Abstract
BACKGROUNDNeuronal hyperexcitability characterizes the early stages of Alzheimer’s disease (AD). In animals, early misfolded tau and amyloid-β (Aβ) protein accumulation — both central to AD neuropathology — promote cortical excitability and neuronal network dysfunction. In healthy humans, misfolded tau and Aβ aggregates are first detected, respectively, in the brainstem and frontomedial and temporobasal cortices, decades prior to the onset of AD cognitive symptoms. Whether cortical excitability is related to early brainstem tau — and its associated neuroinflammation — and cortical Aβ aggregations remains unknown.METHODSWe probed frontal cortex excitability, using transcranial magnetic stimulation combined with electroencephalography, in a sample of 64 healthy late-middle–aged individuals (50–69 years; 45 women and 19 men). We assessed whole-brain [18F]THK5351 PET uptake as a proxy measure of tau/neuroinflammation, and we assessed whole-brain Aβ burden with [18F]Flutemetamol or [18F]Florbetapir radiotracers.RESULTSWe found that higher [18F]THK5351 uptake in a brainstem monoaminergic compartment was associated with increased cortical excitability (r = 0.29, P = 0.02). By contrast, [18F]THK5351 PET signal in the hippocampal formation, although strongly correlated with brainstem signal in whole-brain voxel-based quantification analyses (P value corrected for family-wise error [PFWE-corrected] < 0.001), was not significantly associated with cortical excitability (r = 0.14, P = 0.25). Importantly, no significant association was found between early Aβ cortical deposits and cortical excitability (r = –0.20, P = 0.11).CONCLUSIONThese findings reveal potential brain substrates for increased cortical excitability in preclinical AD and may constitute functional in vivo correlates of early brainstem tau accumulation and neuroinflammation in humans.TRIAL REGISTRATIONEudraCT 2016-001436-35.FUNDINGF.R.S.-FNRS Belgium, Wallonie-Bruxelles International, ULiège, Fondation Simone et Pierre Clerdent, European Regional Development Fund.
Highlights
Alzheimer’s disease (AD) is characterized by a pathogenesis that spreads over decades prior to the onset of cognitive symptoms [1]
We found a significant and positive association between TEP slope and brainstem monoaminergic gray matter (bmGM) mean [18F]THK5351 standardized uptake value ratio (SUVR), both in a simple correlation and after adjusting for transcranial magnetic stimulation (TMS)-EEG stimulation parameters and demographic variables (Pearson’s correlation: r = 0.29, P = 0.02; generalized linear mixed model [GLMM]: F1,57 = 4.76, P = 0.03, R2β* = 0.08; Figure 2A and Table 2)
We first performed voxel-based quantification (VBQ) analyses on standardized [18F]THK5351 SUVR maps to reveal brain regions for which [18F]THK5351 radiotracer uptake correlated with bmGM uptake
Summary
Alzheimer’s disease (AD) is characterized by a pathogenesis that spreads over decades prior to the onset of cognitive symptoms [1]. Aβ senile plaques follow a stereotyped progression pattern, but it is distinct from tau NFT expansion. They are first found in the frontomedial and temporobasal areas, before reaching the rest of the neocortex, allocortical brain regions (e.g., entorhinal cortex and hippocampus), and nuclei from the basal forebrain; they encompass brainstem nuclei and the cerebellum [6, 7]. Early misfolded tau and amyloid-β (Aβ) protein accumulation — both central to AD neuropathology — promote cortical excitability and neuronal network dysfunction. In healthy humans, misfolded tau and Aβ aggregates are first detected, respectively, in the brainstem and frontomedial and temporobasal cortices, decades prior to the onset of AD cognitive symptoms. Whether cortical excitability is related to early brainstem tau — and its associated neuroinflammation — and cortical Aβ aggregations remains unknown
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