(422b) Exploring the Neurological Exposome for Precision Prevention of Neurodegenerative Diseases

The exposome represents the totality of exposures from conception onwards. Unravelling it requires to simultaneously identify, characterize and quantify exogenous and endogenous exposures and modifiable risk factors that predispose to and predict disease throughout the human life span. Exposome science will help us understand the intricate web of relationships between environmental exposures, lifestyle, genetics and disease, contributing significantly to the determination of causal associations between environmental factors and human health. Exposome research tries to reverse the paradigm of “nature versus nurture” and adopt one defined by complex and dynamic interactions between DNA sequence, epigenetic DNA modifications, gene expression and environmental factors that all combine to influence disease phenotypes. The approach outlined herein brings together and organizes environmental, socio-economic, exposure, biomarker and health effect data. The overall methodology is being verified in a series of population studies across Europe, tackling various levels of environmental exposure, age windows and gender differentiation of exposure, and socio-economic and genetic variability. Given the challenges posed by the longer life expectancy of latest generation, healthy aging is a major exposome challenge. As a result, determination of causality would support targeted interventions towards precision prevention, contributing significantly towards healthy aging. Among the adverse health effects related to aging, neurodegenerative diseases hold a key role. Thus, understanding the interplay among environmental and genetic factors, in relation to the respective involved mechanisms, is the cornerstone of targeted interventions, both at individual and community level.

Making this vision come true poses significant scientific and technological challenges in terms of both untangling the complex biological networks that regulate our body’s response to external stressors and processing and analysing the large datasets generated from the use of multiple high throughput analytical platforms, that are based on omics technologies, particularly transcriptomics and metabolomics. In addition, it includes all the procedures and computational sequences necessary for applying advanced bioinformatics coupling advanced data mining, biological and exposure modelling so as to ensure that environmental exposure-health associations are studied comprehensively. For this study, data from human cohort of older population have been evaluated. Human biomonitoring samples have been analysed for identifying metabolomics and transcriptomics fingerprints, while they have also been mapped jointly into metabolic pathways, for identifying the perturbated pathways that are mostly associated with neurodegenerative diseases. Finally, these pathways have been associated with external exposure factors such as exposure to chemicals and air pollution, the role of diet and the presence of beneficial nutrients, as well as the overall health status.

In our study, we confirmed that aging brings a lot of physiological changes, usually accompanied by deterioration in cognitive function as a result of neurological degeneration. However, this process does not follow a standardized course for all individuals, and it comes as the result of the combination of multiple factors. These factors include genetic susceptibility, exposure to environmental chemicals, lifestyle aspects such as poor nutrition, excessive alcohol ingestion, lack of exercise and stress, as well as health status (e.g. hypertension or diabetes). In addition, age-dependent hormonal changes during normal aging contribute to the overall neurological and cognitive declining process.

A major mechanism resulting in this degenerative process, is the one related to oxidative stress and inflammation. Although these processes consist of distinct biochemical cascades, there are common nodes in the respective pathways of disease, especially regarding brain function, which is highly susceptible to oxidative stress. This is evidenced by the concurrent presence of oxidative stress markers found in brain specimens such as ROS and the respective markers of effect, and inflammation markers, such as cytokines and other inflammatory mediators or activated immune cells.

Although diet is considered as a major exogenous factor, up-regulation of the inflammatory metabolic pathway and in many cases the oxidative stress pathway might be triggered by other exogenous exposures such as exposure to man-made chemicals (e.g. industrial chemicals and pesticides), as well as lifestyle parameters. It was also found that health status might predispose for Alzheimer’s disease/dementia and cognitive decline through oxidative stress pathways.

Exploring the role of diet in triggering and accelerating the progression of Alzheimer’s disease, should account for both the beneficial nutrients (vitamin D, ω3, α-carotene, polyphenols, folates) and the neurotoxicants (heavy metals, pesticides) associated with specific food items. High dimension biology tools based on a combination of omics technologies were able to identify the molecular fingerprints and pathway changes (e.g. arachidonic acid cascade in the hippocampus, phospholipid homeostasis) associated with specific dietary patterns, exposure to chemicals inducing oxidative stress (ambient air PM) and neuroinflammation (Hg) and genetic factors (ApoE e4 carriers).

