Neuroprotective Mechanisms of S-Nitrosylated ApoE2 in Aging and Alzheimer's Disease

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Apolipoprotein-E (ApoE) is known to maintain the structure of lipoprotein particles and mediate lipid transport. Among three ApoE isoforms (i.e., ApoE2, ApoE3, and ApoE4) present in human, ApoE4 was identified as a major genetic risk factor for late onset Alzheimer’s disease (AD), whereas ApoE2 is thought to be protective. ApoE3 represents the most prevalent form and is thereby considered the standard for the general population. The three variants of ApoE differ only in their Cys/Arg composition at 112 and 158. Namely, ApoE2 contains Cys112 and Cys158; ApoE3 includes Cys112 and Arg158; and ApoE4 has Arg112 and Arg158. Although how ApoE2 protects against AD onset remains incompletely understood, the presence of two Cys residues in ApoE2 seems to engender a distinct structure (e.g., disulfide-linked dimers) and function, possibly explaining the protective effects that ApoE2 exerts towards AD. Notably, recent studies demonstrated that ApoE acts as a ligand for TREM2, another AD risk factor expressed in microglia, thus facilitating uptake and degradation of aggregated Aβ. In the current application, we propose that the redox-mediated posttranslational modification, S-nitrosylation, contributes to the protective effect of ApoE by forming SNO-ApoE2. Protein S-nitrosylation is a modification involving the covalent addition of a nitric oxide (NO)-related species (e.g., nitrosonium cation [NO+]) to a thiolate anion of cysteine. A critical mechanism for generation of SNO-proteins in vivo entails transfer of an NO-related species (i.e., transnitrosylation) from one protein to another. Our group and others have extensively demonstrated that S-nitrosylation produced by physiological levels of NO mediates neuroprotective effects, while, NO in excess elicits neurotoxicity in models of neurodegeneration via aberrant S-nitrosylation. Notably, our Preliminary Studies and published reports show that, unlike ApoE4, ApoE2 efficiently undergoes S-nitrosylation, leading to conformational changes. The change in SNO-ApoE conformation affects binding to interacting partners such as TREM2 and other receptors. Accordingly, we found that SNO-ApoE formation influences Ab uptake and degradation in hiPSC-derived microglia. Moreover, our Preliminary Studies show that SNO-ApoE2 can transnitrosylate TREM2 (producing SNO-TREM2), implying that ApoE2 may affect TREM2-dependent neuroprotective signaling pathways via this transnitrosylation reaction. In the current R01 application, we propose to investigate the mechanism whereby S-nitrosylation affects the biochemical properties of ApoE2 to regulate TREM2-mediated Ab uptake and degradation (Specific Aim 1). Finally, we will characterize the neuroprotective effects of SNO-ApoE2-TREM2 transnitrosylation on synaptic integrity and neuronal survival in AD transgenic mouse models as well as in hiPSC AD models (Specific Aims 2 and 3).