Just as labs around the globe are racing to start marketing plasma Aβ tests, along come new contenders to rattle the field. At this year’s AAIC, held July 14-18 in Los Angeles, scientists reported that certain amino acids on the protein tau are phosphorylated decades before AD symptom onset. Both Randall Bateman, Washington University, St. Louis, and Henrik Zetterberg, University of Gothenburg, told Alzforum that these tau species might reflect a response to Aβ pathology in the brain.
- Phosphorylated tau residues can be measured in CSF.
- Levels rise years before tau PET scans turn positive.
- Tau phosphorylation may reflect response to Aβ pathology.
They, and other investigators, suspect that as amyloid accumulates in the brain, neurons begin to overproduce tau, which then becomes modified at specific sites, such as threonine 181 and serine 217. Those p-tau isoforms can readily be detected in the CSF—and, even more excitingly, in plasma—potentially making them early markers of AD pathology. Ironically, this even implies that a positive blood test for tau could indicate the presence of amyloid plaques in the brain. If these early findings hold up, they raise the future prospect of a single blood draw giving presymptomatic information on brain changes in Aβ and tau. It would be specific for AD as defined by Alois Alzheimer’s original definition of the disease.
Some of the evidence for this comes from Bateman’s own lab. At AAIC, Nicolas Barthélemy from WashU reported a detailed mass spectroscopy analysis of the various phosphorylated forms of tau in the brain and cerebrospinal fluid. Measuring p-tau in the CSF by mass spectrometry presents a challenge, because concentrations there are 1,000-fold lower than in brain lysates. Nonetheless, Barthélemy thinks the CSF may better reflect the state of tau phosphorylation in the brain because phosphatases that retain activity postmortem can change the profile of phospho-tau in tissue samples.
Phosphorylation Hot Spots. Degree of phosphorylation (circle diameters) varies among the normal brain, normal CSF, and AD CSF. Orange and red circles represent slight and high hyperphosphorylation, respectively, compared with normal brain. [Courtesy of Nicholas Barthélemy.]
Barthélemy’s data bore out his idea. He used nano liquid chromatography/high-resolution mass spectroscopy to improve detection of p-tau species in the CSF. Barthélemy immunoprecipitated total tau, digested it with trypsin, then tested for levels of each hypothesized phospho-tau amino acid. He measured the ratio of phosphorylated to unphosphorylated residues as a readout to adjust for fluctuations in total tau levels that might skew the data.
Barthélemy found 12 tau sites that are phosphorylated in normal human CSF. Some, for example p-T205 and p-S208, were unique to the spinal fluid, i.e. absent in brain lysates. Others, including threonines 111, 217, and 231, did show up in the brain, but were more highly phosphorylated in the CSF. Still other phospho-tau species, namely those at the N- and C-termini, only turned up in the brain. Barthélemy speculated that tau’s endpieces may never make it into the CSF because of how the protein is processed and cleared by proteolytic machinery.
What about AD? Here too, Barthélemy found specific changes. In CSF pooled from seven people who were amyloid-positive and had a clinical dementia rating of 0.5, tau was hyperphosphorylated at threonines 111, 181, 205, and 217, and at serine 208, relative to pooled CSF from five amyloid-negative, cognitively normal controls. Sites equally phosphorylated in AD and control CSF included S199 and T231. Serine 202 was slightly less phosphorylated in AD than in controls.
It was the timing of phosphorylation that drew the most attention in Los Angeles. Barthélemy assayed CSF samples from 639 people in the longitudinal DIAN cohort (for a DIAN update from AAIC, see Aug 2019 conference news). In this cross-sectional analysis, changes in phospho-tau emerged decades before symptoms. The earliest increase, at 21 years prior to onset, was for p-T217, followed at 19 years before onset for p-T181. Tantalizingly, Barthélemy reported that CSF p-T217 tau levels correlated with uptake of PiB in the precuneus, and that it predicted amyloid positivity with an accuracy of 97 percent. If this data holds up, it would place increases in p-tau at around the time of amyloid accumulation in the trajectory of AD pathology.
And hold up it did. Three days later at AAIC, Niklas Mattsson from Lund University, Sweden, painted a similar picture of CSF tau markers in the Swedish BioFinder cohort. Mattsson acknowledged earlier work from Barthélemy and Chihiro Sato, also in Bateman’s lab, who used stable isotope labeling kinetics to determine that tau production rates rise in early AD (Mar 2018 news). Based on this, Mattsson, Oskar Hansson also at Lund, and other colleagues in the BioFINDER group, hypothesized that changes in the metabolism and phosphorylation of soluble tau might mediate the relationship between Aβ fibrils and the subsequent development of tau tangles. Mattsson set out to test this.
First, Mattsson correlated p-tau levels with dementia severity. He found higher levels of CSF p181-tau, p217-tau, and total tau among 40 amyloid PET-positive, cognitively unimpaired people than among 18 amyloid PET-negative, cognitively normal volunteers. Phospho-tau levels crept higher still in 38 amyloid PET-positive people who also showed signs of mild cognitive impairment (MCI), and were highest in 35 people who had a diagnosis of AD dementia.
Levels of either phospho-tau species distinguished all four groups, suggesting the markers might prove useful for staging. That the p-tau markers were able to distinguish amyloid-positive from amyloid-negative individuals who all appeared cognitively normal suggests an advantage over tau PET, which becomes positive only once people become impaired. All told, 55 and 70 percent of the cognitively normal, amyloid-positive volunteers tested positive for p181- and p217-tau, respectively. These percentages rose to 90 and 100 among those who were cognitively impaired.
Having established that these tau species rise early in amyloid pathogenesis, Mattsson next asked how they change over time. He tested CSF samples taken two years apart from 32 amyloid-positive, cognitively unimpaired volunteers whose tau PET scans of the inferior temporal cortex would not turn positive for at least another 3.8 years. Even at this early stage, their CSF p181-tau rose by 8.7 pg/mL per year, and the p217 form rose even faster, at 13.8 pg/ml per year.
How does the timing of this CSF p-tau surge relate to early amyloid pathology? To address this, Mattsson correlated CSF p-tau levels with 18F-flutemetamol binding. Both p181- and p217-tau were already higher when a composite standard uptake value ratio of flutemetamol was still below 0.7. In this composite of brain regions where plaque deposition starts, an SUVR of 0.743 denoted amyloid positivity. In other words, these two p-tau CSF markers started to change before the amyloid PET scans were positive. In contrast, flortaucipir binding in the inferior temporal cortex only registered as positive on PET once the flutemetamol SUVR had reached 0.78. In toto, CSF p181- and p217-tau preceded amyloid PET, which preceded neurofibrillary tangle PET as sequential measures of AD pathology.
Other scientists at AAIC were impressed by Mattsson’s finding. Clifford Jack, Mayo Clinic, Rochester, Minnesota, said the CSF/PET relationship for p-tau was uncannily like that for Aβ, where dips in the CSF Aβ42 level precede positive amyloid PET scans. “It seems to be telling us that CSF may be a sensitive measure of what is going on in the brain,” he said.
Val Lowe, also from Mayo, asked why Mattsson focused on tau in the inferior temporal cortex. Lowe wondered if flortaucipir binding in a more sensitive region, such as the medial temporal, might tell a different story. Mattsson said he focused on the inferior temporal because ligand binding is most robust there, but noted that his group did the same analysis for the entorhinal cortex. “All the relationships with p-tau and tau PET remain very similar,” he said.
To learn how these new findings relate to p-tau species in the blood, see Part 8 of this series.—Tom Fagan
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