The Pursuit of the "Inside" of the Amyloid Hypothesis-Is C99 a Promising Therapeutic Target for Alzheimer's Disease?

Aducanumab, co-developed by Eisai (Japan) and Biogen (U.S.), has received Food and Drug Administration approval for treating Alzheimer's disease (AD). In addition, its successor antibody, lecanemab, has been approved. These antibodies target the aggregated form of the small peptide, amyloid-beta (A beta), which accumulates in the patient brain. The "amyloid hypothesis " based therapy that places the aggregation and toxicity of A beta at the center of the etiology is about to be realized. However, the effects of immunotherapy are still limited, suggesting the need to reconsider this hypothesis. A beta is produced from a type-I transmembrane protein, A beta precursor protein (APP). One of the APP metabolites, the 99-amino acids C-terminal fragment (C99, also called beta CTF), is a direct precursor of A beta and accumulates in the AD patient's brain to demonstrate toxicity independent of A beta. Conventional drug discovery strategies have focused on A beta toxicity on the "outside " of the neuron, but C99 accumulation might explain the toxicity on the "inside " of the neuron, which was overlooked in the hypothesis. Furthermore, the common region of C99 and A beta is a promising target for multifunctional AD drugs. This review aimed to outline the nature, metabolism, and impact of C99 on AD pathogenesis and discuss whether it could be a therapeutic target complementing the amyloid hypothesis.