**5. Wnt signaling in Alzheimer's disease**

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive deterioration of cognitive abilities, cerebral accumulation of extracellular amyloid plaques composed mainly of amyloid-β peptide (Aβ), and synaptic alterations [111]. In addition to the accumulation of Aβ aggregates, which is a product of the processing of the amyloid precursor protein (APP), cytoskeletal alterations associated to the abnormal phosphorylation of the microtubule associated protein tau [112, 113] are early manifestations that lead to aberrant remodeling of dendrites and axons, the appearance of dystrophic neurites, synaptic loss [114], and eventually progressive loss of neuronal populations [112].

During more than a decade, a strong relationship between an impaired Wnt signaling pathway activity and neuronal damage in AD has been raised [31, 115-118]. Different studies have shown that Wnt signaling components are altered in AD [119-124], and in addition, the Wnt signaling pathway has been related to other neurodegenerative disorders such as autism and schizophrenia [30, 125]. Among the Wnt components that are affected in AD, it was shown that β-catenin levels are reduced in AD patients carrying presenilin-1 (PS-1)-inherited mutations [124], while the secreted Wnt antagonist Dickkopf-1 (Dkk1) is elevated in postmor‐ tem AD brains and brains from transgenic mouse models for AD [121, 126]. A variant of the LRP6 has been associated with late-onset AD, which confers low levels of Wnt signaling [119]. In addition, genetic studies show a link between Wnt signaling and AD. Epidemiological data show an increased risk for AD in populations where the allele 4 of apo-lipoprotein E (apoE4) is present. Interestingly apoE4 causes inhibition of the canonical Wnt signaling in PC12 cells upon stimulation with Wnt-7a as determined by luciferase activities and nuclear β-catenin levels [127]. Aβ directly binds to the extracellular CRD of Fz5 at or in close proximity to the Wnt-binding site inhibiting the canonical Wnt signaling pathway [128], linking directly Aβ to Wnt impairment. Moreover, the exposure of cultured rat hippocampal neurons to Aβ results in inhibition of canonical Wnt signaling as determined by destabilization of endogenous levels of β-catenin, increase in GSK-3β activity, and a decrease in the expression of some Wnt target genes [129]. Moreover, acute exposure to Aβ increases Dkk1 mRNA levels in hippocampal brain slices, which seems to be associated to synaptic loss induced by Aβ [130].

brains. Inhibiting this autocrine Wnt signaling increases the number of neurons formed and leads to a loss of multipotency among AHPs indicating that this autocrine pathway may

The Wnt signaling has also been involved in the mechanism of the orphan nuclear receptor TLX (also known as NR2E1), which is an important regulator of neural stem cell maintenance and self-renewal in embryonic and adult brains [106, 107] and is involved in neurogenesis in the SVZ [108] and hippocampus [109]. To stimulate neural stem cell proliferation and selfrenewal TLX activates the Wnt/β-catenin pathway in adult mouse neural stem cells by activating the expression of Wnt-7a, which expression was found to be downregulated in TLXnull mice, through binding to two TLX binding sites present in the Wnt-7a gene promoter [95]. Wnt-7a is important for adult neural stem cell proliferation *in vivo* since there is a decreased BrdU labeling in the SGZ and SVZ of adult Wnt7a knockout mice. In TLX-/- mice, intracranial lentiviral transduction of active β-catenin led to a considerable rescue of cell proliferation in the SVZ, suggesting that Wnt/β-catenin acts downstream of TLX to regulate neural stem cell

It has been shown that low oxygen is associated with increased levels of β-catenin *in vivo*, and that hypoxia inducible factor-1α (HIF-1α) modulates the Wnt/β-catenin signaling in embryonic stem cells exposed to low oxygen [110]. Recently, we determined *in vivo* that hypoxia stimulates the activation of the Wnt/β-catenin signaling pathway in the hippocampus of adult mice (our unpublished results), and stimulates cell proliferation in the SGZ of 2 month old wild-type

Altogether, these findings indicate that the Wnt pathway is relevant not only for the develop‐ ment of the nervous system but also for the development of new neurons in the adult brain, being important for the maintenance and self-renewal of the stem cell pool and for the

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive deterioration of cognitive abilities, cerebral accumulation of extracellular amyloid plaques composed mainly of amyloid-β peptide (Aβ), and synaptic alterations [111]. In addition to the accumulation of Aβ aggregates, which is a product of the processing of the amyloid precursor protein (APP), cytoskeletal alterations associated to the abnormal phosphorylation of the microtubule associated protein tau [112, 113] are early manifestations that lead to aberrant remodeling of dendrites and axons, the appearance of dystrophic neurites, synaptic loss [114],

During more than a decade, a strong relationship between an impaired Wnt signaling pathway activity and neuronal damage in AD has been raised [31, 115-118]. Different studies have shown that Wnt signaling components are altered in AD [119-124], and in addition, the Wnt signaling pathway has been related to other neurodegenerative disorders such as autism and

preserve the balance between neural stem cell maintenance and differentiation [105].

proliferation *in vivo* [95].

