Your Brain on Plastic: Measuring the Physiological to Transcriptomic Responses of Human Microglia to Consumer-Grade Micro- and Nanoplastics

Abstract

The production of plastics has reached record levels, with nearly half a billion metric tonnes produced annually. As such, there have been staggering increases in the amount of mismanaged- along with landfill-bound plastic waste across the globe. This has lead to increasing contamination of microplastics (MPs), or plastics smaller than 5 mm, caused by degradation of consumer plastics. Further break down of MPs can lead to the formation of nanoplastics (NPs), or plastics smaller than 1 µm. Both MPs and NPs (MNPs) have been detected in nearly every human organ, including the brain, leading to an increased public concern about their impact on the body. However, the vast majority of studies that seek to understand the impacts of MNPs rely on industrially manufactured, "pristine" polystyrene (PS) nanobeads to serve as a proxy for the "real-world" plastics that are found in the human body. While these particles can be beneficial to understand some of the basic perturbations of plastics in the body, they are far too heterogeneous in terms of size, shape, and composition to accurately model the polydisperse MNPs that have been found in human organs. The absence of studies using relevant plastic models has led to concern in the field as the general interpretation of the impacts of these ubiquitous contaminants remains loosely connected to the real-life situation. This work aims to bridge this research grasp by not only understanding the impacts of real-world MNPs derived from common plastic consumer items, cups and forks, but also comparing these impacts against pristine PS-NPs that are commonly used in toxicological studies. The impact that these pristine particles have, with diameters ranging from 50 - 100 nm (PS-50), are directly contrasted against the impacts elicited by cup-derived and fork-derived plastics (Cup- and Fork-MNPs) on the in vitro brain target human microglial clone 3 (HMC3) cells. Plastics used for exposure underwent extensive material characterization, including size, morphology, surface charge, trace-metal contamination, and composition. Microglial cells, which comprise nearly 10\% of the brain' cellular composition, are the primary immune cells of the central nervous system (CNS) and are commonly used in neuro-degenerative studies to measure the immune response of the brain. By exposing these cells to physiologically-relevant concentrations of both real-world Cup- and Fork-MNPs and comparing the impacts to exposure of pristine PS-50 NPs, the discrepancies between the two plastic conditions can be identified and used to elucidate potential shortcomings of previous studies in the field.

In the first chapter of this study, a comprehensive review of plastic generation, MNP accumulation, and human exposures are detailed in order to serve as a basis for the study's impact. In the second chapter, HMC3 cells were exposed to 0.1, 1.0, and 10 µg/mL concentrations of real-world and pristine MNPs for 72 hours in order to elucidate the physiological responses of the brain's immune system. By focusing on redox homeostasis, inflammatory signaling, and mitochondrial function, an assessment of the cellular responses to MNPs can be made in order to understand the functional changes that may be seen in the brain. In the third chapter, HMC3 cells were exposed to the same plastics at 0.1 µg/mL, the lowest concentration tested, for 72 hours and had their transcriptome sequenced in order to assess the impacts of MNPs on the brain at the gene-expression level. This allows for a detailed understanding of the cellular response in microglia that may be overshadowed by transcriptional and translational lag time or cellular adaptations. Overall, pristine NPs were seen to elicit a heightened response relative to real-world MNPs through decreased mitochondrial respiration, mitochondrial membrane depolarization, decreases in ATP production, stronger inflammatory signaling, and greater redox perturbation, suggesting distress. Contrarily, real-world MNPs displayed an adaptive cell stress response profile through lessened, often nonsignificant disruptions to many of these same parameters. This was further supported through the pristine NP-exposed cells' enriched endoplasmic reticulum (ER) stress and tRNA aminoacylation pathways relative to real-world MNPs, suggesting significantly different stress profiles even at the lowest concentration of MNPs tested. In both real-world MNP-exposed samples, collagen formation-dominated expression profiles were observed, with both Cup- and Fork- MNPs sharing similar transcriptomic enrichments.

Overall, this works serves to support the necessity of utilizing representative plastic particles to model the effects of MNPs on the human body, specifically the brain. The variations in biological responses between real-world and pristine plastics at both the physiological level as well as the transcriptomic level underscore this fact, as pristine particles were shown to evoke stronger stress responses relative to real-world MNPs. By better understanding the effects caused by MNPs from consumer products, researchers can more accurately assess the risks posed by plastics and develop more effective mitigation strategies.