Strong pathological and genetic evidence supports that amyloid beta and the protein it is derived from, amyloid precursor protein (APP), are the central and distinct molecules in Alzheimer’s disease. Amyloid levels rise before tau pathology is apparent and even decades before clinical symptoms emerge. Further, early-onset forms of Alzheimer’s disease are caused by mutations in APP itself or in secretase enzymes that cleave APP into amyloid beta fragments that build up into plaques in the brain. Designing drugs to prevent the generation of amyloid beta from APP (e.g., by inhibiting secretases) or to remove amyloid plaques from the brain has been the dominant research focus. Finally, in the last few years, antibody-based drugs that remove amyloid plaques were approved for treating Alzheimer’s patients. However, some argue that antibody-based AD drugs provide only modest benefits for cognitive function while posing significant risks of serious side effects for patients. Therefore, the field continues to search for new therapeutic targets to develop more effective and safe drugs.
Amyloid beta is produced following sequential cleavage of full-length APP protein by beta and gamma secretases. In contrast, in most physiological conditions, APP is cleaved by alpha secretase at a position in the middle of the amyloid beta sequence. Therefore, the generation of amyloid beta can be precluded by alpha secretase. Of note, alpha secretase cleavage of APP produces a secreted form of APP that is neuroprotective. In the brain, the enzyme that works as alpha secretase is called ADAM10. The location of APP in the cell is also important, whether it is cleaved by alpha secretase or beta/gamma secretases. ADAM10 cleavage of APP happens at the cell surface, including at synapses where two neurons contact each other. APP’s normal function is an adhesion molecule that helps one cell bind to another or its surroundings. Therefore, in physiological conditions, ADAM10 cleavage of APP can regulate the synaptic interaction and adhesion of neuronal processes, such as axons and dendrites, to the surroundings in the brain.
Dr. Suh, working with Dr. Rudolph Tanzi, previously found rare mutations in ADAM10 that increase one’s risk for late-onset Alzheimer’s disease. He followed this lead and showed—in mice—that the risk mutations in ADAM10 reduced normal alpha-secretase activity, which resulted in increased production of amyloid beta and decreased generation of new neurons in the brain. Dr. Suhhypothesized that increasing ADAM10 expression will reduce amyloid beta levels and, at the same time, boost neuroprotective signals that could prevent or slow down disease progression.
Dr. Suh proposed three experimental aims to test this hypothesis. In the first aim, they set out to study how ADAM10 impacts the processing of APP in mice. They proposed the use of adeno-associated viral tools (AAVs) to experimentally increase or decrease ADAM10 levels in the brains of amyloid mouse models (APP/PS1). Three months later, they plan to measure APP and amyloid in the brain with biochemistry and imaging methods. In preliminary studies, they identified a specific region of the ADAM10 gene that may normally suppress the synthesis (translation) of the ADAM10 protein. So, as part of this aim, they also planned to test this prediction using AAVs to introduce ADAM10 in mice—with and without this potential regulatory sequence—and compare ADAM10 levels across groups. In the second aim, they sought to explore the therapeutic potential of antisense oligonucleotides (ASOs) to increase levels of ADAM10 and reduce amyloid pathology. ASOs are a still novel drug modality—the most famous example is Spinraza—that are of high interest for central nervous system disorders because they can be injected directly into the cerebrospinal fluid (bypassing the BBB). In cells in culture, the team proposed to screen ASOs designed to target the potential regulatory sequence of ADAM10 described above. The idea is that an ASO that recognizes this sequence will increase ADAM10 levels by removing the block on protein synthesis. They then intended to test their top ASO candidates in amyloid mice. Outcome measures include memory tasks, ADAM10 levels, APP fragments and their location at the synapse, amyloid burden, and changes in other proteins known to be cleaved by ADAM10 in the brain. Finally, in the third aim, they wanted to begin experiments to help determine if ADAM10 cleavage of APP contributes to Alzheimer’s pathologies independent from amyloid beta. This is intended to help shed light on other—beneficial or detrimental—potential effects of altering ADAM10 levels therapeutically. They proposed to generate two new mouse models with specific mutations in the APP gene that prevent alpha-secretase cleavage. They plan to then characterize the impact of these mutations on several brain functions for which APP and ADAM10 have been implicated, including neuronal adhesion, synapse formation and function, and cognition. Overall, this project aims to deepen our understanding of the role of ADAM10 in Alzheimer’s disease and its potential as a therapeutic target.
In the first year of funding, Dr. Suh’s team made strong progress across all aims. They successfully demonstrated that ADAM10 cleavage of APP does not depend on brain region, indicating that the effects of AAVs targeting ADAM10 should have similar effects throughout the brain. Dr. Suh reports that the AAV injections in the transgenic mice are expected to be finished in the second year of funding. The team also demonstrated successfully that the pharmacological removal of the protein synthesis block increases ADAM10 levels. Screening for ASOs targeting this block is underway. Finally, Dr. Suh’s team successfully generated the two mouse models proposed in the third aim. Unexpectedly, they also generated a third model that Dr. Suh intends to utilize in future characterization experiments planned during the second year.
