The LDL receptor binding domain of apolipoprotein E directs the relative orientation of its C-terminal segment in reconstituted nascent HDL

Abstract

Apolipoprotein E (apoE) (299 residues) is a highly helical protein that plays a critical role in cholesterol homeostasis. It comprises a four-helix bundle N-terminal (NT) and a C-terminal (CT) domain that can exist in lipid-free and lipid-associated states. In humans, there are two major apoE isoforms, apoE3 and apoE4, which differ in a single residue in the NT domain, with apoE4 strongly increasing risk of Alzheimer’s disease (AD) and cardiovascular diseases (CVD). It has been proposed that the CT domain initiates rapid lipid binding, followed by a slower NT domain helix bundle opening and lipid binding to yield discoidal reconstituted high density lipoprotein (rHDL). However, the contribution of the NT domain on the CT domain organization in HDL remains poorly understood. To understand this, we employed Cys-specific cross-linking and spatially-sensitive fluorophores in the NT and CT domains of apoE3 and apoE4, and in isolated CT domain. We noted that the helices in isolated CT domain are oriented parallel to those in the neighboring molecule in rHDL, whereas full length apoE3 and apoE4 adopt either an anti-parallel or hairpin-like organization. It appears that the bulky NT domain determines the spatial organization of its CT domain in rHDL, a finding that has significance for apoE4, which is more susceptible to proteolytic cleavage in AD brains, showing increased accumulation of neurotoxic NT and CT fragments. We envisage that the structural organization of HDL apoE would have profound functional consequences in its ability to regulate cholesterol homeostasis in AD and CVD.

A Review of ApoE4 Interference Targeting Mitophagy Molecular Pathways for Alzheimer's Disease

“…It is well-known that ApoE4 detrimental pathogenic effects are highly dependent on its peculiar conformation with a more compacted structure and higher oligomerized inclination, which is resulted from two amino acid residues varying from ApoE3 (Chen et al, 2011 ). Accordingly, ApoE4 is more susceptible to cleave than ApoE3 (Huang et al, 2001 ; Kothari et al, 2021 ). Different ApoE4 fragments differentially couple with certain neurotoxicity and induce mitochondrial disorders (Chang et al, 2005 ).…”

Isoform‐specific modification of apolipoprotein E by 4‐hydroxynonenal: protective role of apolipoprotein E3 against oxidative species

“…The lowered phospholipid solubilization rate of HNEmodified apoE3 and apoE4 is attributed to impediment in the initial binding step with the modification affecting the step that engages the protein to the lipid surface. Though the precise mechanism is not known, residues 201-299 are believed to initiate lipid binding by anchoring the protein to the lipid surface, followed by movement of the NT domain away from CT domain, and then opening of the helix bundle in the NT domain [43,73].…”

Are apolipoprotein E fragments a promising new therapeutic target for Alzheimer’s disease?

“…It appears that the bulky N-terminal domain determines the spatial organization of its C-terminal domain in reconstituted HDL, a finding that has significance for ApoE4, which is more susceptible to proteolytic cleavage in AD brains. 191 …”

“…It appears that the bulky N-terminal domain determines the spatial organization of its C-terminal domain in reconstituted HDL, a finding that has significance for ApoE4, which is more susceptible to proteolytic cleavage in AD brains. 191 The ApoE mimetic peptides In nature, potentially therapeutic peptides are present as 100 aa short-chain monomers. Such peptides may bind certain membrane receptors and activate specific signaling pathways.…”