Aging-related neurological diseases impact millions people worldwide, and this number is expected to increase drastically as the global average age continues to increase. While the need to develop new neuroprotective therapeutics has never been higher, our current efforts rely on only a partial understanding of the molecular, cellular, and circuit mechanism that drive changes in the aging brain. One such change is increased oxidative stress arising partially from the brain?s high metabolic needs and exacerbated by brain enrichment of oxidation-catalyzing transition metals and incorporation of peroxidation-sensitive polyunsaturated fatty acid (PUFA) species into phospholipid membranes. Accumulation of peroxidized lipids results in membrane damage and ferroptosis, an iron-dependent, non-apoptotic mode of cell death. Our preliminary data suggest that neurons, astrocytes, and microglia are susceptible to ferroptosis, whereas oligodendrocytes are resistant. However, the mechanisms regulating differential susceptibility to ferroptosis in these cell types remain unknown. One possible connection between ferroptosis and neurodegeneration is apolipoprotein E (apoE), the primary lipid and cholesterol transport protein of the central nervous system (CNS). APOE gene variants modulate the probability of developing neurodegenerative disease, with each apoE4 allele conferring an approximately 2-fold increase in risk for late-onset Alzheimer?s disease compared the more common apoE3 allele. In contrast, the apoE2 variant is enriched in cognitively-intact elders. While many characteristics differentiate apoE4 from the other isoforms, including decreased protein stability and altered lipid transport, no one mechanism is considered responsible for increased risk of neurodegeneration in apoE4 carriers. Recent work shows that apoE mediates transfer of peroxidation-sensitive fatty acids from neurons to astrocytes during periods of enhanced neuronal activity, suggesting that deficient capacity for lipid transport could lead to accumulation of these potentially toxic species in neurons. Interestingly, our preliminary data suggest that astrocytes expressing apoE3 may be protected against some forms of ferroptosis compared to apoE knockout astrocytes, but that this susceptibility can be rescued by exogenous application of plasma high density lipoproteins (HDL). We propose to validate our initial findings using primary culture of CNS cells. Because interactions among cell types in the brain are critical for modeling of brain lipid metabolism, we will use organotypic slice culture to investigate the impact of apoE isoform on ferroptosis sensitivity. To characterize how isoform-dependent differences in apoE-lipid interactions impact CNS ferroptosis sensitivity, we will complex apoE to purified plasma HDL and test how these particles impact ferroptosis sensitivity. Lastly, we will identify the lipid species conferring resistance to ferroptosis in our preliminary data by constructing apoE lipoprotein nanoparticles containing individual HDL lipid species and testing their function in cellular assays of ferroptosis sensitivity.
Despite having known for many years that variants of the APOE gene, which encodes the lipid and cholesterol transport protein apolipoprotein E (apoE), are key mediators of brain health, a conclusive mechanism by which apoE modifies brain health and susceptibility to disease remains elusive. This project seeks to better understand how the apoE4 and apoE2 isoforms respectively confer risk for - or resilience to - unhealthy brain aging, poor recovery following brain injury, and development of neurodegenerative diseases by studying the connection between apoE isoform and a recently-identified mechanism of cell death, ferroptosis. Expanding our current understanding of apoE function brings critical new insight to brain health in aging and disease and has the potential to inform the development of new therapeutics.
