How Mercury Causes Brain Neuron Degeneration
by University of Calgary, Faculty of Medicine, Dept. of Physiology & Biophysics. Video that shows neuron degeneration upon exposure to trace amounts of mercury.
Abstract supporting above video.
How Mercury Causes Brain Neuron Degeneration.
F. L. Lorscheider, C. C-W. Leong, N. I. Syed.
University of Calgary, Faculty of Medicine, Dept. of Physiology & Biophysics.
Mercury has long been known to be a potent neurotoxic substance whether it is inhaled or consumed in the diet as a food contaminant. Over the past 15 years medical research laboratories have established that dental amalgam tooth fillings are a major contributor to mercury body burden.
In 1997, a team of research scientists demonstrated that mercury vapour inhalation by animals produced a molecular lesion in brain protein metabolism which was similar to a lesion seen in 80% of Alzheimer diseased brains.
Recently completed experiments by scientists at the University of Calgary's Faculty of Medicine now reveal, with direct visual evidence from brain neuron tissue cultures, how mercury ions actually alter the cell membrane structure of developing neurons.
To better understand mercury's effect on the brain, let us first illustrate what brain neurons look like and how they grow.
In this animation, we see three brain neurons growing in a tissue culture, each with a central cell body and numerous neurite processes. At the end of each neurite is a growth cone where structural proteins are assembled to form the cell membrane. Two principal proteins involved in growth cone function are actin, which is responsible for the pulsating motions seen here, and tubulin, a major structural component of the neurite membrane.
During normal cell growth, tubulin molecules link together, end to end, to form micro-tubules which surround neurofibrils, another structural component of the neuronal axon.
Shown here is the neurite of a live neuron isolated from snail brain tissue displaying linear growth due to growth cone activity. It is important to note that growth cones in all animal species, ranging from snails to humans, have identical structural and behavioural characteristics and use proteins of virtually identical composition.
In this experiment, neurons also isolated from snail brain tissue were grown in culture for several days after which very low concentrations of mercury were added to the culture medium for 20 minutes. Over the next 30 minutes, the neurite membrane underwent rapid degeneration leaving behind a denuded neurofibril seen here.
In contrast, other heavy metals added to this same concentration such as aluminum, lead, cadmium and manganese did not produce this effect.
To understand how mercury causes this degeneration, let us return to our illustration. As mentioned before, tubulin proteins linked together during normal cell growth form the microtubules which support the neurite's structure. When mercury ions are introduced into the culture medium, they infiltrate the cell and bind themselves to newly synthesized tubulin molecules.
More specifically, the mercury ions attach themselves to the binding site reserved for guanosine triphosphate or GTP, and a beta-subunit of the affected tubulin molecules. Since all? GTP normally allows????? the energy which allows tubulin molecules to attach to one another, mercury ions bound to these sites prevent tubulin proteins from linking together. Consequently, the neurite's microtubules begin to disassemble into free tubule molecules leaving the neurite stripped of its supporting structure.
Ultimately, both the developing neurite and it's growth cone collapse and some denuded neurofibrils from aggregates are tangled, as depicted here.
Shown here is a neurite growth cone stained specifically for tubulin and actin before and after mercury exposure. Note that the mercury has caused disintegration of tubulin's microtubule structure.
These new findings reveal important visual evidence as to how mercury causes neuro-degeneration. More importantly, the study provides the first direct evidence that low-level mercury exposure is indeed a precipitating factor that can initiate this neuro-degenerative process within the brain.
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