The specific changes in the arachidonic acid cascade in the hippocampus may alter phospholipids homeostasis and possibly increase the susceptibility of the aging brain to neuroinflammation. Analysis of human aging brain expression datasets from three frontal cortex regions showed that different pathways undergo transitions at different ages. Ten major classes for TFs (transcription factors) have been found to be associated with neurodegenerative diseases. Unique miRNA (micro RNA) targets have been identified as regulatory markers for Alzheimer's disease (AD). A major vascular susceptibility factor gene is apolipoprotein E, found to be associated with sporadic late-onset AD cases. Another interesting vascular susceptibility gene is angiotensin converting enzyme. Other possible genes include VLDL-R, LRP, NOS3, CST3, OLR1, MTHFR, PON1 and VEGF. It has also been found that genetic variation modifies the association between AD biomarkers and neurodegeneration. Genes that regulate the molecular response in the brain to oxidative stress may be particularly relevant to neural vulnerability to the damaging effects of amyloid-βMany neurodegenerative diseases share oxidative stress and nitrosative stress as common terminal processes. According to the free radical theory of aging, an elevation in reactive oxygen species (ROS) and reactive nitrogen species (RNS) damages neural membranes and induces oxidative and nitrosative stress. The increase in oxidative and nitrosative stress is accompanied by the concomitant decline in cognitive and motor performance in the elderly population, even in the absence of neurodegenerative diseases. Additional perturbations, such as the impaired brain iron homeostasis, have been associated to AD in the elderly; intracellular iron accumulation leads to a rise in oxidative damage, contributing to AD pathophysiology. Considering the reduced counteractive response of elderly to oxidative stress, this mechanism is an additional risk factor related to the underlying susceptibility.

With regard to hormonal status, it was found that although the response of neuroinflammation to estrogen hormones is biphasic, overall, the presence of estrogens seems to act protectively, thus menopause is an interesting time – window for capturing the late onset multiple sclerosis. For men, the protective effect of testosterone starts to decline at a later age, of about 65. Estrone produced by adipocytes may represent an important source of inflammatory signaling in both males and females, while there may also be a potential interaction between obesity and vitamin D status in mediating MS is females. In terms of genetic susceptibility, the presence of HLA-DRB1*1501 allele seems to be a risk factor. The lack of estradiol protective effect of estradiol against neurodegenerative disorders, is eventually enhanced by exposure to environmental insults mediating oxidative stress such as ozone. That these effects can be prevented by estradiol treatment, suggests increased susceptibility to neurodegenerative disorders in aging women may be contributed to by reduced estrogen levels post-menopause.

Overall, multi-omics analysis within the overall exposome paradigm provides biological robustness and plausibility of the results. Interpretation of metabolic pathways allows us to target our interventions to the mechanism that is highly perturbed. Key pathways related to the environmental and dietary origins of neurodegeneration comprise oxidative stress induction and inflammatory mediators. Oxidative stress is induced by several dietary and environmental factors such as PAHs and heavy metals (Pb, Hg, Cd, As), thus, reducing exposure to these stressors through diet is beneficial. On the other hand, consumption of foods with anti-oxidants is essential. The interplay with other adverse outcomes forming the Western disease nexus has to be always accounted for, since diabetes and AD may share several drug targets, including insulin and IGF (insulin-like growth factor) stimulation, inflammation and GSK3 (glycogen synthase kinase 3). Another key finding mainly originated from metabolomics analysis is that loss of lipid homeostasis can contribute to neurodegeneration. Thus, reduction of exposure to endocrine disruptors through diet that affect lipids metabolism should be a priority for prevision prevention. This has implications at both the community and personalized level:

• At community level, the most cost-efficient policies for protecting public health should be prioritized
• At the individual level, targeted dietary and lifestyle recommendations (accounting also for genetic susceptibilities that predispose for neurodegenerative diseases) should be derived seeking to minimize exposure and propagation of adverse outcome pathways.