Studies by researchers at Stanford University School of Medicine and at the University of California, San Francisco, have found that the protein tau, which is implicated in neurodegenerative disorders including Alzheimer’s disease, may be linked to these such disorders in a way that differs greatly from the pathological pathway usually ascribed to it.

Hyperphosphorylation and aggregation of tau are hallmarks of primary and secondary tauopathies including frontotemporal dementia (FTD) and AD, and potentially also Huntington’s disease and Parkinson’s disease. Such disorders also share another common pathology, which is deteriorating performance of the cell’s mitochondria. The powerhouses may number in the dozens or in the tens of thousands within a single cell, depending on the cell type’s energy needs. Nerve cells have especially high mitochondria demand.

To date the connection between tau and mitochondrial pathologies has been unclear. Working in cells and in preclinical animal models, the Stanford and UCSF scientists have now shown that phosphorylated tau can enter mitochondria and interfere with the electron transport chain, initiating a vicious cycle of pathological events and triggering reverse electron transport (RET) and the detriments that ensue.

“This is the first demonstration of exactly what tau does inside mitochondria,” said Bingwei Lu, PhD, Stanford professor of pathology, “Our discovery of a whole new mechanism driving tauopathies renders these disorders amenable to new therapeutic interventions.” Lu is senior author of the researchers’ published paper in Neuron, titled “Tau-induced mitochondrial reverse electron transport drives neurodegeneration,” in which they concluded “Our results suggest that RET may serve as a common pathogenic mechanism linking tau abnormalities to mitochondrial dysfunction across diseases.”

Tau is a soluble protein enriched in neuronal axons but is also found in neuronal dendrites, cell bodies, and non-neuronal cells, the authors explained in their report. Tau is also increasingly viewed as one of the strongest instigators of Alzheimer’s disease.  The appearance of telltale forms of the protein in cerebrospinal fluid or in the bloodstream strongly predicts impending Alzheimer’s symptoms. Neuroimaging studies and postmortem inspections indicate the presence of neurofibrillary tangles—long filaments largely composed of tau—inside Alzheimer’s patients’ nerve cells.

Neurofibrillary tangles and other aspects of tau’s misbehavior—notably, a tendency to rack up chemical modifications that shift that protein’s disposition—have been reported in Parkinson’s disease and Huntington’s diseases and in other tauopathies such as frontotemporal dementia and progressive supranuclear palsy. “Tau is phosphorylated at many sites under normal conditions but becomes hyperphosphorylated in disease,” the team also commented.

Tauopathies share another common pathology, which is deteriorating mitochondrial performance. “Mitochondrial dysfunction is also a common feature of tauopathies,” the investigators stated. “The mechanistic link between tau abnormalities and mitochondrial dysfunction and its relationship to the physiological function of tau, however, is unclear.”

In its healthier manifestation, tau is believed to play a role in stabilizing microtubules, skeletal structures in nerve cells that are critical to these cells’ proper operation. Tau molecules do indeed spend some of their time sitting on microtubules, straddling those structures’ identical subunits. So, not unreasonably, the consensus is that tau’s perch on microtubules helps keep them from falling apart.

The newly discovered pathological pathway is entirely independent of both neurofibrillary tangle formation and microtubule instability. Instead, it involves a switch in the directionality of mitochondria’s energy-production line, with a resulting disruption of mitochondria’s primary function, that being the conversion of calories from glucose or fat to energy by the electron-transport chain. This multiple-component complex passes electrons from one to the next of its components, the last of which converts a precursor molecule into ATP, the cell’s universal energy currency.

The new study shows that when the hyperphosphorylated tau molecule interacts with a key mitochondrial component it jams up the electron transport conveyor belt, causing electrons to flow backward. Aptly named reverse electron transport (RET), this snarl produces large amounts of reactive oxygen species (ROS), with accompanying inflammation and damage to proteins.

Reverse electron transport is an area of intense recent interest in biology. Although it was first discovered in the 1960s, there’s still no clear evidence that it serves any constructive physiological role. “In healthy cells, very little reverse electron transport is happening,” Lu said.

The new study shows that reverse electron transport is activated under stress. It may initially serve some beneficial function—for example, providing short-term adaption to that stress—but nothing like that has been proven. “Here, we demonstrate that tau regulates mitochondrial reverse electron transport (RET), which produces excess reactive oxygen species (ROS), reduces the NAD+/NADH ratio, and is activated by aging or stress,” the investigators noted.

The team carried out an extensive series of experiments in fruit flies, mice, human brain tissue and cultured human nerve cells that in some cases contained mutated genes for tau identical to those found in tauopathy patients. They also employed lab-generated nerve cells carrying a well-studied gene duplication that promotes accelerated acquisition of Alzheimer’s disease.

