Characterization of pathological changes in the olfactory system of mice exposed to methylmercury

Methylmercury (MeHg) is a well-known environmental neurotoxicant that causes severe brain disorders such as Minamata disease. Although some patients with Minamata disease develop olfactory dysfunction, the underlying pathomechanism is largely unknown. We examined the effects of MeHg on the olfactory system using a model of MeHg poisoning in which mice were administered 30 ppm MeHg in drinking water for 8 weeks. Mice exposed to MeHg displayed significant mercury accumulation in the olfactory pathway, including the nasal mucosa, olfactory bulb, and olfactory cortex. The olfactory epithelium was partially atrophied, and olfactory sensory neurons were diminished. The olfactory bulb exhibited an increase in apoptotic cells, hypertrophic astrocytes, and amoeboid microglia, mainly in the granular cell layer. Neuronal cell death was observed in the olfactory cortex, particularly in the ventral tenia tecta. Neuronal cell death was also remarkable in higher-order areas such as the orbitofrontal cortex. Correlation analysis showed that neuronal loss in the olfactory cortex was strongly correlated with the plasma mercury concentration. Our results indicate that MeHg is an olfactory toxicant that damages the central regions involved in odor perception. The model described herein is useful for analyzing the mechanisms and treatments of olfactory dysfunction in MeHg-intoxicated patients.

[A Review of the Book "Minamata Disease and the Responsibility of Medical Authorities"]

Even today, more than 60 years after the discovery of Minamata disease, many controversies continue to arise from various viewpoints. Recently, Dr. Shigeru Takaoka has published a book entitled "Minamata Disease and the Responsibility of Medical authorities" as a summary of his and colleagues' previous works in which he presented their objections to past academic theories. We, who were also engaged in this research at Kumamoto University, would like to address some substantial viewpoints. Drs. Nishimura and Okamoto clarified why a series of cases that were clearly Minamata disease were found only in the Minamata plant from late 1950 to 1975, even though many acetaldehyde plants have been operating in Japan for many years. Dr. Takaoka ignored this very important issue and we point out the lack of reliability of his data from their health examination of "10,000" people. From the pathology perspective, Dr. Takaoka mis-understood the location and plasticity of neurons. From a clinical viewpoint, he mentioned the poor evidence for the characteristics and courses of the patients' clinical symptoms.

Neurotoxicity of β-HgS differs from environmental mercury pollutants (MeHgCl and HgCl2) in Neuro-2a cell

β-HgS, differing from environmental mercury pollutants (MeHgCl and HgCl₂) in chemical form, is used as traditional medicine in Asian countries for thousands of years. In this study, Neuro-2a cells were exposed to β-HgS, MeHgCl and HgCl₂ (5 µM) for 6-24 h. The cell viability of β-HgS was higher than MeHgCl with 25.9% and 72.4% in 12 h and 24 h respectively. As the incubation time increased, MeHgCl had obvious damage to cell morphology, decreased the ratio of Bcl-2 and Bak and increased the expressions of TNF-α, IL-6 and IL-1β significantly. Furthermore, the expressions of IL-1β and IL-6 in HgCl₂ group were increased significantly in 6 h and 24 h. The apoptotic rates in MeHgCl and HgCl₂ group were respectively higher than β-HgS with 32.2% and 7.30% in 24 h. Our findings indicate that β-HgS is much less neurotoxicity than MeHgCl and HgCl₂ in Neuro-2a cells.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Rethinking the Minamata Tragedy: What Mercury Species Was Really Responsible?

Industrial release of mercury into the local Minamata environment with consequent poisoning of local communities through contaminated fish and shellfish consumption is considered the classic case of environmental mercury poisoning. However, the mercury species in the factory effluent has proved controversial, originally suggested as inorganic, and more recently as methylmercury species. We used newly available methods to re-examine the cerebellum of historic Cat 717, which was fed factory effluent mixed with food to confirm the source. Synchrotron high-energy-resolution fluorescence detection-X-ray absorption spectroscopy revealed sulfur-bound organometallic mercury with a minor β-HgS phase. Density functional theory indicated energetic preference for α-mercuri-acetaldehyde as a waste product of aldehyde production. The consequences of this alternative species in the "classic" mercury poisoning should be re-evaluated.

