Environmental factors in pathobiology of dementia: the role of PCB exposure, microbiome, and tissue barrier dysfunction

PROJECT SUMMARY/ABSTRACT Alzheimer’s Disease (AD) is the most common cause of dementia in the elderly, and it is the sixth leading cause of death in the United States. AD currently cannot be prevented, cured, or even slowed, and it has a significant public health impact in terms of health care dollars and quality of life for those affected and their family members. Experimental models of ADRD have implicated the gut microbiome-bile acid-brain axis in the development and progression of ADRD. Neurotoxic environmental toxicants, such as polychlorinated biphenyls (PCBs), alter the function of the microbiome, resulting in an altered bile acid homeostasis; however, it is unknown if PCB-mediated changes in the gut microbiome-bile acid-brain axis play a role in the etiology of ADRD. Furthermore, epidemiological studies have major limitations assessing the complex effects of PCB exposure on the gut microbiome-bile acid-brain axis across the lifespan. Thus, there is a critical need to assess how human-relevant PCB mixtures alter the development and progression of ADRD-like phenotypes in experimental models of ADRD via the gut microbiome-bile acid-brain axis. The long-term goal of the transdisciplinary team assembled for this project is to characterize how environmental exposures contribute to ADRD and ultimately prevent ADRD through a precision environmental health paradigm. The translational objective is to demonstrate with a systems biology approach that exposure to a human-relevant PCBs mixture contributes to and accelerates the etiology of ADRD-type outcomes in vivo. The central hypothesis is that exposure to PCBs adversely affects the ADRD phenotype in rTg4510 and APP/PS1 mice, two experimental models of ADRD, by causing microbiome-mediated alterations in the bile acid homeostasis and affecting vascular function in a dose and exposure time-dependent manner. This hypothesis integrates strong preliminary data from the research team showing that PCBs are present in the human brain, affect the microbiome, alter bile acid homeostasis, and cause vascular dysfunction. The hypothesis will be tested using a systems biology approach by assessing how exposure to a human-relevant PCB mixture affects ADRD-related outcomes in experimental models of ADRD. The Specific Aims are to a) characterize effects of PCB exposure on gut microbiome composition and circulating bile acids; b) study the effects of PCB exposure on vascular function, and c) identify ADRD-type pathological changes and memory loss in the brain of PCB exposed rTg4510 or APP/PS1 mice. To ensure integration across all Aims, mediation analysis will be used to demonstrate that the microbiome and/or vascular dysfunction mediates the effects of PCBs on ADRD-type outcomes. These studies will demonstrate that PCB exposure leads to accelerated progression and more severe disease pathology in experimental ADRD models. Identifying PCBs as environmental risk factors that alter ADRD-related outcomes will lay the groundwork for mechanistic studies and inform translational studies for preventing ADRD mediated by environmental toxicants using a precision environmental health paradigm.