Essential tremor with tau pathology features seeds indistinguishable in conformation from Alzheimer's disease and primary age-related tauopathy

Abstract

Neurodegenerative tauopathies are characterized by the deposition of distinct fibrillar tau assemblies whose rigid core structures correlate with defined neuropathological phenotypes. Essential tremor (ET) is a progressive neurological disease that, in some cases, is associated with cognitive impairment and tau accumulation. Consequently, we explored the tau assembly conformation in ET patients with tau pathology using cytometry-based tau biosensor assays. These assays quantify tau prion seeding activity present in brain homogenates based on conversion of intracellular tau-fluorescent protein fusions from a soluble to an aggregated state. Prions exhibit seeding barriers, whereby a specific assembly structure cannot serve as a template for a native monomer if the amino acids are not compatible. We recently exploited the tau prion species barrier to define tauopathies by systematically substituting alanine (Ala) in the tau monomer and measuring its incorporation into seeded aggregates within biosensor cells. The Ala scan precisely classified the conformation of tau seeds from diverse tauopathies. We next studied 18 ET patient brains with tau pathology. Only one case had concurrent high amyloid-β plaque pathology consistent with Alzheimer's disease (AD). We detected robust tau seeding activity in 9 (50%) of the patients. This predominantly localized to the temporal pole and temporal cortex. We examined 8 ET cases with the Ala scan and determined that the amino acid requirements for tau monomer incorporation into aggregates seeded from these ET brain homogenates were identical to those of AD and primary age-related tauopathy (PART), and completely distinct from other tauopathies such as corticobasal degeneration, chronic traumatic encephalopathy, and progressive supranuclear palsy. Based on these studies, tau assembly cores in a pathologically confined subset of ET cases with high tau pathology are identical to AD and PART. This could facilitate more precise diagnosis and therapy for ET patients with cognitive impairment.

Figure 1:. ET cases exhibit seeding in tau biosensor cells.

Brain homogenates from ET patients were transduced into 3R/4R (A) and 4R/4R (B) biosensor cells. ET cases with tau seeding are reported. Strong seeding was recorded primarily in the temporal pole and the temporal cortex, except in case 16 (parietal cortex). Cases 1, 7, 12–14 had similar seeding in 3R/4R and 4R/4R biosensors. Ctx: Cortex.

Figure 2:. Ala scan diagram.

Biosensor cells (i.e. 3R/4R or 4R/4R) were transduced with brain homogenate. Cells were then transduced with lentivirus containing a library of tau RD Ala point mutants (e.g. Mutant X, Mutant Y). The differential incorporation of monomer containing these point mutants into the aggregates is measured by FRET.

Figure 3:. Ala scan for AD, PART, CTE, PSP and CBD cases.

AD, PART, CTE, CBD and PSP case brain homogenates were incubated with 3R/4R (AD, PART) or 4R/4R (CTE, PSP, CBD) biosensors based on preferential seeding. After 48 hours (to allow inclusion formation) cells were plated in triplicate and incubated with the WT-Tau (246–408)-Ala mutant lentivirus library. (A) Line plots of the tau Ala scans for AD, PART, CTE, CBD and PSP. (B) AD and PART scans were highly correlated. AD scans did not correlate with (C) CTE; (D) CBD; (E) PSP.

Figure 4.. 3R/4R Ala scan of ET cases.

Brain homogenates from the highest seeding regions (cases 1, 7, 11–14, 16) were transduced into 3R/4R biosensors. After 48 hours, the cells were incubated with the WT-Tau (246–408)-Ala mutant lentivirus library. (A) Line plots of the 3R/4R Ala scan replicates indicate effects of substitution for each residue (dot) in the setting of AD vs. ET cases. (B) Each ET scan correlated highly with the AD 3R/4R Ala scan.

Figure 5.. Clustering of ET, AD, PART, CTE, CBD, PSP cases based on the Ala scan.

Cluster analysis of Ala scans for all ET cases (3R/4R) reveals grouping of these cases with admixed AD and PART (3R/4R). CTE, CBD, and PSP cases (4R/4R) are poorly correlated with the ET cases.

Figure 6.. Heatmap incorporation profile of the 3R/4R Ala scan for ET, AD, PART, CTE, CBD, PSP.

Heatmap of the 3R/4R Ala scan indicates similar tau Ala hits in ET, AD and PART cases at residue positions I308, Y310, V318, S320, I328, I354, G366, N368.

Figure 7:. 4R/4R Ala scan of ET cases.

Brain homogenates from the highest seeding regions for cases 1, 7, 12, 13, 14 and an AD case were transduced into 4R/4R biosensors. After 48 hours, the cells were incubated with the WT-Tau (246–408)-Ala mutant lentivirus library. (A) Line plots of the 4R/4R Ala scan indicate the effects of substitution for each residue in AD and in the ET cases. (B) Scatter plot comparison between each ET case 4R/4R Ala Scan and the AD and PSP 4R/4R Ala Scans. The plots showed a good correlation with AD but not with PSP.