Neurodegeneration Induced by Metals in Caenorhabditis elegans

Metals are a component of a variety of ecosystems and organisms. They can generally be divided into essential and nonessential metals. The essential metals are involved in physiological processes once the deficiency of these metals has been associated with diseases. Although iron, manganese, copper, and zinc are important for life, it has been evidenced that they are also involved in neuronal damage in many neurodegenerative disorders. Nonessential metals, which are metals without physiological functions, are present in trace or higher levels in living organisms. Occupational, environmental, or deliberate exposures to lead, mercury, aluminum, and cadmium are clearly correlated with the increase of toxicity and varied kinds of pathological situations. Actually, the field of neurotoxicology needs to satisfy two opposing demands: the testing of a growing list of chemicals and resource limitations and ethical concerns associated with testing using traditional mammalian species. Toxicological assays using alternative animal models may relieve some of this pressure by allowing testing of more compounds while reducing expenses and using fewer mammals. The nervous system is by far the more complex system in C. elegans. Almost a third of their cells are neurons (302 neurons versus 959 cells in adult hermaphrodite). It initially underwent extensive development as a model organism in order to study the nervous system, and its neuronal lineage and the complete wiring diagram of its nervous system are stereotyped and fully described. The neurotransmission systems are phylogenetically conserved from nematodes to vertebrates, which allows for findings from C. elegans to be extrapolated and further confirmed in vertebrate systems. Different strains of C. elegans offer a new perspective on neurodegenerative processes. Some genes have been found to be related to neurodegeneration induced by metals. Studying these interactions may be an effective tool to slow neuronal loss and deterioration.

Mercury induces the externalization of phosphatidyl-serine in human renal proximal tubule (HK-2) cells

The underlying mechanism for the biological activity of inorganic mercury is believed to be the high affinity binding of divalent mercuric cations to thiols of sulfhydryl groups of proteins. A comprehensive analysis of published data indicates that inorganic mercury is one of the most environmentally abundant toxic metals, is a potent and selective nephrotoxicant that preferentially accumulates in the kidneys, and is known to produce cellular injury in the kidneys. Binding sites are present in the proximal tubules, and it is in the epithelial cells of these tubules that toxicants such as inorganic mercury are reabsorbed. This can affect the enzymatic activity and the structure of various proteins. Mercury may alter protein and membrane structure and function in the epithelial cells and this alteration may result in long term residual effects. This research was therefore designed to evaluate the dose-response relationship in human renal proximal tubule (HK-2) cells following exposure to inorganic mercury. Cytotoxicity was evaluated using the MTT assay for cell viability. The Annexin-V assay was performed by flow cytometry to determine the extent of phosphatidylserine externalization. Cells were exposed to mercury for 24 hours at doses of 0, 1, 2, 3, 4, 5, and 6 microg/mL. Cytotoxicity experiments yielded a LD50 value of 4.65 +/- 0.6 microg/mL indicating that mercury is highly toxic. The percentages of cells undergoing early apoptosis were 0.70 +/- 0.03%, 10.0 +/- 0.02%, 11.70 +/- 0.03%, 15.20 +/- 0.02%, 16.70 +/- 0.03%, 24.20 +/-0.02%, and 25.60 +/- 0.04% at treatments of 0, 1, 2, 3, 4, 5, and 6 microg/mL of mercury respectively. This indicates a dose-response relationship with regard to mercury-induced cytotoxicity and the externalization of phosphatidylserine in HK-2 cells.

Effects of mercury vapor exposure on neuromotor function in Chinese miners and smelters

OBJECTIVES

Current risk assessment of elemental mercury vapor is based on the tremor toxicity. To clarify the neuromotor effects of occupational exposure to mercury vapor, hand tremor and postural sway were measured in 27 miners and smelters (i.e., exposed workers) and 52 unexposed subjects.

METHODS

Urine samples were collected and total mercury and creatinine concentrations were determined. Data of the tremor and postural sway were analyzed using the fast Fourier transformation.

