A newly identified mechanism may explain how tau protein helps drive Alzheimer’s disease and other tauopathies, according to research from Stanford Medicine. Rather than acting primarily through the formation of neurofibrillary tangles or the destabilization of microtubules, chemically modified tau appears to enter mitochondria and disrupt the organelles’ energy-generating machinery. The resulting metabolic failure triggers a destructive cycle involving abnormal electron flow, oxidative stress, inflammation and neurodegeneration.
Tau has long been associated with Alzheimer’s disease because abnormal forms of the protein can be detected in cerebrospinal fluid and blood before symptoms become apparent. In affected brain tissue, tau accumulates inside neurons in structures known as neurofibrillary tangles. Under normal conditions, tau binds to and helps stabilize microtubules, the intracellular tracks that support the transport of materials through nerve cells. In disease, however, tau can become excessively phosphorylated, meaning that phosphate groups are attached to numerous sites along the protein. This modification alters tau’s behavior, location and ability to interact with other cellular components.
The Stanford-led study, published online in Neuron on Aug. 6, suggests that hyperphosphorylated tau can cause damage even without forming tangles. Researchers found that particular phosphorylation patterns allow tau to move into mitochondria, the organelles responsible for producing most of a cell’s adenosine triphosphate, or ATP. ATP supplies the energy required for neuronal communication, transport and maintenance. Because neurons have exceptionally high energy demands, mitochondrial dysfunction can rapidly compromise their structure and function.
Inside mitochondria, the modified tau molecules interact with NDUFS3, a component of complex I, the first major enzyme assembly in the mitochondrial electron-transport chain. Under normal conditions, electrons pass through a series of protein complexes embedded in the inner mitochondrial membrane. The energy released during this process pumps protons across the membrane, creating an electrochemical gradient that powers ATP synthase. Tau’s binding to NDUFS3 appears to distort the complex and interfere with the normal direction of electron flow.
The result is a process called reverse electron transport. Instead of moving forward through the respiratory chain, electrons flow backward under conditions that favor the reaction, producing unusually large quantities of reactive oxygen species. These chemically reactive molecules can damage proteins, lipids and nucleic acids, while also activating inflammatory signaling pathways. The researchers found evidence of reverse electron transport in fruit flies and mice with tau-related disease, as well as in human brain tissue affected by tauopathy. Healthy neurons showed little or no evidence of the process.
The findings emerged from experiments involving multiple disease models, including animals carrying tau mutations associated with human tauopathies and laboratory-generated human neurons derived from patient cells. The team also studied neurons with a gene duplication linked to an increased risk of early Alzheimer’s disease. Across these systems, mitochondrial stress was closely associated with phosphorylated tau. Removing or reducing tau genetically prevented the abnormal electron flow, while an experimental compound called CPT blocked the interaction between hyperphosphorylated tau and NDUFS3 without stopping normal electron transport.
Animal experiments provided additional evidence that this interaction contributes directly to neurological decline. Fruit flies lacking tau were protected from the severe nervous-system damage and shortened lifespan normally caused by prolonged heat stress. CPT treatment produced similar protection in tau-producing flies and extended their survival. In mice, tau reduction or CPT treatment helped preserve cognition under stressful conditions. In mice with severe tauopathy and cognitive impairment, longer-term CPT administration reduced reverse electron transport in brain mitochondria and improved performance across several behavioral tests.
The treatment also appeared to reduce biological signs of neurodegeneration. CPT-treated animals showed less nerve-cell inflammation and were protected against changes including reduced cortical thickness and loss of total brain volume. In human neurons generated from induced pluripotent stem cells carrying disease-associated tau mutations, CPT prevented several stress-related cellular abnormalities. The convergence of results from animal models, patient-derived neurons and human brain tissue suggests that the mechanism may operate in the human nervous system, although it does not yet establish that CPT is safe or effective as a treatment for patients.
The researchers describe the process as a self-reinforcing loop. Reverse electron transport generates reactive oxygen species, which can promote still more tau phosphorylation. Newly modified tau molecules may then enter mitochondria, bind additional NDUFS3 and further impair respiration. This cycle could help explain how an initially limited mitochondrial disturbance develops into widespread neuronal dysfunction. It also raises the possibility that blocking the tau–NDUFS3 interaction or preventing reverse electron transport could interrupt disease progression without eliminating tau’s normal functions.
The work broadens the range of tau-related mechanisms under investigation in Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, frontotemporal dementia and progressive supranuclear palsy. It may also have relevance to other conditions involving phosphorylated tau and mitochondrial stress, including stroke, traumatic brain injury and some brain tumors. CPT remains an experimental compound, and substantial research will be required before clinical trials can be considered. Bingwei Lu, the study’s senior author, is a co-founder and advisory-board member of Cerapeut Inc., which is developing CPT for neurodegenerative diseases. The study also involved researchers from the University of California, San Francisco, and was supported by grants from the U.S. National Institutes of Health.
Subject of Research: The role of hyperphosphorylated tau in mitochondrial dysfunction and tauopathies.
Article Title: Hyperphosphorylated Tau Disrupts Mitochondrial Energy Production Through Reverse Electron Transport
News Publication Date: Aug. 6
Web References: Stanford Medicine; Stanford School of Medicine; med.stanford.edu
References: Study published online in Neuron; National Institutes of Health grants R21AG083863, R01NS084412, R01AG089752, R37NS083417 and R01NS120219.
Keywords: Alzheimer’s disease, tau, tauopathies, mitochondria, reverse electron transport, NDUFS3, oxidative stress, neurodegeneration, CPT, mitochondrial dysfunction