124 Trends in Cell Signaling Pathways in Neuronal Fate Decision

commitment and proliferation of new neurons.

**5. Wnt signaling in Alzheimer's disease**

and eventually progressive loss of neuronal populations [112].

mice.

As mentioned, one of the hallmarks of AD brains is the abnormal phosphorylation of the tau protein which accumulates as intraneuronal neurofibrillary tangles [131]. Several kinases can phosphorylate tau *in vitro*; however, the bulk of the information supports that Cdk5, extrac‐ ellular signal-related kinase 2, microtubule affinity-regulating kinase and GSK-3β, a key component of the Wnt cascade, are the most relevant kinases for tau phosphorylation *in vivo* [132, 133]. Cultured neurons exposed to Aβ show an increased GSK-3β activity [134, 135], and active GSK-3β has been found in brains staged for AD neurofibrillary changes, with a con‐ comitant decrease in β-catenin levels and an increase in *tau* hyperphosphorylation [136]. Also, neurodegeneration and spatial learning deficits have been observed in GSK-3β conditional transgenic mice [137, 138]. Interestingly, a study shows that the phosphorylation of tau antagonizes apoptosis by stabilizing β-catenin; therefore, up-regulation of β-catenin during tau phosphorylation prevents the cell from going into apoptosis. Increasing levels of phos‐ phorylated tau was correlated with increased levels of nuclear β-catenin, and the knockdown of β-catenin antagonizes the anti-apoptotic effects of *tau* [139]. These findings support a role of β-catenin as a survival element in AD.

Several studies have shown neuroprotective properties of the Wnt signaling activation against the toxicity of Aβ peptide. In cultured hippocampal neurons, exposure to Aβ aggregates causes a decrease in endogenous β-catenin levels, and this effect was overcome by direct activation of the pathway with Wnt-3a conditioned media [117, 129]. The protective effect of Wnt-3a against the toxicity of Aβ oligomers was shown to be mediated by Fz1 receptor, since this effect is modulated by the expression levels of Fz1 in both, PC12 cells and hippocampal neurons [14]. Overexpression of Fz1 significantly increased cell survival induced by Wnt-3a and diminished caspase-3 activation, while knocking-down the expression of the receptor by antisense oligonucleotides decreased the stabilization of β-catenin induced by Wnt-3a and decreased the neuroprotive effect elicited by this Wnt ligand [14].

In agreement with the effect of Wnt-3a, inhibition of GSK-3β by lithium protects hippocampal neurons from Aβ-induced damage. More importantly, *in vivo* lithium treatment of double transgenic APPswe/PSEN1ΔE9 mice, which is a well characterized *in vivo* model of AD that shows most hallmarks of the disease [140], reduced spatial memory impairment, decreased Aβ oligomers and the activation of astrocytes and microglia [141]. *In vivo*, lithium treatment activated the Wnt signaling as shown by the increase in β-catenin and by the inhibition of GSK-3β [141]. These studies suggest that the loss of normal Wnt/β-catenin signaling activity may be involved in the Aβ-dependent neurodegeneration observed in AD and that the activation of the pathway might have beneficial effects for the treatment of the disease [12].

signaling cascades that regulates the generation of new neurons in neurogenic niches. Importantly, different stimuli that regulate neurogenesis involve the regulation of the Wnt signaling, implicating this pathway as a relevant player in the modulation of this physiological

Wnt Signaling Roles on the Structure and Function of the Central Synapses: Involvement in Alzheimer's Disease

http://dx.doi.org/10.5772/54606

127

Considering all the discussed roles of Wnts, it was expected that alterations in the Wnt cascades leads to diseases associated to the nervous system. In fact, deregulation of the Wnt pathway has been related to mental disorders, mood disorders and neurodegenerative diseases. As we have discussed, a bulk of evidence associate Wnt dysfunction to AD, and strongly point to a neuroprotective potential of the Wnt cascades as a therapeutic approach. Future work should focus on explore the therapeutic benefits of stimulating the Wnt signaling pathway *in vivo*.

