The pathological hallmarks of Alzheimer’s disease emerge in and impair specific brain regions and cell types—a concept termed selective vulnerability. Neurons throughout the brain produce the tau protein, but in Alzheimer’s disease, pathological tangles of tau within neurons are first observed in the entorhinal cortex region and then propagate to other brain regions in a characteristic pattern. Why certain neurons in these brain regions are vulnerable to the development of tau pathology while neighboring neurons and neurons in other regions are not remains unknown. Drs. Duff and Bourdenx believe that understanding what makes these cells vulnerable will help identify novel drug targets that can counteract this process and provide protection against Alzheimer’s disease pathologies.
Cells employ several mechanisms to clear misfolded and toxic proteins and maintain a healthy balance among protein production, use, and recycling (proteostasis). Evidence shows that these proteostasis mechanisms become less effective in neurons with age. Since aging is the most significant risk factor for Alzheimer’s disease, Drs. Duff and Bourdenx hypothesize that aging drives proteostasis impairments faster in certain specific neuron types in the entorhinal cortex, rendering these cells unable to properly clear tau pathology when it develops. They tested the first part of this hypothesis during a one-year project in 2023 using single-cell sequencing methods to map out the molecular signatures of aging in distinct cell types within several brain regions of normal mice. Their results suggested that different cell types lose protein-handling functions at different rates. In this follow-on proposal, they are exploring the second part of their hypothesis—that differences in how cells handle and degrade proteins lead to a cellular environment that allows more tau pathology to accumulate in vulnerable cells.
The Duff/Bourdenx team proposed three experimental aims. Their preliminary data suggested that neurons located in the outermost layers of the cortex are most vulnerable. However, recent large-scale cell-type atlas efforts show that cells in these layers can be more precisely classified into subtypes based on gene expression patterns. To build on this, the lab is using state-of-the-art methods to identify cell subtypes and determine which ones are losing proteostasis functions. Using new tau mouse models pioneered by Dr. Duff, they are then determining whether any of these newly identified cell subtypes are particularly prone to accumulating tau pathology. Their hypothesis predicts that subtypes that lose proteostasis function faster will also accumulate tau tangles faster and in greater amounts than other neuron subtypes. In the second aim, they are performing computational analyses on existing gene expression datasets to identify which molecular signaling pathways are being altered in the different vulnerable cell subtypes identified in the first aim. The top candidate genes and pathways are being further studied to determine if modulating their activity impacts tau pathology. In the third aim, using samples from control individuals and Alzheimer’s patients with different stages of tau pathology, they are confirming whether their mouse results translate to humans by exploring whether the vulnerable cell types and signaling pathways identified above are conserved in the human brain.
At the close of the first year of funding, the team is making steady progress across all three aims. For Aim 1, they have bred mice to the appropriate ages and are refining their analytical pipeline. Their preliminary findings reveal a striking similarity between the neuronal populations showing markers of proteostasis defects during chronological aging and those affected by tau pathology in novel knock-in models of Alzheimer’s disease. Additionally, their data highlight the entorhinal cortex as a key region for phosphorylated tau accumulation, with unexpected evidence that a subpopulation of oligodendrocyte cells also exhibits strong tau phosphorylation. In Aim 2, they are combining data from different advanced imaging techniques to study how aging and changes in the MAPT gene (from which the tau protein derives) affect specific cells. They are also running early analyses to see how these changes impact the cells identified as vulnerable in Aim 1. As they move into the next year of funding, they will continue work on Aim 2 and address Aim 3. They have acquired their first dataset of human AD brains and are adapting their mouse analysis pipeline for human samples. They will continue refining their investigation of molecular signatures using single-nucleus datasets, working to uncover conserved mechanisms linking proteostasis disruption and tau pathology across species.
