Aluminum neurotoxicity: From subtle molecular lesions to neurological diseases

Abstract

Aluminum is the third most abundant element in the Earth’s crust. In many of the previous experimental, epidemiological, pathohistological, biochemical and other research studies, aluminum, accumulated from the environment has been recognized as a very harmful substance to the human body. Aluminum intake usually happens unintentionally due to the fact that people know little about its prevalence in water, factory-processed foods, medicines, cosmetics, etc. When accumulated in human organs, it can cause severe damage, and even lead to chronic neurodegenerative diseases. Both oxidative and nitrosative stress can be the leading cause or contribute to its toxic effects in humans and animals. All of this is supported by the fact that mitochondrial dysfunction is the earliest stage of aluminum neurotoxicity. When oxidative damage occurs under the effects of free radicals, together with the decreased antioxidant protection - due to the decreased production of the chemical energy molecule (adenosine triphosphate) as well as reducing equivalents (both in and out of mitochondria) - then the conditions for the occurrence of a vicious circle in aluminum neurotoxicity are created. Aluminum also significantly interferes with the main steps of the synaptic neurotransmission, which may lead to the progression of neuropathies. The glutamate-glutamine pathway and numerous neurotransmitter transporters are affected as well. Oxidative stress and the disruption of neurotransmission do not only exist when adult individuals are exposed to this neurotoxin, but also in individuals prenatally exposed to it as well, and these are expressed after birth. Numerous research studies, both in animals and humans, ex vivo and in vitro, quite clearly showed that aluminum can be associated with chronic neurodegenerative diseases. Additionally, there is a positive correlation between the exposure to aluminum and the pathophysiology of Alzheimer’s, Parkinson’s, Huntington’s disease, amyotrophic lateral sclerosis, and so on. One of the possible mechanisms for the generation/development of these diseases could be the disturbed homeostasis of essential metals and the appearance of unfolded or misfolded proteins that are mostly specific for a particular disease. In those research studies, the influence of aluminum on the generation of beta-amyloid, alpha synuclein, etc. was satisfactorily examined. It is very difficult, however, to suppress aluminum neurotoxicity, as well as development and progression of the diseases caused by or associated with aluminum. This is the result of some complex mechanisms through which aluminum causes its deleterious effects, and which are also responsible for the existence of multiple targets for aluminum. It is, therefore, necessary to know how these mechanisms induce the damage, in order to be able to prevent or treat the damage once it occurs. A large number of substances, including active components in traditional medicine, medical drugs and substances which are used only experimentally, have been examined so far. The results of studies conducted so far are inconclusive and they require further research. According to all the aforementioned findings, it may be concluded that well-planned, prospective and randomized clinical trials are necessary in order to use any of these substances in humans.