**7. The model of APP signaling**

We performed Gene Ontology (GO) analysis and classified these upregulated and downre‐ gulated genes according to the GO terms [30]; however, we did not find genes that were sig‐ nificantly related to cell death among those with altered expression. Furthermore, we evaluated AICD-induced changes in the expression of genes thought to be involved in cell death in AD [30]; however, we found no significant changes in expression of these genes. Thus, it is likely that AICD does not directly induce the expression of genes involved in cell death, but the extreme dynamic changes in gene expression disrupt the homeostasis of cer‐ tain neurons and thus give rise to neuron-specific cell death. Taken together, these results

**6. Can Aβ clarify all aspects of the onset and progression of AD?**

Autosomal dominant mutations in and around the Aβ region of the *APP* gene, which accel‐ erate proteolytic processing, are responsible for hereditary early-onset AD [3]. The human *APP* gene is located on the long arm of chromosome 21 [53], an extra copy of which is present in individuals with Down's syndrome (trisomy 21). Patients with Down's syndrome develop AD by 40 years of age, most likely due to this gene dosage effect [4]. In addition, both PS1 and PS2, which are catalytic components of the γ-secretase complex, were identi‐ fied by genetic linkage analyses as the genes responsible for FAD [5-7]. In many cases, fami‐ lial diseases can provide an understanding of the sporadic ones. Therefore, both APP itself and its proteolytic processing may be responsible for the onset and progression of not only

As mentioned above, Aβ is the main constituent of amyloid plaque, which is thought to play a major role in the pathogenesis of AD; its presence is a hallmark of the AD brain. Thus, the amyloid hypothesis is generally accepted as the mechanism of the onset and progression of AD. Although an alternative hypothesis has also been proposed, which suggests that solu‐ ble Aβ oligomers rather than insoluble amyloid plaques are responsible for the onset and progression of AD because the soluble form of the Aβ oligomer is toxic for neurons [100,

However, several doubts have recently been raised regarding the amyloid hypothesis that Aβ plays a central role in the onset and progression of AD. One of the most critical argu‐ ments against this hypothesis is the presence of high levels of Aβ deposition in many nondemented elderly people [102], suggesting that Aβ amyloid plaques are not toxic. Indeed, transgenic mice overproducing Aβ show amyloid deposition mimicking that seen in the AD brain but do not show neurodegeneration [61]. Furthermore, several anti-Aβ drugs and vac‐ cines have failed to show efficacy in phase III clinical trials [103]. Surprisingly, long-term fol‐ low-up studies showed unexpected problems [104]. Immunization of AD patients with the anti-Aβ vaccine, AN-1792, cleared Aβ amyloid plaques. Actually, patients with high titers of antibody against Aβ showed virtually complete plaque removal. However, there was no evidence of improvement in survival and/or cognitive function, even in patients with high titers of anti-Aβ antibody [104]. Although several interpretation for this lack of improve‐

strongly suggest the existence of APP signaling.

FAD but also sporadic AD.

76 Understanding Alzheimer's Disease

101], Aβ still plays a central role in this idea.

Through this chapter, we discussed the possibility of the existence of APP signaling. It is likely that disorders of this signaling mechanism are involved in the onset and progression of AD. As AICD is generated at the same time as Aβ, acceleration of proteolytic processing leads to overproduction of not only Aβ but also AICD in AD brain as discussed above. Fur‐ thermore, we showed that AICD alters the expression of certain genes and induces neuronspecific apoptosis [29, 30].

If the APP signaling hypothesis is correct, certain molecules involved in APP signaling may be attractive candidates for the targets of drug discovery for treating AD. Fig.6 is a schemat‐ ic model of APP signaling. As mentioned above, after cleavage within the JM domain by αor β-secretase, AICD is released from the membrane by γ-secretase. Inhibiters for these proteases are being studied extensively.

