Identify brain junk - What are all the known intracellular and extracellular substances which build up with age and clog the brain?
Another example is the paired helical filament of Alzheimer's disease, so-called because neurofibrillary tangles have a ribbon-like structure when viewed under the electron microsocope.

It is now accepted that paired helical filaments consist of a single protein, microtubule-associated protein tau, in a polymerized and hyper-phosphorylated state. (see Lee and co-workers). 

This old answer might also be of interest and contains a fuller reference list. 

Neurofibrillary tangles are one of the two characteristic lesions of brain first described by Alzheimer, and are intracellular. The other characteristic lesion is the extracellular neuritic plaques caused by the build up of amyloid-beta (as the OP points out), which is itself part of a much larger transmembrane protein (of unknown function), the amyloid precursor protein or APP.