**6. Finding a proper animal model to study AD: A lesson from the pin1KO mice**

One of the biggest challenges when studying a disease is to develop the proper animal model that would reproduce the main features of that disease within the animal's biological envi‐ ronment. In the case of AD, this is not an easy goal, since mice do not spontaneously develop the features characteristic of this disease.

The only way to induce AD-like pathology with plaques and/or tangles in mice is by generating genetically altered animals. These may either overexpress aggressive mutants of APP linked to familial forms of AD (FAD) [214-218] and hence produce higher amounts of Abeta peptide, or express either wild type or aggressive mutants of tau [217, 218], leading to sustained tau hyperphosphorylation and tangle formation or may express both [217, 218]. These models may recapitulate plaque (APPTg) or tangle (tauTg) pathologies, or both (APPtg crossed to tauTg) [217, 218], and are extremely useful to understand the molecular pathways involved in AD, however they may [216] or may not [219] undergo neurodegeneration, which is a feature of AD. Moreover, they may not be representative of the way the disease progresses in sporadic AD, which affects the vast majority of AD patients, as they may represent only those familial cases of AD caused by those same mutations. Furthermore, these models may not be all specific for AD, since tau hyperphosphorylation and tangle formation occur also in other neurodege‐ nerative diseases, and some of the tau mutations used to generate animal models for AD do not associate with AD, but with other neurodegenerative diseases, such as frontotemporal dementia associated with parkinsonism FTDP [17, 220, 221].

**5.6. Potential novel cis and trans conformation–specific disease diagnoses and therapies**

Our exciting new insight into the role and regulation of p-tau conformations in AD might have important and novel therapeutic implications. For example, it has been shown that Thr231 phosphorylation is the earliest detectable tau phosphorylation event in human AD [130, 159, 160, 198] and its levels are elevated in cerebrospinal fluids and tracks AD progression, but with large individual variations [199, 200], making it difficult to become a standardized test. Our findings that the *cis* conformation appears earlier in MCI and is pathologically more relevant suggest that *cis* pThr231-tau and especially its ratio with *trans* might be a better and easier standardized diagnostic marker, especially for early diagnosis and patient comparison. Furthermore, the findingsthatPin1overexpressionconverts*cis*to*trans*,promotestaudegradationandinhibitstau pathology and neurodegeneration in AD mouse models [201] and that Pin1 SNPs preventing its inhibitionbybrain-specifictranscriptionfactorAP-4isassociatedwithdelayedonsetofAD[202] suggestthatoverexpressingPin1orpreventingPin1inhibitionmightbeanewapproachtoreduce the *cis* to *trans* pThr231-tau ratio to block tau pathology at early stages. Finally, active or passive immunizationagainstsomepSer/Thr-PromotifsintauincludingthepThr231-Promotifhasbeen shown to reduce tau aggregates and improve memory deficits in mouse models [203-209]. However, we have here shown that only ~10% of regular synthetic pThr231-tau peptides is in the pathologicallyrelevant*cis*conformationandtheremaining90%isin*trans*,whichcanstillpromote MT assembly and is not related to neurofibrillary degeneration. Therefore, immunotherapies either using conformation-specific vaccines or antibodies specifically against the pathological‐ ly relevant *cis* pT231-tau conformation might be more specific and effective and safer in treating AD. Given the critical role of Pin1 and other isomerases in controlling the function of many other key regulators in the pathogenesis of human disease, notable Alzheimer's disease, cancer, viral infection, inflammation and autoimmune disorders [210-213], it would be interesting to deter‐ mine whether prolyl isomerization regulates the cellular function of these proteins and whether these conformational switches might be explored for developing novel diagnoses and therapies.

**6. Finding a proper animal model to study AD: A lesson from the pin1KO**

One of the biggest challenges when studying a disease is to develop the proper animal model that would reproduce the main features of that disease within the animal's biological envi‐ ronment. In the case of AD, this is not an easy goal, since mice do not spontaneously develop

The only way to induce AD-like pathology with plaques and/or tangles in mice is by generating genetically altered animals. These may either overexpress aggressive mutants of APP linked to familial forms of AD (FAD) [214-218] and hence produce higher amounts of Abeta peptide, or express either wild type or aggressive mutants of tau [217, 218], leading to sustained tau hyperphosphorylation and tangle formation or may express both [217, 218]. These models may recapitulate plaque (APPTg) or tangle (tauTg) pathologies, or both (APPtg crossed to tauTg) [217, 218], and are extremely useful to understand the molecular pathways involved in AD,

**mice**

124 Understanding Alzheimer's Disease

the features characteristic of this disease.

