For cells to function properly, they actively transport proteins within cells to where they are needed. One way this occurs is via a system called the endosome-lysosome network (ELN). This network is like a traffic system that shuttles proteins to their destinations or degrades them if they are no longer needed. Cells use the ELN in different ways. For example, neurons use it to transport receptors for communication, which is necessary for proper cognition, learning, and memory. Microglia use it to digest and degrade pathogenic molecules and to secrete factors that mediate immune responses. Early in Alzheimer’s disease (AD), before the appearance of amyloid plaques or tau tangles, the ELN becomes dysfunctional. The endosome, part of the ELN, is where the amyloid precursor protein (APP) is cut into toxic amyloid beta fragments that are prone to forming plaques. The swelling of these endosomes represents one of the earliest cellular pathologies seen in Alzheimer’s brains.
Genetic studies consistently identify genes involved in the ELN as risk factors for late-onset AD. One of these is SORL1, which shuttles APP through the ELN and helps limit its cleavage. Dr. Young’s team and others have demonstrated that when SORL1 levels are reduced, endosome trafficking patterns become disrupted, and the production of toxic amyloid fragments increases.
While common SORL1 variants contribute to late-onset Alzheimer’s risk, emerging evidence suggests that rare SORL1 variants may directly cause early-onset Alzheimer’s. This has led a growing number of researchers to consider SORL1 as the fourth familial Alzheimer’s gene, joining the established trio of APP, PS1, and PS2.
Dr. Young hypothesizes that SORL1 genetic variants disrupt ELN trafficking, leading to changes in neuronal cell function. She believes that interventions targeting the ELN could prevent or reduce pathologies and may be ready for clinical trials in the near term.
To test this hypothesis through three experimental aims, Dr. Young’s lab is utilizing two cell models. The first model uses human stem cells (induced pluripotent stem cells, iPSCs) from patients with rare mutations: either a complete loss of the SORL1 gene or mutations in it. However, since these rare mutations may not fully explain the more common late-onset Alzheimer’s, the lab is also using a second model: a larger collection of iPSC cells from late-onset patients. This set of cells is stratified by their endosomal polygenic risk score (ePRS), which indicates an individual’s overall burden of risk variants in known ELN genes. Rather than only taking into account the effect of a single gene variant, ePRS combines the effects of multiple genetic variants affecting the ELN pathway to create a comprehensive risk profile.
In the first aim, they are identifying the molecules and cell signaling pathways most impacted by gene variants that cause ELN dysfunction. They are converting iPSC cells from different genetic backgrounds (SORL1 variants and low and high ePRS) into neurons or microglia and measuring the expression patterns of all genes using RNA sequencing. In the second aim, they are assessing synaptic function and cell death in neurons, as well as phagocytosis and immune responses in microglia, to determine if ELN dysfunction impacts their normal functions. In the third aim, they are exploring therapeutic interventions. They are testing two methods to enhance ELN trafficking—a small molecule drug and gene therapy—to determine if either treatment returns cells to a healthy, control state based on gene expression patterns.
In the first funding period, Dr. Young’s team completed experiments across all aims. Her team’s investigation of expression patterns in neurons and microglia identified several genes impacted by the loss of SORL1. They are now completing similar experiments on the low and high ePRS cell lines. SORL1-deficient neurons studied in the second aim showed marked hyperexcitability, which appears to be driven by changes in AMPA receptor trafficking. AMPA receptors respond to glutamate at the synapse and play a key role in regulating synaptic function. In microglia, SORL1 deficiency caused microglia to under- or overreact to several key cytokines, suggesting a more nuanced role for SORL1 in mediating inflammation. Further experiments are underway to investigate cells bearing key SORL1 mutations. Finally, Dr. Young’s team showed initial success in reducing amyloid and tau levels by targeting SORL1 with a small-molecule therapeutic. However, the discoveries from their first two aims led her team to consider a wider set of potential targets.
