Title: Metabolic evaluation of genetic and environmental contributors to Alzheimer’s disease
Description: Understanding the effect of the environment on human health has benefited from progress made in measuring the exposome. High resolution mass spectrometry (HRMS) has made it possible to measure small molecules across a large dynamic range, allowing researchers to study the role of low abundance environmental toxicants in causing human disease, including examining their effects on biochemistry. Alzheimer’s disease is the most prevalent neurodegenerative disease in the world. While aging is the largest risk factor of the disease, evidence of risk factors for dementias show that lifestyle choices and the environment may modify disease onset and alter the projected prevalence. Observational epidemiological studies have linked exposure to the persistent pesticide dichlorodiphenytrichloroethane (DDT) with increased risk of Alzheimer’s disease (AD). In Chapter 2, using an aging cohort based in Washington Heights and Inwood in Northern Manhattan, I investigated systemic biochemical changes associated with Alzheimer’s disease (AD).

Small molecules in plasma were measured in 59 AD cases and 60 healthy participants of African American, Caribbean Hispanic, and non-Hispanic white ancestry using untargeted liquid-chromatography–based ultra-high-resolution mass spectrometry. Metabolite differences between AD and healthy, the different ethnic groups and apolipoprotein E gene (APOE) ε allele status were analyzed. Untargeted network analysis identified pathways enriched by AD-associated metabolites. Then, in Chapter 3, using the genetically tractable nematode model Caenorhabditis elegans, I investigated whether DDT can exacerbate AD-related pathology. DDT is a persistent organic pollutant which, despite its ban in 1972, can be detected in the blood of most people in the U.S. I investigated whether DDT can exacerbate AD-related pathology using a transgenic C. elegans strain that expresses a mutant tau protein fragment that is prone to aggregation, as well as a mutant strain expressing a non-aggregating form of tau protein.

DDT restricted the growth in all strains; however, the restriction was more severe in the aggregating tau transgenic strain. Further, I found that DDT exacerbates the inhibitory effects of aggregating tau protein on basal mitochondrial respiration, and increases the amount of time the worms spent curled/coiled. High-resolution metabolomics in the whole worm suggests that DDT reduces levels of several amino acids but increases levels of uric acid and adenosylselenohomocysteine. Surprisingly, developmental exposure to DDT blunts the lifespan reduction caused by aggregating tau protein suggesting a mitohormetic effect of the “double-hit” from DDT and aggregating tau protein or an antagonistic effect which could ultimately turn on lifespan extension pathways. Our data suggest that exposure to DDT likely exacerbates the mitochondrial inhibitory effects of aggregating tau protein in C. elegans. DDT may mimic some of the mitochondrial inhibitory effects induced by increased tau protein aggregation, suggesting that the genetic and environmental insult converge on a common mitochondrial inhibitory pathway, which has been associated with AD in several other studies.

Finally, in Chapter 4, I determined changes in global metabolism associated with aggregating tau protein in both C. elegans and humans. We performed high-resolution metabolomic analysis on cerebrospinal fluid (CSF) and plasma obtained from patients of AD and mild cognitive impairment, and cognitively normal controls. Using a transgenic strain of C. elegans which expresses aggregating tau protein in all neurons, I studied the effect of aggregating tau protein on metabolism using high-resolution metabolomic analysis in the whole worm. In the population study, I found >300 features associated (p < 0.05) with phosphorylated tau levels in CSF. Metabolic pathway enrichment identified alterations in fatty acid and amino acid metabolism. Worms expressing aggregating tau showed >900 features altered. Pathway enrichment suggested alterations in glycerophospholipid, fatty acid and amino acid metabolism pathways. To determine which metabolic features are altered in both species, I analyzed annotated features for overlap.

Five metabolites were concordant between human plasma and C. elegans, and four concordant between human CSF and C. elegans. Thus, in this analysis I provide evidence in support of using C. elegans to study changes in global metabolism associated with Alzheimer’s disease. In conclusion, using liquid and gas-based chromatography coupled with high-resolution mass spectrometry, we can measure levels of endogenous and exogenously derived small molecules in different biological matrices. By using the appropriate study design, we can identify candidate molecules and biochemical pathways associated with environmental exposures or disease in human populations. These candidates can be followed up by exposing an appropriate C. elegans strain: transgenic strains, mutant strains, or strains that are susceptible to RNAi based knockdown. Given their short life cycle and being amenable to high-throughput behavioral assays, they can readily provide functional and molecular readouts of the perturbation.

The findings can provide leads for relevant policy around environmental exposures, understanding mechanisms of toxicity and disease, and identifying potential therapeutic targets.