Alzheimer’s disease (AD) starts with the accumulation of amyloid beta into plaques, followed by tau tangle pathology, synapse loss, and neurodegeneration. In addition to these classic brain pathologies, human genetic and experimental studies strongly implicate microglia and other immune cells in the onset or progression of AD. Microglia are typically in surveillance mode in the brain, constantly monitoring for signs of damage or infection. When they detect threats or damage, they shift into different states to respond. For example, they may become inflammatory in response to infections or phagocytic to engulf and clear pathogens, amyloid beta, or damaged synaptic connections. When microglia encounter disease-associated cell debris, they increase the expression of a set of genes required for phagocytosis. This gene expression signature is referred to as DAM (for disease-associated microglia). Several well-known AD risk genes are part of the DAM response, including APOE and TREM2. The variants of these genes associated with AD risk are the E4 allele of APOE (APOE4) and the R47H mutation of TREM2 (TREM2-R47H). Although how these risk variants affect microglial function is becoming clearer, many questions remain. A better understanding of how AD risk variants directly impact microglial functions and interactions with neurons will likely accelerate progress in translating discoveries to therapies.
Studying how changes in microglia impact neurons requires a model system in which microglia and neurons can interact as they do in an actual brain. Animal studies are an appropriate option for some experiments, but there are benefits to using simplified models based on cells in culture, particularly for high-throughput drug screening. Dr. Ginhoux pioneered the development of a more realistic 3D human neural cell culture model, called a neural organoid, that replicates at least some of the complexity of the brain. Recently, his lab added microglia to better replicate the brain’s local environment and showed that, in this novel system, microglia exported cholesterol to neurons and promoted neuronal maturation. Now, they want to put this validated model system to work by exploring how the microglial risk genes TREM2-R47H and APOE4 impact neurons and microglia. They hypothesize that these risk genes affect essential functions involving cellular organelles, such as mitochondria and lysosomes. They propose to identify these functions and test whether modulating them protects cells from AD-relevant pathologies.
The Ginhoux lab proposed three experimental aims. In the first aim, they are growing neural organoids that contain microglia with either TREM2 or APOE risk variants and studying the impact of these genes on various outcome measures, including gene, protein, and lipid levels. These data will be used to identify impacted pathways. In the second aim, they are investigating the effects of TREM2-R47H and APOE4 on microglia organelles and their functions. In the third aim, the lab is testing whether any of the impacted pathways identified in the first aim, supported by results in the second aim or from other studies, have a functional impact on the development of AD-related pathologies in the organoids. As part of this study, they will try to prevent or limit the effects of APOE4 and TREM2-R47H on AD pathology using existing therapeutics when possible or manipulate gene expression in microglia using CRISPR editing.
Dr. Ginhoux’s team made considerable progress across their aims during the first year of funding. They successfully generated brain organoid models with TREM2-R47H and determined the microglial organelles most affected. They also mapped the metabolic changes caused by the mutation, enabling them to parse out the lysosomal, mitochondrial, and lipid metabolism pathways affected. This level of detail allows them to pinpoint the processes most closely associated with AD phenotypes as they move into the second year. In addition, they are working to perform similar experiments in organoids carrying the APOE4 allele in the coming year.
Overall, the development and characterization of organoids with neurons and microglia expressing key risk factors will provide a valuable experimental tool for further investigation and enable more high-throughput and efficient screening of therapeutics for AD.
