N EUROPATHOLOGICAL HALLMARKS OF A LZHEIMER ' S DISEASE

1.3.1 Amyloid plaques

Amyloid plaques are one of the key neuropathological features in AD. The main proteinaceous components of these extracellular deposits are Aβ peptides. Aβ peptides are generated by enzymatic cleavage of the amyloid precursor protein (APP) (Holtzman et al., 2011; Serrano-Pozo et al., 2011). Amyloid plaques can be divided into two groups:

diffuse and neuritic (Yamaguchi et al., 1988b; Holtzman et al., 2011).

Diffuse plaques (FIGURE 1.1 B) consist of non-fibrilar depositions of Aβ with nearly no detectable neuritic dystrophy. They vary in size from 50 µm to several hundred µm (Yamaguchi et al., 1988a; Duyckaerts et al., 2009). Diffuse plaques are also found in the cortex of cognitive normal aged individuals (Serrano-Pozo et al., 2011).

Neuritic plaques (FIGURE 1.1 A) are one of the major hallmarks of AD and can be detected with β-sheet staining dyes like Thioflavin-S and Congo Red (Serrano-Pozo et al., 2011). These extracellular Aβ plaques consist of highly aggregated fibrillary Aβ and are surrounded by swollen, degenerating axons and dendrites. In close proximity to neuritic

1 Introduction

plaques degenerated neurons, as well as astro- and microgliosis are observed (Holtzman et al., 2011; Selkoe, 2011). The density of amyloid fibrils varies as well as its size (10 to 120 μm) (Thomas and Fenech, 2007). Next to Aβ other proteins including APP, tau and ubiquitin are found in neuritic plaques (Su et al., 1998; Duyckaerts et al., 2009). Plaque pathology commonly starts in the neocortex and later progresses to the hippocampus, basal ganglia and cerebellum (Serrano-Pozo et al., 2011). In the end stages of the disease, neuritic plaques are also found in the brainstem and other subcortical structures (Thal et al., 2002; Aldwin and Gilmer, 2013). However, the plaque load correlates poorly with the cognitive decline and severity of the disease (Billings et al., 2005; Schaeffer et al., 2011; Villemagne et al., 2011).

1.3.2 Neurofibrillary Tangles

One of the neuropathological hallmarks of AD are neurofibrillary tangels (NFTs) that were first described by Alois Alzheimer as 'intraneuronal filamentous inclusions' within the perikaryal region of pyramidal neurons (Alzheimer, 1907). Subsequently, ultrastructural studies revealed that the major component of NFTs are paired helical filaments (PHFs), which are mainly constituted of hyperphosphorylated tau (Goedert and Spillantini, 2006;

Castellani et al., 2008).

Tau is a phosphoprotein that is abundant in neurons and produced in all nucleated cells (Duyckaerts et al., 2009; Galimberti and Scarpini, 2012). The normal function of tau is to bind to tubulin assembling and stabilizing microtubules (Goedert and Spillantini, 2006). However, in AD tau is abnormally hyperphosphorylated (FIGURE 1.2 A). The phosphorylation of tau reduces its microtuble binding activity and supports its

self-FIGURE 1.1 Amyloid plaques. (A) Photomicrograph of a neuritic amyloid plaque. Plaque is marked by a dashed circle. Arrow shows neurofibrillary tangle. Modified after Holtzman et al., 2011. (B) Diffuse Aβ plaques. Modified after Duyckarts et al., 2009. Reprinted with permission of the copyright holder.

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aggregation forming PHFs in cell bodies and dystrophic neurites (Alonso et al., 1996;

Holtzman et al., 2011).

The relevance and contribution of tau dysfunction to the pathogenesis of AD remains unclear. It is well established that the tau pathology appear later in the progression of AD than Aβ deposition (Galimberti and Scarpini, 2012). However, neurofibrillary tangles, unlike the plaque pathology, correlate better with the severity of cognitive deficits (Holtzman et al., 2011).

1.3.3 Inflammation

Inflammatory processes are another pathological characteristic of AD (FIGURE 1.2 B).

Microglia, astrocytes, the complement system as well as cytokines and chemokines are involved in the inflammatory reaction of the brain. Activated astrocytes and microglia are found in close proximity to neuritic plaques in AD, suggesting that Aβ is a major trigger of glial activation (Itagaki et al., 1989; Pike et al., 1995a; Krause and Müller, 2010). Following activation, glial cells produce proinflammatory signal molecules including complement molecules as well as cytokines and chemokines (Tuppo and Arias, 2005; Rubio-Perez and Morillas-Ruiz, 2012)

FIGURE 1.2 Tau and inflammation. (A) Hyperphosphorylated tau accumulation in neuronal cell bodies. Modified after Holtzman et al., 2011. (B) Confocal image of astrocyted labeled with GFAP (green) and plaques (red). Modified after Verkhratsky et al., 2010. Reprinted with permission of the copyright holder.

1 Introduction

1.3.4 Neuron loss

In addition to plaques and tangles, neuron loss is a main pathological hallmark of AD.

Areas that are particular affected by neuron loss are the pyramidal layers of the hippocampus, the layer II of the entorhinal cortex, and some areas of the temporal, parietal and frontal neocortex (Holtzman et al., 2011; Serrano-Pozo et al., 2011). For example, stereology showed a significant neuron loss in the entorhinal cortex (EC) of patients with very mild AD (Gomez-Isla et al., 1996). While no neuron loss was observed in the CA1 in preclinical AD a profound neuron loss was reported in AD patients (West et al., 2004). Early studies suggested a correlation between the number of NFT in a region and the loss of neurons within the same region (Cras et al., 1995). Using unbiased stereology, it could be shown that the neuron loss in the superior temporal sulcus in fact partly correlates with the formation of NFT but exceeds it eminently. Strikingly, more than 50 % of neurons in the superior temporal sulcus are lost in patients with AD (Gomez-Isla et al., 1997). Most recent reports suggest that intraneuronal or oligomeric Aβ are instead crucial for cell death and neuron loss in AD (Bayer and Wirths, 2011; Larson and Lesné, 2012).

Cortical atrophy, that is mainly caused by neuron loss, is the most evident macroscopic characteristic of AD. Atrophy affects mainly the hippocampus, amygdale and entorhinal cortex and can be measured by MRI (Bottino et al., 2002). AD can be diagnosed with 80 to 90 % accuracy through hippocampal atrophy measured by MRI (Jagust, 2006). Hippocampal atrophy also allows to predict the progression from MCI to AD to a certain degree (Jack et al., 2005).

Synapse loss also contributes to the cortical atrophy of the AD brain. The number of lost synapses exceeds the decrease of neurons in the cortical area. Therefore, it can be assumed that synapse loss occurs before neuron loss (Serrano-Pozo et al., 2011).

Synapse loss is an early indicator of the pathological processes in AD. It could be shown that patients with mild AD have fewer synapses in the CA1 of the hippocampus than individuals with MCI or healthy controls (Scheff et al., 2007). Decreased synaptic density correlates directly with the severity of AD. Actually, synaptic density is a better correlate of cognitive decline than NFTs or neuron loss. (DeKosky and Scheff, 1990; Scheff et al., 1990; Scheff and Price, 1993; Ingelsson et al., 2004).