ApoE4, which is strongly linked to AD (Roses, 1996), uniquely possesses an arginine at residue 112, whereas both apoE2 and apoE3 have cysteine at this site. This GDC-0068 molecular weight single amino acid difference in

apoE4 is associated with protein instability and domain interaction (Dong et al., 1994; Zhong and Weisgraber, 2009). ApoE3, the most common isoform in humans, has a cysteine residue at position 112 and an arginine at position 158, is more stable than apoE4, and is less likely to display domain interaction than apoE4. As illustrated in Figure 4, in apoE4 the arginine at residue 112 causes the side chain of arginine-61 to be solvent exposed, to extend away from the helical bundle in the N-terminal domain, and to interact with glutamic acid-255 in the C-terminal domain through ionic bonding. Although all isoforms may display domain interaction to some extent, the amino acid substitutions of apoE4 encourage domain interaction much more than the apoE3 and apoE2 isoforms (apoE4 > apoE3 > apoE2). Importantly, this apoE4 property has been established biophysically by fluorescence resonance

energy transfer (FRET) and electron paramagnetic resonance spectroscopy (Dong et al., PI3K inhibitors in clinical trials 1994; Hatters et al., 2005; Xu et al., 2004; Zhong and Weisgraber, 2009). The distance between arginine-61 and glutamic acid-255 was ∼10 Å in apoE4 and greater than 22 Å in apoE3 (Hatters et al., 2005). Recently, the nuclear magnetic resonance structure for full-length apoE3 was determined (Chen et al., 2011b). The structure reveals a unique topology, which is postulated to shield the LDL receptor-binding region by the C-terminal domain and prevent binding to the receptor in the nonlipid form. However, in order to prevent tetramer formation and aggregation, it was necessary to introduce five nonconservative mutations into the C-terminal

domain (F257A/W264R/V269A/L279Q/V287E). Phosphoprotein phosphatase Unfortunately, these rather severe changes undoubtedly alter the C-terminal domain and likely distort the conformation and intramolecular interactions with this domain. Thus, the nuclear magnetic resonance structure may not represent a physiological and functional structure of apoE. In all, these data show how relatively minor changes in apoE sequence yield significant differences in apoE’s ability to promote neuronal health versus neuronal damage, especially when apoE synthesis is increased after injury. The apoE hypothesis therefore suggests that the isoform-dependent neuronal response to increased apoE expression is a critical step in laying the groundwork for future pathology. In the following sections, we describe the apoE4-specific effects on neuronal function and how they affect neuronal integrity at the cellular level. Fluorescence recovery after photobleaching experiments revealed impaired intracellular trafficking of apoE4 through the ER and Golgi apparatus in cultured cells (Brodbeck et al., 2011).