal loss in basal forebrain (BF) region that was related to cognitive dysfunction induced after single and repeated Mn treatment. All processes that generate cholinergic neuronal loss in BF remain to be understood. Mn exposure may produce the reduction of BF cholinergic neurons by increasing amyloid beta (Aβ) and phosphorylated Tau (pTau) protein levels, altering heat shock proteins' (HSPs) expression, disrupting proteasome P20S activity and generating oxidative stress. These mechanisms, described to be altered by Mn in regions different than BF, could lead to the memory and learning process alteration produced after Mn exposure. The research performed shows that single and repeated Mn treatment of SN56 cholinergic neurons from BF induces P20S inhibition, increases Aβ and pTau protein levels, produces HSP90 and HSP70 proteins expression alteration, and oxidative stress generation, being the last two effects mediated by NRF2 pathway alteration. The increment of Aβ and pTau protein levels was mediated by HSPs and proteasome dysfunction. All these mechanisms mediated the cell decline observed after Mn treatment. Our results are relevant because they may assist to reveal the processes leading to the neurotoxicity and cognitive alterations observed after Mn exposure.

Chronic Manganese Administration with Longer Intervals Between Injections Produced Neurotoxicity and Hepatotoxicity in Rats

Abstract

Subacute exposure to manganese (Mn) produced Parkinson’s disease-like syndrome called Manganism. Chronic onset and progression are characteristics of Manganism, therefore, this study aimed to examine Mn toxicity following chronic exposures. Male Sprague-Dawley rats were injected Mn²⁺ 1 and 5 mg/kg, every 10 days for 150 days (15 injections). Animal body weight and behavioral activities were recorded. At the end of experiments, the brain and liver were collected for morphological and molecular analysis. Chronic Mn exposure did not affect animal body weight gain, but the high dose of Mn treatment caused 20% mortality after 140 days of administration. Motor activity deficits were observed in a dose-dependent manner at 148 days of Mn administration. Immunofluorescence double staining of substantia nigra pars compacta (SNpc) revealed the activation of microglia and loss of dopaminergic neurons. The chronic neuroinflammation mediators TNFα, inflammasome Nlrp3, Fc fragment of IgG receptor IIb, and formyl peptide receptor-1 were increased, implicating chronic Mn-induced neuroinflammation. Chronic Mn exposure also produced liver injury, as evidenced by hepatocyte degeneration with pink, condensed nuclei, indicative of apoptotic lesions. The inflammatory cytokines TNFα, IL-1β, and IL-6 were increased, alone with stress-related genes heme oxygenase-1, NAD(P)H:quinone oxidoreductase-1 and metallothionein. Hepatic transporters, such as multidrug resistant proteins (Abcc1, Abcc2, and Abcc3) and solute carrier family proteins (Slc30a1, Slc39a8 and Slc39a14) were increased in attempt to eliminate Mn from the liver. In summary, chronic Mn exposure produced neuroinflammation and dopaminergic neuron loss in the brain, but also produced inflammation to the liver, with upregulation of hepatic transporters.

Graphic Abstract

Electronic supplementary material

The online version of this article (10.1007/s11064-020-03059-2) contains supplementary material, which is available to authorized users.

Characterization of bone aluminum, a potential biomarker of cumulative exposure, within an occupational population from Zunyi, China

OBJECTIVES

Aluminum (Al) is a neurotoxicant; however, efforts to understand Al toxicity are limited by the lack of a quantitative biomarker of cumulative exposure. Bone Al measurements may address this need. Here, we describe and compare non-invasive bone Al measurements with fingernail Al and Al cumulative exposure indices (CEIs).

METHODS

We completed a cross-sectional study of 43 factory workers in Zunyi, China. Bone Al measurements were taken with a compact in-vivo neutron activation analysis system (IVNAA). Fingernail samples were analyzed using inductively coupled plasma mass spectrometry. CEIs, based on self-reported work history and prior literature, were calculated for the prior 5, 10, 15, 20 years and lifetime work history. Linear regressions adjusted for age and education compared fingernail Al and Al CEIs with bone Al.

RESULTS

Median (interquartile range (IQR)) Al measurements were: 15 μg/g dry bone (IQR = 28) for bone Al; 34.9 μg/g (43.3) for fingernail; and 24 (20) for lifetime CEI. In adjusted regression models, an increase in 15-year CEI was significantly associated with increased bone Al (β = 0.91, 95% confidence interval (CI): 0.16, 1.66). Associations of bone Al with 10- and 20-year CEI were approaching statistical significance (β = 0.98, 95% CI: -0.14, 2.1; β = 0.59, 95% CI: -0.01, 1.18, respectively). Other models were not statistically significant.

CONCLUSIONS

Bone Al was significantly associated with 15-year Al CEI, but not other Al CEIs or fingernail Al. Bone Al may be a useful measure of cumulative, rather than short-term, Al exposure. Additional refinement of this method is ongoing.

