Different Populations of Human Locus Ceruleus Neurons Contain Heavy Metals or Hyperphosphorylated Tau: Implications for Amyloid-β and Tau Pathology in Alzheimer's Disease.

A marked loss of locus ceruleus (LC) neurons is a striking pathological feature of Alzheimer's disease (AD). LC neurons are particularly prone to taking up circulating toxicants such as heavy metals, and hyperphosphorylated tau (tau(HYP)) appears early in these neurons. In an attempt to find out if both heavy metals and tau(HYP) could be damaging LC neurons, we looked in the LC neurons of 21 sporadic AD patients and 43 non-demented controls for the heavy metals mercury, bismuth, and silver using autometallography, and for tau(HYP) using AT8 immunostaining. Heavy metals or tau(HYP) were usually seen in separate LC neurons, and rarely co-existed within the same neuron. The number of heavy metal-containing LC neurons did not correlate with the number containing tau(HYP). Heavy metals therefore appear to occupy a mostly different population of LC neurons to those containing tau(HYP), indicating that the LC in AD is vulnerable to two different assaults. Reduced brain noradrenaline from LC damage is linked to amyloid-β deposition, and tau(HYP) in the LC may seed neurofibrillary tangles in other neurons. A model is described, incorporating the present findings, that proposes that the LC plays a part in both the amyloid-β and tau pathologies of AD.