What transforms tau from a well-behaved protein into a rogue? While much attention has focused on hyperphosphorylation, Michal Novak and Branislav Kovacech at the Slovak Academy of Sciences in Bratislava propose that truncation is the culprit. They have found truncated tau fragments in neurofibrillary tangles in human AD brain. Echoing the amyloid-β story, they claim that these are stickier than the full-length protein, they seed tau aggregation, exit cells, and may kick off the neurodegenerative cascade seen in AD. The truncated proteins damage synapses in a rat model. If all this proves to be independently replicated, it could change scientists’ view of tau and their approach to tau-based therapies by providing more specific targets. The researchers collaborate with a small R&D biotech company co-founded by Novak called Axon Neuroscience in Vienna, Austria. The company plans to test a vaccine against pathologically truncated tau in humans.
The work is not yet widely known in the tau field, but has caught the interest of several scientists. Erik Portelius at the University of Gothenburg, Sweden, who heard the researchers present these data at the 8th International Winter Conference on Alzheimer’s Disease, held 7-10 December 2012 in Zuers, Austria, found the data intriguing. He noted that with Aβ, researchers narrowed down pathological effects mainly to the Aβ40 and Aβ42 fragments. “Now we have the same case with tau: that you can find different fragments that are more specific for AD,” Portelius told Alzforum. He believes these tau fragments deserve further investigation and could make promising drug targets. Other researchers contacted by Alzforum said the findings are interesting, but they are holding out judgment until they see more data.
Novak first discovered truncated pieces of tau 25 years ago (see Wischik et al., 1988; Novák, 1994). More recently, the researchers isolated soluble and insoluble tau fractions from human brain and compared their composition. In Zuers, Kovacech reported that the insoluble fraction contains tau pieces with their N-terminal ends cleaved off. This changes the shape of the protein and exposes sticky, proline-rich domains, Kovacech said. The repeat domains bind each other, forming paired helical fragments, which are prone to clump. By contrast, soluble tau fragments are clipped from the C-terminal end, right through the middle of the repeat domains, thus preventing their aggregation. This type of truncation may protect the cell from tau tangles, Kovacech suggested. In response to an audience question, Kovacech said they are not yet certain which proteases cleave tau, although they are investigating this.
The researchers next wanted to know if truncated tau could initiate disease. They made a transgenic rat that expressed a human insoluble tau fragment, 3R tau 151-391. It consists of residues 151-391, including three repeat domains and a proline-rich region. The scientists chose the rat as a model because, unlike mice, rats express the same six isoforms of tau that humans do (see Hanes et al., 2009). The transgenic rat recapitulated the neurofibrillary cascade seen in AD (see Zilka et al., 2006; Filipcik et al., 2012). The animals developed tau tangles, their cerebrospinal fluid (CSF) tau rose, and they showed signs of oxidative stress and inflammation in the brain (see Cente et al., 2006; Zilka et al., 2009). They died at 15 months of age compared to rats’ normal lifespan of two to three years. The transgenic rat brain contains the same mix of soluble C-truncated pieces and insoluble N-truncated tau species seen in human AD brain, making them a good model for the disease, Kovacech claimed. Novak noted that, by contrast, most current animal models of tau pathology use mutated tau, which does not occur in human AD disease.
In Zuers, Novak also presented data suggesting that the pathological N-terminally truncated tau may harm synaptic transmission. His group isolated presynapses and postsynapses from the transgenic and normal rats. In the transgenic presynapse, endogenous tau levels were higher than in controls. This correlated with changes in microtubules that made them more stable, and therefore less able to remodel synapses, he noted. The postsynaptic density normally contains no tau, but the transgenics were full of the protein. The mislocalized tau damaged neurofilaments, and therefore synaptic transmission, Novak said. In addition, the number of synaptic vesicles dropped, and levels of several synaptic proteins changed in the transgenic synapses. The rat model revealed another interesting feature: Most of the endogenous, full-length tau in these animals was not hyperphosphorylated, while truncated tau was. “Truncated tau produces an attractive substrate for kinases,” Novak told Alzforum.
Some researchers remain agnostic on the issue of whether tau truncation plays a major role in AD pathology. “At this stage, there are still many open questions regarding the exact molecular basis of tau truncation and how it could possibly be used for therapeutic intervention,” Hansruedi Loetscher at Hoffmann-La Roche, Basel, Switzerland, wrote to Alzforum.
Khalid Iqbal at the New York State Institute for Basic Research in Developmental Disabilities, Staten Island, pointed out that Novak’s work clearly shows truncated tau exists in human neurofibrillary tangles, but not whether the truncation occurred before or after the tangles formed. The amount of truncated tau in tangles is small. Nonetheless, Iqbal noted that the transgenic rat model demonstrates that even a small amount of truncated tau can serve as a nucleus for aggregation of endogenous tau into tangles. Therefore, tau truncation could trigger tangles in a subset of AD cases, Iqbal told Alzforum. All told, however, he favors the idea that in most cases of human disease, hyperphosphorylation occurs first. This modification causes tau to unfold from its normal paperclip shape, exposing its sticky repeat regions and promoting aggregation, Iqbal noted.
Novak believes that truncated tau fragments not only seed tangles, but may also spread pathology through the brain. Pathological tau fragments get out of cells. Novak claims to have found them in blood and CSF in the transgenic rats, and cites preliminary evidence that they exist in human CSF as well. Intriguingly, researchers at iPierian, a biotech company in South San Francisco, California, recently reported finding toxic tau fragments in human CSF, although it remains unclear if these are the same ones seen by Novak (see ARF related news story). If tau fragments do transmit disease from cell to cell, then antibodies against these fragments might help clean up the brain and arrest the progression of the disease. Axon Neuroscience plans to test a vaccine against truncated tau in a Phase 1 AD trial. Company representatives will present their preclinical data at the 11th International Conference on Alzheimer’s and Parkinson’s Diseases, to be held 6-10 March 2013 in Florence, Italy.
Several other groups are currently testing tau immunotherapies in animal models (see Chai et al., 2011; Boutajangout et al., 2011; and Troquier et al., 2012). If successful, a tau therapy might also help people who suffer from other tauopathies such as frontotemporal dementia, a devastating disorder with no treatment. The tau field could use some good news. Tau-based therapy davunetide (see ARF related news story) recently failed in a Phase 3 trial of the tauopathy progressive supranuclear palsy. The GSK-3β inhibitor tideglusib failed in a Phase 2 trial of the same disease. The blue dye derivative Rember® recently started a Phase 3 program in AD and frontotemporal dementia (see ARF related news story). Novak noted that his group has not yet looked at whether truncated tau species occur in other tau-based diseases, but plans to do so in the future.––Madolyn Bowman Rogers.
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