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Introduction

Human APOE is a 34 kDa glycoprotein1 that is composed of 299 amino acids after cleavage of the 18-amino-acid signal peptide. In the CNS, APOE is abundantly expressed in astrocytes, microglia, vascular mural cells and choroid plexus cells, and to a lesser extent in stressed neurons2, 3. Once APOE has been secreted from the cells, the cell surface ATP-binding cassette transporters ABCA14 and ABCG15 transfer cholesterol and phospholipids to nascent APOE to form lipoprotein particles. The size and density of APOE lipoprotein particles in the cerebrospinal fluid (CSF) are similar to those of HDL6. APOE plays a critical role in redistributing cholesterol and other lipids to neurons through binding to cell-surface APOE receptors. The LDL receptor (LDLR) family members, including LDLR and LDLR-related protein 1 (LRP1), are major APOE receptors involved in APOE-mediated lipid metabolism. APOE has two main functional regions: the receptor-binding region in the amino-terminal domain and the lipid-binding region in the carboxy-terminal domain7, 8.

Computer-assisted modelling has been performed to examine the conformational changes in APOE that potentially occur on lipid binding. Structural modelling of human APOE3 was accomplished using data on APOE from the Protein Data Bank9-16. The modelling technique has  been well documented17-23 and enables composite protein structures to be built from multiple structural templates. The resulting lipid-free and lipid-bound structures are shown in figure below24-27. The structures show that the lipid-binding region (residues 244272) of APOE interacts directly with the lipid particle. Lipid binding to APOE increases the accessibility of the receptor-binding region (residues 136150), thereby enabling cellular lipid delivery. The structural changes to APOE that are predicted to take place on lipid binding are consistent with those documented in the existing structural literature for this protein9, 10, 13, 14, 16, 28-33.

The APOE isoforms encoded by the three corresponding gene alleles differ from one another only at positions 112 and 158 (APOE2: Cys112, Cys158; APOE3: Cys112, Arg158; APOE4: Arg112, Arg158). However, these single amino acid polymorphisms substantially alter the structure and function of APOE, thereby modulating its binding properties with regard to both lipids and receptors. For example, the binding of APOE2 to LDLR is more than 50 times weaker than the binding  of APOE3 or APOE4 to this receptor34. In the periphery, the relatively low affinity of APOE2 for LDLR impairs the clearance of triglyceride-rich lipoprotein remnant particles and, as a consequence, APOE2 contributes to the onset of type III hyperlipoproteinaemia1, 35. By contrast, enhanced binding of APOE4 to VLDL particles, as compared with the other APOE isoforms34, impairs lipolytic processing of VLDL in the periphery; thus, APOE4 is associated with pro-atherogenic changes in lipoprotein distribution35.

In addition to their role in lipid homeostasis in the periphery, APOE isoforms differentially modulate multiple pathways in the brain, including lipid transport, synaptic integrity and plasticity, glucose metabolism and cerebrovascular function. However, the correlation between the structure of individual APOE isoforms and their modulation of these pathways in the brain is less clear than for their actions on peripheral lipid metabolism. Possible mechanisms by which structural differences between APOE isoforms could modulate these brain pathways include differences in protein conformation, post-translational modification, lipoprotein preference and binding affinity for receptors8.

Studies have also shed light on the potential role of plasma APOE in brain homeostasis. Synaptic dysfunction in Apoe-knockout mice can be partially restored, and cognition improved, by genetic restoration of peripheral APOE36. These effects occur despite the presence of the bloodbrain barrier (BBB), which blocks APOE influx from the periphery. The relative ratio of APOE4 to APOE3 in plasma positively correlates with the loss of regional grey matter volume and abnormal cerebral glucose metabolism in cognitively healthy APOE*ε3/ε4 carriers37, linking the isoform composition of plasma APOE to structural and metabolic changes of the brain. Therefore, plasma APOE is a potential determinant of brain structure and function.

In APOE*ε3/ε4 individuals, the APOE4:APOE3 ratio is >1 in the brain and CSF but <1 in the plasma38, 39, suggesting that the metabolic pathways of APOE isoforms differ between the CNS and plasma. Thus, further studies exploring the similarities and differences in APOE metabolism between the CNS and periphery, and the functional crosstalk between brain and plasma APOE, would aid a better understanding of the impact of APOE isoforms on brain physiology.

