The human apolipoprotein (APOE) gene has three variants: APOE2, APOE3, and APOE4. Carrying the APOE4 variant increases the risk of developing sporadic late-onset Alzheimer’s disease compared to the neutral APOE3 variant. In contrast, the APOE2 variant is protective, significantly reducing the likelihood of an Alzheimer’s diagnosis. The exact mechanisms behind these opposing effects remain unclear, although APOE4 carriers exhibit greater brain amyloid buildup.
Complicating this picture further, each person inherits two copies of the APOE gene—one from each parent—resulting in six possible genotype combinations: APOE2/2, APOE2/3, APOE2/4, APOE3/3, APOE3/4, and APOE4/4. A key question is what happens when a person carries both a protective and a risk variant (APOE2/4)? Research suggests that APOE4’s negative impact on AD risk is greater than APOE2’s positive impact. However, a recent brain imaging study showed that APOE2/4 carriers had less amyloid pathology than APOE3/4 or APOE4/4 carriers, suggesting that APOE2 still provides some protection in the presence of APOE4. This finding supports the idea that APOE2 partially counteracts APOE4’s harmful effects. Drs. Zhao and Ren propose that understanding how APOE2 influences Alzheimer’s pathologies in the presence of APOE4 will be crucial for developing therapies that harness APOE2’s protective properties.
Drs. Zhao and Ren are testing the hypothesis that APOE2 slows the accumulation of pathological amyloid in the brain but does not prevent other harmful processes triggered by APOE4, such as tau accumulation. To investigate this, they have two experimental aims to determine whether—and how—APOE2 mitigates APOE4-driven pathology. As part of their experimental approach, they are using CureAlz-funded mouse models that express human APOE variants and crossing them with an amyloid mouse model. They will then compare APOE2/3, APOE2/4, and APOE3/4 models to a “neutral” APOE3/3 model, all on a transgenic amyloid background.
In their first aim, they are further characterizing how APOE2/4 influences amyloid pathology, using a range of methods to assess its effects on lipid metabolism, neuroinflammation, and brain vasculature integrity—key pathways in which APOE plays a role. In their second aim, they are investigating whether APOE2 protects against tau pathology in amyloid mouse models. Since mice do not naturally produce the form of tau that tangles in Alzheimer’s, Drs. Zhao and Ren will induce mouse neurons to produce human tau protein by introducing it using a viral vector. They will then compare how APOE2 affects tau accumulation, synapse and neuron loss, and other outcomes in mice expressing APOE2/3, APOE3/3, and APOE3/4 using the same measures as in Aim 1 and compared to APOE3/3 mice.
They made significant progress in the first year of funding. For Aim 1, Drs. Zhao and Ren completed the comparisons among APOE2/3, APOE3/3, and APOE3/4 with APOE3/3 mice, all on a 5xFAD amyloid background. Their findings suggest that APOE2 prevents APOE4-driven increases in amyloid beta accumulation and neuritic damage. Transcriptomic analysis revealed that APOE4 promotes harmful microglial activation, while APOE2 helps maintain a protective microglial state and supports normal microglial-plaque interactions. They have begun working with an additional amyloid mouse model (APP-SAA) to ensure replicability and expand on these results. They are also advancing Aim 2, having developed and tested the viruses for tau delivery and expression, and have started breeding APP-SAA mice for tau-related experiments to be done in the next funding year. Moving forward, they will validate their findings in the APP-SAA amyloid model, further investigate how APOE2 influences microglial responses, and advance their tau pathology studies by initiating tau virus injections in APP-SAA mice.
