Researchers at the Massachusetts Institute of Technology have uncovered an association between APOE4 and erroneous cholesterol processing. This genetic variant is associated with the onset of Alzheimer’s disease but the exact reasons why remain partly unknown. The study, published in Nature, examined post-mortem human brains, cultured brain cells and mice models to investigate the disease-causing mechanisms and potential treatments.
A strong association
The APOE gene encodes a protein that is involved in the transport of lipids, such as cholesterol, around the body. Three APOE alleles exist – E2, E3 (the dominant allele in the human population) and E4.
Over the years, research has uncovered associations between the presence of the E4 allele and the onset of Alzheimer’s disease. Found in nearly half of all those diagnosed with the condition, inheritance of one copy of APOE4increases an individual’s chance of Alzheimer’s disease up to four-fold when compared to those who do not possess the variant. In addition, the presence of two copies increases risk up to twelve-fold.
The reasons behind this correlation are not fully understood. It has long been known that the allele causes alteration to the lipid binding region of the APOE protein, but many questions remain as to how exactly this causes the cognitive decline that characterises the disease.
Li-Huei Tsai and her team at MIT began by analysing 32 human brains, both with and without the APOE4 variant, using single-nucleus RNA sequencing (snRNA-seq). The team characterised different cell types within the prefrontal cortex and determined pathways and processes which were altered in the presence of the APOE4 variant. Among these dysregulated processes were immune responses and neuron signalling.
The work uncovered an association between the APOE4 variant and upregulation of cholesterol-producing genes in cells called oligodendrocytes. These cells are responsible for the myelination of the axonal portion of neurons – “insulating” them and facilitating the travel of electrical impulses in the central nervous system.
To further analyse this relationship and its consequences, the team cultured oligodendrocytes from induced pluripotent stem cells (iPSC), both with and without copies of the APOE4 variant. Gene expression was seen to be similar in these cells to the post-mortem brain tissues, and analysis revealed that cholesterol erroneously accumulated within the endoplasmic reticulum of the oligodendrocytes, disrupting cellular processes and causing stress. The presence of the APOE4 variant and this aberrant cholesterol localisation were also correlated with a reduction in myelination in both the iPSC cells and the post-mortem tissue.
Figure 1: Graphs showing expressing of cholesterol-producing genes. Researchers compared cholesterol production in homozygous APOE3 and APOE4 samples, and in heterozygous APOE3/4 samples.
Cyclodextrin – a new hope?
Armed with this new knowledge, the team set about finding a way to combat the presence of cholesterol in the endoplasmic reticulum of oligodendrocytes. The iPSC derived cells were cultured in the presence of cyclodextrin – a molecule which aids in the reduction of cholesterol accumulation. Following treatment, the APOE4 cells were not significantly dissimilar to those with the E3 variant. This was also seen to correlate with an increase in myelination in the APOE4 cells.
Subsequent in vivo analysis in mice mirrored what was seen in the cyclodextrin treated cells, furthering the idea that improved cholesterol trafficking could have positive impacts on myelination. Moreover, cyclodextrin treated APOE4-positive mice exhibited increased cognitive function and memory than their untreated counterparts.
A look to the future
Alzheimer’s is currently uncurable, and although treatments exist to alleviate symptoms of the condition, it is vital that we further our understanding of the mechanisms behind the disease.
The work by Tsai and her team builds on previous research and adds to what is known about APOE4 and its long-standing association with Alzheimer’s. The use of cyclodextrin and its positive results in mice points towards a potential way forward in the use of cholesterol-trafficking drugs to combat the disease. However, further work is needed to determine the feasibility of this treatment – with co-author Leyla Akay describing cyclodextrin as a “sledgehammer” which “just depletes cholesterol from cells.”