Both APP and tau are phosphorylated by protein kinase (PKs) as part of their normal function. The trans-conformation of phosphorylated APP and tau may present the physiological conformation that promotes their normal function (green boxes). Pin1 expression is induced during neuron differentiation and necessary to maintain normal neuronal function by preventing the unscheduled activation of mitotic events and/or controlling the function of phosphopro‐ teins in the event that they become abnormally phosphorylated. For example, by catalyzing isomerization of the cis to trans conformation, Pin1 might promote non-amyloidogenic APP processing reducing Abeta production, as well as promote tau dephosphorylation and restore tau function. However in AD, a loss of Pin1 function, either through downregulation of Pin1 function, oxidative inactivation, phosphorylation or possible genetic alterations, can lead to build-up of cis-pS/T –P motifs. Cis-p-tau and cis-p-APP are proposed to represent pathological conformations (red ovals). Cis-p-APP is processed by the amyloidogenic pathway, which lead to a build-up of amyloid beta-42 (Abeta42), decreased levels of neurotropic alphaAPPs and the resultant formation of amyloid plaques. Cis-p-tau is resistant to protein phosphatases, which leads to a loss of MT binding, hyperphosphorylated tau an the formation of neurofibril‐ lary tangles. The formation of tangles and plaques might further reduce Pin1 function by sequestering Pin1 and induc‐ ing oxidative modifications, respectively, in a positive feedback loop. In addition, a lack of proper Pin1 function leads to activation of kinases such as GSK3beta, which further increases both phosphorylation of tau and APP and also in‐ hibits APP turnover, contributing to both tau and Abeta pathologies and causing neuronal death. Therefore, Pin1 de‐ regulation might act on multiple pathways to contribute to AD development.

**Figure 3.** The regulation of APP processing and tau function by Pin1 in healthy and Alzheimer's neuron.

In addition, also animal models developed to understand a specific pathway even in absence of plaques [118], show limitations in the interpretation of the results.

pathologic protein functions, and highlight Pin1 as a successful regulator of such toxic conformations, opening new avenues in the medical field of AD. In fact, if a therapeutic target, Pin1 could block both tau and Abeta pathologies early in the disease, also resolving the eternal

Pin1 Protects Against Alzheimer's Disease: One Goal, Multiple Mechanisms

http://dx.doi.org/10.5772/55085

127

Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School,

[1] Holtzman DM, Morris JC, Goate AM. Alzheimer's disease: the challenge of the sec‐

[2] Bateman RJ, Aisen PS, De Strooper B, Fox NC, Lemere CA, Ringman JM, et al. Auto‐ somal-dominant Alzheimer's disease: a review and proposal for the prevention of

[3] Aisen PS, Cummings J, Schneider LS. Symptomatic and nonamyloid/tau based phar‐ macologic treatment for Alzheimer disease. Cold Spring Harb Perspect Med. 2012

[4] Winblad B, Engedal K, Soininen H, Verhey F, Waldemar G, Wimo A, et al. A 1-year, randomized, placebo-controlled study of donepezil in patients with mild to moder‐

[5] Winblad B, Poritis N. Memantine in severe dementia: results of the 9M-Best Study (Benefit and efficacy in severely demented patients during treatment with meman‐

[6] Tariot PN, Farlow MR, Grossberg GT, Graham SM, McDonald S, Gergel I. Meman‐ tine treatment in patients with moderate to severe Alzheimer disease already receiv‐

[7] Schneider LS, Insel PS, Weiner MW. Treatment with cholinesterase inhibitors and memantine of patients in the Alzheimer's Disease Neuroimaging Initiative. Arch

ing donepezil: a randomized controlled trial. JAMA. 2004 Jan 21;291(3):317-24.

and unsolvable conflict: What happens first?

Lucia Pastorino, Asami Kondo, Xiao Zhen Zhou and Kun Ping Lu

ond century. Sci Transl Med. 2011 Apr 6;3(77):77sr1.

Alzheimer's disease. Alzheimers Res Ther. 2011;3(1):1.

ate AD. Neurology. 2001 Aug 14;57(3):489-95.

tine). Int J Geriatr Psychiatry. 1999 Feb;14(2):135-46.

\*Address all correspondence to: lpastori@bidmc.harvard.edu

\*Address all correspondence to: klu@bidmc.harvard.edu

**Author details**

Boston, USA

**References**

Mar;2(3):a006395.

Neurol. 2011 Jan;68(1):58-66.

In contrast, the model offered by knocking out the Pin1 gene in our Pin1KO model may recapitu‐ late some of the features characteristic of both tau and Abeta pathology in sporadic AD, and therefore could serve as a valid tool to investigate the pathways that can be targeted to prevent or halt the disease progression. In fact, Pin1KO mice 1) develop age-dependent Abeta pathology associated with early neuronal deficit that leads to neurodegeneration (elevated Abeta levels associated with increased intracellular deposition) [41], 2) are characterized by age-dependent tau hyperphosphorylation, stabilization and PHF formation [42], and 3) show age-dependent neurodegeneration in selected areas [42]. Because genetic and proteomic findings link de‐ creasedPin1levelsand/oractivitytoAD[56,61],wecouldspeculatethatthePin1KOanimalmodel be very close to recapitulating the features that characterize AD in humans, and therefore may serve as a valid model to study the molecular pathways involved in AD.
