**8. Conclusion**

dependent Shc or IRS1 peptide complexes, where the phosphate moiety is read out by a set of

The NPTY sequence is followed by the 688KFFEQMQN695 sequence, which forms the C-termi‐ nus of the AICD (Figures 4A and 4E). The conformation of this region is slightly different and not always present in the structures, as the complexes have mostly been formed with truncated synthetic peptides. In the Fe65-PTB2 (which contains the entire C-terminus) and X11α com‐ plexes, the region is part of the C-terminal helix αC. The helix is fixed to the PTB domains by hy‐ drophobic interactions of the two phenylalanines (Phe689 and Phe690) with the C-terminal helices of the respective PTB domains. These helices are three turns longer than those of Shc [79] and IRS1 [80] PTBs, and therefore the total interaction surfaces are significantly larger.

In most PTB domain complexes bound to an NPXY motif the described surfaces comprise the entire interaction, however, there is a single exception to the rule: the Fe65-PTB2/AICD complex, where the interface is about three times as large and includes an additional α helix (helix αN, 669PEERHLSKMQQ679) N-terminal to the 681GYENPTY sequence (Figure 4C) [30]. This helix is N-terminally capped by the 667VTPEER motif comprising the phosphorylatable Thr668 as already described. Like helix αC, helix αN is of amphipathic character and binds on a hydrophobic patch on the Fe65-PTB2 surface located in between strand β5 and the Nterminus of the C-terminal helix, which is almost perpendicularly crossed by helix αN. Whereas Leu674 and Met677 cover the hydrophobic patch, Glu670, His673, and Gln678 are involved in polar interactions with the PTB domain. With the exception of Glu670, the 667VTPEER capping box is not touching the PTB domain, which is somewhat astonishing, as it was afore known that phosphorylation of Thr668 is detrimental to complex formation [20]. As described for free AICD, the side chain of Thr668 is hydrogen-bonded to the main chain of Glu671, and Pro668 is *in trans* configuration. Furthermore, the side chain of Glu671 is tied back

to the main chain nitrogen of Thr668, and thus completing the rigid helix cap.

tion: what determines stability, lifetime, and eventually toxicity of the AICD?

**7. AICD turnover**

14 Understanding Alzheimer's Disease

The most important question arising from structural data is how phosphorylation is able to regulate Fe65-PTB2/AICD complex formation in a process that is critically involved in Aβ gen‐ eration and AD pathogenesis? Phosphorylation induces a *cis* configuration of Pro669 [46], which is incompatible with the formation of helix αN. As found by mutational studies [30], the destruction of the helix cap increases the entropy of the system and reduces the binding affini‐ ty, and once the helix is dissolved, the remaining interfaces are not sufficient for maintaining the complex. This molecular switch model is only valid for the Fe65-PTB2/AICD interaction, as all other PTB domains do not contact Thr668 and phosphorylation does therefore not alter their binding affinity [20]. Intriguingly, the Fe65-PTB2/AICD interface spans almost the entire AICD-C31 fragment, which has been implicated in apoptotic events. This raises the next ques‐

The turnover of APP is very fast (with a half life of cell surface APP of about 30-40 minutes only [81] and only about 10% of APP are estimated to reach the cellular membrane, whereas

conserved arginine residues and the binding pocket is much more pronounced [36].

Despite enormous efforts to develop an efficient treatment for AD, only symptomatic treat‐ ments with modest impact on the progress of the disease are available [6]. Drugs currently approved for the treatment of AD are either acetylcholine esterase inhibitors to increase the level of the neurotransmitter, which is depleted in AD brains, or antagonize the NMDA receptor to prevent abnormal neuronal stimulation [91]. None of them directly targets the amyloid cascade and would thereby allow for a disease-modifying treatment. Many current therapeutic approaches for AD focus on the reduction of the Aβ load either by inhibiting the involved secretases BACE and γ-secretase, or by augmenting the elimination of amyloid peptides, e.g. by active or passive immunotherapy [6]. Finally, a smaller number of trials have targeted ApoE4 levels or either tau phosphorylation or tau aggregation. None of the ap‐ proaches was successful so far, which means that either there were not enough clinical trials or the ideas were too simplistic to be potent for a complex disease. Like for other complex diseases (i.e. hypertension or AIDS), a combination of drugs that have different modes of action could be the key to success.

In this sense, the AICD might be re-evaluated as a potential drug target. In contrast to Aβ, the AICD is a physiological highly relevant protein domain modulating a diverse set of important APP functions including trafficking and signal transduction. As both proc‐ esses are also directly affecting Aβ production, upstream targeting of AICD might be beneficial as the Aβ pathology is prevented *a priori*. Moreover, the pathophysiology of the AICD and its breakdown product AICD-C31 has come into the focus of AD research and would be tackled directly. As the AICD by its nature is created intracellular, effi‐ cient compounds need to be able to pass the plasma membrane and to accumulate with‐ in neurons, as is i.e. the case for the NMDA receptor antagonist memantine [92]. However, the AICD is intrinsically disordered, and therefore the protein interaction net‐ work around the AICD might be the crucial target rather than the AICD itself. Major binding partners are the PTB domains, with their known ability to modulate Aβ produc‐ tion (like Fe65, ShcA, and X11α) and to specifically recognize and fold the AICD. Al‐ though protein-protein interactions are notoriously difficult to be targeted, the urgent need for a disease-modifying and efficient treatment for this devastating disease seems worth the trial.

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