Functional diversity of apolipoprotein E: from subcellular localization to mitochondrial function

Abstract

Human apolipoprotein E (APOE), originally known for its role in lipid metabolism, is polymorphic with three major allele forms, namely, APOEε2, APOEε3, and APOEε4, leading to three different human APOE isoforms. The ε4 allele is a genetic risk factor for Alzheimer's disease (AD); therefore, the vast majority of APOE research focuses on its role in AD pathology. However, there is increasing evidence for other functions of APOE through the involvement in other biological processes such as transcriptional regulation, mitochondrial metabolism, immune response, and responsiveness to dietary factors. Therefore, the aim of this review is to provide an overview of the potential novel functions of APOE and their characterization. The detection of APOE in various cell organelles points to previously unrecognized roles in mitochondria and others, although it is actually considered a secretory protein. Furthermore, numerous interactions of APOE with other proteins have been detected, providing indications for new metabolic pathways involving APOE. The present review summarizes the current evidence on APOE beyond its original role in lipid metabolism, to change the perspective and encourage novel approaches to future research on APOE and its isoform-dependent role in the cellular metabolism.

Keywords: Interactome; Mitochondria-associated ER membranes; Protein–protein interactions; Proteolytic fragmentation.

Fig. 1

Predicted spatial protein structure of human APOE (AlphaFold) and intracellular localisation of APOE. a The APOE isoproteins differ at one and two amino acid (AA) positions, respectively (112 and 158). This affects the structure and thus the function of the protein, e.g., APOE2 shows a significantly reduced affinity for the low-density lipoprotein receptor (LDLR) and the lipid binding preference differs between the isoforms. The different colours of the protein structure indicate the model confidence of the structure prediction (dark blue, very high; light blue, confident; yellow, low; orange, very low). Protein structure image downloaded from https://alphafold.ebi.ac.uk/. HDL high-density lipoprotein, VLDL very low-density lipoprotein. b Schematic illustration of an eukaryotic cell and the individual cell organelles and compartments. APOE is known to be a secreted protein, but has been detected inside the cell in, e.g., mitochondria, peroxisomes, and the nucleus. MAMs mitochondria-associated ER membranes

Fig. 2

Illustration of the assembly of mitochondria and mitochondria–ER contacts and the inherent biochemical pathways with suggested APOE involvement or regulation by the APOE isoform. a Mitochondria-associated ER membrane complex (MAM). b Mitochondrial and MAM pathways in which APOE may be involved. (I) The impact of APOE isoforms on mitochondrial function depends on the cell type and species, but consistently decreased neuronal OXPHOS protein and ATP levels were observed in APOE4. (II) Mitochondrial accumulation and stress-induced translocation are increased in APOE4. (III) The different steps in phospholipid synthesis take place through consecutive exchange of substrates from the ER membrane (synthesis of phosphatidylserine; PS) to the mitochondrion (conversion of PS to phosphatidylethanolamine (PE), which is increased in APOE4) and back. The final methylation step by PEMT is accomplished in the ER membrane, yielding phosphatidylcholine (PC). (IV) Calcium is released from the ER through the IP3R1–GRP75–VDAC1 complex and shuttled to the mitochondrion through the mitochondrial calcium uniporter protein (MCU) into the inner matrix. Increased calcium flux was found in APOE4 Neuro2a cells, and higher mitochondrial swelling was observed in APOE4-treated H9c2 cells, which was caused by the interaction of APOE (derived from the lysosomal degradation of L5-LDL) with VDAC1. The protein–protein interaction of APOE with MAM proteins such as VDAC1 and GRP75 is one explanation for the presence of APOE in MAMs. (V) MFN2 dimers connect the OMM with the ER membrane, acting as MAM tethering proteins. Depending on the tissue and species, APOE affects the expression of MFN1 and MFN2. (VI) MAMs are involved in cholesterol metabolism, and proteomic analyses provide evidence for a possible role of APOE in VLDL assembly in MAMs

Fig. 3

Potential APOE protein–protein interactions in human cells and tissue. An in silico analysis performed with BioGRID 4.4 software revealed that there is evidence for 145 potential protein binding partners whose physical interaction with APOE has been demonstrated in at least one study in each case. The larger the blue circle of the corresponding protein is, the stronger the connectivity with APOE, and thicker binding lines represent stronger evidence supporting the association. Image downloaded from https://thebiogrid.org/