We thank to Felipe G. Serrano for his contribution in the artwork. This work was supported by Grants from FONDECYT (N°1120156) and the Basal Center of Excellence in Science and Technology (CONICYT-PFB12/2007) to NCI and FONDECYT (N°11110012) and Insertion of

Center for Aging and Regeneration (CARE), Department of Cell and Molecular Biology, Fac‐

[1] Nusse, R, & Varmus, H. Three decades of Wnts: a personal perspective on how a sci‐

[2] Van Amerongen, R, & Nusse, R. Towards an integrated view of Wnt signaling in de‐

[3] Gordon, M. D, & Nusse, R. Wnt signaling: multiple pathways, multiple receptors,

[4] Schulte, G. International Union of Basic and Clinical Pharmacology. LXXX. The class

velopment. Development (Cambridge, England) (2009). , 136(19), 3205-14.

and multiple transcription factors. J Biol Chem (2006). , 281(32), 22429-33.

ulty of Biological Sciences, Pontifical Catholic University of Chile, Santiago, Chile

entific field developed. EMBO J (2012). , 31(12), 2670-84.

Frizzled receptors. Pharmacol Rev (2010). , 62(4), 632-67.

Postdoctoral Researchers in the Academy (CONICYT-79090027) to LV-N.

process.

**Acknowledgements**

**Author details**

**References**

Nibaldo C. Inestrosa and Lorena Varela-Nallar

APPswe/PSEN1ΔE9 mice show decreased levels of adult neurogenesis [142]. In these mice, we evaluated the effect of hypoxia on the generation of new neurons in the hippocampus. As previously mentioned hypoxia induces the activation of the Wnt/β-catenin signaling pathway in the hippocampus of wild-type mice. Mice were exposed to low oxygen and neurogenesis was evaluated by incorporation of BrdU and double staining with DCX. It was determined that hypoxia is a strong stimulator of neurogenesis in AD mice (our unpublished results). Currently we are evaluating whether this effect is related to the activation of the canonical Wnt pathway. Also, we have observed that voluntary wheel running strongly increased neuro‐ genesis in APPswe/PSEN1ΔE9 mice and also decreased Aβ burden and tau phosphorylation (our unpublished results). As previously mentioned, voluntary running was found to increase *de novo* expression of Wnt-3 [101], suggesting that the effects observed in runner AD mice could involve the activation of the Wnt signaling pathway.

In addition to the role of the canonical Wnt signaling, we have studied whether Wnt-5a is able to protect neurons against Aβ oligomers synaptotoxicity [143]. Synaptic failure is an early event in AD, and soluble Aβ oligomers are proposed to be responsible for the synaptic pathology that occurs before the plaque deposition and neuronal death [74, 144]. Electrophysiological analysis of Schaffer collaterals-CA1 glutamatergic transmission in hippocampal slices dem‐ onstrated that Wnt-5a prevents the decrease in the amplitude of fEPSP and EPSCs induced by Aβ oligomers, indicating that Wnt-5a prevents the synaptic damage triggered by Aβ [143]. Moreover, Wnt-5a prevented the decrease in the postsynaptic density scaffold protein PSD-95 and synaptic loss in cultured hippocampal neurons [143], supporting that Wnt-5a improves synaptic function in the presence of Aβ.

Additionally, the activation of several signaling pathways that crosstalk with the Wnt pathway also supports the neuroprotective potential of the Wnt cascades in AD [12].

## **6. Conclusions**

As we have discussed throughout this Chapter, the Wnt signaling pathway has fundamental roles in the development and function of the CNS. As discussed, the canonical and noncanonical Wnt signaling cascades have shown to be important for the formation and structure of central synapses, and in addition to the structural effects, Wnt ligands acutely modulate synaptic transmission and plasticity. Also, in the adult brain the Wnt pathway is one of the signaling cascades that regulates the generation of new neurons in neurogenic niches. Importantly, different stimuli that regulate neurogenesis involve the regulation of the Wnt signaling, implicating this pathway as a relevant player in the modulation of this physiological process.

Considering all the discussed roles of Wnts, it was expected that alterations in the Wnt cascades leads to diseases associated to the nervous system. In fact, deregulation of the Wnt pathway has been related to mental disorders, mood disorders and neurodegenerative diseases. As we have discussed, a bulk of evidence associate Wnt dysfunction to AD, and strongly point to a neuroprotective potential of the Wnt cascades as a therapeutic approach. Future work should focus on explore the therapeutic benefits of stimulating the Wnt signaling pathway *in vivo*.
