Alzheimer’s disease (AD) is defined by the accumulation of amyloid plaques and tau tangles in the brain, but it also profoundly impacts cellular functions. A growing body of research implicates dysfunction in specific cellular organelles, tiny structures that coordinate diverse cell functions, as contributors to AD. These include the mitochondria, the “powerhouses” of the cell that produce all the energy required for the body to function. Healthy mitochondria are especially important for the brain, which, despite making up only 2% of the body’s weight, consumes about 20% of the body’s energy. Unfortunately, damaged mitochondria accumulate in brain cells with age and under different disease conditions, including AD.
Defective mitochondria can impact cell health indirectly, when not enough energy is produced to support normal functions, and directly, when damaged or old mitochondria start to release toxic molecules. For example, mitochondrial DNA (mtDNA) can leak out into the cell when mitochondria are damaged. When DNA is detected in the wrong place, a DNA-sensing pathway called the cGAS-STING pathway is activated, which stimulates an inflammatory response. This activation is generally protective, as it recognizes and eliminates the presence of DNA-containing pathogens; however, like other immune responses, it may be detrimental if left unchecked. Along these lines, previous work supported by CureAlz showed that pharmacological inhibitors of cGAS improve learning and memory in a tau mouse model.
Cells manage damaged mitochondria through mitophagy, the process of breaking down and clearing defective mitochondria from the cell. Mitophagy of sick mitochondria prevents the activation of the cGAS-STING pathway by clearing the mitochondria before they release mtDNA. Unfortunately, mitophagy also becomes impaired during aging and in AD, and defective mitophagy may increase the risk of AD. Maintaining this important mitochondrial quality control mechanism is essential for brain health and may also offer new avenues for therapeutic intervention in AD.
Drs. Nilsson and Fang are interested in investigating how mitophagy and damaged mitochondria contribute to inflammation. Their preliminary data point to a key molecule called TBK1 (TANK-binding kinase 1 protein) that plays a role in both processes: (1) it helps regulate the production of inflammatory molecules like interferons after cGAS-STING activation, and (2) when it comes in contact with damaged mitochondria, it activates proteins required for mitophagy. Changes in TBK1 activity are linked with neurodegenerative disease in humans and in mouse models of AD. The team also found that cGAS-STING and TBK1 are activated in microglia near amyloid plaques in an amyloid mouse model (APP-NL-F-G). They hypothesize that an imbalance in TBK1’s dual roles in mitophagy and inflammation contributes to Alzheimer’s pathologies. They are specifically exploring whether TBK1 shifts from promoting mitophagy in a healthy brain to increasing inflammation in an AD brain.
They proposed three experimental aims. In aim one, they are testing their hypothesis that increasing TBK1 levels boosts mitophagy and reduces amyloid-related pathologies in amyloid mice. They are using modern methods (viral vectors) to increase or decrease TBK1 levels in neurons or microglia and measuring mitochondrial health, inflammatory gene expression, amyloid pathology, and cognitive performance. In the second aim, they are studying TBK1’s role in mitophagy using a novel mouse line (mito-QC) engineered to express a fluorescent marker that makes mitochondria glow red when damaged and undergoing mitophagy and green when healthy. They are breeding these mice with the amyloid mouse model and monitoring changes in mitophagy levels in different brain cell types starting at two months of age. The mice will also be treated with a pharmacological TBK1 inhibitor to test its impact on mitophagy. As part of this aim, they are studying specific TBK1 protein modifications in cultured human microglia cells to identify changes linked to mitophagy versus inflammation. In the third aim, they are investigating how TBK1 levels and activity change as AD progresses by analyzing blood, cerebrospinal fluid (CSF), and autopsy brain tissue from patients in Nordic countries spanning preclinical and clinical stages.
At the end of the first year of funding, the team has made significant progress across all three aims. In Aim one, using amyloid mouse models, they confirmed that damaged mitochondria accumulate with age and activate inflammation through the cGAS-STING pathway. Proteomic and transcriptomic analyses revealed age-related increases in inflammatory and mitochondrial stress pathways, along with disruptions in lipid metabolism. The team also found that TBK1 becomes highly active in microglia surrounding amyloid plaques, and that loss of TBK1 in neurons led to greater amyloid accumulation, underscoring its importance for maintaining brain health. In Aim two, they used mito-QC reporter mice to visualize mitochondrial turnover in living brain tissue and found reduced mitophagy in regions containing amyloid plaques and activated microglia, suggesting that impaired mitochondrial clearance directly contributes to neuroinflammation and plaque buildup. In the coming year, the team will turn to Aim two, testing whether pharmacological inhibition or overexpression of TBK1 can modulate mitophagy and inflammation in amyloid mice. They are also collaborating with proteomics experts to map TBK1’s post-translational modifications in human microglia and how those modifications affect TBK1’s interactions with other proteins. Further, they are analyzing human brain and cerebrospinal fluid samples to determine how TBK1 activity changes across different disease stages. Together, these studies are defining how TBK1 links mitochondrial dysfunction to inflammation in AD and may establish it as a promising therapeutic target.
