**4. Animals models for neuropsychiatric diseases**

Owing to the importance that Reelin have in the correct structuration and lamination of the brain during development and in neuronal connectivity and synaptogenesis in the adult brain, its dysfunction has been directly related to the generation or susceptibility to acquire neuropsychiatric conditions such as depression and schizophrenia, or neurodegenerative diseases such as Alzheimer's disease (AD) [45].

The most tangible evidence supporting these putative relationships was obtained through studies of human brains derived from neuropsychiatric and neurodegenerative conditions. Decreased levels of Reelin are shown in postmortem samples from prefrontal cortex of pa‐ tients with schizophrenia and bipolar disorders [47]. This decrease may be explained in schizophrenic patients by an abnormal hypermethylation of the *reelin* promoter, an epige‐ netic modification involved in gene silencing [48]. Furthermore, immunohistochemistry ex‐ periments in depressive and schizophrenic patients show decreased Reelin expression at the hippocampus [49]. On the other hand, diminished Reelin levels in the hippocampus of pa‐ tients with AD had been reported, suggesting a direct correlation between the severity of the disease and the extent of decreased Reelin expression [50]. All of these antecedents provide evidence enough to feature a molecular link between decreased Reelin levels and neurode‐ generative/psychiatric diseases.

In order to understand the etiology of neurodegenerative/psychiatric diseases, different ani‐ mal models had been developed. A widely paradigm is the "two hit" model, which suggests that genetic and environmental factors may affect the development of central nervous sys‐ tem, acting as "the first hit". These early disorders are linked to long-term vulnerability, which after a "second hit" could cause the symptoms for a disease [51-52]. For diseases such as depression, autism and schizophrenia, the heterozygous *reeler* mice had been used as the genetic "first hit", while stress events after the birth or in adulthood are used as the environ‐ mental "second hit". The results indicate that heterozygous *reeler* mice, after a stressful event, such as maternal deprivation or corticosterone injection, exhibit significantly in‐ creased depressive or schizophrenic behaviors as compared with wild type littermates [53-54]. Indeed, *reeler* heterozygous animals in the absence of a stressful event, display a phenotype indistinguishable from control animals [55].

Finally, Reelin had been associated with the pathological hallmarks for AD, the senile pla‐ ques and the neurofibrillary tangles (NFT). Reelin can modulate tau phosphorylation, the core protein of NFT [38]. It is also associated with senile plaques, large extracellular aggre‐ gates mainly formed for β-amyloid peptide (Aβ). Immunohistochemical studies revealed that Reelin colocalizes with the amyloid precursor protein (APP) in the neuritic component of typical AD plaques, at the hippocampus and cortex of mice expressing a mutant version of APP [64]. Additionally, a reduction of Reelin-producing cells had been observed in older mice and primates. This reduction is accompanied by the presence of Reelin aggregates and memory deficits. Mice harboring APP with AD-associated mutations also showed Reelin ag‐ gregates, which co-localized with non-fibrillar amyloid plaques [65]. In addition, Reelin forms oligomeric or protofibrillary deposits during aging, potentially creating a precursor

The Amyloidogenic Pathway Meets the Reelin Signaling Cascade: A Cytoskeleton Bridge Between...

http://dx.doi.org/10.5772/54038

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A direct relationship between decreased Reelin expression and increased levels of Aβ peptide and plaque accumulation was provided by studies using transgenic mice carry‐ ing the APP Swedish and *reeler* mutation. The absence of Reelin expression resulted in an age-dependent exacerbation of plaque pathology and increased NFTs in double mutants as compared with the single APPsw mutant [67]. Finally, recent studies demonstrated a feedforward mechanism by which Reelin would favor the formation of senile plaques; and the subsequent Aβ peptide production would increase the Reelin levels by altering its

Neurofibrillary tangles are amongst the standard characteristics of AD brains. These struc‐ tures were firstly described by Alois Alzheimer more than a century ago and are com‐ posed of a densely packed array of fibers of 20 nm in diameter, called paired helical filaments (PHF), which at the core are mainly composed by the microtubule-associated protein, tau [69-70]. Tau protein stabilizes and enhances microtubule polymerization. It is a heterogeneous protein giving rise to 6 isoforms derived from alternative splicing [71]. It contains 3 or 4 imperfect repeats of 31 or 32 amino acids each in tandem which confers the microtubule-binding properties of the protein. These repeats are enriched in basic aminoacids that interact electrostatically with the mostly acidic C-terminal of β-tubulin subunit [72]. Tau protein is highly phosphorylated in fetal brain [73], but minimally phos‐ phorylated in normal adult brain [74]. The abnormal phosphorylation state of several res‐ idues in tau protein plays an important role modulating the affinity to microtubules and promoting its aggregation [75] forming the core of PHFs [69,76-77]. Tau protein can be phosphorylated by many protein kinases such as calcium-calmodulin dependent kinase [78]; PKA [79-81] and PKC [82-83]. Interestingly, many of these residues are hyperphos‐ phorylated in AD brains mainly due to an imbalance in the activity of kinases belongs to the family of proline-directed Ser/Thr protein kinases (PDPKs), such as mitogen-activated

proteolytic processing in the cortex of mice and humans with AD [68].