Yamazaki, Y., et al. (2019). “Apolipoprotein E and Alzheimer disease: pathobiology and targeting strategies.” Nat Rev Neurol 15(9): 501-518.

Literature cited

  1. Mahley RW. Apolipoprotein E: cholesterol transport protein with expanding role in cell biology. Science 1988;240:622-630.
  2. Xu Q, Bernardo A, Walker D, Kanegawa T, Mahley RW, Huang Y. Profile and regulation of apolipoprotein E (ApoE) expression in the CNS in mice with targeting of green fluorescent protein gene to the ApoE locus. J Neurosci 2006;26:4985-4994.
  3. Kang SS, Ebbert MTW, Baker KE, et al. Microglial translational profiling reveals a convergent APOE pathway from aging, amyloid, and tau. J Exp Med 2018;215:2235-2245.
  4. Wahrle SE, Jiang H, Parsadanian M, et al. ABCA1 is required for normal central nervous system ApoE levels and for lipidation of astrocyte-secreted apoE. J Biol Chem 2004;279:40987-40993.
  5. Karten B, Campenot RB, Vance DE, Vance JE. Expression of ABCG1, but not ABCA1, correlates with cholesterol release by cerebellar astroglia. J Biol Chem 2006;281:4049-4057.
  6. LaDu MJ, Gilligan SM, Lukens JR, et al. Nascent astrocyte particles differ from lipoproteins in CSF. J Neurochem 1998;70:2070-2081.
  7. Bu G. Apolipoprotein E and its receptors in Alzheimer’s disease: pathways, pathogenesis and therapy. Nat Rev Neurosci 2009;10:333-344.
  8. Kanekiyo T, Xu H, Bu G. ApoE and Abeta in Alzheimer’s disease: accidental encounters or partners? Neuron 2014;81:740-754.
  9. Dong J, Peters-Libeu CA, Weisgraber KH, et al. Interaction of the N-terminal domain of apolipoprotein E4 with heparin. Biochemistry 2001;40:2826-2834.
  10. Segelke BW, Forstner M, Knapp M, et al. Conformational flexibility in the apolipoprotein E amino-terminal domain structure determined from three new crystal forms: implications for lipid binding. Protein Sci 2000;9:886-897.
  11. Wilson C, Mau T, Weisgraber KH, Wardell MR, Mahley RW, Agard DA. Salt bridge relay triggers defective LDL receptor binding by a mutant apolipoprotein. Structure 1994;2:713-718.
  12. Dong LM, Wilson C, Wardell MR, et al. Human apolipoprotein E. Role of arginine 61 in mediating the lipoprotein preferences of the E3 and E4 isoforms. J Biol Chem 1994;269:22358-22365.
  13. Wilson C, Wardell MR, Weisgraber KH, Mahley RW, Agard DA. Three-dimensional structure of the LDL receptor-binding domain of human apolipoprotein E. Science 1991;252:1817-1822.
  14. Dong LM, Parkin S, Trakhanov SD, et al. Novel mechanism for defective receptor binding of apolipoprotein E2 in type III hyperlipoproteinemia. Nat Struct Biol 1996;3:718-722.
  15. Guttman M, Prieto JH, Handel TM, Domaille PJ, Komives EA. Structure of the minimal interface between ApoE and LRP. J Mol Biol 2010;398:306-319.
  16. Chen J, Li Q, Wang J. Topology of human apolipoprotein E3 uniquely regulates its diverse biological functions. Proc Natl Acad Sci USA 2011;108:14813-14818.
  17. Richter JE, Robles HG, Mauricio E, Mohammad A, Atwal PS, Caulfield TR. Protein molecular modeling shows residue T599 is critical to wild-type function of POLG and description of a novel variant associated with the SANDO phenotype. Hum Genome Var 2018;5:18016.
  18. Puschmann A, Fiesel FC, Caulfield TR, et al. Heterozygous PINK1 p.G411S increases risk of Parkinson’s disease via a dominant-negative mechanism. Brain : a journal of neurology 2017;140:98-117.
  19. Kayode O, Wang R, Pendlebury DF, et al. An Acrobatic Substrate Metamorphosis Reveals a Requirement for Substrate Conformational Dynamics in Trypsin Proteolysis. J Biol Chem 2016;291:26304-26319.
