In this project, the Faundez lab is exploring a hypothesis that would upend the field’s current thinking about the role of mitochondrial dysfunction in Alzheimer’s disease onset and progression. Mitochondria are the powerhouses of the cell, producing energy needed for essential functions like protein transport and secretion using a system termed the electron transport chain. Mitochondria in AD become dysfunctional, and dysfunctional mitochondria accumulate in cells during the disease. Several gene variants tied to mitochondrial function have been identified for their impact on Alzheimer’s risk. The current dominant hypothesis is that the risk gene APOE4 increases the production of classic AD pathologies—amyloid beta aggregates and tau pathology—leading to mitochondrial dysfunction and cellular stress and loss. However, the Faundez lab generated surprising data suggesting a different order of events: when the electron transport chain was disrupted, cells produced and secreted 49 times more APOE protein and increases in APOE levels in AD mouse model brains occurred after the rise in beta amyloid and APP levels.
Cells of many types and locations in the body produce APOE protein and then secrete it into the surrounding environment, where it binds to receptors on nearby cells of multiple types, triggering a variety of pathways. APOE has three main variants with important risk impacts for sporadic (classic) Alzheimer’s disease for people of Western European and Asian descent: APOE4 is the strongest adverse risk genetic factor; APOE3 is considered neutral risk; and APOE2 is the strongest protective genetic risk factor. APOE plays many roles in the brain and body, but researchers still do not fully understand which functions are influenced by different APOE variants or how strongly they contribute to the variant’s effect on disease risk. Thus, the Faundez discovery ties two well-established aspects of Alzheimer’s together in a new way. Given their findings, the Faundez team hypothesizes that mitochondrial dysfunction triggers an increase in cellular APOE production as a protective mechanism for the cell but that these increasing APOE levels have negative AD-related consequences in turn.
To test their hypotheses, the Faundez lab is assessing different components of the electron transport chain to determine which ones regulate APOE expression. They are doing so in multiple cell types that produce APOE in the brain—astrocytes, microglia, and neurons—to see whether the disruption has different effects on APOE levels and secretion by cell type. Early results suggest that this response may be specific to astrocytes, which, under normal conditions, produce more APOE than neurons or microglia.
In their second aim, the Faundez lab is testing whether the increase in APOE caused by disruption of the electron transport chain is a protective response for astrocytes in cell culture that reduces further impairment of energy production and whether the protection is affected by the astrocyte’s genotype: APOE2, APOE3, APOE4, or no APOE gene at all. This aim is critical to understanding why mitochondrial dysfunction leads to increased APOE expression and secretion and the consequences on astrocyte health and function in the Alzheimer’s context. The results of the Faundez project will indicate whether interventions to support the electron transport chain and, thus, mitochondrial health might effectively reduce the negative role of APOE4 in Alzheimer’s disease.
In their first year of funding, the Faundez lab created multiple induced pluripotent stem cell (iPSC) lines with key electron transport chain proteins knocked out, as well as APOE knockout (KO) lines and lines for each APOE isoform. They have successfully differentiated these iPSCs into astrocytes and are now working to generate microglia and neurons as well in pursuit of Aim 1. As part of the second aim, Dr. Faundez’s team found that deleting APOE can protect cells from electron transport chain damage. To understand the molecular changes behind this protective effect, the lab conducted a detailed protein analysis. They found that APOE deletion changed how cells process energy and respond to stress but did not affect any proteins directly involved in the electron transport chain. The lab also discovered that astrocytes with the APOE3 variant are more resistant to electron transport chain disruptions than those with APOE4. Dr. Faundez intends to investigate these changes in more depth in future studies.
