Title: Hexavalent Chromium Neurotoxicity: Metabolic Origins and Therapeutic Interventions Significance: The burden of neurocognitive disorders in military veterans is significant. This includes high rates of anxiety, depression, and post-traumatic stress disorder as well as a suicide rate that is 3-fold higher than the civilian population. Critically, there is also a significant neuropsychiatric treatment gap that leaves many veterans inadequately treated. Importantly, essential omega-3 fatty acids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are enriched in the brain where they exert multiple anti-inflammatory, anti-oxidant, and neuroprotective effects. Conversely, omega-3 fatty acid deficiency is associated with depressive symptoms. As such omega-3 fatty acid supplementation represents a promising avenue of therapy for multiple neurocognitive disorders; however, traditional means of supplementation such as free fatty acids or triglycerides (e.g., fish oil) have proven disappointing, likely due to their inability to penetrate the blood brain barrier (BBB).
In contrast, lysophosphatidylcholine (LPC) is a unique phospholipid with a specific transporter at the BBB, Mfsd2a. As such, omega-3 fatty acid supplementation using the LPC form (i.e., LPC-DHA/EPA) offers a novel avenue to enrich brain essential fatty acid levels and potentially improve neurocognitive function. Such an approach may be especially relevant to address the impact of environmental toxicants that induce oxidative stress and are linked to neurocognitive disorders. Indeed, our preliminary data suggest that developmental arsenic exposure depletes brain DHA levels, while concurrent dietary supplementation with LPC-DHA preserves them. Whether other pro-oxidant metals induce similar effects that can be reversed by LPC-DHA/EPA remains an understudied but potentially clinically significant area of investigation.
One such metal is hexavalent chromium [Cr6+; Cr(VI)], which is used commonly in the military for its anti-corrosive properties. Animal models show that Cr(VI) induces lipid oxidation in the brain. How Cr(VI) specifically affects brain omega-3 fatty acid levels as well as its impact on neurocognitive outcomes are largely unknown; moreover, little is known about ways to reverse the pro-oxidant impact of Cr(VI) on the brain. Specific Aims: The central hypothesis of this application is that Cr(VI) depletes the brain of the essential fatty acids DHA and EPA, resulting in anxiety- and depression-like behaviors as well as impairments in learning and memory; while targeted post-exposure dietary supplementation with LPC-DHA/EPA [but not DHA and EPA in the triglyceride form (TG-DHA/EPA)] restores brain omega-3 fatty acid balance and improves neurocognition.
Specific Aim 1: Impact of Cr(VI) on Brain Lipid Metabolism and Neurocognition. We will use an extended low dose Cr(VI) exposure model in which C57BL/6J mice receive Cr(VI) via their drinking water during the period of late adolescence through early adulthood. This exposure period reflects a typical tour of military service. After exposure, mice will undergo a comprehensive battery of behavioral studies to evaluate measures of anxiety/depression and learning/memory as per our established protocols. In addition, lipidomic analyses will be performed to determine lipid speciation in select brain regions, adipose tissue, and liver to determine levels of the essential fatty acids DHA and EPA as well as their specific phospholipid compartment. Additional molecular markers to be examined will include lipid adducts, cytokines, brain-derived neurotrophic factor, and chromium levels (by ICP-MS).
Specific Aim 2: Rescue of Brain Lipids after Cr(VI) Exposure using a Novel Lipid Species. After Cr(VI) exposure as in Aim 1, mice will be randomized to one of three diets (control, TG-DHA/EPA, or LPC-DHA/EPA) for 4 weeks. Behavioral, molecular, and lipidomic analyses will be conducted as in Aim 1. These studies will ascertain whether LPC-DHA/EPA uniquely restores essential fatty levels in the brain and if this restoration diminishes markers of oxidative stress and inflammation while improving neurocognition, potentially opening up LPC-DHA/EPA as a therapy for those suffering from service-associated Cr(VI) exposure. Relevance to US Military Veterans’Health: US military veterans suffer disproportionately from neurocognitive disorders. While multiple factors may be involved, exposure to environmental toxicants remains an important-but-understudied area of risk. Cr(VI) is used extensively in the military for its hardness and corrosion esistance. Military activities associated with Cr(VI) exposure include aircraft maintenance, metal working, metal treatment and machining, and coating and painting operations. Cr(VI) is particularly critical for aircraft coatings, where concerns persist regarding non-Cr(VI) substitutes. Currently, at least 34 US Army installations are reported to have Cr(VI) contamination, while studies suggest that military activities employing Cr(VI) lead to exposures that often exceed the permissible exposure limit (PEL). Overall Impact: Military-associated exposures impose lifelong risks on health, likely contributing to higher neurocognitive morbidity among US veterans. This project goes beyond simply documenting that risk to begin to explore whether a targeted, scalable, lipid-based intervention can reverse the adverse neurocognitive effects of service-related Cr(VI) exposure in order to restore mental health in US military veterans.