Tau pathology in the brainstem monoaminergic neurons reflect resilience to Alzheimer’s disease pathology in the Nun study cases

The presence of amyloid and tau pathologies is the pathological hallmark of Alzheimer's disease (AD). However, the presence of non-demented individuals with sufficient AD pathology indicates that AD-linked pathology does not always lead to dementia. The current view is that a non-demented (ND) individual with sufficient AD pathology represents an individual resilient to AD pathology. To gain insight about resilience to AD pathology, we examined the neuropathology in the brainstem monoaminergic (MAergic) neurons in the Nun Study participants with equally high Braak AD stage (V-VI) with dementia and without clinical dementia. Because MAergic pathology is thought to occur in response to cortical AD pathology, any differences in MAergic pathology between the AD and ND groups with similarly advanced AD pathology could reflect the resilience of MAergic neurons to cortical AD pathology. Examination of Locus Coeruleus (LC) and/or Raphe for the presence of tau pathology showed that, despite the similar forebrain pathology, relative levels of perikaryal and neuritic tau pathology were significantly lower in ND than in AD subjects. The ND subjects exhibit greater pathology than control subjects without AD pathology, indicating that cortical AD pathology does impact subcortical neurons in both AD and ND cases. Significantly, the extent of neurodegenerative pathology in LC and Raphe neurons correlated with cognitive performance in AD cases, while no such correlation was seen in ND cases. Our results show that while cortical AD pathology is associated with increased MAergic neuropathology, quantitative differences in the extent of MAergic pathology in the brainstem may reflect underlying resistance to AD pathology.