Association between long-term occupational manganese exposure and bone quality among retired workers

Despite well documents for manganese-induced neurological deficits, limited researches are available for effects of manganese (Mn) exposure on the bone. Here we aimed to explore the associations between long-term occupational Mn exposure and bone quality among retired workers. We conducted a cross-sectional study of 304 exposed subjects (n, male = 161 and female = 143) and 277 control retired workers (n, male = 65 and female = 212) recruited from a ferromanganese refinery. Self-reported occupation types were used as exposure classification confirmed by expert consultation. Bone quality was measured by quantitative ultrasound (QUS). In sex-stratified analyses throughout, stiffness index (SI) and T-score levels of the participants in the highest exposed group [tertile 3 of Mn cumulative exposure index (Mn-CEI)] were significantly lower as compared with the control group among female workers (SI, mean, 61.60 vs. 68.17; T-score, mean, -3.01 vs. -2.34, both P < 0.05). In addition, SI and T-score were found to be negatively associated with Mn-CEI only in the highest exposure group as compared with the female controls (both P = 0.01). However, we did not find the significant difference for SI or T-score among the male subjects in exposure models and the male controls (P > 0.05). Our results suggest that female retired workers in the highest Mn-exposed model (tertile 3 of Mn-CEI) potentially experience a higher risk of developing osteoporosis compared with the female controls. Further investigations on possible mechanisms on bone quality alteration are needed in the future.

Manganese exposure and working memory-related brain activity in smallholder farmworkers in Costa Rica: Results from a pilot study

Main sources of manganese (Mn) in the general population are diet and drinking water. Mn is also found in ethylene bisdithiocarbamate (EBDC) fungicides used in agriculture or emitted into the air by ferromanganese plants and welding fumes, which can be additional environmental and occupational sources of exposure. High occupational Mn exposure has been linked with motor, behavioral, and cognitive impairment, but its effects on neural function remain poorly understood. We conducted a functional neuroimaging study in a sample of 48 farmworkers in Zarcero County, Costa Rica, an agricultural region where EBDC fungicides are sprayed. We measured Mn concentrations in farmworkers' toenails (n = 40 farmworkers) and hair (n = 33 farmworkers), and recorded brain activity in the dorsolateral prefrontal cortex during a letter-retrieval working memory task using functional near-infrared spectroscopy (fNIRS). We estimated exposure-outcome associations using multivariable linear regression models adjusted for age and education level. Geometric mean (geometric standard deviation) toenail and hair Mn concentrations were 0.40 μg/g (3.52) and 0.24 μg/g (3.54), respectively. We did not find strong evidence that Mn concentrations were associated with working memory-related brain activity in this sample of farmworkers; we also found null associations between working memory task accuracy and brain activity. However, our small sample size may have limited our ability to detect small effect sizes with statistical precision. Our study demonstrates that fNIRS can be a useful and feasible tool in environmental epidemiology for examining the effects of toxicants, like Mn, on neural function. This may prove to be important for elucidating neuropathological pathways that underlie previously reported associations of elevated Mn exposure with neurotoxic effects.

The association of bone, fingernail and blood manganese with cognitive and olfactory function in Chinese workers

Occupational manganese (Mn) exposure has been associated with cognitive and olfactory dysfunction; however, few studies have incorporated cumulative biomarkers of Mn exposure such as bone Mn (BnMn). Our goal was to assess the cross-sectional association between BnMn, blood Mn (BMn), and fingernail Mn (FMn) with cognitive and olfactory function among Mn-exposed workers. A transportable in vivo neutron activation analysis (IVNAA) system was designed and utilized to assess BnMn among 60 Chinese workers. BMn and FMn were measured using inductively coupled plasma mass spectrometry. Cognitive and olfactory function was assessed using Animal and Fruit Naming tests, World Health Organization/University of California-Los Angeles Auditory Verbal Learning Test (AVLT) and the University of Pennsylvania Smell Identification Test (UPSIT). Additional data were obtained via questionnaire. Regression models adjusted for age, education, factory of employment, and smoking status (UPSIT only), were used to assess the relationship between Mn biomarkers and test scores. In adjusted models, increasing BnMn was significantly associated with decreased performance on average AVLT scores [β (95% confidence interval (CI)) = -0.65 (-1.21, -0.09)] and Animal Naming scores [β (95% CI) = -1.54 (-3.00, -0.07)]. Increasing FMn was significantly associated with reduced performance measured by the average AVLT [β (95% CI) = -0.35 (-0.70, -0.006)] and the difference in AVLT scores [β (95% CI) = -0.40 (-0.77, -0.03)]. BMn was not significantly associated with any test scores; no significant associations were observed with Fruit Naming or UPSIT tests. BnMn and FMn, but not BMn, are associated with cognitive function in Mn-exposed workers. None of the biomarkers were significantly associated with olfactory function.