Nucleus basalis of Meynert degeneration signals earliest stage of Alzheimer's disease progression

Abstract

The Nucleus Basalis of Meynert (NbM) is the main source of cholinergic projection to the entorhinal cortex (EC) and hippocampus (HC), where acetylcholine plays a key role in their function. Both cholinergic cells of NbM and their receptive targets in the EC and HC show sensitivity to neurofibrillary degeneration during the early stages of Alzheimers disease. Precise delineation of the NbM region on T1w scans is challenging due to limited spatial resolution and contrast. Using deformation-based morphometry (DBM), we sought to quantify NbM degeneration along the Alzheimers disease trajectory and confirm recent studies suggesting that NbM degeneration happens before degeneration in the EC or HC. MRI scans of 1447 participants from the Alzheimers Disease Neuroimaging Initiative (ADNI1, 2 and GO) were up-sampled to 0.5 mm voxel-size in each direction (0.125 mm3) before non-linear registration to an ADNI-based unbiased symmetric average MRI template. A subgroup of 677 amyloid-positive participants were chosen for volume time course analysis where the resulting deformation fields were used to compute a high-resolution Jacobian determinant map for each participant as a proxy for local volume change within NbM, EC, and HC masks. Scores of volume change were corrected for age and sex and z-scored using normative data from cognitively healthy amyloid-negative (CN-) participants (n = 219). Two-sample t-tests were then used to compare the baseline differences between NbM, EC, and HC for amyloid-positive cognitively normal controls (CN+), amyloid-positive early mild cognitive impairment (eMCI), and amyloid-positive late mild cognitive impairment (lMCI). Longitudinal linear mixed-effect models were used to compare trajectories of volume change after realigning all participants into a common timeline based on their cognitive decline. Results showed a significant cross-sectional difference at baseline between CN+ and eMCI, and eMCI and lMCI for both left and right Z-scored NbM volumes. There was no difference in z-scored EC and HC volumes between CN+ and eMCI groups but results from eMCI and lMCI differed significantly in these regions. Longitudinal analysis, with a focus on the early disease stages showed the earliest deviation from the CN- trajectory in the NbM and HC in the subject-specific time realigned data. Contrary to the notion that Alzheimers disease originates in EC, we found that NbM volume changes earlier in the disease trajectory than EC or HC. Converging evidence from cross-sectional and longitudinal models suggest that the NbM may be a focal target of early AD progression, which is often obscured by normal age-related decline.