RESULTS

The geometric means of the urinary mercury level (UHg) were 228 (range 22.6-4,577) microg/g creatinine for the exposed workers and 2.6 (1.0-17.4) microg/g creatinine for the unexposed subjects. Total tremor intensity and frequency-specific tremor intensities at 1-6 and 10-14 Hz were significantly larger in the exposed workers than in the unexposed subjects (P < 0.05), but they were not significantly related to the UHg among the exposed workers (P > 0.05). In contrast, there were no significant differences in any postural sway parameters between the above two groups (P > 0.05), but the transversal sway with eyes open was positively related to the UHg among the exposed workers in using multiple regression analysis (P < 0.05).

CONCLUSIONS

These findings suggest that postural sway, as well as hand tremor, may be affected by elemental mercury vapor exposure, but the former test seems to be less sensitive to mercury than the latter one.

Industrial and agricultural pollution: a threat to the health of children living in the Arctic region

Various environmental pollutants of industrial or agricultural origin such as persistent organic pollutants (POCs) are causing great concern owing to their toxicity to humans and animals. At the Stockholm Convention on POCs in 2001, 12 of these pollutants, i.e. dioxins, PCBs (polychlorinated biphenyls) and DDT were referred to as "the dirty dozen".

CONCLUSION

Collaborative studies by scientists from Canada, Russia, Scandinavia and other countries representing different fields such as environmental chemistry, ecology and medical sciences may increase our knowledge about the present threat of toxic chemicals to ecology and human health in the Arctic region. It is hoped that improved understanding will promote preventive political decisions.

[Health and socio-economic condition of certified Minamata disease patients, 1999]

OBJECTIVES

To clarify the needs and to consider establishing a social support system for patients with Minamata disease (MD), or methylmercury poisoning, by investigating their health and socioeconomic conditions.

METHODS

The total number of people certified as having MD in May 1999 by the Kumamoto and Kagoshima Prefecture Government Committees on MD was 2265. We sent two questionnaires to 917 individuals who were surviving at that time, which corresponded to 40.5% of the total number of MD patients. The first survey sought information on the individual's health-seeking behavior, and the second survey was about their socio-economic conditions and requirements for welfare and medical care in the future.

RESULTS

The average age among male patients was 68.0 +/- 13.2 yrs (n = 477) and that among female patients was 71.2 +/- 13.0 yrs (n = 440). The response rates were 45.7% (n = 416) for the first questionnaire and 38.6% (n = 354) for the second questionnaire. Among the MD patients, 71.7% judged their health condition to be 'bad' or 'very bad', and 97.4% received medical treatments that included acupuncture or moxacautery and massage. Regarding the activity of daily living (ADL), which includes 'communicating', 'walking', 'eating', 'use of toilet', 'dressing' and 'taking a bath', the rates of 'independent' were relatively low among those under 49 yrs and those over 75 yrs compared with the other age groups. Many individuals emphasized that they had anxiety about their health and health care in the future.

CONCLUSION

We concluded that the quality of life (QOL) of MD patients was low. It is important to consider developing a social support system for MD patients.

Cases of mercury exposure, bioavailability, and absorption

Mercury is a unique element that, unlike many metals, has no essential biological function. It is liquid at room temperature and is 13.6 times heavier than water. Its unique physical properties have been exploited for a variety of uses such as in mercury switches, thermostats, thermometers, and other instruments. Its ability to amalgamate with gold and silver are used in mining these precious metals and as a dental restorative. Its toxic properties have been exploited for medications, preservatives, antiseptics, and pesticides. For these reasons there have been many industrial uses of mercury, and occupational exposures of workers and industrial emissions and effluents contaminating air, water, soil, and ultimately food chains have long been a matter of great public health concern. This paper examines briefly six cases representing various forms of exposure to different species of mercury, and indicates the methodological issues in estimating exposure, bioavailability and absorption; these cases include Minamata disease in Japan, organic mercury poisoning in Iraq, methylmercury (MeHg) exposure in the Amazon, dimethylmercury (PMM) in the laboratory, an elemental mercury spill in Cajamarca, Peru, and a mercury-contaminated building in Hoboken, NJ, USA. Other scenarios that are not described include occupational exposure to mercury salts, mercurial preservatives in vaccines, cultural and ritualistic uses of mercury, and mercury in dental amalgams.