Thesis (Ph.D.)--University of Washington, 2019Developmental neurotoxicity (DNT) is one of the least tested health effects of the more than 80,000 chemicals registered for use today. In order to more efficiently test chemicals for DNT, more knowledge is needed on mechanisms that trigger adverse neurodevelopmental outcomes. Lipids are critical for neurodevelopment; therefore, disruption of lipid homeostasis by chemicals is expected to have detrimental effects on this process. However, few have investigated this as a possible mechanism of DNT. In preliminary studies, we demonstrated that benzalkonium chloride compounds (BACs) alter cholesterol biosynthesis and lipid homeostasis in neuronal cells. BACs are the most commonly used quaternary ammonium compound (QAC) disinfectants. They are applied in food processing lines, health care facilities, residential settings, and are common ingredients in over-the-counter cosmetics, hand sanitizers, and pharmaceutical products. Therefore, exposure to BACs is prevalent given the diversity of applications and may occur through dermal/eye contact, inhalation, and ingestion. Recent studies demonstrate that BAC exposure leads to an increased incidence of neural tube defects in utero and increased apoptosis of neural progenitor cells (NPCs). However, the effects of BACs on neurodevelopment as a result of altered lipid homeostasis has not been investigated. In my dissertation work, I characterized the effects of BACs on lipid homeostasis and neurodevelopmental processes. First, I showed that BACs potently inhibit cholesterol biosynthesis in mouse and human neuronal cells. Building on this work, I showed that BACs can cross the blood-placental barrier and enter the developing mouse brain following in utero exposure via maternal diet. Further transciptomic analyses of the developing brain elucidated key signaling pathways affected by BACs, including cholesterol biosynthesis, liver X receptor-retinoid X receptor (LXR/RXR) signaling, and glutamate receptor signaling. Mass spectrometry analysis revealed decreases in total sterol levels and downregulation of triglycerides and diglycerides, which were consistent with the upregulation of genes involved in sterol biosynthesis and uptake, as well as inhibition of LXR signaling. Finally, I investigated the effects of BACs on neurospheres, free floating structures of NPCs, which are used as a three-dimensional (3-D) in vitro model of neurodevelopment. I found that BACs depleted the pool of NPCs and increased apoptosis, which contributed to a reduction in neurosphere growth. Transcriptome analysis revealed that BACs activated the integrated stress response, a mechanism used by cells to adapt to a variety of stressors including oxidative stress, mitochondrial dysfunction, nutrient deficiency, or hypoxia. Altogether, the findings of this dissertation demonstrate that a class of commonly used disinfectants alter lipid homeostasis and impact neurodevelopmental processes. These data add to a growing body of literature reporting the adverse effects of BACs on neurodevelopment. Importantly, this work supports a novel mechanism by which environmental chemicals may target neurodevelopment
