Effect of α-synuclein on amyloid ß-induced toxicity: relevance to Lewy body variant of Alzheimer disease.

Alzheimer's disease, the most prevalent age-related neurodegenerative disease, is characterized by the presence of extracellular senile plaques composed of amyloid-beta (Aß) peptide and intracellular neurofibrillary tangles. More than 50 % of Alzheimer's disease (AD) patients also exhibit abundant accumulation of α-synuclein (α-Syn)-positive Lewy bodies. This Lewy body variant of AD (LBV-AD) is associated with accelerated cognitive dysfunction and progresses more rapidly than pure AD. In addition, it has been suggested that Aß and α-Syn can directly interact. In this study we investigated the effect of α-Syn on Aß-induced toxicity in cortical neurons. In order to mimic the intracellular accumulation of α-Syn observed in the brain of LBV-AD patients, we used valproic acid (VPA) to increase its endogenous expression levels. The release of α-Syn from damaged presynaptic terminals that occurs during the course of the disease was simulated by challenging cells with recombinant α-Syn. Our results showed that either VPA-induced α-Syn upregulation or addition of recombinant α-Syn protect primary cortical neurons from soluble Aß1-42 decreasing the caspase-3-mediated cell death. It was also found that neuroprotection against Aß-induced toxicity mediated by α-Syn overexpression involves the PI3K/Akt cell survival pathway. Furthermore, recombinant α-Syn was shown to directly interact with Aß1-42 and to decrease the levels of Aß1-42 oligomers, which might explain its neuroprotective effect. In conclusion, we demonstrate that either endogenous or exogenous α-Syn can be neuroprotective against Aß-induced cell death, suggesting a cell defence mechanism during the initial stages of the mixed pathology.

Epigenetics in neurodegeneration: a new layer of complexity.

Several diseases are known to have a multifactorial origin, depending not only on genetic but also on environmental factors. They are called "complex disorders" and include cardiovascular disease, cancer, diabetes, and neuropsychiatric and neurodegenerative diseases. In the latter class, Alzheimer's (AD) and Parkinson's diseases (PD) are by far the most common in the elderly and constitute a tremendous social and economical problem. Both disorders present familial and sporadic forms and although some polymorphisms and risk factors have been associated with AD and PD, the precise way by which the environment contributes to neurodegeneration is still unclear. Recent studies suggest that environmental factors may contribute for neurodegeneration through induction of epigenetic modifications, such as DNA methylation, and chromatin remodeling, which may induce alterations in gene expression programs. Epigenetics, which refers to any process that alters gene activity without changing the actual DNA sequence, and leads to modifications that can be transmitted to daughter cells, is a relatively novel area of research that is currently attracting a high level of interest. Epigenetic modulation is present since the prenatal stages, and the aging process is now accepted to be associated with a loss of phenotypic plasticity to epigenetic modifications. Since aging is the most important risk factor for idiopathic AD and PD, it is expected that epigenetic alterations on DNA and/or chromatin structure may also accumulate in neurodegeneration, accounting at least in part to the etiology of these disorders.

Alzheimer's disease: the quest to understand complexity.

Alzheimer's disease (AD) is a complex disorder of the central nervous system that affects an increasing number of people worldwide due to the overall aging of the human population. In addition to genetics, which accounts for a small fraction of all cases, the etiology is multifactorial with other currently unknown triggers. It is crucial to unravel the physiological mechanisms that, being disrupted, could lead to neurodegeneration, as this knowledge could ultimately lead to the identification of novel neuroprotective strategies that could be used as therapeutics. Although mitochondrial dysfunction and the resultant oxidative stress are believed to play a major role in the pathogenesis of both early- and late-onset AD, it is conceivable that the altered physiological state of the cells leading to sporadic AD could involve additional mechanisms. Much evidence suggests that epigenetic modification of gene expression can accumulate with age leading to an altered response to stress and to an enhanced susceptibility to diseases. Since aging has a major impact in different late-onset, complex diseases and, in particular, in the late-onset forms of AD, epigenetic alterations might play an important role in the pathophysiology of this disorder. Studies exploring this idea are underway and suggest that both methylation abnormalities in AD-related genes due to disruption of mechanisms that regulate the availability of methyl groups (SAM/HCY cycle) and changes of global histone acetylation levels might play a role in neurodegeneration. Thus, it is essential to undertake novel global approaches, which may lead to the development of new avenues for therapeutic intervention.