Continuity of neuropil threads with tangle-bearing and tangle-free neurons in Alzheimer disease cortex

Neuropil threads (NTs) are abnornal processes that are associated with tangle-bearing neurons in gray matter areas of Alzheimer disease (AD) brains. Although NTs contain paired helical filaments (PHFs) and share multiple tau epitopes with neurobrillary tangles (NFTs), the relationship between NTs and tangle-bearing neurons is unclear. For this reason, we assessed the continuity of NTs with tangle-bearing and tangle-free neurons. Since astrocytes express low levels of tau and rarely have been shown to contain PHFs, we also examined the relationship of NTs to cortical astrocytes. This was done using histochemical and immunochemical methods in conjunction with confocal laser scanning microscopy to examine NTs in amygdala and entorhinal cortex of seven AD brains. Only a small fraction of NTs (<1%) in 3.5×10⁶ μm³ of amygdala and entorhinal cortex could betraced to local neurons with NFTs or to neurons that did not contain NFTs, and no NTs were continuous with cortical astrocytes. These results indicate that only a very small percentage of NTs in entorhinal cortex and amygdala occur in the most proximal segments of processes that emanate from tangle-bearing or tangle-free neurons. This implies that the majority of NTs reside in the distal parts of dendrites and/or the terminal arborizations of axons or that NTs are discontinuous abnormalities. Taken together, these data suggest that NTs could disrupt local and long distance neuronal circuitry and thereby contribute to the cognitive impairments seen in AD patients.