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Uptake of mercury species by fruit flies

(10.11.2015)


Background:
The heavy metal mercury has been known as a toxic element for centuries. From the mass poisoning events of Minamata and Iraq poison grain disaster it is well known that the toxicity is significantly depending on the chemical species being present. Although the toxicology of mercury is a highly investigated topic, much less is known about the transport, distribution and metabolism of mercury species in biological organisms.

It is well understood that mercury shows a high affinity to sulfhydryl groups, which are omnipresent in proteins serving as binding partners in adduct formation. For this reason, mercury accumulates in those parts of the organism rich in the amino acid cysteine. Further, it was shown that organomercury compounds such as methylmercury and ethylmercury are able to cross the blood-brain barrier when bound to particular amino acids acting as transporters. In order to study such processes in more detail, there is increasing interest to visualize the mercury distribution in relevant biological tissues. While laser ablation with inductively coupled plasma mass spectrometric detection (LA-ICP-MS) has been developed as a versatile tool for bioimaging purposes during recent years, quantification of mercury in LA-ICP-MS studies remains challenging. Major problems are analyte losses by volatilization and adsorption as well as matrix effects.

The new study:
German researchers from the Universities of Münster and Potsdam developed a method for the quantitative bioimaging of mercury in fruit flies used as a model organism. The use of this model organism allows for a high number of experiments with different mercury species and improved statistical conclusions for both practical and ethics reasons.

Wild type fruit flies (Drosophila melanogaster) were exposed to mercury through feeding- stuff contaminated with methylmercury, thimerosal or mercury chloride of various concentrations. Ten female and male parental animals were raised up in tubes containing the mercury spiked feed in incubators at 25 °C with artificially generated 12 h/12 h light rhythm. After 3 days of breeding and sufficient oviposition, the parental generation was removed. L3 state larvae were collected after 9 days and the adult F1 generation after 14 days of breeding.

For later bioimaging purpose the sacrificed flies and larvae were infiltrated in sucrose solution and embedded in gelatin. The samples were cut with a cryotome into sections having a thickness of 10 or 20 µm.

For external calibration, gelatin-based matrix-matched standards were prepared. Gelatin proved to be a suitable matrix as it is easy to handle and consists of proteins so that the laser ablation properties can be compared with the biological tissues of larvae or adult flies. Mercury volatilization losses were avoided by adding DMSA as complexing agent.

The analysis of samples and standards via LA-ICPMS was performed with a commercially available laser ablation system (LSX 213 G2+, CETAC Technologies, Omaha) coupled to a quadrupole based inductively coupled plasma mass spectrometer (Agilent 7500 ce, Santa Clara). Due to the known neurotoxicity of some mercury species, one of the main targets for further analysis was the spatial distribution of mercury in the brain and the selectivity of the blood-brain barrier.

Microscopic and LA-ICPMS mercury maps of a fruit fly larva fed with methylmercury chloride (0.2 µg Hg/g).
Fig. 2
(a) Microscopic and (b) LA-ICPMS mercury maps of a fruit fly larva fed with methylmercury chloride (0.2 µg Hg/g). The lobes of the brain are marked with arrows. (c) Microscopic and (d) LA-ICPMS image of the head of an adult fruit fly fed with methylmercury chloride (0.2 µg Hg/g). (e) Microscopic and (f) LA-ICPMS image of the head of an adult fruit fly fed with thimerosal (0.2 µg Hg/g). The 202Hg signal is used for data evaluation. Images were recorded with a laser spot diameter of 10 µm and a cutting thickness of 20 µm.

The LA-ICP-MS mercury maps for the larva clearly show a high enrichment of 10- 20 µg/g mercury in the brain, corresponding to a factor of 50 compared to the mercury concentration in feed. Thus, the expected transfer of methylmercury chloride over the blood-brain barrier can be confirmed.

The different accumulation behavior of methylmercury and ethylmercury could be observed in the brains of adult flies. For both, methylmercury chloride and thimerosal, mercury was detected within the whole head. Again, this indicates that the used organic mercury species are able to pass the blood-brain barrier. The enrichment of methylmercury in the brain is approximately 3 times higher than for thimerosal. The investigation of even higher feed concentrations of mercury(II) chloride also showed the absence of mercury in the fly’s head indicating that this inorganic species cannot cross the blood-brain barrier.



The original studies

Ann-Christin Niehoff, Oliver Bolle Bauer, Sabrina Kröger, Stefanie Fingerhut, Jacqueline Schulz, Sören Meyer, Michael Sperling, Astrid Jeibmann, Tanja Schwerdtle, Uwe Karst, Quantitative Bioimaging to Investigate the Uptake of Mercury Species in Drosophila melanogaster, Anal. Chem., 87 (2015) 10392-10396. DOI: 10.1021/acs.analchem.5b02500



Analytical instrumentation:

Agilent ICP-MS 7500-CE
Teledyne CETAC laser ablation system LSX-213 G2+


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Related EVISA Resources

Brief summary: ICP-MS: A versatile detection system for speciation analysis
Link Database: Toxicity of Organo-mercury compounds
Link Database: Mercury exposure through the diet
Link Database: Environmental cycling of methylmercury
Link Database: Environmental cycling of inorganic mercury
Link Database: Environmental pollution of methylmercury
Link Database: Environmental pollution of inorganic mercury
Link Database: Toxicity of mercury
Link Database: Research projects related to organo-mercury compounds
Link Database: All about thimerosal (thiomersal)



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last time modified: November 10, 2015



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