UPS of the CNS: Longitudinal profiling of mitochondrial DNA in neuronal‐enriched packages from plasma of individuals with cognitive decline

Abstract

Background Alzheimer’s disease (AD) is a complex neurodegenerative disorder that disproportionately burdens Mexican Americans (MAs) due to a combination of population‐specific environmental exposures and genetic risk factors. In addition to AD, MAs have a higher prevalence of metabolic comorbidities such as type 2 diabetes (T2D) that have been implicated in accelerated age‐related cognitive impairment. Tissue‐specific mitochondrial dysfunction and oxidative stress are early and prominent features of both AD and T2D that propagate systemic inflammation as these diseases progress. Despite the shared pathophysiology, we do not fully understand the underlying molecular mechanisms linking cognitive decline and metabolic stress, especially in understudied minority populations. Mitochondrial dysfunction may serve as the biological basis of the AD health disparity in MAs who experience earlier onset of symptoms and have an overrepresentation of T2D compared to non‐Hispanic whites (NHWs). This project aims to study T2D‐related risk for cognitive impairment by assessing the packages that neurons secrete to communicate with cells in the periphery. These neuronal‐enriched exosomes (NEEs) can cross the blood‐brain barrier and are thus capable of mediating systemic inflammation in response to CNS‐sourced stress through their mitochondrial DNA (mtDNA) cargo. We hypothesize that plasma NEE mtDNA load will reflect cognitive status and T2D comorbidity over time. Methods Exosomes of neuronal origin were enriched via CD171‐based immunocapture in longitudinal plasma samples (two timepoints, 5 years apart) from 348 Texas Alzheimer’s Research and Care Consortium (TARCC) participants stratified by race/ethnicity (MA/NHW), cognitive status (AD/MCI/NC), and T2D comorbidity (+/‐). The success of NEE isolation was assessed using nanoparticle tracking analysis (NTA) and immunoblotting against CD171 and CD81 (canonical exosome surface marker). Following extraction of total DNA from NEEs, absolute mtDNA load and deletion ratio were estimated using a real‐time qPCR assay targeting sites within the minor (D‐loop) and major (ND4) arcs. Results Herein, we demonstrate the ability to isolate NEEs from plasma and quantify their mtDNA cargo. Resultant values were analyzed for longitudinal differences and correlation with disease status based on various neuropsychological measures. Conclusion As the first effort to assess mtDNA in NEEs, this project provides novel insight into how T2D confers risk for AD across different populations over time.