Overview

The connection between the APOE gene and Alzheimer’s disease has received a great deal of interest, because having one of the variants of the gene-APOE4-can increase risk of developing the disease by a factor of 10x. Amyloid, which develops as abnormal protein bundles in the Alzheimer’s brain, builds up most readily in the presence of APOE4, but the reasons that this buildup occurs are not completely understood.

Once amyloid plaques form in the brain, a destructive process takes place and the disease begins to develop. Tau, a critical protein in the brain, begins to form abnormal tangles and the brain interprets this as a sign of infection. This triggers the brain’s natural immune system, microglia, to fight the infection, but the microglia over-respond and begin to spray damaging chemicals, such as cytokines and free radicals, into the brain, causing further damage to the tissue.

Evidence for APOE’s impact on other components of Alzheimer’s pathology, including tau, inflammation, and the surrounding cerebral vessels, is starting to emerge. David Holtzman, MD, Co-Chair of the Cure Alzheimer’s Fund Research Leadership Group, reported in the science journal, Nature, that mice expressing APOE4 show far more tau tangle growth and inflammation than mice expressing either of the other two APOE variants, APOE2, or APOE3. APOE clearly has an important role in Alzheimer’s disease.

Aside from the threat of amyloid, tau, and inflammation-the hallmarks of Alzheimer’s pathology-co-existing conditions, such as diabetes and hypertension, can compromise blood vessels further raising risk of developing Alzheimer’s disease. Among the brain’s blood vessel is the blood-brain barrier, a semi-permeable membrane that serves to protect the brain by blocking harmful invaders and pathogens from entering the brain, while allowing chemicals to exit. We already know that APOE4 weakens the blood-brain barrier thereby lowering the brain’s defense against unwanted visitors.

The studies in the consortium include the following investigations:

APOE and Tau

To reduce APOE’s harmful impact, Dr. Holtzman, the lead investigator of the APOE consortium, will separately expose mice to two different compounds designed to reduce the amount of APOE in just the astrocytes-the cells that manufacture APOE. These chemicals will be delivered at various time points in the disease course to determine if there is an optimal time to deliver the treatments, as well as to study if there is a time at which it is “too late” to slow tangle formation. His team will test both female and male mice to determine if APOE related effects are gender specific.

APOE and Inflammation

It turns out that microglia, the brain’s immune cells, can change from healthy to unhealthy with just a flip of an APOE-dependent switch. Dr. Butovsky will determine whether selective removal of APOE from just the microglia will allow conversion back to its healthy form. In collaboration Dr. Paul Greengard and Dr. Holtzman, Dr. Butovsky will systematically remove all APOE variants from the microglia in two types of genetically modified mice-those that overexpress amyloid and those that overexpress tau-to see whether deleterious protein production slows after microglia are reverted back to their healthy form.

Protective Blood Brain Barrier

Dr. Wellington developed a bioengineered blood-brain barrier that allows her to simulate flow through the blood-brain barrier. She is now using these bioengineered vessels to test whether APOE4 is the least able of the APOE variants to clear amyloid from the brain.

There is a subset of monocytes that can travel to the blood-brain barrier to aid amyloid clearance; therefore it would be of benefit to gain a better understanding of the relationship between APOE and these monocytes. Drs. Butovsky, Wellington, and Bu are collaborating to determine whether APOE (and which variant) affects monocyte migration to the blood-brain barrier as well as any associated amyloid clearing abilities.

Peripheral APOE

It is also possible that peripheral APOE (made outside the brain) impacts brain function and could be used as a target for therapies. To examine this possibility, we need to be able to identify the origin of the APOE; did it come from inside the brain or outside?

Dr. Bu and his group have developed a technique to engineer mice that selectively express APOE either in just the periphery or in just the brain. In mice with peripheral-expressing APOE, they will investigate whether peripheral APOE3 (generally know as the good form of APOE) and APOE4 (the bad form of APOE) affect cerebral blood vessels and blood flow differently both before and after amyloid plaques have formed.
Pathway to Therapies

Some brain regions are more susceptible to Alzheimer’s pathology than others, so it is important to understand what biological mechanism(s) might be contributing to this vulnerability. Dr. Greengard’s group will look at gene expression in the neurons of three brain regions integral to memory-two of which are vulnerable to Alzheimer’s disease (the entorhinal cortex and CA1 region of hippocampus) and one of which is not vulnerable to the disease (dentate gyrus region of the hippocampus). The mice will be modified to either express APOE2 (Also a good form of APOE) or APOE4.

Using methods to identify the responsible proteins, and their quantities, Dr. Greengard’s group will be able to detect, for each APOE variant, whether there are any genes that show greater or lesser protein expression in vulnerable vs. non-vulnerable regions. Further, based on existing findings that the APOE4 gene impacts women more than men, Dr. Greengard will study mice of both genders noting any gender-specific differences that can be incorporated into therapeutic design. Once proteins associated with risk are identified, therapies can be developed to alter them so that they behave more like the proteins produced in non-vulnerable regions. Conversely, if proteins rendering regions less vulnerable are identified, Dr. Greengard’s group can try to use them as treatment for vulnerable regions in the hopes that they will become more resilient to the impact of Alzheimer’s pathology.

This process of identifying protective and risk genes within specific APOE variants and brain regions may eventually lead to effective therapies.

Dr. Bateman will apply his innovative method SILK (stable isotope labeling kinetics) to APOE. The method allows for labeling proteins to directly measure how fast they are synthesized and break down. Slight abnormalities in this protein turnover can lead to deleterious protein fragments that promote amyloid buildup and neurodegeneration. His team will evaluate whether any deleterious fragments result from APOE4 break down. Assuming they do, two different chemical treatments will be used to try to break down amyloid or to help it clear. The findings can be used to develop therapeutic targets against APOE.