ESRF's nanoprobe experiment uses synchrotron-based X-ray fluorescence to perform chemical-element imaging with a spatial resolution of better than 90 nm. This exceptional resolution, in combination with an extremely high X-ray flux (up to 1012 photons/s), enables imaging within subcellular compartments, such as mitochondria, lysosomes or neurosecretory vesicles.
Dopamine - a neurotransmitter that relays signals between nerve cells in the brain - can form stable complexes with iron in vitro. Thus it has been suggested that it may exert a protective effect by chelating iron in dopaminergic neurons, and that this system may be defective in patients with Parkinson's disease.
To test this hypothesis, the researchers - from the University of Bordeaux (France), the University of Seville (Spain), INSERM Grenoble Institute of Neurosciences (France) and the ESRF - used the nanoprobe imaging station to study iron distribution in an in vitro model of dopamine-producing neuronal cells.
Fine focus
The synchrotron-based nanoprobe works by exciting samples with a highly focused X-ray beam and recording the emitted fluorescence with a Si(Li) detector. By scanning the beam across a sample, a map of the trace-element composition at each point - as indicated by characteristic fluorescence signals - is created. The system is sensitive enough to detect as little as 10-18 g of iron within a cellular structure of 100 nm diameter.
The researchers examined neuronal cells that had been exposed to sub-cytotoxic levels of iron and/or alpha-methyltyrosine (AMT, an inhibitor of dopamine synthesis). The fluorescence images showed that iron is stored within dopamine vesicles inside the neuronal cells. The study also demonstrated that iron storage in the vesicles decreased when dopamine production was inhibited (via exposure of the cells to AMT).
This function of dopamine vesicles in iron storage is critical in understanding the molecular mechanisms involved in Parkinson's disease, in which dopamine vesicular storage is impaired. According to these results, this irregularity could result in increased levels of toxic iron-dopamine complexes outside of the neurovesicles, resulting in an enhanced death of dopaminergic neurons and a shortage of dopamine in the brain.
As well as enabling the study of iron distribution in cellular models of Parkinson's disease, the ESRF's new nanoimaging station will allow researchers to investigate the subcellular distribution of any metal ion involved in neurodegenerative diseases or physiological neuronal functions. What's more, the tool is ideal for researchers investigating the subcellular distribution of metal ions in other fields, including metal toxicology, chemical carcinogenesis and cellular pharmacology of inorganic compounds.
According to ESRF scientist Peter Cloetens, the research team decided to publish the results in an open-access journal in part to publicize the new imaging facility. "We want the different scientific communities to know that this machine is available," he explained. "And the best way is by letting everyone have access to the results."