The Intracellular Domain of Amyloid Precursor Protein is a Potential Therapeutic Target in Alzheimer's Disease

Amyloid-β (Aβ) is widely believed to cause Alzheimer's disease (AD), as it is the major constituent of the amyloid plaques observed in the brains of people with AD (the so-called amyloid hypothesis). Based on this hypothesis, therapies utilizing immune responses against Aβ have been performed and have succeeded in effectively removing amyloid plaques, but have shown no evidence of improvements in survival and/or cognitive function. Thus, it may be necessary to think about this problem from a different viewpoint. γ-Secretase was initially identified as an enzyme that cleaves amyloid precursor protein (APP) and produces Aβ. Although the primary function of γ-secretase has not been fully clarified, this enzyme is well known to play a central regulatory role in Notch signaling. After the shedding of the Notch ectodomain by metalloproteases, γ-secretase releases the intracellular domain (ICD) of Notch, which immediately translocates to the nucleus to modify the expression of certain genes. Recently, many type 1 transmembrane proteins have also been reported as substrates for γ-secretase. Interestingly, several of these substrates may share a γ-secretase-regulated signaling mechanism similar to that of Notch. Indeed, we have demonstrated that the ICD of APP (AICD) induces dynamic changes in gene expression and neuron-specific apoptosis, suggesting that APP also has a signaling mechanism that is closely linked with AD. In this review, we first summarize the evidence that γ-secretase-regulated mechanisms similar to Notch signaling may play wide-ranging roles in signaling events involving type 1 transmembrane proteins, including APP. We also focus on the possibility that APP signaling is involved in the onset and progression of AD. Based on these ideas, we hypothesize that APP signaling, especially AICD, may be an attractive therapeutic target in AD.