The presence, origin, and significance of A beta peptide in the cell bodies of neurons.

Interest in the A beta amyloid in Alzheimer disease (AD) has largely focused on the A beta in the neuropil, an extracellular site. Here much attention has been given to the possibility that A beta acts as a neurotoxin. However, increasing emphasis is now being given to the relationship between neurofibrillary tangles (NFT) and the degree of cognitive decline, as opposed to the relationship between decline and senile plaques, the sites of extracellular A beta deposition. This review focuses attention on the existence and significance of A beta in the cell body of the neuron. The review brings together diverse strands of literature indicating: (1) the tau-positive, paired helical filaments that are the main component of NFT are not themselves the source of the amyloid-like staining (Congo red birefringence) of PHF, and are not, in fact, an "amyloid"; (2) there is A beta within the cytoplasm of neurons affected by AD and in other conditions characterized by tau-positive neurofibrillary tangles; (3) peptides derived from portions of the A beta precursor can bind to PHF; (4) the affected neurons are the source of extracellular A beta in their vicinity and are also unable to maintain the synaptic structures that depend upon the integrity of the neuronal cell body; and (5) debates about whether the intracellular A beta is an amyloid depend upon beliefs about its tertiary structure and assumptions concerning the relationship between the size of self-aggregating A beta molecules, their tertiary structure, and their ability to generate Congo red birefringence without necessarily being detected as ultrastructural filaments 5-10 nm wide. Based upon this literature, it is suggested that the Congo red birefringence generated by NFT is caused by A beta, intimately bound to the NFT. Moreover, whether defined as an amyloid or not, the A beta in the neuronal cell body indicates an abnormal processing of the precursor molecule on the way to its ultimate transmembrane domain. Deranged neuronal functioning, which leads to this abnormal processing and/or the intracellular A beta itself, may be the cause of subsequent functional and morphologic abnormalities in the brain.