Through their studies they demonstrated that reverse electron transport was occurring in animal models of tauopathy as well as in tauopathy-afflicted human brain tissues. Healthy nerve cells, largely spared of hyperphosphorylated tau’s malevolent presence, showed no sign of reverse electron transport or its downside effects.

Next, the investigators showed how reverse electron transport is activated, finding that tau molecules enter mitochondria, but only when they’re phosphorylated. There, they can bind to a component of the electron-transport chain called NDUSF3, warping that protein’s shape. When this happens, electrons drop off the conveyor belt and start flowing backward. “Mechanistically, tau enters mitochondria and directly interacts with the complex I subunit NDUFS3 to promote RET in a phosphorylation-dependent manner,” they continued. “Elevated RET further drives tau hyperphosphorylation, establishing a self-perpetuating pathological loop.”

Studies showed that genetically or pharmacologically depleting tau halted this defection. An experimental drug called CPT prevented hyperphosphorylated tau from binding to NDUSF3, blocking reverse electron transport without impairing normal electron flow. Experimental animals that were genetically altered to produce no or little tau, suffered none of the cognitive or other behavioral deficits or brain pathophysiology exhibited by tau-producing, but otherwise genetically identical animals under stress conditions. “In flies, mice, and human induced pluripotent stem cell (hiPSC)-derived neurons, tau depletion eliminates stress-induced RET and confers resilience,” the authors stated.

Deleting the gene for tau, for instance, protected fruit flies from the severe, life-shortening nervous-system damage that normally results from prolonged exposure to elevated temperatures. CPT treatment of the tau-producing normal flies not only protected them against heat stress but extended their lifespan.

The study showed equivalent findings in mic engineered to not produce tau. Cognition in these animals was protected by CPT treatment from the detrimental effect of heat stress. CPT also protected tau-producing normal mice subjected to heat stress.

Tau hyperphosphorylation proved critical for promoting reverse electron transport. Only tau molecules that had undergone particular phosphorylation events could get inside mitochondria, bind to NDUFS3, and induce reverse electron transport.

In tauopathy mice with severe cognitive deficiencies, an extended CPT regimen inhibited reverse electron transport in the brain mitochondria. This significantly improved the animals’ performance on a wide range of behavioral tests and prevented nerve-cell inflammation as well as several characteristic markers of neurodegeneration, such as diminished cortical thickness and total brain volumes.

“Crucially, therapeutic inhibition of RET mitigates tau-induced neurotoxicity in multiple models, without observable detrimental effects on normal animals,” the authors noted. “The RET inhibitor CPT effectively disrupts the pathological loop between RET and tau phosphorylation, ameliorating neurotoxicity across species. In fly and mouse models of tauopathy, CPT treatment rescues behavioral deficits, reduces neuroinflammation, and mitigates neurodegeneration.”

Reverse electron transport is a textbook example of a vicious circle, Lu said. The massive release of highly reactive chemicals dramatically boosts the odds that individual tau molecules will get hyperphosphorylated, leading to additional activation of reverse electron transport. “Once this gets started, it can become self-perpetuating,” he commented. Reverse-electron-transport inhibition holds promise as a therapeutical strategy for tauopathies and, potentially, other maladies characterized by aberrant tau phosphorylation and mitochondrial dysfunction, such as brain tumors, stroke and traumatic brain injuries, Lu suggested.

“RET regulation thus represents a previously unrecognized normal function of tau that becomes pathological in disease, providing a therapeutic target for various conditions characterized by tau abnormalities and mitochondrial dysfunction,” the authors stated in their paper. “These findings suggest that RET inhibition holds promise as a therapeutic strategy for not only tauopathies but potentially other brain diseases characterized by aberrant tau phosphorylation and mitochondrial dysfunction.”

Lu added, “The main results we observed in our animal models were also seen in patient brain tissues and in the laboratory generated nerve-cell models we derived from tauopathy-patients’ cells. This suggests that what we learned from this study is applicable to the human nervous system. In fly and mouse models of tauopathy, CPT treatment rescues behavioral deficits, reduces neuroinflammation and mitigates neurodegeneration. In hiPSC-derived neurons carrying pathogenic tau mutations, CPT protects against stress-induced cellular abnormalities.”

These are early days for this compound’s clinical development, he stated. “Much more work remains to be done before it can undergo clinical trials.” Lu is co-founder and sits on the advisory board of Cerapeut, a company that is developing CPT as a therapeutic drug for the treatment of neurodegenerative diseases.

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