**5. Cytoskeletal abnormalities in Alzheimer´s disease**

condition for Aβ plaque formation [66].

**5.1. Tau protein and neurofibrillary tangles**

The "two hit" model has also been used to study the molecular mechanisms leading to the AD [56]. It is proposed that both oxidative stress and failures in mitotic signaling can inde‐ pendently triggers the onset of the disease; however both are necessary for their progression [57]. In addition, a correspondence had been established between the Reelin expression in the entorhinal cortex of aged rats with their cognitive abilities. A study revealed that aged "cognitively disabled" rats show a significant decreased of Reelin in neurons on layer II of the entorhinal cortex. Such a reduction in Reelin expression was not observed in juvenile or elderly "cognitively able" rats [58].

Since Reelin is expressed from development to adult stages, is conceivable that alterations in Reelin expression, induced by genetic or environmental factors generate a vulnerable stage, and a secondary factor, present in normal aging, may trigger the onset and progression of a pathological condition.

The Reelin-activated signaling pathways, which may be involved in the generation and de‐ velopment of AD are still unclear and will be discussed in next sections. In the last part of this section, we present some of the evidences that correlate altered levels of Reelin and AD. Pyramidal neurons placed in layer II of the entorhinal cortex and the hippocampus derived from AD patients brains exhibit decreased Reelin expression [50]. On the other hand, an in‐ crease in the full length and 180 kD proteolytic fragment of Reelin had been observed in the frontal cortex of AD derived samples [59]. The increase of this proteolytic fragment is attrib‐ uted to problems with the proteolysis of Reelin, associated with decreased Rab11-endocyto‐ sis of full length Reelin [60]. In the other hand, an increase of Reelin is also observed in the frontal cortex of AD patients, which may involve a compensatory mechanism in response to the lower expression in disease-related most vulnerable areas like the entorhinal cortex and hippocampus [50].

The CR neurons participation in AD is a controversial issue. While electronic microscopy analysis suggested that CR neurons of the temporal cortex were dramatically reduced in AD patients [61], another study showed no difference between AD patients and normal, healthy subjects [62]. On the other hand, there are some polymorphisms in the Reelin gene which had been associated with AD. Seripa and colleagues reported significant differences in two analyzed polymorphisms in the Reelin gene, in a group of 223 Caucasians AD patients. These differences were exacerbated in female patients [63].

Finally, Reelin had been associated with the pathological hallmarks for AD, the senile pla‐ ques and the neurofibrillary tangles (NFT). Reelin can modulate tau phosphorylation, the core protein of NFT [38]. It is also associated with senile plaques, large extracellular aggre‐ gates mainly formed for β-amyloid peptide (Aβ). Immunohistochemical studies revealed that Reelin colocalizes with the amyloid precursor protein (APP) in the neuritic component of typical AD plaques, at the hippocampus and cortex of mice expressing a mutant version of APP [64]. Additionally, a reduction of Reelin-producing cells had been observed in older mice and primates. This reduction is accompanied by the presence of Reelin aggregates and memory deficits. Mice harboring APP with AD-associated mutations also showed Reelin ag‐ gregates, which co-localized with non-fibrillar amyloid plaques [65]. In addition, Reelin forms oligomeric or protofibrillary deposits during aging, potentially creating a precursor condition for Aβ plaque formation [66].

A direct relationship between decreased Reelin expression and increased levels of Aβ peptide and plaque accumulation was provided by studies using transgenic mice carry‐ ing the APP Swedish and *reeler* mutation. The absence of Reelin expression resulted in an age-dependent exacerbation of plaque pathology and increased NFTs in double mutants as compared with the single APPsw mutant [67]. Finally, recent studies demonstrated a feedforward mechanism by which Reelin would favor the formation of senile plaques; and the subsequent Aβ peptide production would increase the Reelin levels by altering its proteolytic processing in the cortex of mice and humans with AD [68].