As mentioned in section 4.3, non-phosphorylated AICD can bind to the nuclear adaptor pro‐ tein Fe65 [92, 93], which is essential for translocation of AICD to the nucleus. However, phosphorylated AICD cannot bind to Fe65. These results suggest the possibility that a cer‐ tain stimulus controls APP signaling through phosphorylation and dephosphorylation of AICD. It has also been shown that the majority of cell membrane-associated APP is phos‐ phorylated specifically at Thr668 in neurons [107]. Therefore, phosphorylated AICD, which is released from the cell membrane to the cytoplasm by γ-secretase, cannot bind to Fe65 and thus cannot translocate to the nucleus. Phosphorylated AICD left in the cytosol is rapidly degraded, probably by the proteasome and/or IDE [88]. However, if AICD is dephosphory‐ lated by certain phosphatase, AICD can binds to Fe65. Thus, AICD/Fe65 complexes may im‐ mediately translocate to the nucleus, where they mediate expressions of certain target genes in association with histone acetyltransferase Tip60 [27]. Besides dephosphorylation of AICD, if phosphorylation of membrane-associated APP is inhibited, non-phosphorylated AICD may also increase. Therefore, it is likely that non-phosphorylated AICD is involved in the onset and progression of AD.

involved in the pathogenesis in the AD brains. However, the physiological functions of this

γ-Secretase–Regulated Signaling and Alzheimer's Disease

http://dx.doi.org/10.5772/54230

79

The signaling hypothesis for γ-secretase suggesting that its primary function is to regulate the signaling of type 1 membrane proteins was proposed by analogy of Notch signaling. In the canonical Notch signaling pathway, ligands bind to the extracellular domain of Notch expressed on neighboring cells, and trigger sequential proteolytic cleavage. Finally, NICD is released from the cell membrane by γ-secretase and translocates into the nucleus where it modulates gene expression through binding to transcription factors. Thus, γ-secretase plays

While APP is thought to play central roles in the onset and progression of AD, the physio‐ logical functions of this protein also have not yet been fully elucidated. However, it has been shown that AICD, which is released from the cell membrane by γ-secretase, also translocates to the nucleus and may function as a transcriptional regulator. These observations suggest

In this chapter, we focused on the signaling aspects of APP and its pathological roles in AD. Indeed, we showed that AICD alters gene expression and induces neuron-specific apoptosis. Thus, it is likely that APP has a signaling mechanism similar to that of Notch and that APP signaling is at least partially responsible for the onset and progression of AD. If the APP sig‐ naling hypothesis is correct, several molecules involved in APP signaling may be attractive candidates for the targets of drug discovery for treating AD. Thus, extensive studies about

protease remain to be clarified.

this issue are expected.

AD, Alzheimer's disease;

Dll, Delta-like protein

FAD, familial AD;

APP, amyloid precursor protein;

AICD, the intracellular domain of APP; Aph-1, anterior pharynx defective-1; CAA, cerebral amyloid angiopathy;

Dll1IC, the intracellular domain of Dll1;

EGF, epidermal growth factor;

Hes, Hairy/Enhancer of split; ICD, intracellular domain;

**Abbreviations**

Aβ, amyloid-β;

a central regulatory role in Notch signaling.

the existence of a signaling mechanism similar to that of Notch.

**Figure 6.** Model of APP signaling pathway. The majority of cell membrane-associated APP is phosphorylated within its ICD in neurons. After cleavage of JM domain by α- or β-secretase, AICD is released from the membrane by γ-secretase. Phosphorylated AICD cannot bind to the nuclear adaptor protein Fe65, which is thought to be essential for transloca‐ tion of AICD to the nucleus, and thus cannot translocate to the nucleus. Phosphorylated AICD left in the cytosol is rap‐ idly degraded, probably by the proteasome and/or insulin-degrading enzyme (IDE). On the other hand, dephosphorylated AICD binds to Fe65. Therefore, dephosphorylated AICD/Fe65 complexes immediately translocate to the nucleus, where they meidate up- and downregulation of certain target genes in association with Tip60.

In addition to these possibilities, it is also likely that AICD is ineffective in the normal brain, because almost all AICD is degraded rapidly, and APP signaling cannot be transmitted. However, both AICD and Aβ are overproduced in the AD brain compared to normal brain. Thus, although the majority of AICD is degraded, a small amount of the remaining AICD may play a role in signaling and cause neuron-specific cell death in the AD brain. In addi‐ tion, if the degrading activity of AICD is reduced or lost in the AD brain, APP signaling, which leads to neuron-specific cell death, may be enhanced. Thus, compounds that inhibit translocation of AICD to the nucleus will be good candidates for AD therapy. From this point of view, protein phosphatase inhibitors and chemicals that impair the interaction be‐ tween AICD and Fe65 may be potential ones.