  20. Caulfield TR, Fiesel FC, Springer W. Activation of the E3 ubiquitin ligase Parkin. Biochem Soc Trans 2015;43:269-274.
  21. Caulfield TR, Fiesel FC, Moussaud-Lamodière EL, Dourado DFAR, Flores SC, Springer W. Phosphorylation by PINK1 Releases the UBL Domain and Initializes the Conformational Opening of the E3 Ubiquitin Ligase Parkin. PLoS Comput Biol 2014;10:e1003935.
  22. Zhang YJ, Caulfield T, Xu YF, et al. The dual functions of the extreme N-terminus of TDP-43 in regulating its biological activity and inclusion formation. Hum Mol Genet 2013;22:3112-3122.
  23. Caulfield T, Devkota B. Motion of transfer RNA from the A/T state into the A-site using docking and simulations. Proteins 2012;80:2489-2500.
  24. Altschul SF, Madden TL, Schaffer AA, et al. Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acids Res 1997;25:3389-3402.
  25. Hooft RW, Sander C, Scharf M, Vriend G. The PDBFINDER database: a summary of PDB, DSSP and HSSP information with added value. Computer applications in the biosciences : CABIOS 1996;12:525-529.
  26. Hooft RW, Vriend G, Sander C, Abola EE. Errors in protein structures. Nature 1996;381:272.
  27. Krieger E, Joo K, Lee J, et al. Improving physical realism, stereochemistry, and side-chain accuracy in homology modeling: Four approaches that performed well in CASP8. Proteins 2009;77 Suppl 9:114-122.
  28. Frieden C, Wang H, Ho CMW. A mechanism for lipid binding to apoE and the role of intrinsically disordered regions coupled to domain-domain interactions. Proc Natl Acad Sci USA 2017;114:6292-6297.
  29. Mondal T, Wang H, DeKoster GT, Baban B, Gross ML, Frieden C. ApoE: In Vitro Studies of a Small Molecule Effector. Biochemistry 2016;55:2613-2621.
  30. Frieden C. ApoE: the role of conserved residues in defining function. Protein Sci 2015;24:138-144.
  31. Garai K, Verghese PB, Baban B, Holtzman DM, Frieden C. The binding of apolipoprotein E to oligomers and fibrils of amyloid-beta alters the kinetics of amyloid aggregation. Biochemistry 2014;53:6323-6331.
  32. Frieden C, Garai K. Concerning the structure of apoE. Protein Sci 2013;22:1820-1825.
  33. Ji ZS, Dichek HL, Miranda RD, Mahley RW. Heparan sulfate proteoglycans participate in hepatic lipaseand apolipoprotein E-mediated binding and uptake of plasma lipoproteins, including high density lipoproteins. J Biol Chem 1997;272:31285-31292.
  34. Hatters DM, Peters-Libeu CA, Weisgraber KH. Apolipoprotein E structure: insights into function. Trends Biochem Sci 2006;31:445-454.
  35. Phillips MC. Apolipoprotein E isoforms and lipoprotein metabolism. IUBMB life 2014;66:616-623.
  36. Lane-Donovan C, Wong WM, Durakoglugil MS, et al. Genetic Restoration of Plasma ApoE Improves Cognition and Partially Restores Synaptic Defects in ApoE-Deficient Mice. J Neurosci 2016;36:10141-10150.
  37. Nielsen HM, Chen K, Lee W, et al. Peripheral apoE isoform levels in cognitively normal APOE epsilon3/epsilon4 individuals are associated with regional gray matter volume and cerebral glucose metabolism. Alzheimer’s research & therapy 2017;9:5.
  38. Martinez-Morillo E, Hansson O, Atagi Y, et al. Total apolipoprotein E levels and specific isoform composition in cerebrospinal fluid and plasma from Alzheimer’s disease patients and controls. Acta Neuropathol 2014;127:633-643.
  39. Baker-Nigh AT, Mawuenyega KG, Bollinger JG, et al. Human Central Nervous System (CNS) ApoE Isoforms Are Increased by Age, Differentially Altered by Amyloidosis, and Relative Amounts Reversed in the CNS Compared with Plasma. J Biol Chem 2016;291:27204-27218.