Final answer:
High concentrations of APP fragments lead to the formation of toxic oligomers, which are detrimental to neuronal cells and associated with neurodegenerative diseases.
Step-by-step explanation:
At high concentrations, APP fragments are known to form toxic oligomers. These oligomers can disrupt cellular functions and have been implicated in neurodegenerative diseases like Alzheimer's. In contrast to promoting cell division or enhancing neurotransmitter release, the accumulation of APP fragments into oligomers contributes to neuronal damage and is a subject of significant research interest due to its implications for understanding and treating diseases.